Physics-Based Relationship for Pore Pressure and Vertical Stress Monitoring Using Seismic Velocity Variations
Round 1
Reviewer 1 Report
Dear authors,
the manuscript (MS hereafter) “Physics-based relationship for pore pressure monitoring using seismic velocity variations” by Fokker et al., proposes an intriguing methodology dealing with the analysis and the monitoring of seismic velocity changes as a proxy for the study of possible pore pressure variations. The methodology, theoretically sounding, has been tested on the Groningen site by adopting its dedicated seismic network, which is very important for the monitoring of seismic activity related to the exploitation of the gas field of the area.
The manuscript is very well written; the introduction is complete, even if some references related to the study of the spatial and time-lapse variations of elastic properties of the subsoil should be provided. The mathematical derivations of the adopted equations are clear and right. The figures are clear and easy to be read. The interpretation is justified by the results. I would like to point out only an issue concerning the methodology:
- You validated the robustness of the proposed approach by comparing the surface wave velocity variations inferred from interferometry with the ones modeled by means of the adjoint technique by Hawkins (2018), in its turn depending on the stress-induced shear wave velocity variations. In particular, your estimations from interferometry come from the stretching method, “based on the assumption that velocity change is homogeneous. This assumption is valid for velocity changes due to fluctuations in the pore pressure, because the causes of these fluctuations, precipitation and evaporation, are similar for the whole area”. Then you compare these estimations with the ones coming from your modelling, in which the “effect of the vertical compressional stress” is neglected (eq. 12) and only the pore pressure is taken into account. In conclusion, the estimations of velocity variations retrieved from the modelling and the ones coming from interferometry are both based on the assumption of a negligible effect of the vertical compressional stress. My question is: “Do you think that the agreement between these estimations are somehow influenced by the assumptions underlying the methods adopted by you or not? And, if not, are you able to provide with a justification?”
I will provide you the manuscript with some minor “line-by-line” comments, underlining typos and requests of clarifications.
Best regards
Comments for author File: 
Comments.pdf
Author Response
Dear anonymous reviewer,
Thank you for your constructive comments and suggestions. Please find our point-to-point responses in the attachment.
Kind regards,
Eldert Fokker, Elmer Ruigrok, Rhys Hawkins and Jeannot Trampert
Author Response File: 
Author Response.docx
Reviewer 2 Report
In this paper the authors propose a physics-based model to monitor the pore pressure using seismic velocity variation. They test this model by using a subset of data acquired by a dense seismic network in Groningen (Netherlands). The paper is well written and the English is fluent. Anyhow, in my opinion, minor revisions are needed before its publication in this journal according to my comments given below:
- I propose to add in the title the words “near surface” between “for” and “pore pressure” (i.e. … relationship for near surface pore pressure monitoring …). Indeed, when I read the title I hypothesized that the paper was focused on the pore pressure monitoring related to the seismicity induced by gas storage. Since such area is a well-known case study of induced seismicity, I think that several researchers could make the same mistake.
- I suggest to better emphasize the objective of the work both in the abstract and in the last part of the introduction as well as to explicit the investigation depth of your work.
- Section 3.2, page 5, line 157: substitute “until” with “down to”
- Figure 3d: for depths below 20 m I observe clear vertical bars down to 850 m. Please, could you better explain and clarify why we observe these bars in the figure?
- Figure 4: Why the model of the shear-wave velocity change produce an abrupt change at 50 m depth? Can you explain this observation?
- Figure 3a, Figure 7: I suggest to use different line types in addition to different colors in order to make the figures understandable also in gray scale.
- Figure 9: Please label with “(a)” the panel for RR, with “(b)” the panel for ZZ, and with “(c)” the panel for TT. It is not mandatory, but I suggest to plot also the first overtone dispersion curve at least on the RR component.
- Discussions: I understand that your forward model has been validated by matching the results with the observed variation of the shear-wave velocity. I think that providing an example of results expressed directly in terms of pore pressure variations instead of variation of the shear-wave velocity would be beneficial for your paper.
- Conclusions: Conclusions are too short. I suggest to add a list of items reporting the main findings of your work. This will help the reader to focus the key points of your work.
Author Response
Dear anonymous reviewer,
Thank you for your constructive comments and suggestions. Please find our point-to-point responses in the attachment.
Kind regards,
Eldert Fokker, Elmer Ruigrok, Rhys Hawkins and Jeannot Trampert
Author Response File: Author Response.docx
Reviewer 3 Report
Manuscript entitled "Physics-based relationship for pore pressure monitoring using seismic velocity variation" presents development of physics-based model
relating fluctuation in the groundwater table and the pore pressure with seismic velocity variation through change in effective stress using a subset of Groningen seismic network.
I like study, well written manuscript.
General comment - abstract and conclusions should be more concise with more details on the study: motivation, aim, method, results, conclusion, future work.
Minor comments:
- put Figure 1 below paragraph of chapter 3, prior to 3.1. It is confusing to be in Theory chapter. In Figure 1 put scale bar and north arrow (RS guidelines - Images and maps must have a scale, a north arrow and coordinates).
- In general it is better to put figures after main body text. Put Figure 2 prior to 3.2. and after paragraph that deals with this figure. Please check for all other figures.
- On Figure 9 can you show all three dispersion curves with thicker red line, not black - it is barely seen. Z axis is missing for colored amplitude spectra. RR and ZZ are strangely fitted at very low frequencies?! Can you explain it? Usually dispersion is fitted through high amplitude part of spectra.
- Z legend for colors in figure 10, althogh is mentioned in caption.
Author Response
Dear anonymous reviewer,
Thank you for your constructive comments and suggestions. Please find our point-to-point responses in the attachment.
Kind regards,
Eldert Fokker, Elmer Ruigrok, Rhys Hawkins and Jeannot Trampert
Author Response File: Author Response.docx
