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: The contact author has declared that neither they nor their co-authors have competing interests. However, one of the authors (MM) is a member of the editorial board of Solid Earth.
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 preprint. The responsibility to include appropriate place names lies with the authors.- Preprint
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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 -
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 - 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 -
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
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