Reflection seismic imaging of the manganese mineralisation in the Griqualand West Basin, South Africa

Sihoyiya, Mpofana; Manzi, Musa Siphiwe Doctor; James, Ian; Westgate, Michael

doi:https://doi.org/10.5194/egusphere-2025-3117

Preprints

https://doi.org/10.5194/egusphere-2025-3117

Preprints

07 Jul 2025

| 07 Jul 2025

Reflection seismic imaging of the manganese mineralisation in the Griqualand West Basin, South Africa

Mpofana Sihoyiya, Musa Siphiwe Doctor Manzi, Ian James, and Michael Westgate

Abstract. The Kalahari Manganese Field (KMF) in the Northern Cape Province of South Africa hosts some of the world’s richest manganese deposits, largely concealed beneath thick Cretaceous to Cenozoic Kalahari Group sediments. To improve imaging of the concealed Transvaal Supergroup strata, a high-resolution 2D reflection seismic survey was conducted in November 2023 across the Severn farm area. The survey comprised five profiles totalling 18.9 km, acquired using 5 Hz 1C geophones connected to wireless nodes, enabling effective burial beneath loose aeolian sand for improved coupling. A compact 500 kg drop hammer, mounted on a Bobcat, served as the seismic source, offering excellent manoeuvrability across challenging sandy terrain. Shot spacing was 10 m, with four vertical stacks per shot to enhance signal-to-noise ratio, yielding nearly five million seismic traces. Refraction tomography using first-break travel times provided near-surface P-wave velocity models, revealing variable Kalahari sediment thicknesses ranging from 20 to 70 m and bedrock velocities of ~5500 m/s associated with Karoo Supergroup strata. Despite the challenges posed by the thick sand cover, lithified calcrete horizons within the Kalahari sediments significantly aided seismic energy propagation. Post-stack Kirchhoff time migration imaging revealed nine laterally continuous high-amplitude reflectors between 0.05 and 3.42 km depth, corresponding to major stratigraphic boundaries from the Kalahari Group down to the Ghaap Group. Of particular interest is the moderate-amplitude reflection pair at 1.05–1.35 km depth, interpreted as the Hotazel Formation, the primary manganese host. This study demonstrates that, when appropriately designed, reflection seismic imaging can be a powerful tool for delineating deep mineralized strata beneath thick sedimentary cover in arid environments.

Received: 30 Jun 2025 – Discussion started: 07 Jul 2025

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Mpofana Sihoyiya, Musa Siphiwe Doctor Manzi, Ian James, and Michael Westgate

Status: final response (author comments only)

RC1:
'Comment on egusphere-2025-3117', Anonymous Referee #1, 29 Jul 2025
In this manuscript, authors presented that five 2D reflection seismic profiles deployed in the Kalahari Manganese Field to improve imaging of the concealed Transvaal Supergroup strata. Nine reflectors and refraction tomography velocity models are revealed through this study.
General comments
Despite actuality and significant work done by the authors, I think that the manuscript should be required major revisions in its current shape for two reasons as follows.
Main geological interpretation results (Nine reflectors) were presented in the paper titled “Reflection seismic imaging of the manganese mineralisation in the Griqualand West Basin, South Africa” by Joggee et al.(2025), the first and the second authors are also co-authors of the published paper. The difference of the manuscript andtheirprior paper and new findings must be presented in the manuscript.

Authors stated a lot about data acquisition method to obtain high-quality and -resolution seismic data, not reflection seismic imaging in the conclusion section. They do not correspond to the title of the manuscript. Authors should focus on what you want to present in the manuscript.

Special comments
Line 65, Figure 1a is not clear. It is important to give regional and local geological maps to help readers understand geological background. A typical lithostratigraphic section is preferable. And the location and layout of seismic survey lines do not need to be shown in Figure 1b, there are displayed in Figure 2.

Line 181, the recording time of raw shot gather in Figure 5c is 1000 ms，not same with other shot gathers. Is it right? Reflections should be marked in Figure 5c-5e.

Line 186, main data processing methods and key parameters should be given in a table and examples of raw shot gathers dominated by reflection waves or with clear reflections waves should be shown after data processing in the processing section.

Line 188, pre-stack migration was applied in this study, but I cannot see any pre-stack migration results in the manuscript. The Kirchhoff post-stack time-migrated seismic sections is stated in the caption of Figure 7. What kind of migration methods did you use?

Line 199, You may consider to focus on near-surface structure with refraction tomography results.

Line3 251-270, nine reflectors were interpreted through reflection seismic profiles. However, reflectors numbered 1 and 2 do not agree with the results of your prior results (Joggee et al., 2025). I am wondering if there are your new results .

Line 274, “stud”should be “study” ?

Line 292, I don’t understand why you didn’t provide profile 3 result.

Line 320, profile 3 was also missed in Figure 8.
Citation: https://doi.org/10.5194/egusphere-2025-3117-RC1
- AC1:
  'Reply on RC1', Mpofana Sihoyiya, 17 Sep 2025
  Commented: Main geological interpretation results (Nine reflectors) were presented in the paper titled “Reflection seismic imaging of the manganese mineralisation in the Griqualand West Basin, South Africa” by Joggee et al. (2025), the first and the second authors are also co-authors of the published paper. The difference of the manuscript and their prior paper and new findings must be presented in the manuscript.
  We thank the reviewer for this important observation. We acknowledge that the nine reflectors and their geological significance were initially reported in Jogee et al. (2025), in which two of the present authors were co-authors. However, the current manuscript differs in several key aspects:
  
  Near-Surface Imaging: The present work provides detailed tomographic refraction velocity models and near-surface P-wave velocity characterization, which were not covered in the earlier publication. This new contribution sheds light on the effect of the thick Kalahari cover on seismic energy penetration and resolution.
  
  Different Focus: Jogee et al. (2025) placed greater emphasis on geological interpretation and regional stratigraphy. In contrast, the present manuscript focuses more on reflection seismic techniques, including acquisition, processing, and velocity modeling, thereby complementing the earlier study.
  
  We will revise the manuscript to highlight these differences more explicitly in the Introduction and Discussion sections to clarify the novelty and focus of this work.
  
  In addition, the authors will also present and compare the pre-stack and post-stack migration approaches that were undertaken, which were not presented in our first submission and in the paper by Jogee et al. (2025).
  
  Commented: Authors stated a lot about data acquisition method to obtain high-quality and -resolution seismic data, not reflection seismic imaging in the conclusion section. They do not correspond to the title of the manuscript. Authors should focus on what you want to present in the manuscript.
  We appreciate the reviewer’s observation. We agree that the Conclusion section placed more emphasis on the acquisition methods than on reflection seismic imaging, which is central to the manuscript. The focus of the paper is indeed reflection seismic imaging of the Kalahari Manganese Field. While the acquisition discussion is important to highlight the challenges of imaging beneath the thick Kalahari cover, we will revise the Conclusion to emphasize the main imaging results, namely:
  
  The successful delineation of nine prominent reflectors corresponding to key lithostratigraphic contacts from the Kalahari Group down to the crystalline basement.
  
  The improved seismic imaging of the Hotazel Formation, which hosts manganese mineralisation, despite the thick near-surface sedimentary cover.
  
  The integration of refraction tomography with reflection seismic imaging to enhance near-surface velocity characterization and reflection quality.
  
  We will reframe the Conclusion so that it aligns more directly with the title and scope of the manuscript, focusing on reflection seismic imaging results and their implications while briefly noting the acquisition strategy as a supporting factor. We will also revise the title to not only focus on imaging to reflect the importance of this study, “delineation of critical mineral deposits beneath the thick cover,” using a cost-effective source, a 500 kg drophammer.
  
  Commented: Line 65, Figure 1a is not clear. It is important to give regional and local geological maps to help readers understand geological background. A typical lithostratigraphic section is preferable. And the location and layout of seismic survey lines do not need to be shown in Figure 1b, there are displayed in Figure 2.
  Corrected and updated on Line 65 of Sihoyiya et al. 2025_Track_Changes.
  
  Commented: Line 181, the recording time of raw shot gather in Figure 5c is 1000 ms,not same with other shot gathers. Is it right? Reflections should be marked in Figure 5c-5e.
  We thank the reviewer for this observation. Figure 5c represents results from a shorter profile (Profile 3) compared to the other profiles. To enhance clarity, only the top 1000 ms is displayed for better visualization.
  
  Regarding the marking of reflections, these are not indicated in Figures 5c–5e because they are masked by strong low-frequency signal in the raw data, which makes clear identification difficult at this stage. However, the reflections become more evident after subsequent processing steps, as illustrated in the later figures.
  
  Commented: Line 186, main data processing methods and key parameters should be given in a table and examples of raw shot gathers dominated by reflection waves or with clear reflections waves should be shown after data processing in the processing section.
  We appreciate the reviewer’s suggestion. We will provide a summary table in the processing section that outlines the main data processing steps along with the key parameters used in each stage. This will improve clarity and allow readers to better follow the workflow.
  
  In addition, we will include examples of shot gathers after processing, where reflections are more clearly visible compared to the raw data. This will highlight how the applied processing methods improved the signal-to-noise ratio and enhanced the continuity of reflection events.
  
  Commented: Line 188, pre-stack migration was applied in this study, but I cannot see any pre-stack migration results in the manuscript. The Kirchhoff post-stack time-migrated seismic sections is stated in the caption of Figure 7. What kind of migration methods did you use?
  We thank the reviewer for pointing this out. We acknowledge the inconsistency in the text and figure captions. In this study, we tested both Kirchhoff pre-stack time migration (PreSTM) and post-stack time migration (PostSTM). In the paper we only presented PreSTM results, however in the revised version we plan to include the comparisons between the two migration approaches. . The caption in Figure 7 will be corrected to reflect that the seismic sections are pre-stack time migrated.
  
  No post-stack migration results were included in the manuscript, but we will include them in the revised version for comparisons with PostSTM and state that the final interpretation only focused on PreSTM results.
  
  Commented: Line 199, You may consider to focus on near-surface structure with refraction tomography results.
  We appreciate the reviewer’s suggestion. More interpretation on the refraction tomography results will be incorporated in the revised manuscript to improve the understanding of the near-surface structure, which will also provide complementary constraints to the reflection seismic imaging.
  
  Commented: Line 251-270, nine reflectors were interpreted through reflection seismic profiles. However, reflectors numbered 1 and 2 do not agree with the results of your prior results (Joggee et al., 2025). I am wondering if there are your new results.
  Thank you for pointing this out. You are correct that reflectors 1 and 2 should match the results from Joggee et al. (2025). This discrepancy was due to an error in labeling, and we will correct it in the revised manuscript to ensure consistency with the previous study.
  
  Commented: Line 274, “stud” should be “study”?
  Thank you for pointing that out. We will correct “stud” to “study” in the revised manuscript.
  
  Commented: Line 292, I don’t understand why you didn’t provide profile 3 result.
  Thank you for your comment. Profile 3 was not included because its data quality was significantly lower than that of the other profiles, with high noise levels and poor reflector continuity, which made interpretation unreliable. We focused on the profiles with robust and interpretable signals to ensure accuracy. We will clarify this in the revised manuscript for transparency.
  
  Commented: Line 320, profile 3 was also missed in Figure 8.
  Thank you for pointing this out. Profile 3 was omitted from Figure 8 because its data quality was insufficient for reliable interpretation. We will add a note in the figure caption and the main text to explain why Profile 3 is not included, ensuring clarity for the readers.
  
  Citation: https://doi.org/10.5194/egusphere-2025-3117-AC1
RC2:
'Comment on egusphere-2025-3117', Anonymous Referee #2, 18 Aug 2025
Overall, the manuscript is clearly written and mostly well structured (see comments in the pdf). However, its novelty appears rather limited compared to Jogee et al. (2025). Most of the processing and interpretation steps are essentially identical to the earlier publication. The only substantial addition seems to be the tomographic component, while even the perspective 3D view and horizon picking were already shown previously—yet these are not included here. To strengthen the contribution, I recommend a more explicit comparison with the earlier work and a clearer demonstration of what is new in this study.
Remarks
Section 3.1:
Please add a table summarizing the acquisition parameters.

Please include examples of the seismic data both before and after processing. This would allow readers to better evaluate the impact of your workflow (see also the comments in the pdf).

Provide a more detailed explanation of how guided waves were handled during processing.

Currently you are showing less information compared to Jogee et al. (2025)

Section 3.3:
When describing sediment thicknesses across the profiles, consider including a map of thickness distribution. This would help clarify where the sedimentary cover (low-velocity zone) is thickest and how it may impact image quality.

Figures 7 and 8:
Profile 3 appears to be missing.

Ensure that the profiles are displayed with the correct aspect ratio (Profiles 1 and 2 are twice as long as Profile 5).

Clarify whether different CDP bin sizes were used across the profiles.

Section 4:
You provide a detailed overview of the expected boundaries. Have you computed a synthetic seismogram from the series of reflection coefficients? This would allow for a direct comparison with the seismic images and could significantly strengthen the interpretation.

Why is no 3D interpretation or perspective view included here, given that one was already presented in Jogee et al. (2025)?

Please add interpretation figures for each profile (highlighting the main geological units, etc.), similar to Profile 1 in Figure 12 of Jogee et al. (2025).
Citation: https://doi.org/10.5194/egusphere-2025-3117-RC2
- AC2:
  'Reply on RC2', Mpofana Sihoyiya, 17 Sep 2025
  Commented: Overall, the manuscript is clearly written and mostly well structured (see comments in the pdf). However, its novelty appears rather limited compared to Jogee et al. (2025). Most of the processing and interpretation steps are essentially identical to the earlier publication. The only substantial addition seems to be the tomographic component, while even the perspective 3D view and horizon picking were already shown previously—yet these are not included here. To strengthen the contribution, I recommend a more explicit comparison with the earlier work and a clearer demonstration of what is new in this study.
  Thank you for your constructive feedback. We appreciate your point regarding the novelty of the manuscript relative to Joggee et al. (2025). To address this, we will explicitly highlight in the revised manuscript: the new contributions, comparison with previous work, and Clarification of previously shown elements.
  
  These revisions will better demonstrate the novelty and value of the current study, ensuring readers can clearly see what is new and significant.
  
  In addition, we will compare the prestack and poststack migration results as we tested both before we decided to present the prestack time migrated results as the method offered us better results.
  
  Commented: Please add a table summarizing the acquisition parameters.
  Thank you for the suggestion. We will add a table summarizing all acquisition parameters, including source type, receiver spacing, number of shots, line lengths, and recording parameters, in the revised manuscript to provide a clear and concise overview for the readers.
  
  Commented: Please include examples of the seismic data both before and after processing. This would allow readers to better evaluate the impact of your workflow (see also the comments in the pdf ).
  Thank you for the suggestion. We will include examples of the seismic data both before and after processing in the revised manuscript. This will clearly illustrate the improvements achieved through our workflow and allow readers to better assess the impact of the processing steps.
  
  Commented: Provide a more detailed explanation of how guided waves were handled during processing.
  Thank you for the clarification. We will update the manuscript to explain that guided waves were removed using a radial trace transform filter, where the guided waves were characterized in the radial trace domain, transformed back to the spatial–temporal domain, and subtracted from the data. Additionally, FK filtering was applied to further attenuate any remaining guided-wave energy. This will provide a clear and detailed description of the processing approach.
  
  Commented: Currently you are showing less information compared to Jogee et al. (2025.
  Thank you for your observation. We acknowledge that some elements presented in Joggee et al. (2025), such as full 3D perspectives and horizon picking across all profiles, are not fully shown here. In the revised manuscript, we will clarify the focus of the current study, emphasize the new contributions, and provide additional figures or supplementary material where appropriate to present more comprehensive information while maintaining clarity.
  
  Commented: When describing sediment thicknesses across the profiles, consider including a map of thickness distribution. This would help clarify where the sedimentary cover (low-velocity zone) is thickest and how it may impact image quality.
  Thank you for the suggestion. We will include a map showing the distribution of sediment thickness across the profiles in the revised manuscript. This will highlight areas with the thickest sedimentary cover (low-velocity zones) and help readers understand how variations in thickness may affect seismic image quality.
  
  Commented: Profile 3 appears to be missing
  Thank you for pointing this out. As noted earlier, Profile 3 was omitted due to poor data quality, which made reliable interpretation difficult. We will clarify in the manuscript that Profile 3 is not included and explain the reason for its exclusion to avoid any confusion.
  
  Commented: Ensure that the profiles are displayed with the correct aspect ratio (Profiles 1 and 2 are twice as long as Profile 5.
  Thank you for the comment. We will ensure that all profiles are displayed with the correct aspect ratio in the revised manuscript so that the relative lengths and scales of Profiles 1, 2, and 5 are accurately represented.
  
  Commented: Clarify whether different CDP bin sizes were used across the profiles.
  Thank you for the clarification. We will update the manuscript to explain that the CDP bin sizes were calculated as half the receiver spacing (CDP bin size = ½ × receiver spacing). Because the receiver spacing varied between profiles, the CDP bin sizes also differed accordingly. This will be clearly stated to avoid any confusion.
  
  Commented: You provide a detailed overview of the expected boundaries. Have you computed a synthetic seismogram from the series of reflection coefficients? This would allow for a direct comparison with the seismic images and could significantly strengthen the interpretation.
  Thank you for the suggestion. We have not yet computed a synthetic seismogram in this study. However, we acknowledge that generating one from the series of reflection coefficients would allow a direct comparison with the seismic images and strengthen the interpretation. We will include this step in the revised manuscript to clearly demonstrate the consistency between the geological model and the seismic response.
  
  Commented: Why is no 3D interpretation or perspective view included here, given that one was already presented in Jogee et al. (2025).
  Thank you for the comment. In this study, we focused on presenting the 2D seismic interpretations and the new tomographic results, which represent the contributions beyond Joggee et al. (2025). For this reason, we did not include a 3D interpretation or perspective view. However, we recognize that including a 3D perspective could provide additional context and continuity with the earlier work. We will therefore add a 3D perspective view to the revised manuscript to enhance comparison with the previous study.
  
  Commented: Please add interpretation figures for each profile (highlighting the main geological units, etc.), similar to Profile 1 in Figure 12 of Jogee et al. (2025)
  Thank you for the suggestion. We will add interpretation figures for each profile in the revised manuscript, highlighting the main geological units and reflectors, similar to the presentation of Profile 1 in Figure 12 of Joggee et al. (2025). This will provide a clearer visual summary of the subsurface geology across all profiles.
  
  Commented: see also the comments in the pdf
  Thank you for the note. We have reviewed all the comments in the PDF and will address them systematically in the revised manuscript to ensure that all suggestions and corrections are incorporated.
  
  Citation: https://doi.org/10.5194/egusphere-2025-3117-AC2
EC1:
'Comment on egusphere-2025-3117', Christopher Juhlin, 23 Aug 2025

Dear Authors,
Two reviewers have carefully read the manuscript and consider that required revision is necessary before publication. Please fprovide point by point responses to the comments by the reviewers. It is particularly important to emphasize what is new in the paper compared to previous work.
Best Regards,
Chris Juhlin (Guest editor)

Citation: https://doi.org/10.5194/egusphere-2025-3117-EC1
- AC3: 'Reply on EC1', Mpofana Sihoyiya, 17 Sep 2025
  
  Dear Prof. Juhlin,
  We thank you and the reviewers for the constructive feedback on our manuscript “Reflection seismic imaging of the manganese mineralisation in the Griqualand West Basin, South Africa.” We will revise the paper accordingly and provide a detailed point-by-point response.
  The revised version will emphasize what is new compared to Jogee et al. (2025), including: (i) a stronger focus on reflection seismic imaging, (ii) comparison of pre-stack and post-stack migration approaches not shown previously, and (iii) improved discussion of refraction tomography and near-surface structures. We will also refine the conclusions, add a processing table with key parameters, and improve figures with clearer stratigraphy and marked reflections.
  Best regards,
  
  Mpofana Sihoyiya (on behalf of all co-authors)
  
  Citation: https://doi.org/10.5194/egusphere-2025-3117-AC3

Mpofana Sihoyiya, Musa Siphiwe Doctor Manzi, Ian James, and Michael Westgate

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Short summary

We used sound waves to look beneath the surface in the Northern Cape of South Africa to search for hidden layers of rock that may contain valuable manganese deposits. Our study revealed nine distinct underground layers, including one likely to host manganese. This method helps locate buried resources without drilling, showing how advanced imaging can support mineral exploration in areas covered by thick sand.


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