Comment on se-2021-37

This paper is a relatively brief focused discussion on methods for how a DAS array can be used to 1) map subsurface heterogeneity (“exploration”) and 2) detect and locate seismic events (“monitoring”). The methods are validated and compared with geophone (“node”) data co-located with the array in the publicly available PoroTomo data set. The paper achieves its goals and is a useful contribution to understanding how to best to use DAS data. The paper should be particularly valuable to anyone planning a surface DAS array for one or both of the targeted applications.

for one or both of the targeted applications.
While I recommend publication, I do have a list of editorial comments, suggestions, and questions. The most significant one is for readability and it is to consider combining the methods and results for the two categories of results ("exploration" and "monitoring"). I do also prefer that "exploration" be called "subsurface site effects" or "subsurface heterogeneity mapping" and the "monitoring" be called "seismic event and location detection". These titles are clumsy but they don't overpromise. After all, the basic result in the "exploration" category is relative site effect time delays of +/-0.12 seconds. Therefore, I might also suggest a more specific title, e.g., Methods for subsurface mapping and event detection using Distributed Acoustic Sensing: Examples from Brady Hot Springs PoroTomo Experiment Specific Comments, Suggestions, Questions. l. 1. "PoroTomo" is short for "Poroelastic Tomography" so there is no reason to have the "TOMO" all caps. l. 118-150 -These paragraphs from "Methods" would most seamlessly be followed by the "Location" results (see l. 219).
L.207-209. Can you suggest a reason why the criterion for removing channels needs to be increased to 2 gage lengths from a corner? l. 212-216 -Can you relate this result to local site effects presented in Parker et al. or Zeng et al.? l. 219 -Note how out-of-context this statement is because its antecedent goes back to l. 118-135. It is best to discuss this result right after l. 135. Even though the paper is short, I don't think the reader can be expected to keep this in mind. Consider keeping the "data and methods" and their corresponding results together.
l. 225 -Similarly to above, EDT was referred to back in "methods" section. Also, "Conversely" is a term of logic. The meaning here is "In comparison".
l. 270 -Don't understand "(2) with a relatively poor areal coverage with respect to a standard seismic network" when DAS arrays up to 50-km in length can be deployed.
l. 271-272 -The statement about "old style" neglects recent deployment of large N node (e.g., Long Beach). So the proper comparison is between large N nodes and DAS.
l. 276 -How does scattering of late arriving surface waves affect P? In general, the wavelengths of P mean that segment-scale heterogeneity or topography should not be cause poor wavefield coherence. Variable coupling of the cable seems more likely.
l. 283-285 -Are "not-aligned segments" the same or different than "three separated segments." i.e., is the difference 2 vs. 3, or parallel vs. not parallel? What is the point here?
l. 289-290 -Maybe add that teleseismic waves coming into low-velocity, near-surface sediments are nearly vertical and horizontal DAS sensitivity is attenuated by its cos^2 dependence. Also, emphasize that this is a consequence of the PoroTomo geology and is not necessarily a generalization about DAS.
l. 302 -Clarify meaning of "intra-profile channels". Statement seems contradictory. How can very coherent channels be dominated by locally-scattered waves, unless the profile length is small compared with scatterers? l. 311-315. Did not try to estimate magnitude of events, but maybe ML can be applied if there are a large number.