Preprint Article Version 4 Preserved in Portico This version is not peer-reviewed

Investigation of the Time-Lapse Changes with the DAS Borehole Data at the Brady Geothermal Field Using Deconvolution Interferometry

Version 1 : Received: 30 April 2021 / Approved: 3 May 2021 / Online: 3 May 2021 (16:42:03 CEST)
Version 2 : Received: 26 June 2021 / Approved: 29 June 2021 / Online: 29 June 2021 (11:49:00 CEST)
Version 3 : Received: 5 November 2021 / Approved: 8 November 2021 / Online: 8 November 2021 (13:15:26 CET)
Version 4 : Received: 27 December 2021 / Approved: 29 December 2021 / Online: 29 December 2021 (12:39:03 CET)

How to cite: Chang, H.; Nakata, N. Investigation of the Time-Lapse Changes with the DAS Borehole Data at the Brady Geothermal Field Using Deconvolution Interferometry. Preprints 2021, 2021050014 (doi: 10.20944/preprints202105.0014.v4). Chang, H.; Nakata, N. Investigation of the Time-Lapse Changes with the DAS Borehole Data at the Brady Geothermal Field Using Deconvolution Interferometry. Preprints 2021, 2021050014 (doi: 10.20944/preprints202105.0014.v4).

Abstract

The distributed acoustic sensing (DAS) has great potential for monitoring natural-resource reservoirs and borehole conditions. However, the large volume of data and complicated wavefield add challenges to processing and interpretation. In this study, we demonstrate that seismic interferometry based on deconvolution is a convenient tool for analyzing this complicated wavefield. We extract coherent wave from the observation of a borehole DAS system at the Brady geothermal field in Nevada. Then, we analyze the coherent reverberating waves, which are used for monitoring temporal changes of the system. These reverberations are tirelessly observed in the vertical borehole DAS data due to cable or casing ringing. The deconvolution method allows us to examine the wavefield at different boundary conditions. We interpret the deconvolved wavefields using a simple 1D string model. The velocity of this wave varies with depth, observation time, temperature, and pressure. We find the velocity is sensitive to disturbances in the borehole related to increasing operation intensity. The velocity decreases with rising temperature, which potentially suggests that the DAS cable or the casing are subjected to high temperature. This reverberation can be decomposed into distinct vibration modes in the spectrum. We find that the wave is dispersive, and the the fundamental mode propagate with a large velocity. The method can be useful for monitoring borehole conditions or reservoir property changes. For the later, we need better coupling than through only friction in the vertical borehole to obtain coherent energy from the formation.

Keywords

Distributed Acoustic Sensing; Borehole; Time-Lapse

Subject

EARTH SCIENCES, Geophysics

Comments (1)

Comment 1
Received: 29 December 2021
Commenter: Hilary Chang
Commenter's Conflict of Interests: Author
Comment: We have revised the manuscript based on reviewers' comments. The revised version has been accepted for publication on the Remote Sensing journal on Dec. 27, 2021.  Below are our responses the the comments and a description of the corresponding changes:

===== Response to Reviewer 1 Comments =====

Point 1:
Firstly, it is best to add a flowchart to the methods section.
Response 1:
We have added a flowchart in Figure 3 to summarize the procedure and link the text and the equation.

Point 2:
Secondly, the conclusion in Abstract section needs to be supported by specific experimental data.
Response 2:
We have revised the abstract to link to our results of the DAS experiment and processing.

Point 3:
Finally, the discussion section needs to be augmented by some more specific experiments.
Response 3:
We have linked the discussions with specific results as citing figures. We have changed the discussion sections and discussed our processing (lower part), model (appropriateness of the 1D string model), and results (symmetry, wave propagation, velocities).

===== Response to Reviewer 2 Comments =====

Point 1:
I would suggest that the phenomenon the authors are describing is “cable ringing” and not casing ringing. “Casing ringing” is caused by energy traveling in the casing where the casing is free to move independently because of insufficient cement bonding to the formation. Similarly, insufficient coupling to the casing of the cable with the embedded fiber, allows the cable to move independent of the casing.
Response 1:
Indeed, two coupling problems exist in this case, one is between the cable and the casing, the other one is between the casing and the formation. Both cases would cause the ringing signals. We cannot distinguish between them from the current data as both the cable and the casing are made of steel and are in similar velocity range, as similar to the discussion in Miller et al. (2018) and the description in Hartog (2017). However, it is possible that both contribute to the ringing. Also, the ringing shows features that suggest the existence of a separate ringing occasionally (Figure 6). We have changed the mentions of “casing ringing” to “casing and cable ringing” to avoid confusion.

Hartog, A. H. (2017). An introduction to distributed optical fibre sensors. CRC press, pp.360.
Miller, D. E., Coleman, T., Zeng, X., Patterson, J. R., Reinisch, E., Wang, H., ... & Feigl, K. (2018, February). DAS and DTS at Brady Hot Springs: observations about coupling and coupled interpretations. In Proceedings of the 43rd Workshop on Geothermal Reservoir Engineering, Stanford, CA, USA (pp. 12-14).

Point 2:
With such a model, one may have expected the authors to attempt to estimate the limited number of parameters directly from the data using a sort of least-squares estimation process. One may speculate that a good description of the “cable ringing” noise may allow this to be estimated and removed to reveal data that are more interesting to the owner of the well. This might have allowed the authors to do an analysis along the shallower part of the well, like what they do in Appendix B for the deeper part of the well. However, as it now stands, I do not consider the material in Appendix B to be essential to the main topic of this paper, which appears to be describing the “cable ringing” phenomenon.
Response 2:
Using the model for denoising would be difficult for this data set. Because this requires more parameters in addition to the velocity parameter we estimated (e.g., attenuation…) and the low quality of data prevent precise estimation for these additional parameters. We have added discussion on this limitation in the first paragraph of the Discussion. We note that the actual conditions can be more complicated than our model and the solutions are not unique. Therefore, this model may not be appropriate if one wants to analyze full-wavefield inversion, but this model might be the simplest the simplest that can explain our extracted waves well.
We have clarified the structure of the paper at the last paragraph of Introduction where we explain the link between Appendices and the main text.

Point 3:
What is the standard deviation on the slope of -17.1 m/s/◦C (in lines 234-235)? Was this slope estimated from the raw (temperature, velocity) data tuples, or from the average velocity for each temperature?
Response 3:
The slope is estimated from the raw (temperature, velocity; i.e., the data points). To clarify this point, we have added this explanation to the caption of Figure 7. The black curve in Figure 7a is the mean value. The standard deviation (the blue shades) above and below the mean show the variability of data at each depth.

Point 4:
In Figure 8, the velocities of the dominant modes vary between about 4730 m/s and 5220 m/s. Is this consistent with the range of velocities shown in Figures 6?
Response 4:
The phase velocities in Figure 9 (original Figure 8) are at the higher end of the group velocity in Figure 7 (original Figure 6). The velocities are dispersive, and the lower modes appear to have higher velocities in this case. We have discussed this in the 5th paragraph of the Discussion section.

Point 5:
Specific comments, mostly on formulation and language:
Line 7: Replace “tirelessly” with (e.g.) “frequently”.
Line 16: Replace “the the” by “the”.
Line 25: Replace “stream” by “string” or “array”.
Line 41: The terms “artificial stripes” is not very descriptive. This could be replaced by “noise that occurs simultaneously and same amplitude on all sensor channels”.
Lines 41-42: Replace “casing ringing” with “cable ringing”.
Since both cable and casing ringing can contribute to the ringing, we have changed “casing ringing” to “cable and casing ringing” to avoid confusion.
Line 69: Replace “took this advantage” by “used this”.
Line 70: Replace “the building” with “a building”.
Line 74: Replace “with a simple model” with “using a simple model”.
Line 77: Replace “In below” by “In the following”.
Line 80: Replace “this signal” by “the recorded signal”.
Line 87: Replace “that spans about 380 m” with “that is about 380 m deep”(?)
Line 97: Replace “wellhead” with “wellheads”.
Line 125: Replace “retrieved” by “retrieve”.
Line 126-127: Replace “monthly and annually shear wave velocity changes between the” with “monthly and annual changes in shear-wave velocity using”.
Line 167: Replace “i is the imaginary number” with “i = ”.
Line 169: Define “Q”.
Line 180: Replace “source” with “sources” (two occurrences).
Line 186: Do you mean to say “with two sources on both boundaries”, or do tou rather mean “with one source on either boundary” (which would be consistent with Figure 4, and equation 3?
We mean both sources should have comparable amplitude. To clarify this point, we have modified the first few sentences of that paragraph:
“Figure 4 shows that we can reproduce the wavefields using model 1. In Figure 4, the same time window is examined using three different virtual sources (the red lines). The dominant waves exhibit symmetry between causal and acausal times (i.e., left and right to the blue lines) regardless of the virtual source. To achieve this symmetry regardless of the virtual source, the two sources in model 1 need to have comparable amplitudes (Appendix A1).”
Line 191: Replace “Figure 3d-3f show the model” with “Figure 3d-3f show that the model”.
Line 193-194: I would suggest replacing “We reproduce it by putting the dominant source on the other side of the system to the source” with “We reproduce it by letting the source S1 at the depth of the virtual source, be much weaker than S2, the source at the opposite end of the interval”.
Line 195: Replace “We reproduce it…” with “We reproduce this…”.
Line 211: Replace “between” with “in” or “within”.
Line 215: Descriptive inconsistency between text and figure caption. Referring to Figure 6, the text says “grey dot”, whereas the caption of Figure 6 says ”black dot”.
We have unified them to “gray dot”.
Line 376: Replace “1s” with “1” (??).
Line 417: Replace “these waves only present” with “these waves are only present”.
Line 424: Replace “is consisted of” with “consists of”.
Lines 427-428. The statement “The slower apparent velocities might be due to incident angles” is wrong. The apparent velocities are always larger than the formation velocities.
Lines 429-430: Take out “if we could extract them more often in time”.
Page 18: In the author list of references 2, 3, 4, 6, 39, 46, the term “other” appear, I assume by error?
Response 5:
We have fixed these sentences and renewed the references. Points without comments are points we modified as suggested.
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