the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 06 Mar 2025
2D Seismic Imaging of the Koillismaa Layered Igneous Complex, North-Eastern Finland
Abstract. The Seismic and Electromagnetics Methods for Deep mineral exploration (SEEMS DEEP) project is associated with the Koillismaa Layered Igneous Complex (KLIC) in north-eastern Finland. The KLIC is characterized by a Bouguer positive gravity and magnetic anomaly zone connecting the two exposed ends of the KLIC i.e. the Koillismaa intrusion and the Näränkävaara intrusion. The KLIC has the potential to host several critical raw minerals, like nickel and cobalt which are included in the European Union’s critical raw material list. For this purpose, two regional seismic profiles were acquired to map the regional reflectivity in the area, constraining the large-scale information about the geological architecture of KLIC. Seismic imaging delineated reflectivity up to a depth of ~5–6 km with several reflective packages at various depths which may be representative of the presence of dykes, faults, and major lithological contacts present in the area. Several regional faults were also imaged. The top of the magma conduit associated with KLIC was successfully mapped with hints of fault-like events cross-cutting the intrusion revealing a more complex internal structure that was earlier assumed of as a single lithological unit. It was interpreted that a second magma conduit might exist between the Koillismaa intrusion and the Näränkävaara Intrusion. Results were compared against the available petrophysical data and a preliminary available geological model based on the density model of the gravity inversion with constraints from the drillhole data.
Competing interests: At least one of the (co-)authors is a member of the editorial board of Solid Earth. The peer-review process was guided by an independent editor, and the authors also have no other competing interests to declare.
Publisher's note: Copernicus Publications remains neutral with regard to jurisdictional claims made in the text, published maps, institutional affiliations, or any other geographical representation in this paper. While Copernicus Publications makes every effort to include appropriate place names, the final responsibility lies with the authors. Views expressed in the text are those of the authors and do not necessarily reflect the views of the publisher.-
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The requested preprint has a corresponding peer-reviewed final revised paper. You are encouraged to refer to the final revised version.
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CC1: 'Comment on egusphere-2025-496', Giacomo Medici, 11 Mar 2025
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AC3: 'Reply on CC1', Brij Singh, 25 Jun 2025
Along with the two anonymous reviewer, we are thankful to the community member for taking the effort to review our manuscript and providing us with the constructive comments. In the revised manuscript, we have considered those suggestions, e.g., a better structured description of the aim of the 2D seismic survey, more details on the geology of the area, a revised discussion section etc. We have revised the Figure 1, provided an enlarged Figure 7, and a compact Figure 8. We are once again thankful for his contribution.
Citation: https://doi.org/10.5194/egusphere-2025-496-AC3
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AC3: 'Reply on CC1', Brij Singh, 25 Jun 2025
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RC1: 'Comment on egusphere-2025-496', Anonymous Referee #1, 12 Apr 2025
The paper presents two new seismic profiles over the Koillismaa layered igneous complex, a prospective target for raw materials (Ni-PGE). The data is novel, and the area is of particular interest given the current demand for raw materials to support the transition to a green economy. Therefore, the paper is worth publishing. However, it currently has several weaknesses that must be addressed prior to publication. My main points are detailed below.
Curvelet denoising applied to prestack data:
Coherency enhancement methods are part of the standard toolbox in seismic data processing and are typically applied to sections at the end of the processing sequence. In this paper, however, the authors apply a curvelet-denoising tool to shot gathers. This is unusual and not part of the standard hardrock processing workflow. The main concern is that artefacts may be introduced into the data and final sections. Artefacts are clearly visible on the shot gathers presented in Figure 4c and 4f, before the first breaks (especially when zoomed in). While this part of the data is muted and does not affect processing, it is reasonable to assume that similar artefacts could have been introduced in the useful portions of the shot gathers as well. The data is challenging and requires strong processing to enhance reflections, but the curvelet-denoised data appears over-filtered. If possible, I strongly recommend using a less aggressive parametrization. At the very least, I suggest presenting sections without the curvelet denoising and comparing them to the ones shown in Figure 5. This would allow readers to assess the actual impact of this denoising approach on the final results.Geology of the area:
The section on the geology of the area is very sparse and should be expanded. Many elements shown in Figure 1a are not discussed in the text—especially the areas exposed on the western side of the figure. Conversely, some features are mentioned in the text without being properly introduced in the geology section (e.g., the Korpua iron ore). A brief overview of faulting in the area would also strengthen this section and provide important context for interpretation.Interpretation:
The interpretation feels hesitant. While many reflections are labelled and annotated, few are given a geological interpretation. The authors should attempt to interpret these reflections based on what is known about the KLIC area and from physical rock properties. Even if speculative, this would be a valuable starting point. I also recommend synthesizing all interpretation results into Figure 8, with proper identification of key features (e.g., Korpua iron ore, regional and local faults, second conduit). The current version of Figure 8 is useful only for the conduit.Additional comments and edits:
- Line 28:
“Therefore, it is required to explore newer and deeper targets to sustain the supply and demand equilibrium.”
→ I suggest replacing this with: “As a result, there is a growing need to explore new and deeper targets to maintain the balance between supply and demand.” - Line 30:
“…ultramafic igneous rocks hold great potential to fulfil this requirement (Ripley and Li, 2018).”
→ Please specify which raw materials are associated with ultramafic rocks (e.g., Ni, Cr, PGE, etc.). - Line 43:
“Petrophysical and lab studies were done on the collected core samples from the drillhole, supplemented by limited wireline-logging data (Heinonen et al., 2022; Nousiainen et al., 2022).”
→ This sentence should be moved to Section 2, as it is not particularly informative in the introduction. Alternatively, clarify how this work supports the inference of a conduit. - Line 55:
“…also serve as a good instrument”
→ Please replace with: “are also valuable” - Line 56:
“It is cheaper”
→ Please revise to: “Two-dimensional surveys are cheaper…” - Line 60:
“With this aim,”
→ The aim is not clearly defined in the preceding paragraphs. Please consider rephrasing or expanding earlier content to better establish the objective. - Line 69:
“…the magmatism that led to the breakup of one or more Archaean cratons”
→ Please revise to: “…the magmatism associated with an extensional phase that led to the breakup of Archean cratons.” - Line 83:
“…thus verifying the presence of a deepseated magma conduit system connecting the exposed intrusions of the complex”
→ What specific evidence supports the existence of a conduit system, rather than an additional intrusion possibly related to the two exposed ones? Could all features be part of a single layered intrusion subsequently broken into parts during regional deformation? More detail should be added to the geology section to clarify these points. - Line 92:
“a dominant frequency of 35 Hz.”
→ What is the rationale for selecting this dominant frequency? Please provide justification or context. - Line 95:
“These faults/fractures and lithology changes…”
→ Which specific faults and fractures are being referred to? Please be more precise. - Line 121 (in Data Acquisition section):
→ Could you include some information on surface conditions (e.g., soil type), road surface types, and topography? This would help readers understand the environment and the data acquisition constraints. - Line 130:
“The data is of better quality for shotpoints located in the southern and western end of the survey for which the firstbreak energy was visible for the full-offset range, otherwise, an effective offset of ~4 – 5 km is generally observed for the rest of the shots.”
→ Please revise to: “The data is of better quality for shotpoints located in the southern and western end of the survey where the first-break energy is visible for the full-offset range. For the remaining shotpoints, clear first breaks are generally limited to approximately 4–5 km.” - Line 136:
“…standard processing workflow applied to the hardrock Seismics”
→ It is unclear whether a standardized processing workflow for hardrock seismics exists. In any case, please provide relevant references to support this claim. - Table 2:
→ What is the rationale for using a floating datum at a constant elevation of 300 m when the final datum is also at 300 m? Please clarify.
→ Also, why is FX-decon used instead of curvelet denoising? - Line 175:
“we performed KPreSDM in 3D mode”
→ What is the rationale for using 3D mode given that the profiles are relatively straight? Only the eastern part of L2000 appears sufficiently crooked to potentially justify this approach. Please provide additional evidence or justification for using the 3D mode over 2D on these profiles. Also, what pre-processing steps were applied prior to the Kirchhoff prestack depth migration? - Line 193:
“Reflectors mapped between CDPs 725 – 1200 (SF2, SF3, SF4) and depth up to ~2 – 3km have an up-dip movement towards the surface.”
→ The phrase “up-dip movement” is unclear in this context. Could you please clarify what is meant here? Are you referring to the geometry or apparent dip of the reflectors? - Line 195:
“Compared to the KOSE2018 profile (Fig. 2), the new acquisition and processing produced abundant reflectivity…”
→ To what extent is this increased reflectivity due to the curvelet denoising applied on prestack data versus differences in acquisition parameters between the KOSE2018 and the new survey? This distinction is important and should be clarified. - Line 227:
“The top of the magma conduit was successfully imaged with reflections mapped until the depth of ~5 – 6 km.”
→ This statement is unclear and difficult to reconcile with sections. Are you referring to the top or to the outer shell of the conduit? According to Figure 5, the top appears to be at approximately 2 km depth. Also, which reflectors define the bottom of the conduit? Please clarify. - Line 228:
“The internal structure of the magma conduit seems more complex than its present understanding with hints of fault-like structures cross-cutting the intrusion.”
→ The phrase “hints of fault-like structures” is vague. What are the implications of faults within the conduit? Additionally, the reflector interpreted as the top of the conduit appears discontinuous and cross-cut (e.g., it does not appear like not a smooth surface), which further supports the idea of internal complexity. This should be discussed more clearly. - Line 229:
“The obtained results were correlated with the initial (i.e. pre-seismic) geological model.”
→ Please provide more details about this geological model. Is it the surface shown in Figure 8? How was it derived—through modeling, potential field inversion, or another method? Clarification is needed. - Line 247:
“…but this higher package of reflectivity could be associated with the Korpua Iron Ore Intrusion (Makkonen, 1972).”
→ What is the Korpua Iron Ore, and what is its orientation and location relative to the profile? I suggest introducing the Korpua Iron ore in the geology section since it is a feature discussed in the interpretation. Additionally, what evidence or inferences support this interpretation? - Line 258:
“It can be inferred that there might be a second magma conduit between the exposed Koillismaa intrusion and Näränkävaara intrusion of the KLIC.”
→ Where is this second conduit visible in the seismic sections? What evidence or geological inference supports this interpretation? - Line 280:
“hints of fault-like structures…”
→ This phrase is ambiguous. Please clarify what these features are. - Line 280:
“It is interpreted that there might be a second magma conduit between the exposed Koillismaa intrusion and Näränkävaara intrusion of the KLIC.”
→ Again, where exactly is this second conduit located in the seismic data? Please point to the relevant features in the sections. - Line 283:
“This will tremendously help build a more detailed geological model of the Koillismaa area.”
→ Possibly—but how will this be achieved if most of the reflectors remain uninterpreted? Clarification or a more cautious statement may be warranted. - Figure 1:
→ Please add a legend for the geological units shown in 1a. - Figure 2:
→ Indicate the location of this profile on Figure 1.
→ Increase the size of the borehole symbol—it is currently too small.
→ Add a legend for the lithologies intersected in the borehole.
→ The color scheme used for rock types does not match that of Figure 3. Please harmonize these.
→ Also, why does the amplitude colorbar only show positive values? Seismic data usually consists of both peaks and troughs—please explain. - Figure 8:
→ The geological model is difficult to interpret. Figure 8a suggests a 3D surface, while 8b appears entirely contained within profile L1000, suggesting it represents the outer shell rather than just the top of the conduit. Please clarify.
→ Also, specify what the red arrows represent in the figure caption. - Final comments:
→ The manuscript would benefit from a general language revision. The English is generally understandable but could be improved in several places for clarity and flow.
Citation: https://doi.org/10.5194/egusphere-2025-496-RC1 -
AC1: 'Reply on RC1', Brij Singh, 25 Jun 2025
First of all, we are very thankful to the reviewer for dedicating one's time and effort for reviewing the manuscript and providing us with the detailed comments. This has helped us enormously in revising our manuscript with more information. All the relevant textual changes made in the revised manuscript has been highlighted in red. Below we provide the one-to-one response to all the points raised by the reviewer. We hope we had been able to satisfactorily address all of them. They are as below:
Curvelet denoising applied to prestack data:
Coherency enhancement methods are part of the standard toolbox in seismic data processing and are typically applied to sections at the end of the processing sequence. In this paper, however, the authors apply a curvelet-denoising tool to shot gathers. This is unusual and not part of the standard hardrock processing workflow. The main concern is that artefacts may be introduced into the data and final sections. Artefacts are clearly visible on the shot gathers presented in Figure 4c and 4f, before the first breaks (especially when zoomed in). While this part of the data is muted and does not affect processing, it is reasonable to assume that similar artefacts could have been introduced in the useful portions of the shot gathers as well. The data is challenging and requires strong processing to enhance reflections, but the curvelet-denoised data appears over-filtered. If possible, I strongly recommend using a less aggressive parametrization. At the very least, I suggest presenting sections without the curvelet denoising and comparing them to the ones shown in Figure 5. This would allow readers to assess the actual impact of this denoising approach on the final results.Response: Thank you very much for your comment. Yes, it is rightly pointed out that coherency enhancements are usually applied at the end of the processing sequence. We initially followed the same approach but it did not produce any significant improvements without very aggressive thresholding of the curvelet coefficients – which in turn was damaging the signal and introducing artefacts. This indicates that the characteristics of the scattered energy in the final stacked section were difficult to separate from some of the migrated signals of weak coherency. Therefore, we attempted the approach of curvelet filtering on the pre-processed shot gathers before stacking. A similar approach was presented by Górszczyk et al. (2015) for pre-stack CIGs conditioning which was further used for the moveout picking for reflection traveltime tomography. This approach helped us in enhancing the coherency of the signals which produced more coherent depth images.
Reference: Górszczyk, A., M. Cyz, and M. Malinowski. "Improving depth imaging of legacy seismic data using curvelet-based gather conditioning: A case study from Central Poland." Journal of Applied Geophysics 117 (2015): 73-80.
We acknowledge that there are artefacts visible in the shot gathers due to curvelet filtering. As correctly pointed out that the data is challenging, parameterization for curvelet filtering was chosen such that it effectively removes the incoherent noise and enhances the signal-to-noise ratio. We tested various levels of thresholding to avoid damaging the coherent signals. In fact, we used relatively mild filtering in this case to minimize the problem of artefacts while improving the signal coherency of the weak reflections.
Although the curvelet filtering was applied before stacking, any of the introduced artefacts were cancelled during the stacking of the CIG gathers. As a result, we did not observe any artefacts due to the curvelet filtering on the final migrated stacks. This is because their energy is relatively low with respect to the coherent signal, as well as because they are introduced in very different (random) time-offset positions from one gather to another. Hence, in contrast to coherent reflections, they stacked destructively. A comparison of migrated stacks, both pre-stack & post-stack, with and without curvelet filtering is provided in the attached supplement document.
Lastly, as curvelet filtering, or data processing per se was not the main highlight of the article, and we used curvelet-denoising merely as a signal enhancement tool, therefore we did not provide any comparison for the same. An article on the application of curvelet filtering on hardrock seismic data is cited (Górszczyk et al., 2015) [Row 178].
Geology of the area:
The section on the geology of the area is very sparse and should be expanded. Many elements shown in Figure 1a are not discussed in the text—especially the areas exposed on the western side of the figure. Conversely, some features are mentioned in the text without being properly introduced in the geology section (e.g., the Korpua iron ore). A brief overview of faulting in the area would also strengthen this section and provide important context for interpretation.Response: Thank you very much for this comment. We have now simplified Fig. 1a and 1b, and have significantly expanded the geology of the area in more detail taking note of all the suggestions that have been made. More information on the regional geology, structural setting of the KLIC and associated mineralization has been added in this section. [Row 76-98]
The term ‘Korpua Intrusion’ has been altogether removed from the manuscript for better clarity as it was part of the same Banded Iron Formation (BIF) present on the southern end of the Koillismaa Deep Anomaly. [Row 91-93]
Interpretation:
The interpretation feels hesitant. While many reflections are labelled and annotated, few are given a geological interpretation. The authors should attempt to interpret these reflections based on what is known about the KLIC area and from physical rock properties. Even if speculative, this would be a valuable starting point. I also recommend synthesizing all interpretation results into Figure 8, with proper identification of key features (e.g., Korpua iron ore, regional and local faults, second conduit). The current version of Figure 8 is useful only for the conduit.Response: We acknowledge that the article lacked a geological explanation of many reflectors delineated in the depth image. Therefore, we have completely revised the section 6 ‘Interpretation and discussion’ with more focus on the interpretation, structural geology and mineralization. Figures 8a and 8b have been combined into a single image with discussions on the mapping of magma conduit, its internal reflectivity and other reflectors that are observed in the seismic image. [Row 253-304]
Additional comments and edits:
Line 28:
“Therefore, it is required to explore newer and deeper targets to sustain the supply and demand equilibrium.”
→ I suggest replacing this with: “As a result, there is a growing need to explore new and deeper targets to maintain the balance between supply and demand.”Response: This sentence has been modified [Rows 32-33].
Line 30:
“…ultramafic igneous rocks hold great potential to fulfil this requirement (Ripley and Li, 2018).”
→ Please specify which raw materials are associated with ultramafic rocks (e.g., Ni, Cr, PGE, etc.).Response: This information has been added [Rows 34-35]. Also, more information related to this is provided in Rows 36-39.
Line 43:
“Petrophysical and lab studies were done on the collected core samples from the drillhole, supplemented by limited wireline-logging data (Heinonen et al., 2022; Nousiainen et al., 2022).”
→ This sentence should be moved to Section 2, as it is not particularly informative in the introduction. Alternatively, clarify how this work supports the inference of a conduit.Response: This sentence has been removed from the introduction section.
The inference of the magma conduit has been mainly based on the drilling information and the age determination of the mafic-ultramafic lithologies encountered during the drilling. The latter information has now been added. [Rows 108-110]
Line 55:
“…also serve as a good instrument”
→ Please replace with: “are also valuable”Response: This sentence has been modified [Row 58].
Line 56:
“It is cheaper”
→ Please revise to: “Two-dimensional surveys are cheaper…”Response: This sentence has been modified [Row 59].
Line 60:
“With this aim,”
→ The aim is not clearly defined in the preceding paragraphs. Please consider rephrasing or expanding earlier content to better establish the objective.Response: The paragraph has been expanded with a more structured description of the aim of the 2D seismic survey. [Rows 66-72]
Line 69:
“…the magmatism that led to the breakup of one or more Archaean cratons”
→ Please revise to: “…the magmatism associated with an extensional phase that led to the breakup of Archean cratons.”Response: This sentence has been modified [Row 75].
Line 83:
“…thus verifying the presence of a deepseated magma conduit system connecting the exposed intrusions of the complex”
→ What specific evidence supports the existence of a conduit system, rather than an additional intrusion possibly related to the two exposed ones? Could all features be part of a single layered intrusion subsequently broken into parts during regional deformation? More detail should be added to the geology section to clarify these points.Response: Thank you very much for this comment. We acknowledge that limited information was provided on this. Therefore, this section has been significantly expanded. [Rows 76-98]
Line 92:
“a dominant frequency of 35 Hz.”
→ What is the rationale for selecting this dominant frequency? Please provide justification or context.Response: The Ricker wavelet has been used to produce the synthetic seismogram shown in Figure 3d. The peak frequency of 35 Hz is chosen so that the seismic response from the key geological units observed in the borehole can be generated to verify whether those lithologies will be detected in the surface seismic study. For example, at this frequency we can easily see the response for the impedance contrast observed for the events shown by black arrows in Fig. 3b. As the highest frequency in our survey goes up to 160 Hz compared to 35 Hz dominant frequency Ricker wavelet meaning these rocktypes should produce a clear seismic signal at the surface. This information has been added [Rows 113-114].
Line 95:
“These faults/fractures and lithology changes…”
→ Which specific faults and fractures are being referred to? Please be more precise.Response: Information on these specific events is now provided [Rows 120-122]. These events are also indicated by black arrows in Figure 3b, which was imaged in the walkaway VSP survey.
Line 121 (in Data Acquisition section):
→ Could you include some information on surface conditions (e.g., soil type), road surface types, and topography? This would help readers understand the environment and the data acquisition constraints.Response: The acquisition mainly took place along the existing gravel roads. We have moved this information to the start of the paragraph [Rows 149-150]. Other than that, only the elevation changes was the other main component that had been mentioned in Table 1 (last row). For more clarification, we have simplified Fig. 1b, i.e. the topographic map. The surrounding lakes and swamps in the surrounding area are now distinctly visible.
Line 130:
“The data is of better quality for shotpoints located in the southern and western end of the survey for which the firstbreak energy was visible for the full-offset range, otherwise, an effective offset of ~4 – 5 km is generally observed for the rest of the shots.”
→ Please revise to: “The data is of better quality for shotpoints located in the southern and western end of the survey where the first-break energy is visible for the full-offset range. For the remaining shotpoints, clear first breaks are generally limited to approximately 4–5 km.”Response: This sentence has been revised. [Rows 157-159]
Line 136:
“…standard processing workflow applied to the hardrock Seismics”
→ It is unclear whether a standardized processing workflow for hardrock seismics exists. In any case, please provide relevant references to support this claim.Response: The time-domain processing of hardrock seismic data is pretty straightforward and has been demonstrated in previous 2D case studies (semi-)regional scale. We have added relevant citations. [Row 164]
Table 2:
→ What is the rationale for using a floating datum at a constant elevation of 300 m when the final datum is also at 300 m? Please clarify.
→ Also, why is FX-decon used instead of curvelet denoising?Response: Thank you very much for pointing it out. This has now been corrected in the main text [Row 172] and also in Table 2. As mentioned earlier, curvelet denoising was applied to the pre-stack shot gathers, while FX-decon was applied post-stacking to enhance the coherent signal. It further helped us in obtaining a more focussed image.
Line 175:
“we performed KPreSDM in 3D mode”
→ What is the rationale for using 3D mode given that the profiles are relatively straight? Only the eastern part of L2000 appears sufficiently crooked to potentially justify this approach. Please provide additional evidence or justification for using the 3D mode over 2D on these profiles. Also, what pre-processing steps were applied prior to the Kirchhoff prestack depth migration?Response: Yes, it is true that the profiles are relatively straight for the most part but are crooked towards the end (eastern end of profile L2000 and also northern end of profile L1000). Interpretation of 2D data often suffers from the inability to distinguish reflections originating outside the image plane. Given the regional length of the profiles, a narrow 3D KPreSDM accommodates out-of-plane reflections by effectively positioning them at their true positions. It also better handles the complex geological setting as it accounts for changes in velocity both vertically and horizontally compared to time-domain imaging which only accounts for changes in velocity vertically. Relevant citations are provided. [Row 205]
No additional preprocessing was applied to the data for KPreSDM; the same preprocessed data used for DMO-PoSTM was used. This information is now mentioned in the main text [Row 210]
Line 193:
“Reflectors mapped between CDPs 725 – 1200 (SF2, SF3, SF4) and depth up to ~2 – 3km have an up-dip movement towards the surface.”
→ The phrase “up-dip movement” is unclear in this context. Could you please clarify what is meant here? Are you referring to the geometry or apparent dip of the reflectors?Response: Here, we simply meant the direction of the reflectors which is in the updip direction towards the surface. This has been corrected. [Rows 223-224]
Line 195:
“Compared to the KOSE2018 profile (Fig. 2), the new acquisition and processing produced abundant reflectivity…”
→ To what extent is this increased reflectivity due to the curvelet denoising applied on prestack data versus differences in acquisition parameters between the KOSE2018 and the new survey? This distinction is important and should be clarified.Response: As mentioned earlier, curvelet-denoising was merely used as a signal enhancement tool (see the comparison in the attached document). The abundant reflectivity observed in the depth images is mainly due to the well-optimised acquisition. We agree that more information is required and it is a better fit in the discussion section rather than the results section. Therefore, we have moved this to section 6. [Rows 258-263]
Line 227:
“The top of the magma conduit was successfully imaged with reflections mapped until the depth of ~5 – 6 km.”
→ This statement is unclear and difficult to reconcile with sections. Are you referring to the top or to the outer shell of the conduit? According to Figure 5, the top appears to be at approximately 2 km depth. Also, which reflectors define the bottom of the conduit? Please clarify.Response: Thank you very much for this comment. This section has now been completely revised.
Regarding the magma conduit, yes, we again agree that it was a bit confusing and to remove this ambiguity, we have removed the term ‘top’ while discussing the magma conduit in the manuscript. We now clearly state that the top of the arcuate reflectivity that is observed in profile L1000 [Row 217-218], are representative of the mafic cumulates exposed in the drillhole (shown in Fig. 3e) and discussed in rows 270-272.
Regarding the bottom of the conduit, intuitively, it appears that the reflector marked as DR2 and its adjacent cross-dipping reflector in Fig. 5 constitute the bottom of the magma conduit. As the drillhole does not reach this depth, or in the absence of any other geophysical or direct evidence – we could not verify at this stage whether it is the bottom of the magma conduit. Therefore, we have avoided interpreting the bottom of the magma conduit at this stage; it is only speculative. More detailed interpretation of the 2D seismic results along with 3D seismics and electromagnetics study which are also alos part of the project are underway.
Line 228:
“The internal structure of the magma conduit seems more complex than its present understanding with hints of fault-like structures cross-cutting the intrusion.”
→ The phrase “hints of fault-like structures” is vague. What are the implications of faults within the conduit? Additionally, the reflector interpreted as the top of the conduit appears discontinuous and cross-cut (e.g., it does not appear like not a smooth surface), which further supports the idea of internal complexity. This should be discussed more clearly.Response: Yes, we acknowledge that the article had lacked proper interpretation of these events. Therefore we have revised the entire section 6.
Line 229:
“The obtained results were correlated with the initial (i.e. pre-seismic) geological model.”
→ Please provide more details about this geological model. Is it the surface shown in Figure 8? How was it derived—through modeling, potential field inversion, or another method? Clarification is needed.Response: The information on the geological model has been provided at various places in the manuscript [Rows 24-26, 122-123, 294-295].
Yes, the modelled magma conduit shown in Fig. 8 is a 3D surface, it is now clearly mentioned in the main text. [Row 294]
Line 247:
“…but this higher package of reflectivity could be associated with the Korpua Iron Ore Intrusion (Makkonen, 1972).”
→ What is the Korpua Iron Ore, and what is its orientation and location relative to the profile? I suggest introducing the Korpua Iron ore in the geology section since it is a feature discussed in the interpretation. Additionally, what evidence or inferences support this interpretation?Response: Thank you very much for pointing this out. In the revised manuscript, we have removed the term ‘Korpua Iron ore’ as it is part of the same banded iron formation (BIF) shown in Fig. 1c, which has a specific name for this part of the BIF only. We have added more geological information on BIF in section 2 [Row 91-93] and in section 6 [Row 280-292]. Accordingly, Fig 1c has been updated where Korpua Iron is replaced with BIF and is clearly marked by red arrows.
Line 258:
“It can be inferred that there might be a second magma conduit between the exposed Koillismaa intrusion and Näränkävaara intrusion of the KLIC.”
→ Where is this second conduit visible in the seismic sections? What evidence or geological inference supports this interpretation?Response: Yes, we acknowledge that this information is not supported by the depth images, therefore we have removed this claim from the manuscript. The justification for it was related to the overall geological setting of the KLIC with respect to the Tornio-Naarankavaraa belt and the observed gravity high anomaly. Therefore, only a speculative description was mentioned in the manuscript. But as the second conduit is not visible in the seismic section, we have removed it from the article.
Line 280:
“hints of fault-like structures…”
→ This phrase is ambiguous. Please clarify what these features are.Response: This phrase has been altogether removed from the article due to its ambiguity.
Line 280:
“It is interpreted that there might be a second magma conduit between the exposed Koillismaa intrusion and Näränkävaara intrusion of the KLIC.”
→ Again, where exactly is this second conduit located in the seismic data? Please point to the relevant features in the sections.Response: As pointed out earlier, this claim has been removed due to the lack of evidence in the seismic depth images.
Line 283:
“This will tremendously help build a more detailed geological model of the Koillismaa area.”
→ Possibly—but how will this be achieved if most of the reflectors remain uninterpreted? Clarification or a more cautious statement may be warranted.Response: We now believe that the more detailed interpretation provided in section 6 validates this statement. We have removed the word ‘tremendously’. [Row 329]
Figure 1:
→ Please add a legend for the geological units shown in 1a.Response: Fig 1 has been significantly revised with a more simplified geology and topographic map. All the legends have been described. Accordingly, relevant information has been added in the main text in section 2.
Figure 2:
→ Indicate the location of this profile on Figure 1.
→ Increase the size of the borehole symbol—it is currently too small.
→ Add a legend for the lithologies intersected in the borehole.
→ The color scheme used for rock types does not match that of Figure 3. Please harmonize these.
→ Also, why does the amplitude colorbar only show positive values? Seismic data usually consists of both peaks and troughs—please explain.Response: KOSE2018 almost entirely overlaps the profile L1000; therefore it is not shown in Fig. 1b or 1c. Instead, the overlapping has been shown in the top panel of Fig. 2 (KOSE2018 profile in red colour overlaps the profile L1000 in black). The same information has been provided in the description of Fig. 2. Figure 2 has been modified with the addition of depth labels and a borehole with lithologies having the same colorscheme as shown in Fig 3e. It is mentioned that the legends for the borehole lithologies are mentioned in Fig. 3 [Row 140]
Yes, the seismic data indeed has both peaks and troughs. Here, to make the reflectors more distinctly visible, the scale has been skewed towards black color as +ve (peaks), and white as troughs (almost around the zero).
Figure 8:
→ The geological model is difficult to interpret. Figure 8a suggests a 3D surface, while 8b appears entirely contained within profile L1000, suggesting it represents the outer shell rather than just the top of the conduit. Please clarify.
→ Also, specify what the red arrows represent in the figure caption.Response: Yes, it is rightly pointed out that it was not mentioned whether it is a 3D surface of the whole magma conduit and not just the top of the conduit. This section has been altogether revised for a much clearer description. [Rows 294 – 304]
Red arrows mark the internal reflectivity observed within the magma conduit. It has been added to the caption of Fig. 8 [Row 311-312]. Also, Figures 8a and 8b are combined into a single figure.
Final comments:
→ The manuscript would benefit from a general language revision. The English is generally understandable but could be improved in several places for clarity and flow.Response: In the revision, we have simplified several sentences throughout the manuscript for a smoother flow of information.
- Line 28:
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RC2: 'Comment on egusphere-2025-496', Anonymous Referee #2, 28 May 2025
This work deals with processing of two crosscutting reflection seismic profiles acquired as part of a Seismic and Electromagnetics Methods for Deep mineral exploration (SEEMS DEEP) project above a known gravity/magnetic anomaly of the Koillismaa Layered Igneous Complex (KLIC) in north-eastern Finland. The work is well introduced and elaborated, processing steps applied are relevant and aim at improving the coherency and continuity of the events seen on raw seismic data, along with showing others revealed only after processing. Both pre- and post-stack imaging strategies employed shed new light on the geological complexity in the area, especially internal reflectivity within previously interpreted extent of the Koillismaa intrusion. This is obvious both from the current results and when comparing with the results of earlier surveys conducted at the same site. The authors also suggest a possibility of various magma conduits and/or diabase veins as the source of reflectivity.
Generally, I have only a few comments which are rather of cosmetic form, and one major one for the authors. The major one follows. As one of the main aims of SEEMS DEEP and the work presented itself, as the authors are claiming, is to aid in understanding of the critical raw materials relevant for the EU, why is there no discussion of the results of this study in that respects? How the abstract and Introduction are formulated, I was expecting to see a discussion of the results obtained in a mineral systems context where the source of the magnetic/gravity anomaly of the Koillismaa intrusion, or the iron band formation would be addressed, along with connection of the petrophysical information as encountered in the drillhole with the deeper source. At present, this part is entirely absent. As the drillhole does not reach the deeper reflections, even a brief speculative discussion part placing the results in a larger context, similar to as applied in the mineral systems concept, might suffice.
Minor comments follow:
Rows 14-15: How this sentence is formulated, it is emphasized that the main goal behind the survey is to understand the mineralization potential of KLIC, but the main results are on the geological context, not the mineralization. Perhaps adding to this sentence something in th form of: "hence it is important to understand the geological setting that might have resulted in emplacement of mineralization in a broader mineral systems context "
Row 63: Perhaps "driven" might be more appropriate instead of "playground"
Rows 81-83. Here, the word "diamond" is redundant and confusing, hence would be better to remove it. Would also be good to add the final borehole depth as it is unclear at present.
Row 91: Given the sweep parameters of the survey being 20-160 Hz, could you elaborate on the reasoning to use a wavelet centered at 35 Hz for the synthetics? Is it related to the consistency across various datasets acquired previously at the same site or some other effect?
Row 105, Figure 1 comments. It is very difficult to distinguish between the color code used for Korpua Intrusion and the dykes. Maybe some different colors could improve this? Also, as a non-geologist, the color of the Archean basement is shown but the figure also shows a texture that is absent in the legend, besides the "pink" unit that is not in the legend at all...
Row 110, Figure2: Perhaps it would be beneficial to the reader to show a depth/time axis.
Table 1: Would be good to know also the peak force used. Also, a comma between "20-160 Hz" and "+1dB" might be good to distinguish sweep bandwidth versus type.
Rows 138-139: A curiosity driven question. Are these CDP bins or they are common midpoint (CMP) bins at this stage?
Row 148: Are not surface wave and ground roll the same thing? Perhaps the authors meant shear-wave and ground roll?
Row 161: What does the abbreviation PoSTM stands for?
Rows 185-186: I guess this is a combined effect of the lower fold at shallower depths due to 15/30 m shot/rec spacing, tougher with the effect of spherical divergence correction applied? Would be happy to hear your comment on this.
Row 220, Figure 6: To me, it feels like the time grid lines just make the interpretation more complicated, hence might be better to remove them.
Rows 247-249: Perhaps this might be a good place to discuss the mineralization potential of this horizon or the KLIC in general, as this is the scientific question raised by the SEEMS DEEP and this work...
Rows 253-254: Could this also be the source of mineralization and what kind of critically important EU minerals can be expected or were mapped within the core samples in the borehole?
Citation: https://doi.org/10.5194/egusphere-2025-496-RC2 -
AC2: 'Reply on RC2', Brij Singh, 25 Jun 2025
Firstly, we are very thankful to the reviewer for dedicating one's time and effort for reviewing the manuscript and providing us with the comments. Below we provide the one-to-one response to all the points raised by the reviewer. All the relevant textual changes made in the revised manuscript has been highlighted in red. We hope we had been able to satisfactorily address all of them. They are as below:
Comments from Reviewer-2
This work deals with processing of two crosscutting reflection seismic profiles acquired as part of a Seismic and Electromagnetics Methods for Deep mineral exploration (SEEMS DEEP) project above a known gravity/magnetic anomaly of the Koillismaa Layered Igneous Complex (KLIC) in north-eastern Finland. The work is well introduced and elaborated, processing steps applied are relevant and aim at improving the coherency and continuity of the events seen on raw seismic data, along with showing others revealed only after processing. Both pre- and post-stack imaging strategies employed shed new light on the geological complexity in the area, especially internal reflectivity within previously interpreted extent of the Koillismaa intrusion. This is obvious both from the current results and when comparing with the results of earlier surveys conducted at the same site. The authors also suggest a possibility of various magma conduits and/or diabase veins as the source of reflectivity.
Generally, I have only a few comments which are rather of cosmetic form, and one major one for the authors. The major one follows. As one of the main aims of SEEMS DEEP and the work presented itself, as the authors are claiming, is to aid in understanding of the critical raw materials relevant for the EU, why is there no discussion of the results of this study in that respects? How the abstract and Introduction are formulated, I was expecting to see a discussion of the results obtained in a mineral systems context where the source of the magnetic/gravity anomaly of the Koillismaa intrusion, or the iron band formation would be addressed, along with connection of the petrophysical information as encountered in the drillhole with the deeper source. At present, this part is entirely absent. As the drillhole does not reach the deeper reflections, even a brief speculative discussion part placing the results in a larger context, similar to as applied in the mineral systems concept, might suffice.
Response: Firstly, we are thankful to the reviewer for reviewing the article and for the constructive comments. We acknowledge the reviewer’s comment on the lack of discussion on the mineralization potential associated with KLIC, which can be interpreted based on the new seismic results obtained within the SEEMS DEEP project. Therefore, we have provided more information on the “geology of the area” in section 2 [Rows 76-98]. Now, there is a larger context provided related to the overall geological setting and the associated mineralisation (changes are made to Figure 1a accordingly). Also, the whole section 6 – ‘Interpretation and Conclusion’ is rewritten with more detailed information on the structural setting and discussion on the potential of the mineralisation based on seismic imaging [Rows 253-304]. Accordingly, the ‘Abstract’ and ‘Conclusion’ sections are also updated [Rows 14-24, Rows 323-327]. All the textual changes made in the revised manuscript are highlighted in red.
Minor comments follow:
Rows 14-15: How this sentence is formulated, it is emphasized that the main goal behind the survey is to understand the mineralization potential of KLIC, but the main results are on the geological context, not the mineralization. Perhaps adding to this sentence something in th form of: "hence it is important to understand the geological setting that might have resulted in emplacement of mineralization in a broader mineral systems context "
Response: We have revised this sentence [Rows 14-16], and more structured information on the main aim of the survey has been added in the ‘Introduction’ section [Rows 66-72]
Row 63: Perhaps "driven" might be more appropriate instead of "playground"
Response: In our opinion ‘playground’ is a better word to describe the various technical developments that took place in both electromagnetics and seismics within the SEEMS DEEP project. [Rows 64-66]
Rows 81-83. Here, the word "diamond" is redundant and confusing, hence would be better to remove it. Would also be good to add the final borehole depth as it is unclear at present.
Response: The word ‘diamond’ has been removed and information on the end depth of the borehole has been added [Rows 103-105].
Row 91: Given the sweep parameters of the survey being 20-160 Hz, could you elaborate on the reasoning to use a wavelet centered at 35 Hz for the synthetics? Is it related to the consistency across various datasets acquired previously at the same site or some other effect?
Response: The Ricker wavelet has been used to produce the synthetic seismogram shown in Figure 3d. The peak frequency of 35 Hz is chosen so that the seismic response from the key geological units observed in the borehole can be generated to verify whether those lithologies will be detected in the surface seismic study. For example, at this frequency we can easily see the response for the impedance contrast observed for the events shown by black arrows in Fig. 3b. As the highest frequency in our survey goes up to 160 Hz compared to 35 Hz dominant frequency Ricker wavelet meaning these rocktypes should produce a clear seismic signal at the surface. This information has been added [Rows 113-114].
Row 105, Figure 1 comments. It is very difficult to distinguish between the color code used for Korpua Intrusion and the dykes. Maybe some different colors could improve this? Also, as a non-geologist, the color of the Archean basement is shown but the figure also shows a texture that is absent in the legend, besides the "pink" unit that is not in the legend at all...
Response: Yes. Thank you very much for pointing this out. Figure 1 has been reproduced with more simplification of Figure 1a and a new Figure 1b. All the appropriate legends have been added to describe the geology. Also, the term ‘Korpua Intrusion’ has now been removed from the figure and the manuscript as this is the same as the banded iron formation (BIF) which runs at depth parallel to the KLIC (marked by red arrow in Fig. 1a). We realised that the specific name of ‘Korpua’ was a bit misleading as if it might be inferring to a different rock-type. The term ‘Korpua’ has been altogether removed from the article. It is now been confined to BIF (in section 2 [Row 91-93], and in section 6 [Row 280-292].). We have kept the same colorscheme for BIF and dykes, as one is a surface and other is a line, so it is easily distinguishable in our opinion.
Row 110, Figure2: Perhaps it would be beneficial to the reader to show a depth/time axis.
Response: Figure 2 has been modified with the addition of depth scale and the borehole lithology colorscheme is now the same as shown in Fig. 3e.
Table 1: Would be good to know also the peak force used. Also, a comma between "20-160 Hz" and "+1dB" might be good to distinguish sweep bandwidth versus type.
Response: Peak force information has been added to main text [Row 155], and Table 1 is corrected.
Rows 138-139: A curiosity driven question. Are these CDP bins or they are common midpoint (CMP) bins at this stage?
Response: Very often the terms, CDP and CMP are used interchangeably. For a flat reflector both are the same and for a dipping reflector CDP is a more accurate word. Therefore, we used the term “CDP”.
Row 148: Are not surface wave and ground roll the same thing? Perhaps the authors meant shear-wave and ground roll?
Response: Yes, thank you very much for pointing it out.. It is corrected in the article [Row 176].
Row 161: What does the abbreviation PoSTM stands for?
Response: Thanks again for pointing it out. The full form of the term is now added [Row 187].
Rows 185-186: I guess this is a combined effect of the lower fold at shallower depths due to 15/30 m shot/rec spacing, tougher with the effect of spherical divergence correction applied? Would be happy to hear your comment on this.
Response: The same processing has been applied to both profiles as they are acquired in close proximity over a similar geological setting and with the same acquisition parameters. We do not think that the lower fold at shallower depth and spherical divergence correction could have the effect that deep as our nominal CDP is ~7.5 m, it is sufficiently dense for hardrock seismics. If it were the case, the effect should have been more or less similar for both profiles which is not the case. Also, KPreSDM results have better focussing than the DMO-PoSTM results, so it is also a bit dependent on the imaging principle. We have observed similar results for 3D seismics as well which is currently in development for submission for journal publication. In my opinion, it is probably more related to the low impedance contrast between the overlying Archean Gneisses and the granite (see Fig. 3).
Row 220, Figure 6: To me, it feels like the time grid lines just make the interpretation more complicated, hence might be better to remove them.
Response: This point is a bit unclear to us. Do you mean the grid lines at depth or the red line used for the profile intersection? The grid lines at depth provide a better sense of depth to the readers when looking at the reflections far away from the axis. The intersection of the profiles is shown because in Figs 5 and 6, we are looking at 2D DMO-PoSTM results vs 3D PreSDM which is extracted along an inline (see Fig. 1c). It is important for the readers to have this information for a qualitative comparison. [Row 226]
Rows 247-249: Perhaps this might be a good place to discuss the mineralization potential of this horizon or the KLIC in general, as this is the scientific question raised by the SEEMS DEEP and this work...
Response: The section 6 has been completely rewritten. More information on the banded iron formation and the mineralization has been provided.
Rows 253-254: Could this also be the source of mineralization and what kind of critically important EU minerals can be expected or were mapped within the core samples in the borehole?
Response: A more detailed explanation on the mineralization potential has been added in section 6.
Regarding the borehole: the mineral system studies focused on Ni–Cu–PGE mineralization, and according to a visual log of the drill core, the mafic part of the core does not contain any indications of sulphide mineralization. The core only includes some trace amounts of Cu–Ni–sulphide minerals, and the Ni–Cu concentrations are therefore relatively low. According to geochemical analyses, the core is not PGE mineralized (Tirroniemi et al., 2024).
Citation: https://doi.org/10.5194/egusphere-2025-496-AC2
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AC2: 'Reply on RC2', Brij Singh, 25 Jun 2025
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EC1: 'Comment on egusphere-2025-496', Christopher Juhlin, 29 May 2025
Dear Authors,
We now have reviews from the two referees. Based on their reviews and my reading of the MS I judge you have moderate revision with additional review afterwards. When revising the paper please pay close attention to the general comments of the referees.
Best Regards,
Chris Juhlin
Citation: https://doi.org/10.5194/egusphere-2025-496-EC1
Interactive discussion
Status: closed
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CC1: 'Comment on egusphere-2025-496', Giacomo Medici, 11 Mar 2025
General comments
Very good geophysical research. Please, follow my specific comments to improve the manuscript.
Specific comments
Lines 49-52. “Geophysical surveys for mineral exploration have been routinely accomplished by potential field methods, electromagnetics, electrical methods....depths required to distinguish different lithologies. In comparison, seismic reflection surveys can provide higher-resolution images of the subsurface and have now emerged as an established method for deep mineral exploration in the hardrock environment”. Insert recent papers that discuss the use of electromagnetics, electrical methods and seismic surveys for deep exploration in the hardrock lithologies.
- Medici, G., Ling, F., Shang, J. 2023. Review of discrete fracture network characterization for geothermal energy extraction. Frontiers in Earth Science, 11, 1328397.
- Eberhart‐Phillips, D., Stanley, W. D., Rodriguez, B. D., Lutter, W.J. 1995. Surface seismic and electrical methods to detect fluids related to faulting. Journal of Geophysical Research: Solid Earth, 100(B7), 12919-12936.
Line 67. Clearly state the 3 to 4 specific objectives of your research by using numbers (e.g., i, ii, and iii).
Line 69. “Layered intrusions”. Please, describe in detail the geometries of these geological bodies. The geological scenario is currently unclear.
Lines 69-100. This section needs more detail on the regional stratigraphy.
Lines 69-100. Provide more detail on the presence of faults that I can observe in the geological maps.
Lines 225-262. A discussion should also incorporate literature to show that your research has advanced the field.
Lines 318-409. Improve the literature for either introduction or discussion.
Figures and tables
Figure 1a, c. Make evident the type of faults.
Figure 1b. What the contours represent? Unclear, specify in the caption.
Figure 1b. Increase the graphic resolution.
Figures 7 and 8. These are very important figures, you can make them larger.
Citation: https://doi.org/10.5194/egusphere-2025-496-CC1 -
AC3: 'Reply on CC1', Brij Singh, 25 Jun 2025
Along with the two anonymous reviewer, we are thankful to the community member for taking the effort to review our manuscript and providing us with the constructive comments. In the revised manuscript, we have considered those suggestions, e.g., a better structured description of the aim of the 2D seismic survey, more details on the geology of the area, a revised discussion section etc. We have revised the Figure 1, provided an enlarged Figure 7, and a compact Figure 8. We are once again thankful for his contribution.
Citation: https://doi.org/10.5194/egusphere-2025-496-AC3
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AC3: 'Reply on CC1', Brij Singh, 25 Jun 2025
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RC1: 'Comment on egusphere-2025-496', Anonymous Referee #1, 12 Apr 2025
The paper presents two new seismic profiles over the Koillismaa layered igneous complex, a prospective target for raw materials (Ni-PGE). The data is novel, and the area is of particular interest given the current demand for raw materials to support the transition to a green economy. Therefore, the paper is worth publishing. However, it currently has several weaknesses that must be addressed prior to publication. My main points are detailed below.
Curvelet denoising applied to prestack data:
Coherency enhancement methods are part of the standard toolbox in seismic data processing and are typically applied to sections at the end of the processing sequence. In this paper, however, the authors apply a curvelet-denoising tool to shot gathers. This is unusual and not part of the standard hardrock processing workflow. The main concern is that artefacts may be introduced into the data and final sections. Artefacts are clearly visible on the shot gathers presented in Figure 4c and 4f, before the first breaks (especially when zoomed in). While this part of the data is muted and does not affect processing, it is reasonable to assume that similar artefacts could have been introduced in the useful portions of the shot gathers as well. The data is challenging and requires strong processing to enhance reflections, but the curvelet-denoised data appears over-filtered. If possible, I strongly recommend using a less aggressive parametrization. At the very least, I suggest presenting sections without the curvelet denoising and comparing them to the ones shown in Figure 5. This would allow readers to assess the actual impact of this denoising approach on the final results.Geology of the area:
The section on the geology of the area is very sparse and should be expanded. Many elements shown in Figure 1a are not discussed in the text—especially the areas exposed on the western side of the figure. Conversely, some features are mentioned in the text without being properly introduced in the geology section (e.g., the Korpua iron ore). A brief overview of faulting in the area would also strengthen this section and provide important context for interpretation.Interpretation:
The interpretation feels hesitant. While many reflections are labelled and annotated, few are given a geological interpretation. The authors should attempt to interpret these reflections based on what is known about the KLIC area and from physical rock properties. Even if speculative, this would be a valuable starting point. I also recommend synthesizing all interpretation results into Figure 8, with proper identification of key features (e.g., Korpua iron ore, regional and local faults, second conduit). The current version of Figure 8 is useful only for the conduit.Additional comments and edits:
- Line 28:
“Therefore, it is required to explore newer and deeper targets to sustain the supply and demand equilibrium.”
→ I suggest replacing this with: “As a result, there is a growing need to explore new and deeper targets to maintain the balance between supply and demand.” - Line 30:
“…ultramafic igneous rocks hold great potential to fulfil this requirement (Ripley and Li, 2018).”
→ Please specify which raw materials are associated with ultramafic rocks (e.g., Ni, Cr, PGE, etc.). - Line 43:
“Petrophysical and lab studies were done on the collected core samples from the drillhole, supplemented by limited wireline-logging data (Heinonen et al., 2022; Nousiainen et al., 2022).”
→ This sentence should be moved to Section 2, as it is not particularly informative in the introduction. Alternatively, clarify how this work supports the inference of a conduit. - Line 55:
“…also serve as a good instrument”
→ Please replace with: “are also valuable” - Line 56:
“It is cheaper”
→ Please revise to: “Two-dimensional surveys are cheaper…” - Line 60:
“With this aim,”
→ The aim is not clearly defined in the preceding paragraphs. Please consider rephrasing or expanding earlier content to better establish the objective. - Line 69:
“…the magmatism that led to the breakup of one or more Archaean cratons”
→ Please revise to: “…the magmatism associated with an extensional phase that led to the breakup of Archean cratons.” - Line 83:
“…thus verifying the presence of a deepseated magma conduit system connecting the exposed intrusions of the complex”
→ What specific evidence supports the existence of a conduit system, rather than an additional intrusion possibly related to the two exposed ones? Could all features be part of a single layered intrusion subsequently broken into parts during regional deformation? More detail should be added to the geology section to clarify these points. - Line 92:
“a dominant frequency of 35 Hz.”
→ What is the rationale for selecting this dominant frequency? Please provide justification or context. - Line 95:
“These faults/fractures and lithology changes…”
→ Which specific faults and fractures are being referred to? Please be more precise. - Line 121 (in Data Acquisition section):
→ Could you include some information on surface conditions (e.g., soil type), road surface types, and topography? This would help readers understand the environment and the data acquisition constraints. - Line 130:
“The data is of better quality for shotpoints located in the southern and western end of the survey for which the firstbreak energy was visible for the full-offset range, otherwise, an effective offset of ~4 – 5 km is generally observed for the rest of the shots.”
→ Please revise to: “The data is of better quality for shotpoints located in the southern and western end of the survey where the first-break energy is visible for the full-offset range. For the remaining shotpoints, clear first breaks are generally limited to approximately 4–5 km.” - Line 136:
“…standard processing workflow applied to the hardrock Seismics”
→ It is unclear whether a standardized processing workflow for hardrock seismics exists. In any case, please provide relevant references to support this claim. - Table 2:
→ What is the rationale for using a floating datum at a constant elevation of 300 m when the final datum is also at 300 m? Please clarify.
→ Also, why is FX-decon used instead of curvelet denoising? - Line 175:
“we performed KPreSDM in 3D mode”
→ What is the rationale for using 3D mode given that the profiles are relatively straight? Only the eastern part of L2000 appears sufficiently crooked to potentially justify this approach. Please provide additional evidence or justification for using the 3D mode over 2D on these profiles. Also, what pre-processing steps were applied prior to the Kirchhoff prestack depth migration? - Line 193:
“Reflectors mapped between CDPs 725 – 1200 (SF2, SF3, SF4) and depth up to ~2 – 3km have an up-dip movement towards the surface.”
→ The phrase “up-dip movement” is unclear in this context. Could you please clarify what is meant here? Are you referring to the geometry or apparent dip of the reflectors? - Line 195:
“Compared to the KOSE2018 profile (Fig. 2), the new acquisition and processing produced abundant reflectivity…”
→ To what extent is this increased reflectivity due to the curvelet denoising applied on prestack data versus differences in acquisition parameters between the KOSE2018 and the new survey? This distinction is important and should be clarified. - Line 227:
“The top of the magma conduit was successfully imaged with reflections mapped until the depth of ~5 – 6 km.”
→ This statement is unclear and difficult to reconcile with sections. Are you referring to the top or to the outer shell of the conduit? According to Figure 5, the top appears to be at approximately 2 km depth. Also, which reflectors define the bottom of the conduit? Please clarify. - Line 228:
“The internal structure of the magma conduit seems more complex than its present understanding with hints of fault-like structures cross-cutting the intrusion.”
→ The phrase “hints of fault-like structures” is vague. What are the implications of faults within the conduit? Additionally, the reflector interpreted as the top of the conduit appears discontinuous and cross-cut (e.g., it does not appear like not a smooth surface), which further supports the idea of internal complexity. This should be discussed more clearly. - Line 229:
“The obtained results were correlated with the initial (i.e. pre-seismic) geological model.”
→ Please provide more details about this geological model. Is it the surface shown in Figure 8? How was it derived—through modeling, potential field inversion, or another method? Clarification is needed. - Line 247:
“…but this higher package of reflectivity could be associated with the Korpua Iron Ore Intrusion (Makkonen, 1972).”
→ What is the Korpua Iron Ore, and what is its orientation and location relative to the profile? I suggest introducing the Korpua Iron ore in the geology section since it is a feature discussed in the interpretation. Additionally, what evidence or inferences support this interpretation? - Line 258:
“It can be inferred that there might be a second magma conduit between the exposed Koillismaa intrusion and Näränkävaara intrusion of the KLIC.”
→ Where is this second conduit visible in the seismic sections? What evidence or geological inference supports this interpretation? - Line 280:
“hints of fault-like structures…”
→ This phrase is ambiguous. Please clarify what these features are. - Line 280:
“It is interpreted that there might be a second magma conduit between the exposed Koillismaa intrusion and Näränkävaara intrusion of the KLIC.”
→ Again, where exactly is this second conduit located in the seismic data? Please point to the relevant features in the sections. - Line 283:
“This will tremendously help build a more detailed geological model of the Koillismaa area.”
→ Possibly—but how will this be achieved if most of the reflectors remain uninterpreted? Clarification or a more cautious statement may be warranted. - Figure 1:
→ Please add a legend for the geological units shown in 1a. - Figure 2:
→ Indicate the location of this profile on Figure 1.
→ Increase the size of the borehole symbol—it is currently too small.
→ Add a legend for the lithologies intersected in the borehole.
→ The color scheme used for rock types does not match that of Figure 3. Please harmonize these.
→ Also, why does the amplitude colorbar only show positive values? Seismic data usually consists of both peaks and troughs—please explain. - Figure 8:
→ The geological model is difficult to interpret. Figure 8a suggests a 3D surface, while 8b appears entirely contained within profile L1000, suggesting it represents the outer shell rather than just the top of the conduit. Please clarify.
→ Also, specify what the red arrows represent in the figure caption. - Final comments:
→ The manuscript would benefit from a general language revision. The English is generally understandable but could be improved in several places for clarity and flow.
Citation: https://doi.org/10.5194/egusphere-2025-496-RC1 -
AC1: 'Reply on RC1', Brij Singh, 25 Jun 2025
First of all, we are very thankful to the reviewer for dedicating one's time and effort for reviewing the manuscript and providing us with the detailed comments. This has helped us enormously in revising our manuscript with more information. All the relevant textual changes made in the revised manuscript has been highlighted in red. Below we provide the one-to-one response to all the points raised by the reviewer. We hope we had been able to satisfactorily address all of them. They are as below:
Curvelet denoising applied to prestack data:
Coherency enhancement methods are part of the standard toolbox in seismic data processing and are typically applied to sections at the end of the processing sequence. In this paper, however, the authors apply a curvelet-denoising tool to shot gathers. This is unusual and not part of the standard hardrock processing workflow. The main concern is that artefacts may be introduced into the data and final sections. Artefacts are clearly visible on the shot gathers presented in Figure 4c and 4f, before the first breaks (especially when zoomed in). While this part of the data is muted and does not affect processing, it is reasonable to assume that similar artefacts could have been introduced in the useful portions of the shot gathers as well. The data is challenging and requires strong processing to enhance reflections, but the curvelet-denoised data appears over-filtered. If possible, I strongly recommend using a less aggressive parametrization. At the very least, I suggest presenting sections without the curvelet denoising and comparing them to the ones shown in Figure 5. This would allow readers to assess the actual impact of this denoising approach on the final results.Response: Thank you very much for your comment. Yes, it is rightly pointed out that coherency enhancements are usually applied at the end of the processing sequence. We initially followed the same approach but it did not produce any significant improvements without very aggressive thresholding of the curvelet coefficients – which in turn was damaging the signal and introducing artefacts. This indicates that the characteristics of the scattered energy in the final stacked section were difficult to separate from some of the migrated signals of weak coherency. Therefore, we attempted the approach of curvelet filtering on the pre-processed shot gathers before stacking. A similar approach was presented by Górszczyk et al. (2015) for pre-stack CIGs conditioning which was further used for the moveout picking for reflection traveltime tomography. This approach helped us in enhancing the coherency of the signals which produced more coherent depth images.
Reference: Górszczyk, A., M. Cyz, and M. Malinowski. "Improving depth imaging of legacy seismic data using curvelet-based gather conditioning: A case study from Central Poland." Journal of Applied Geophysics 117 (2015): 73-80.
We acknowledge that there are artefacts visible in the shot gathers due to curvelet filtering. As correctly pointed out that the data is challenging, parameterization for curvelet filtering was chosen such that it effectively removes the incoherent noise and enhances the signal-to-noise ratio. We tested various levels of thresholding to avoid damaging the coherent signals. In fact, we used relatively mild filtering in this case to minimize the problem of artefacts while improving the signal coherency of the weak reflections.
Although the curvelet filtering was applied before stacking, any of the introduced artefacts were cancelled during the stacking of the CIG gathers. As a result, we did not observe any artefacts due to the curvelet filtering on the final migrated stacks. This is because their energy is relatively low with respect to the coherent signal, as well as because they are introduced in very different (random) time-offset positions from one gather to another. Hence, in contrast to coherent reflections, they stacked destructively. A comparison of migrated stacks, both pre-stack & post-stack, with and without curvelet filtering is provided in the attached supplement document.
Lastly, as curvelet filtering, or data processing per se was not the main highlight of the article, and we used curvelet-denoising merely as a signal enhancement tool, therefore we did not provide any comparison for the same. An article on the application of curvelet filtering on hardrock seismic data is cited (Górszczyk et al., 2015) [Row 178].
Geology of the area:
The section on the geology of the area is very sparse and should be expanded. Many elements shown in Figure 1a are not discussed in the text—especially the areas exposed on the western side of the figure. Conversely, some features are mentioned in the text without being properly introduced in the geology section (e.g., the Korpua iron ore). A brief overview of faulting in the area would also strengthen this section and provide important context for interpretation.Response: Thank you very much for this comment. We have now simplified Fig. 1a and 1b, and have significantly expanded the geology of the area in more detail taking note of all the suggestions that have been made. More information on the regional geology, structural setting of the KLIC and associated mineralization has been added in this section. [Row 76-98]
The term ‘Korpua Intrusion’ has been altogether removed from the manuscript for better clarity as it was part of the same Banded Iron Formation (BIF) present on the southern end of the Koillismaa Deep Anomaly. [Row 91-93]
Interpretation:
The interpretation feels hesitant. While many reflections are labelled and annotated, few are given a geological interpretation. The authors should attempt to interpret these reflections based on what is known about the KLIC area and from physical rock properties. Even if speculative, this would be a valuable starting point. I also recommend synthesizing all interpretation results into Figure 8, with proper identification of key features (e.g., Korpua iron ore, regional and local faults, second conduit). The current version of Figure 8 is useful only for the conduit.Response: We acknowledge that the article lacked a geological explanation of many reflectors delineated in the depth image. Therefore, we have completely revised the section 6 ‘Interpretation and discussion’ with more focus on the interpretation, structural geology and mineralization. Figures 8a and 8b have been combined into a single image with discussions on the mapping of magma conduit, its internal reflectivity and other reflectors that are observed in the seismic image. [Row 253-304]
Additional comments and edits:
Line 28:
“Therefore, it is required to explore newer and deeper targets to sustain the supply and demand equilibrium.”
→ I suggest replacing this with: “As a result, there is a growing need to explore new and deeper targets to maintain the balance between supply and demand.”Response: This sentence has been modified [Rows 32-33].
Line 30:
“…ultramafic igneous rocks hold great potential to fulfil this requirement (Ripley and Li, 2018).”
→ Please specify which raw materials are associated with ultramafic rocks (e.g., Ni, Cr, PGE, etc.).Response: This information has been added [Rows 34-35]. Also, more information related to this is provided in Rows 36-39.
Line 43:
“Petrophysical and lab studies were done on the collected core samples from the drillhole, supplemented by limited wireline-logging data (Heinonen et al., 2022; Nousiainen et al., 2022).”
→ This sentence should be moved to Section 2, as it is not particularly informative in the introduction. Alternatively, clarify how this work supports the inference of a conduit.Response: This sentence has been removed from the introduction section.
The inference of the magma conduit has been mainly based on the drilling information and the age determination of the mafic-ultramafic lithologies encountered during the drilling. The latter information has now been added. [Rows 108-110]
Line 55:
“…also serve as a good instrument”
→ Please replace with: “are also valuable”Response: This sentence has been modified [Row 58].
Line 56:
“It is cheaper”
→ Please revise to: “Two-dimensional surveys are cheaper…”Response: This sentence has been modified [Row 59].
Line 60:
“With this aim,”
→ The aim is not clearly defined in the preceding paragraphs. Please consider rephrasing or expanding earlier content to better establish the objective.Response: The paragraph has been expanded with a more structured description of the aim of the 2D seismic survey. [Rows 66-72]
Line 69:
“…the magmatism that led to the breakup of one or more Archaean cratons”
→ Please revise to: “…the magmatism associated with an extensional phase that led to the breakup of Archean cratons.”Response: This sentence has been modified [Row 75].
Line 83:
“…thus verifying the presence of a deepseated magma conduit system connecting the exposed intrusions of the complex”
→ What specific evidence supports the existence of a conduit system, rather than an additional intrusion possibly related to the two exposed ones? Could all features be part of a single layered intrusion subsequently broken into parts during regional deformation? More detail should be added to the geology section to clarify these points.Response: Thank you very much for this comment. We acknowledge that limited information was provided on this. Therefore, this section has been significantly expanded. [Rows 76-98]
Line 92:
“a dominant frequency of 35 Hz.”
→ What is the rationale for selecting this dominant frequency? Please provide justification or context.Response: The Ricker wavelet has been used to produce the synthetic seismogram shown in Figure 3d. The peak frequency of 35 Hz is chosen so that the seismic response from the key geological units observed in the borehole can be generated to verify whether those lithologies will be detected in the surface seismic study. For example, at this frequency we can easily see the response for the impedance contrast observed for the events shown by black arrows in Fig. 3b. As the highest frequency in our survey goes up to 160 Hz compared to 35 Hz dominant frequency Ricker wavelet meaning these rocktypes should produce a clear seismic signal at the surface. This information has been added [Rows 113-114].
Line 95:
“These faults/fractures and lithology changes…”
→ Which specific faults and fractures are being referred to? Please be more precise.Response: Information on these specific events is now provided [Rows 120-122]. These events are also indicated by black arrows in Figure 3b, which was imaged in the walkaway VSP survey.
Line 121 (in Data Acquisition section):
→ Could you include some information on surface conditions (e.g., soil type), road surface types, and topography? This would help readers understand the environment and the data acquisition constraints.Response: The acquisition mainly took place along the existing gravel roads. We have moved this information to the start of the paragraph [Rows 149-150]. Other than that, only the elevation changes was the other main component that had been mentioned in Table 1 (last row). For more clarification, we have simplified Fig. 1b, i.e. the topographic map. The surrounding lakes and swamps in the surrounding area are now distinctly visible.
Line 130:
“The data is of better quality for shotpoints located in the southern and western end of the survey for which the firstbreak energy was visible for the full-offset range, otherwise, an effective offset of ~4 – 5 km is generally observed for the rest of the shots.”
→ Please revise to: “The data is of better quality for shotpoints located in the southern and western end of the survey where the first-break energy is visible for the full-offset range. For the remaining shotpoints, clear first breaks are generally limited to approximately 4–5 km.”Response: This sentence has been revised. [Rows 157-159]
Line 136:
“…standard processing workflow applied to the hardrock Seismics”
→ It is unclear whether a standardized processing workflow for hardrock seismics exists. In any case, please provide relevant references to support this claim.Response: The time-domain processing of hardrock seismic data is pretty straightforward and has been demonstrated in previous 2D case studies (semi-)regional scale. We have added relevant citations. [Row 164]
Table 2:
→ What is the rationale for using a floating datum at a constant elevation of 300 m when the final datum is also at 300 m? Please clarify.
→ Also, why is FX-decon used instead of curvelet denoising?Response: Thank you very much for pointing it out. This has now been corrected in the main text [Row 172] and also in Table 2. As mentioned earlier, curvelet denoising was applied to the pre-stack shot gathers, while FX-decon was applied post-stacking to enhance the coherent signal. It further helped us in obtaining a more focussed image.
Line 175:
“we performed KPreSDM in 3D mode”
→ What is the rationale for using 3D mode given that the profiles are relatively straight? Only the eastern part of L2000 appears sufficiently crooked to potentially justify this approach. Please provide additional evidence or justification for using the 3D mode over 2D on these profiles. Also, what pre-processing steps were applied prior to the Kirchhoff prestack depth migration?Response: Yes, it is true that the profiles are relatively straight for the most part but are crooked towards the end (eastern end of profile L2000 and also northern end of profile L1000). Interpretation of 2D data often suffers from the inability to distinguish reflections originating outside the image plane. Given the regional length of the profiles, a narrow 3D KPreSDM accommodates out-of-plane reflections by effectively positioning them at their true positions. It also better handles the complex geological setting as it accounts for changes in velocity both vertically and horizontally compared to time-domain imaging which only accounts for changes in velocity vertically. Relevant citations are provided. [Row 205]
No additional preprocessing was applied to the data for KPreSDM; the same preprocessed data used for DMO-PoSTM was used. This information is now mentioned in the main text [Row 210]
Line 193:
“Reflectors mapped between CDPs 725 – 1200 (SF2, SF3, SF4) and depth up to ~2 – 3km have an up-dip movement towards the surface.”
→ The phrase “up-dip movement” is unclear in this context. Could you please clarify what is meant here? Are you referring to the geometry or apparent dip of the reflectors?Response: Here, we simply meant the direction of the reflectors which is in the updip direction towards the surface. This has been corrected. [Rows 223-224]
Line 195:
“Compared to the KOSE2018 profile (Fig. 2), the new acquisition and processing produced abundant reflectivity…”
→ To what extent is this increased reflectivity due to the curvelet denoising applied on prestack data versus differences in acquisition parameters between the KOSE2018 and the new survey? This distinction is important and should be clarified.Response: As mentioned earlier, curvelet-denoising was merely used as a signal enhancement tool (see the comparison in the attached document). The abundant reflectivity observed in the depth images is mainly due to the well-optimised acquisition. We agree that more information is required and it is a better fit in the discussion section rather than the results section. Therefore, we have moved this to section 6. [Rows 258-263]
Line 227:
“The top of the magma conduit was successfully imaged with reflections mapped until the depth of ~5 – 6 km.”
→ This statement is unclear and difficult to reconcile with sections. Are you referring to the top or to the outer shell of the conduit? According to Figure 5, the top appears to be at approximately 2 km depth. Also, which reflectors define the bottom of the conduit? Please clarify.Response: Thank you very much for this comment. This section has now been completely revised.
Regarding the magma conduit, yes, we again agree that it was a bit confusing and to remove this ambiguity, we have removed the term ‘top’ while discussing the magma conduit in the manuscript. We now clearly state that the top of the arcuate reflectivity that is observed in profile L1000 [Row 217-218], are representative of the mafic cumulates exposed in the drillhole (shown in Fig. 3e) and discussed in rows 270-272.
Regarding the bottom of the conduit, intuitively, it appears that the reflector marked as DR2 and its adjacent cross-dipping reflector in Fig. 5 constitute the bottom of the magma conduit. As the drillhole does not reach this depth, or in the absence of any other geophysical or direct evidence – we could not verify at this stage whether it is the bottom of the magma conduit. Therefore, we have avoided interpreting the bottom of the magma conduit at this stage; it is only speculative. More detailed interpretation of the 2D seismic results along with 3D seismics and electromagnetics study which are also alos part of the project are underway.
Line 228:
“The internal structure of the magma conduit seems more complex than its present understanding with hints of fault-like structures cross-cutting the intrusion.”
→ The phrase “hints of fault-like structures” is vague. What are the implications of faults within the conduit? Additionally, the reflector interpreted as the top of the conduit appears discontinuous and cross-cut (e.g., it does not appear like not a smooth surface), which further supports the idea of internal complexity. This should be discussed more clearly.Response: Yes, we acknowledge that the article had lacked proper interpretation of these events. Therefore we have revised the entire section 6.
Line 229:
“The obtained results were correlated with the initial (i.e. pre-seismic) geological model.”
→ Please provide more details about this geological model. Is it the surface shown in Figure 8? How was it derived—through modeling, potential field inversion, or another method? Clarification is needed.Response: The information on the geological model has been provided at various places in the manuscript [Rows 24-26, 122-123, 294-295].
Yes, the modelled magma conduit shown in Fig. 8 is a 3D surface, it is now clearly mentioned in the main text. [Row 294]
Line 247:
“…but this higher package of reflectivity could be associated with the Korpua Iron Ore Intrusion (Makkonen, 1972).”
→ What is the Korpua Iron Ore, and what is its orientation and location relative to the profile? I suggest introducing the Korpua Iron ore in the geology section since it is a feature discussed in the interpretation. Additionally, what evidence or inferences support this interpretation?Response: Thank you very much for pointing this out. In the revised manuscript, we have removed the term ‘Korpua Iron ore’ as it is part of the same banded iron formation (BIF) shown in Fig. 1c, which has a specific name for this part of the BIF only. We have added more geological information on BIF in section 2 [Row 91-93] and in section 6 [Row 280-292]. Accordingly, Fig 1c has been updated where Korpua Iron is replaced with BIF and is clearly marked by red arrows.
Line 258:
“It can be inferred that there might be a second magma conduit between the exposed Koillismaa intrusion and Näränkävaara intrusion of the KLIC.”
→ Where is this second conduit visible in the seismic sections? What evidence or geological inference supports this interpretation?Response: Yes, we acknowledge that this information is not supported by the depth images, therefore we have removed this claim from the manuscript. The justification for it was related to the overall geological setting of the KLIC with respect to the Tornio-Naarankavaraa belt and the observed gravity high anomaly. Therefore, only a speculative description was mentioned in the manuscript. But as the second conduit is not visible in the seismic section, we have removed it from the article.
Line 280:
“hints of fault-like structures…”
→ This phrase is ambiguous. Please clarify what these features are.Response: This phrase has been altogether removed from the article due to its ambiguity.
Line 280:
“It is interpreted that there might be a second magma conduit between the exposed Koillismaa intrusion and Näränkävaara intrusion of the KLIC.”
→ Again, where exactly is this second conduit located in the seismic data? Please point to the relevant features in the sections.Response: As pointed out earlier, this claim has been removed due to the lack of evidence in the seismic depth images.
Line 283:
“This will tremendously help build a more detailed geological model of the Koillismaa area.”
→ Possibly—but how will this be achieved if most of the reflectors remain uninterpreted? Clarification or a more cautious statement may be warranted.Response: We now believe that the more detailed interpretation provided in section 6 validates this statement. We have removed the word ‘tremendously’. [Row 329]
Figure 1:
→ Please add a legend for the geological units shown in 1a.Response: Fig 1 has been significantly revised with a more simplified geology and topographic map. All the legends have been described. Accordingly, relevant information has been added in the main text in section 2.
Figure 2:
→ Indicate the location of this profile on Figure 1.
→ Increase the size of the borehole symbol—it is currently too small.
→ Add a legend for the lithologies intersected in the borehole.
→ The color scheme used for rock types does not match that of Figure 3. Please harmonize these.
→ Also, why does the amplitude colorbar only show positive values? Seismic data usually consists of both peaks and troughs—please explain.Response: KOSE2018 almost entirely overlaps the profile L1000; therefore it is not shown in Fig. 1b or 1c. Instead, the overlapping has been shown in the top panel of Fig. 2 (KOSE2018 profile in red colour overlaps the profile L1000 in black). The same information has been provided in the description of Fig. 2. Figure 2 has been modified with the addition of depth labels and a borehole with lithologies having the same colorscheme as shown in Fig 3e. It is mentioned that the legends for the borehole lithologies are mentioned in Fig. 3 [Row 140]
Yes, the seismic data indeed has both peaks and troughs. Here, to make the reflectors more distinctly visible, the scale has been skewed towards black color as +ve (peaks), and white as troughs (almost around the zero).
Figure 8:
→ The geological model is difficult to interpret. Figure 8a suggests a 3D surface, while 8b appears entirely contained within profile L1000, suggesting it represents the outer shell rather than just the top of the conduit. Please clarify.
→ Also, specify what the red arrows represent in the figure caption.Response: Yes, it is rightly pointed out that it was not mentioned whether it is a 3D surface of the whole magma conduit and not just the top of the conduit. This section has been altogether revised for a much clearer description. [Rows 294 – 304]
Red arrows mark the internal reflectivity observed within the magma conduit. It has been added to the caption of Fig. 8 [Row 311-312]. Also, Figures 8a and 8b are combined into a single figure.
Final comments:
→ The manuscript would benefit from a general language revision. The English is generally understandable but could be improved in several places for clarity and flow.Response: In the revision, we have simplified several sentences throughout the manuscript for a smoother flow of information.
- Line 28:
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RC2: 'Comment on egusphere-2025-496', Anonymous Referee #2, 28 May 2025
This work deals with processing of two crosscutting reflection seismic profiles acquired as part of a Seismic and Electromagnetics Methods for Deep mineral exploration (SEEMS DEEP) project above a known gravity/magnetic anomaly of the Koillismaa Layered Igneous Complex (KLIC) in north-eastern Finland. The work is well introduced and elaborated, processing steps applied are relevant and aim at improving the coherency and continuity of the events seen on raw seismic data, along with showing others revealed only after processing. Both pre- and post-stack imaging strategies employed shed new light on the geological complexity in the area, especially internal reflectivity within previously interpreted extent of the Koillismaa intrusion. This is obvious both from the current results and when comparing with the results of earlier surveys conducted at the same site. The authors also suggest a possibility of various magma conduits and/or diabase veins as the source of reflectivity.
Generally, I have only a few comments which are rather of cosmetic form, and one major one for the authors. The major one follows. As one of the main aims of SEEMS DEEP and the work presented itself, as the authors are claiming, is to aid in understanding of the critical raw materials relevant for the EU, why is there no discussion of the results of this study in that respects? How the abstract and Introduction are formulated, I was expecting to see a discussion of the results obtained in a mineral systems context where the source of the magnetic/gravity anomaly of the Koillismaa intrusion, or the iron band formation would be addressed, along with connection of the petrophysical information as encountered in the drillhole with the deeper source. At present, this part is entirely absent. As the drillhole does not reach the deeper reflections, even a brief speculative discussion part placing the results in a larger context, similar to as applied in the mineral systems concept, might suffice.
Minor comments follow:
Rows 14-15: How this sentence is formulated, it is emphasized that the main goal behind the survey is to understand the mineralization potential of KLIC, but the main results are on the geological context, not the mineralization. Perhaps adding to this sentence something in th form of: "hence it is important to understand the geological setting that might have resulted in emplacement of mineralization in a broader mineral systems context "
Row 63: Perhaps "driven" might be more appropriate instead of "playground"
Rows 81-83. Here, the word "diamond" is redundant and confusing, hence would be better to remove it. Would also be good to add the final borehole depth as it is unclear at present.
Row 91: Given the sweep parameters of the survey being 20-160 Hz, could you elaborate on the reasoning to use a wavelet centered at 35 Hz for the synthetics? Is it related to the consistency across various datasets acquired previously at the same site or some other effect?
Row 105, Figure 1 comments. It is very difficult to distinguish between the color code used for Korpua Intrusion and the dykes. Maybe some different colors could improve this? Also, as a non-geologist, the color of the Archean basement is shown but the figure also shows a texture that is absent in the legend, besides the "pink" unit that is not in the legend at all...
Row 110, Figure2: Perhaps it would be beneficial to the reader to show a depth/time axis.
Table 1: Would be good to know also the peak force used. Also, a comma between "20-160 Hz" and "+1dB" might be good to distinguish sweep bandwidth versus type.
Rows 138-139: A curiosity driven question. Are these CDP bins or they are common midpoint (CMP) bins at this stage?
Row 148: Are not surface wave and ground roll the same thing? Perhaps the authors meant shear-wave and ground roll?
Row 161: What does the abbreviation PoSTM stands for?
Rows 185-186: I guess this is a combined effect of the lower fold at shallower depths due to 15/30 m shot/rec spacing, tougher with the effect of spherical divergence correction applied? Would be happy to hear your comment on this.
Row 220, Figure 6: To me, it feels like the time grid lines just make the interpretation more complicated, hence might be better to remove them.
Rows 247-249: Perhaps this might be a good place to discuss the mineralization potential of this horizon or the KLIC in general, as this is the scientific question raised by the SEEMS DEEP and this work...
Rows 253-254: Could this also be the source of mineralization and what kind of critically important EU minerals can be expected or were mapped within the core samples in the borehole?
Citation: https://doi.org/10.5194/egusphere-2025-496-RC2 -
AC2: 'Reply on RC2', Brij Singh, 25 Jun 2025
Firstly, we are very thankful to the reviewer for dedicating one's time and effort for reviewing the manuscript and providing us with the comments. Below we provide the one-to-one response to all the points raised by the reviewer. All the relevant textual changes made in the revised manuscript has been highlighted in red. We hope we had been able to satisfactorily address all of them. They are as below:
Comments from Reviewer-2
This work deals with processing of two crosscutting reflection seismic profiles acquired as part of a Seismic and Electromagnetics Methods for Deep mineral exploration (SEEMS DEEP) project above a known gravity/magnetic anomaly of the Koillismaa Layered Igneous Complex (KLIC) in north-eastern Finland. The work is well introduced and elaborated, processing steps applied are relevant and aim at improving the coherency and continuity of the events seen on raw seismic data, along with showing others revealed only after processing. Both pre- and post-stack imaging strategies employed shed new light on the geological complexity in the area, especially internal reflectivity within previously interpreted extent of the Koillismaa intrusion. This is obvious both from the current results and when comparing with the results of earlier surveys conducted at the same site. The authors also suggest a possibility of various magma conduits and/or diabase veins as the source of reflectivity.
Generally, I have only a few comments which are rather of cosmetic form, and one major one for the authors. The major one follows. As one of the main aims of SEEMS DEEP and the work presented itself, as the authors are claiming, is to aid in understanding of the critical raw materials relevant for the EU, why is there no discussion of the results of this study in that respects? How the abstract and Introduction are formulated, I was expecting to see a discussion of the results obtained in a mineral systems context where the source of the magnetic/gravity anomaly of the Koillismaa intrusion, or the iron band formation would be addressed, along with connection of the petrophysical information as encountered in the drillhole with the deeper source. At present, this part is entirely absent. As the drillhole does not reach the deeper reflections, even a brief speculative discussion part placing the results in a larger context, similar to as applied in the mineral systems concept, might suffice.
Response: Firstly, we are thankful to the reviewer for reviewing the article and for the constructive comments. We acknowledge the reviewer’s comment on the lack of discussion on the mineralization potential associated with KLIC, which can be interpreted based on the new seismic results obtained within the SEEMS DEEP project. Therefore, we have provided more information on the “geology of the area” in section 2 [Rows 76-98]. Now, there is a larger context provided related to the overall geological setting and the associated mineralisation (changes are made to Figure 1a accordingly). Also, the whole section 6 – ‘Interpretation and Conclusion’ is rewritten with more detailed information on the structural setting and discussion on the potential of the mineralisation based on seismic imaging [Rows 253-304]. Accordingly, the ‘Abstract’ and ‘Conclusion’ sections are also updated [Rows 14-24, Rows 323-327]. All the textual changes made in the revised manuscript are highlighted in red.
Minor comments follow:
Rows 14-15: How this sentence is formulated, it is emphasized that the main goal behind the survey is to understand the mineralization potential of KLIC, but the main results are on the geological context, not the mineralization. Perhaps adding to this sentence something in th form of: "hence it is important to understand the geological setting that might have resulted in emplacement of mineralization in a broader mineral systems context "
Response: We have revised this sentence [Rows 14-16], and more structured information on the main aim of the survey has been added in the ‘Introduction’ section [Rows 66-72]
Row 63: Perhaps "driven" might be more appropriate instead of "playground"
Response: In our opinion ‘playground’ is a better word to describe the various technical developments that took place in both electromagnetics and seismics within the SEEMS DEEP project. [Rows 64-66]
Rows 81-83. Here, the word "diamond" is redundant and confusing, hence would be better to remove it. Would also be good to add the final borehole depth as it is unclear at present.
Response: The word ‘diamond’ has been removed and information on the end depth of the borehole has been added [Rows 103-105].
Row 91: Given the sweep parameters of the survey being 20-160 Hz, could you elaborate on the reasoning to use a wavelet centered at 35 Hz for the synthetics? Is it related to the consistency across various datasets acquired previously at the same site or some other effect?
Response: The Ricker wavelet has been used to produce the synthetic seismogram shown in Figure 3d. The peak frequency of 35 Hz is chosen so that the seismic response from the key geological units observed in the borehole can be generated to verify whether those lithologies will be detected in the surface seismic study. For example, at this frequency we can easily see the response for the impedance contrast observed for the events shown by black arrows in Fig. 3b. As the highest frequency in our survey goes up to 160 Hz compared to 35 Hz dominant frequency Ricker wavelet meaning these rocktypes should produce a clear seismic signal at the surface. This information has been added [Rows 113-114].
Row 105, Figure 1 comments. It is very difficult to distinguish between the color code used for Korpua Intrusion and the dykes. Maybe some different colors could improve this? Also, as a non-geologist, the color of the Archean basement is shown but the figure also shows a texture that is absent in the legend, besides the "pink" unit that is not in the legend at all...
Response: Yes. Thank you very much for pointing this out. Figure 1 has been reproduced with more simplification of Figure 1a and a new Figure 1b. All the appropriate legends have been added to describe the geology. Also, the term ‘Korpua Intrusion’ has now been removed from the figure and the manuscript as this is the same as the banded iron formation (BIF) which runs at depth parallel to the KLIC (marked by red arrow in Fig. 1a). We realised that the specific name of ‘Korpua’ was a bit misleading as if it might be inferring to a different rock-type. The term ‘Korpua’ has been altogether removed from the article. It is now been confined to BIF (in section 2 [Row 91-93], and in section 6 [Row 280-292].). We have kept the same colorscheme for BIF and dykes, as one is a surface and other is a line, so it is easily distinguishable in our opinion.
Row 110, Figure2: Perhaps it would be beneficial to the reader to show a depth/time axis.
Response: Figure 2 has been modified with the addition of depth scale and the borehole lithology colorscheme is now the same as shown in Fig. 3e.
Table 1: Would be good to know also the peak force used. Also, a comma between "20-160 Hz" and "+1dB" might be good to distinguish sweep bandwidth versus type.
Response: Peak force information has been added to main text [Row 155], and Table 1 is corrected.
Rows 138-139: A curiosity driven question. Are these CDP bins or they are common midpoint (CMP) bins at this stage?
Response: Very often the terms, CDP and CMP are used interchangeably. For a flat reflector both are the same and for a dipping reflector CDP is a more accurate word. Therefore, we used the term “CDP”.
Row 148: Are not surface wave and ground roll the same thing? Perhaps the authors meant shear-wave and ground roll?
Response: Yes, thank you very much for pointing it out.. It is corrected in the article [Row 176].
Row 161: What does the abbreviation PoSTM stands for?
Response: Thanks again for pointing it out. The full form of the term is now added [Row 187].
Rows 185-186: I guess this is a combined effect of the lower fold at shallower depths due to 15/30 m shot/rec spacing, tougher with the effect of spherical divergence correction applied? Would be happy to hear your comment on this.
Response: The same processing has been applied to both profiles as they are acquired in close proximity over a similar geological setting and with the same acquisition parameters. We do not think that the lower fold at shallower depth and spherical divergence correction could have the effect that deep as our nominal CDP is ~7.5 m, it is sufficiently dense for hardrock seismics. If it were the case, the effect should have been more or less similar for both profiles which is not the case. Also, KPreSDM results have better focussing than the DMO-PoSTM results, so it is also a bit dependent on the imaging principle. We have observed similar results for 3D seismics as well which is currently in development for submission for journal publication. In my opinion, it is probably more related to the low impedance contrast between the overlying Archean Gneisses and the granite (see Fig. 3).
Row 220, Figure 6: To me, it feels like the time grid lines just make the interpretation more complicated, hence might be better to remove them.
Response: This point is a bit unclear to us. Do you mean the grid lines at depth or the red line used for the profile intersection? The grid lines at depth provide a better sense of depth to the readers when looking at the reflections far away from the axis. The intersection of the profiles is shown because in Figs 5 and 6, we are looking at 2D DMO-PoSTM results vs 3D PreSDM which is extracted along an inline (see Fig. 1c). It is important for the readers to have this information for a qualitative comparison. [Row 226]
Rows 247-249: Perhaps this might be a good place to discuss the mineralization potential of this horizon or the KLIC in general, as this is the scientific question raised by the SEEMS DEEP and this work...
Response: The section 6 has been completely rewritten. More information on the banded iron formation and the mineralization has been provided.
Rows 253-254: Could this also be the source of mineralization and what kind of critically important EU minerals can be expected or were mapped within the core samples in the borehole?
Response: A more detailed explanation on the mineralization potential has been added in section 6.
Regarding the borehole: the mineral system studies focused on Ni–Cu–PGE mineralization, and according to a visual log of the drill core, the mafic part of the core does not contain any indications of sulphide mineralization. The core only includes some trace amounts of Cu–Ni–sulphide minerals, and the Ni–Cu concentrations are therefore relatively low. According to geochemical analyses, the core is not PGE mineralized (Tirroniemi et al., 2024).
Citation: https://doi.org/10.5194/egusphere-2025-496-AC2
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AC2: 'Reply on RC2', Brij Singh, 25 Jun 2025
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EC1: 'Comment on egusphere-2025-496', Christopher Juhlin, 29 May 2025
Dear Authors,
We now have reviews from the two referees. Based on their reviews and my reading of the MS I judge you have moderate revision with additional review afterwards. When revising the paper please pay close attention to the general comments of the referees.
Best Regards,
Chris Juhlin
Citation: https://doi.org/10.5194/egusphere-2025-496-EC1
Peer review completion
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Andrzej Górszczyk
Michał Malinowski
Suvi Heinonen
Uula Autio
Tuomo Karinen
Marek Wojdyła
The requested preprint has a corresponding peer-reviewed final revised paper. You are encouraged to refer to the final revised version.
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(3106 KB) - Metadata XML
General comments
Very good geophysical research. Please, follow my specific comments to improve the manuscript.
Specific comments
Lines 49-52. “Geophysical surveys for mineral exploration have been routinely accomplished by potential field methods, electromagnetics, electrical methods....depths required to distinguish different lithologies. In comparison, seismic reflection surveys can provide higher-resolution images of the subsurface and have now emerged as an established method for deep mineral exploration in the hardrock environment”. Insert recent papers that discuss the use of electromagnetics, electrical methods and seismic surveys for deep exploration in the hardrock lithologies.
- Medici, G., Ling, F., Shang, J. 2023. Review of discrete fracture network characterization for geothermal energy extraction. Frontiers in Earth Science, 11, 1328397.
- Eberhart‐Phillips, D., Stanley, W. D., Rodriguez, B. D., Lutter, W.J. 1995. Surface seismic and electrical methods to detect fluids related to faulting. Journal of Geophysical Research: Solid Earth, 100(B7), 12919-12936.
Line 67. Clearly state the 3 to 4 specific objectives of your research by using numbers (e.g., i, ii, and iii).
Line 69. “Layered intrusions”. Please, describe in detail the geometries of these geological bodies. The geological scenario is currently unclear.
Lines 69-100. This section needs more detail on the regional stratigraphy.
Lines 69-100. Provide more detail on the presence of faults that I can observe in the geological maps.
Lines 225-262. A discussion should also incorporate literature to show that your research has advanced the field.
Lines 318-409. Improve the literature for either introduction or discussion.
Figures and tables
Figure 1a, c. Make evident the type of faults.
Figure 1b. What the contours represent? Unclear, specify in the caption.
Figure 1b. Increase the graphic resolution.
Figures 7 and 8. These are very important figures, you can make them larger.