The authors of this manuscript present a solid example of the application of HVSR to map near-surface structures (<100 m) in a dry and arid region in Chili. Overall, the processing of the HVSR is sound, and the resulting 3D volume of Vs velocities is well done. Two points were puzzling to me concerning the HSVR. The first was the presence of > 100 Hz in the spectrum, and the second was the resolution of the method below bedrock. Details of these comments are given below.

Where I think the manuscript can be improved is around the story of weathering-induced fractures versus tectonically created fractures. I think this is a fantastic way to frame this data set because there is significant value in understanding if fractures are created by weathering or formed from other tectonic stresses (maybe even during formation) for the Esurf community, so I believe it’s in the authors best interest to clarify some of these points. The authors argued two points. The first point is that there is no saprolite at the site, and the second is that the fractures in the fractured bedrock are from tectonic fractures and are not created by weathering. More details are given below, but the first point is based on the idea that the velocity is greater than 0.8 km/s, which doesn’t appear until the discussion section around L352. Was there saprolite observed in the core shown in Figure 5?  It doesn’t look like it, but it was never clearly stated. Can the authors say with certainty that there was no saprolite at the site from that core? Did the authors have to set casing somewhere? A velocity of 1000 m/s is not all that much higher than 800 m/s, so although I agree with the authors that the Vs velocities are quite fast, I think there needs to be some more evidence supporting the idea that there is no saprolite at the site.

The second argument was that the fractured bedrock is a result of tectonic or inherited fractures and not weathering. From a geophysical standpoint, you can only determine if there are fractures, which is questionable. However, after reading through this, I think that I pieced together this argument: the authors believe these re tectonically or inherited fractures because this area is arid (but the data are not shown or given), if there was water present, then the velocities would be much lower than 0.8 m/s and there would be saprolite at the site.  The second part of this argument references another two sites (but not shown here) that weathering is deeper elsewhere. I think these are valid points, but they need to be articulated a bit better in this paper so that non-geophysicists, someone from regolith/critical zone science, will recognize the significance of the findings. Overall, I think the authors have all the information there to support this argument but can re-order and change the emphasis in some locations to really increase the impact of this paper, which I believe is critical to the ESurf community.

I do much seismic refraction and have done a lot of MASW. I have not worked with HVSR data myself, but as I was reading through this paper the first time, I had a few questions crop up: How important is setting the bedrock velocity in the HSVR inversion? For example, if you select a bedrock velocity that’s too fast, what effect does that have on the inverted results? The authors could address this by giving an example or something, “Selecting bedrock velocities is critical because if we overestimate the velocity, we get a slightly shallower first layer”. Or is this point irrelevant because the authors use an MCMS type of inversion and sample a full range of velocities? Either way, it was a bit confusing, and I thought I would raise it to help the readers know how someone familiar with inversion theory and geophysical methods, but not nceisarrly HVSR read through this section.

L150: “For the inversion workflow, we followed the scheme described by Bodin (2010), which had also been applied to the HVSR curve inversion (Cipta et al., 2018; Trichandi et al., 2023).” From the standpoint of this paper, I don’t know what the authors did without going back and reading Bodin (2010). Can the authors summarize the paper in 1-2 sentences? What made this flow important? Why is the ratio between the Horizontal and Vertical important? The authors never actually give a single-sentence explanation for how HSVR works. It would be nice for readers more interested in the weathering story, as opposed to the geophysical inversion if that makes sense. I would suggest something like, “HVSR works by comparing the frequency spectrum between the vertical and horizontal components because of the way surface waves get trapped in the boundary”

L144: “Furthermore, reliably fixing the bedrock velocity would necessitate accurate P- and S-wave velocity information.” Doesn’t this exist from Tridanci? How far away was that site from this one? They have MASW and p-wave refraction data. Why not use the bedrock velocities from this study?

L150: “For the inversion workflow, we followed the scheme described by Bodin (2010), which had also been applied to the HVSR curve inversion (Cipta et al., 2018; Trichandi et al., 2023).” From the standpoint of this paper, I don’t know what the authors did without going back and reading Bodin (2010). Can the authors summarize the paper in 1-2 sentences? What was special about this method? Why did the authors choose to follow this authors processing flow?

L155: “For the number of layers, we also enabled the inversion to explore models with a number of layers between 2 and 20 layers (including half space).” If you know the sis a weathered granite why have more than 4: soil/saprolite/fractured bedrock/bedrock?

L159: So even though you let it vary to 20 layers the inversion results zeroed in on 3-4 layers? That’s quite remarkable if all the uncertainties above are correct. I would suggest highlighting and emphasizing this point. For example, Although we allowed our inversion to fit up to 20 layers, in most weathering systems developed over crystalline rocks have 3-4 layers: soil, saprolite, fractured bedrock, and bedrock (your pick to a large number of available references). Despite the large range of possibilities that we allowed our model to search, the best fitting models are consistent with the weathering idea of 3-4 layers without us adding additional constraints.

L169: “It is also important to note that we did not invert the density value in our inversion and used the inverted Vp value for the density approximation.” What relationship did you use? Is it empirical is it linear? Density = m*Vp ?? Or is it exponential, density = a*Vp^x?

L194: “he HVSR curve already brings an insight into the weathering zone depth as the peak frequency from the HVSR curve is often linked to the top of the bedrock depth” For the non-experienced HSVR folks can you add a bit more to this? For example, if the peak is at a higher frequency, the bedrock is shallower, or if the peak is lower its’ deeper. Essentially, how is the peak of the spectrum related to the bedrock depth without having to go and read these three citations? In other words, in one to two sentences can you sum up those references? I also this would be a good place to put the first equation. This equation answers my question above, but right now the equation is in a different sections so can the authors say something like, “if the Vs of the overburden and bedrock are known or assumed, then the peak of the frequencies is related to the depth of bedrock through Eq. 1:”

Figure 2. How does the peak around 2800 s affect your HSVR inversion? Is that an earthquake? Is it an active source? I assume that’s responsible for the peak around 100 Hz in panel G. What would have happened if you didn’t have that spike. For example, in Figure 4, are the spikes of linear features a result of where the ambient energy is higher frequencies? It kind of looks like that spike at 100 Hz is not consistent across all the soundings in Fig. 3. Which makes me wonder if the dikes are a result of the presence of this high-frequency energy. I look at the final figure and believe the authors especially because you can see the dikes in figure 1, but presenting the data with this large spike really makes me question the presence of those high-velocity dikes.

L207: “The most distinctive feature is the two southwest–northeast stripes of high peak frequencies” Is this a result of whatever the noise spike is? This is tied into my question on Figure 2 above. What do the authors think would if you didn’t have that spike? Without it would you have enough energy in the 100 Hz range? Even for someone who doesn’t to HVSR, 100 Hz seems high for a passive survey. With my active source sledgehammer data, I usually get peak frequencies around 65-85 Hz. I agree with the authors that the dikes are real because they structurally line up in the survey, but some additional explanation for the presence of those higher frequencies is certainly necessary in the proceeding sections. Maybe think about changing Figure 2. Can you show a sounding that doesn’t have a huge spike that still has the 100 Hz peak present?  In other words can you make figure two so that it shows what woul appear to be noise and still have the 100 Hz energy? This really makes me think at this stage in the manuscript (maybe you address it later) that high bedrock is a result of some external source creating higher frequencies.

L250: “and hydrothermally altered granite” How do the authors know this is hydrothermally altered? Is this coming from the borehole logs? There is no way to know this simply from the velocity information. Please add some support for this interpration.

What’s’ the random symbols on L293?

L308 should be difficult to prove not difficult to proof.

L315: “From the bottom, we have the granite bedrock, which was intruded by two mafic dikes. Then, from ca. 30 meters depth to the surface, we have an overlying altered and fractured granite. The mafic dikes in our deployment area also do not seem to intrude up to the surface as we did not find any exposed dike in the middle of the profile during the data acquisition.” Is it possible that the interpreted dikes are just lateral heterogeneity in the weathering profile, something like Tor coming to the surface? Does the HVSR confirm a fast velocity at depth? Or does the sensitivity of the HSVR method stop at the top of the bedrock? I understand that you get a peak because of energy bouncing around in a low-velocity layer. According to the EPA, “By measuring three components of seismic noise over time, the HVSR method can determine the fundamental resonance frequency (f₀) of the overburden at a point location. This frequency is related to the layer thickness (h) (i.e., depth to bedrock) and average shear-wave seismic velocity (Vs) of the corresponding unconsolidated materials. Thus, the HVSR method can be used to estimate the thickness or shear-wave velocity of the sediments that sharply overlie bedrock.”

In other words, those peaks resonate in the overburden layer, which would suggest you can’t know anything below this boundary right? So another alternative was to look at Figure 8 is a mask below bedrock, which would make your section show a rise in bedrock with a higher velocity (maybe a tor that’s unfractured), as opposed to the dikes. I agree with the authors that the dikes shown in Figure 1 support their interpretation but the way Figure 8 is drawn does not sit will with my conceptual understanding of HSVR.

 L335: I am not sure how this is supported; you say tectonic stress or lithostatic decompression, but the way the manuscript is currently written, it’s tough to know where and why the authors arrived at this conclusion.

L338: “However, our study site in Pan de Azúcar provides an excellent observation of conditions preceding chemical weathering” How do you know? Is this relying on the cross-cutting relationships of the mafic dikes? Is it because of the timing of magma emplacement? It’s really not clear to me why this is a good example and how you even know those are tectonic fractures versus weathering fractures. Does climate come into play? This is what’s said on L340, “These, however, were absent in Pan de Azúcar given the absence of water and vegetation”. This needs to be moved up in the paragraph—the authors should lead with this.

L334-L359: This paragraph is complicated to follow because I haven’t read Krone et al. I think this is a critical point to make in terms of the weathering story, but you need to include some visual representation. Can you take the average depth to bedrock and plot it against precipitation or temperature (two climate variables). I think the authors are trying to say that the thickness of bedrock is consistent with a climate-controlled system, thus, at this site that is dry and hot weathering is thin despite being fractured. This argument doesn’t really come off all that clearly, or doesn’t feel all that supported as written, though I can see the logic coming through. The authors are pulling from a lot of observations elsewhere that are not shown in this paper, so they need to describe some specifics from those cases to help hit this point home. In other words, please include relevant information so that this paper can stand alone.

Overall, I think that this paper provides a unique and critical observation of the shallow near-surface on a scale that is not traditionally done. I agree with the authors that the framing of these fantastic results in terms of weathered versus inherited fractures is of great interest to the ESurf community, and hope that my comments provide the authors some insight into how a CZ geophysicist understood the paper so that the authors can help clarify some things. I do believe all the pieces are there, just some slight re-organization and inclusion of some details from the cited papers will make this a much improved and high-impact paper. Well done.

Review

3D shear wave velocity imaging of the subsurface structure of granite rocks in the arid climate of Pan de Azúcar, Chile, revealed by Bayesian inversion of HVSR curves

By Rahmantara Trichandi et al.

General Comments:

This paper evaluates the 3D structure of monzogranite bedrock near the Pan de Azúcar National Park (Atacama Desert, Northern Chile), using Bayesian inversion of horizontal-to-vertical spectral ratios (HVSR). The authors analyzed a large number of HVSR curves, derived from ambient noises, recorded at over a dozen stations in the study region. The authors used their best HVSR curves to invert for 1D shear wave velocity models, which later used to generate 3D model beneath the study region. The results are summarized in 7. Conclusion and outlook:

Based on the Vs model, we identified fractured and altered granite in Pan de Azúcar to reach 30 – 40 meters deep. We found no Vs signature of a saprolite layer that is indeed absent in Pan de Azúcar due to the arid climate. Mafic dike structures were identified at depth.

The technical quality of the paper is good although there are considerable grammar/writing mistakes that should be corrected before publishing. Authors have clearly analyzed and interpreted the results. Novelty of the research is also acceptable. However, I have some suggestions for authors to improve the quality of the manuscript before publishing it. Therefore, I recommend accepting this article with major revisions. My suggestions are as below:

The authors assembled a large data set and rigorously processed HVSR curves to generate 3D shear wave velocity model. However, it is somewhat disappointing that the presented results do not include resolution test (e.g., checkerboard test. See Wang et al., 2021) to estimate the horizontal and vertical resolution of the model. I would suggest adding that into the results section which will improve the quality of the manuscript.

It would be great to compare shear wave velocity profile in the borehole with the 1D profiles extracted at nearby stations. If any information available, please add that comparison into the manuscript.

Specific comments:

1. Line 24: Add “shear wave velocity (Vs)”.

1. Line 44-45: Not clear what authors wanted to report here.

1. Line 50-53: Not clear. Re-write the sentence.

1. Line 89-90: Use subscript (e.g., Na₂O)

1. Line 104-106: Remove sentences start with “ Initially…” and “However…”

1. Line 203: Remove “quickly”

1. Line 233: Define “masl”

1. Line 116: Define “SESAME”
2. Line 238: Please add velocity contour lines into figure 7b to show 3 layers discussed in the text.

1. Line 307: Replace “also ask” with “assume”. I noted similar types of wording at many places in the manuscript (e.g., sentence start with “We found…” in line 377) and I would recommend fixing those.  

1. Line 293: What is “βpo”

Figures:

Fig 1: Caption- Replace “Red dots” with “Black dots”.

Fig 2: Please add Y-axis labels for figures A-F. Where is this receiver located? Add receiver location into figure 1. Are the records in velocity or displacement or raw data?

Fig 4: Add shading (e.g., gray color) to the area where you have less data coverage (white region in your plots) or lower resolution (see example figures in Wang et al., 2021).

Fig 5: Add data point location into figure 1. If Vs profile beneath the borehole is available, pleas add that into figure B for comparison.

Fig 6: Add shading as I mentioned above for Fig 4. Please add color scales corresponding to each slice. If you use same color scale for all figures, then, put only one-color scale bottom of the figure. Fig A-C needs x-axis labels. Remove words top on each figure (e.g., Depth Slice DS: XX) and add depth value upper right corner of each figure as “10 m or 20 m etc.” (see example figures in Wang et al., 2021). Keep black solid line (cross section) within the area where you have the best resolution.

Fig 7 & 8: For Fig A; similar comment as Fig 6.

                   For Fig B; Zoom into the region where you have the best resolution. It seems like X axis is not correct. Remove text above the figures B and C. Mark ‘NW’ and ‘SE’ corner of the profile (see example figures in Wang et al., 2021). Remove depth axis in Fig B and C.

Reference suggested:

Wang, W., Nyblade, A., Mount, G., Moon, S., Chen, P., Accardo, N., et al. (2021). 3D seismic anatomy of a watershed reveals climate-topography coupling that drives water flowpaths and bedrock weathering. Journal of Geophysical Research: Earth Surface, 126, e2021JF006281. https://doi. org/10.1029/2021JF006281