|I would like to thank the authors for taking the time to address the concerns raised during my first assessment of the manuscript. I think that the redefinition of this study’s objectives and clarification of some misunderstandings on my part have improved the quality of the manuscript. I agree with most of the comments made by the authors in response to my assessment, but I still see one major issue that I think should be addressed. (The page/line numbers mentioned below refer to the tracked changes document.)|
I still do not agree with the interpretation that DAS amplitudes “robustly correlate with near surface geology” (abstract, L22). In their rebuttal, the authors suggest that their interpretation is based on a somewhat arbitrary amplitude threshold of 0.1. If I apply the same threshold for other sections of the cable, then I would expect to see numerous subsurface features in Fig. 7 and one feature in Fig. 8, which do not seem to be present. The authors also mention that the correlation with the velocity model is contradictive. To explain this contradiction, they argue that the injection wells are concentrated in one particular area with high Vp, “and thus the fracture network is expected to be well developed” (P8 L244-245). I don’t quite follow this reasoning, since in this case I would expect to see a low instead of a high Vp.
Looking at Fig. 21 of Wang et al. (2018), which shows P/S-wave SNR for the entire DAS array, a similar picture appears. Even though there exist array-scale trends in the SNR, these trends do not seem to correlate immediately with mapped geological features (faults, warm grounds, fumaroles). Of course, there could be deeper geological features with no surface expression that could be the cause for the trends in site effects, which should then manifest themselves in the 3D velocity models.
Given that fluctuations in the DAS amplitudes are seen in several places across the array that do not consistently correlate (in my opinion) with mapped near-surface geological features, I suspect that the correlation between the amplitude variation and one feature seen in Fig. 10 is due to chance. If the authors want to maintain their claim that DAS amplitudes robustly correlate with near-surface geology, they will have to quantify how robust this correlation is for the entire cable, and show that the null-hypothesis (no correlation) can be rejected with reasonable confidence. By giving only 1 example that qualitatively correlates well (and 1 example that doesn’t show a direct correlation), the reader cannot assess the significance of the correlation and evaluate what “robust” really means.
Alternatively, if the authors agree with me that the correlation is potentially due to chance, they could modify the abstract/main text to indicate that “the DAS and nodal arrays are in mutual agreement when it comes to site amplifications, but that it is not immediately clear which geological features are responsible for these amplifications. DAS could therefore hold potential for detailed mapping of shallow subsurface heterogeneities, but with the currently available information of the Brady Hot Springs subsurface geology, this potential cannot be quantitatively verified”. In this way, it is clear to the reader that more work remains to be done, instead expecting that DAS is a “robust” tool for subsurface mapping.
Martijn van den Ende
P2 L33, “derivative of strain”: maybe a more general way of phrasing would be that DAS measures the spatial derivative of particle motion. The interrogator used in the PoroTomo experiment measured strain rate (the temporal derivative of strain), but other interrogators measure just strain.
P2 L41: as Reviewer #2 already pointed out, Marra et al. did not use DAS
P10 L 315: DAS is not horizontal by definition, but by configuration.
Fig 11: I think the caption is mislabelled. Panel (a) shows the nodal array, panel (b) shows the DAS array.
PWF method: I think we can debate for quite some time about the (dis)similarities between PWF and beamforming, but it would be better to do this over a coffee at a conference than through peer-review reports/rebuttals.