the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Investigation of Seismic Anisotropy in the Undisturbed Rotondo Granite
Abstract. The hypothesis of stress-induced seismic anisotropy was tested in the Bedretto Lab, a deep underground rock laboratory in the Swiss alps. Several comprehensive crosshole seismic surveys were acquired to analyze the directional dependency of seismic wave velocities in the undisturbed host rock. This required precise knowledge on the source and receiver positions as well as a good data quality that allow the determination of traveltimes for different wave types. A tilted transverse isotropic (TTI) model could be established that explains the measured data to a first order. All relevant model parameters could be well constrained using P- and S-wave arrival times. However, a systematic misfit distribution indicates that a more complex anisotropy model might be required to fully explain the measurements. This is consistent with our hypothesis that seismic anisotropy has a significant stress-induced component. More controlled laboratory experiments on the centimeter to decimeter scale were performed to validate our field measurements. These measurements show a comparable order of P- and S-wave anisotropy in the rock volume. The knowledge on the driving mechanism for anisotropy in igneous rocks can potentially help to enhance the monitoring of stress field variations during geothermal operations, thereby improving hazard assessment protocols.
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RC1: 'Comment on egusphere-2024-1919', Wojciech Gajek, 07 Oct 2024
The paper discusses the hypothetic impact of stress-induced anisotropy on the velocity of seismic waves. Importantly, the analysis addresses the igneous type of rocks and includes a simple yet directly addressing problem case study with good-quality data. This unique case study examines stress-induced anisotropy due to the impact of overburden stress on igneous rocks in an in-situ condition that falls between reservoir and lab scales.The paper is well-written, clearly presents the data and contains a comprehensive discussion. The paper may have been better if it contained: a wider picture of stress-induced anisotropy in the introduction together with a more complete discussion of previous and similar works in stress-induced anisotropy topic. Also, Authors should carefully revise the text to eliminate phrases like "slightly better" and eliminate typos. A nice addition, that might have been used by the Authors in future is including fractures in the inversion of the stiffness tensor for more complete inversion.Nevertheless, according to the provided discussion, the presented material is sufficient for drawing presented conclusions. However, one the cited materials (namely "Schneider, C.: Seismic Velocity Anisotropy and Microstructural Characterization of Ductile Shear Zones in the Rotondo Granite, Ph.D. thesis, 2022." ), to my understanding is the only and crucial source to verify the claims of the Authors regarding the absence of intrinsic anisotropy in discussed rocks: "The undeformed Rotondo granite is also fully isotropic under high pressures in the lab, indicating that the anisotropy is not intrinsic from the material itself but must be controlled by external factors (Schneider, 2022)." Unfortunately, despite my attempts, I was unable to find this publication (thesis) to verify the claims above.Therefore, I would recommend this paper undergo a minor revision, during which I would require the Authors to update the reference to be complete (including e.g. a link to university repository) to the cited thesis that would be in agreement with Solid Earth's recommendations on citing theses, as a condition to be accepted. Alternatively, Authors should provide another reasoning (e.g., a modified figure from that thesis if possible) showcasing a lack of intrinsic anisotropy. Without these, the results is too speculative to be published in current form.Some specific comments, corrections etc are included in the annotated pdf.
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AC2: 'Reply on RC1', Kathrin Behnen, 11 Dec 2024
Thank you for your detailed and constructive feedback on the manuscript. Your comments are greatly appreciated and will definitely help to further improve the paper. We understand your concerns that the paper should include a wider picture of stress-induced anisotropy and lacks the comparison with other studies on this topic. We will revise the introduction and discussion section to incorporate relevant studies.
We also highly appreciate your comment, to include fractures in the inversion of the stiffness tensor. You are right that this could lead to a more complete inversion. We will keep this in mind for further studies on this topic.
Thank you for indicating that the work of Schneider et al. (2022) is not available online. You are right that the material you mentioned is essential for the analysis of the intrinsic anisotropy of the Rotondo granite analyzed in this study. It’s the only work so far, measuring seismic anisotropy under controlled conditions in the lab for different (confining) pressures.
The thesis is submitted to the research collection of ETH Zurich and will be available soon. The DOI will then be added to the revised manuscript. Unfortunately, there is a mistake in the reference list of the manuscript, it’s not a PhD thesis but a master thesis. We will of course correct this and I’m very sorry for this mistake.
We also appreciate your comments and correction in the manuscript itself. Responses to the comments are directly included in the attached pdf.
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AC2: 'Reply on RC1', Kathrin Behnen, 11 Dec 2024
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RC2: 'Comment on egusphere-2024-1919', Leon Thomsen, 03 Nov 2024
- AC1: 'Reply on RC2', Kathrin Behnen, 11 Dec 2024
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