Peer Review: CO2 Sequestration Monitoring via Surface Waves

Review #1:
The article is dedicated to geophysical monitoring of CO2 sequestration, which I
would consider as a very relevant topic for the Journal. In principle it is quite nicely
structured and reasonably well written. However it does have one principle problem
in it, which does steer my recommendation regarding the publication to either major
revision or rejection of the article (with an opportunity of resubmission). The article
describes a feasibility study of CO2 sequestration monitoring using surface waves
analysis and the model which is used for the study contain a relatively thin CO2
reservoir at the large depth. Authors run a comprehensive and complicated workflow
to simulate the TL signal in the form of changes in the phase velocities of surface
Rayleigh waves. Changes in the reservoir level are related to changes in saturation
only; any changes related to pore pressure (in reservoir) and stress state
(reservoir/overburden/underburden) are ignored. As a result they come to a quite
logical conclusion - no significant changes are will occur. The main problem I have
here is that there's no point of running the workflow to predict the null result in this
particular case, it is quite obvious, providing the model used. CO2 saturation is
unlikely to affect significantly shear wave velocity (and MASW is in principle
sensitive to shear wave velocities, no to P waves), thin reservoir at a big depth is
unlikely to contribute to the phase velocity curve determined by properties of the
whole overburden, one also need a very low frequencies to get to that depth anyway.
The other important issue is that even though poor applicability of MASW is derived
on the basis of the reasonably well done simulations, I do challenge this statement.
The cartoon model used (41 m only reservoir at ~1 km depth, pressure changes
ignored) is unlikely to be a viable for large-scale CO2 sequestration projects
mentioned in the introduction. It is probably a more or less Ok model for a site which
authors had in mind, but it does not allow to come up to any conclusive
recommendation on applicability of the method worldwide. I would advise to make
one of the two efforts:
1. While still ignoring the pressure effect, try to estimate conditions at which phase
velocity curves will became sensitive (consider thicker reservoirs; vary elastic
properties, reservoir depth).
2. Consider coupled seismic/geomechanics approach (see 2011, Herwanger &
Koutsabeloulis for a comprehensive set of references). There are even published
examples of application of surface waves analysis (ocean bottom) for reservoir
depletion monitoring. I understand that this might be a much more labour intensive
suggestion if you would consider doing some actual modelling, but mentioning this at
least in the discussion part would be fair.
-We authors appreciate the valuable critiques provided by the reviewer. It is true that
in our original simulations not every factor was considered and we reached
conclusions that are pretty logical. We accepted the reviewer's suggestion and we
extended our endeavors in the revised version by:
(1) Completed a comprehensive survey of more than 155 CCS projects (typical cases
are listed in Appendix table of the revision) and got the physical parameter ranges

associated with the CCS projects in the real world.
(2) Based on the survey, an additional rock formation, sandstone, was added in both
the rock sample study (Section 2.2) and the CO2 reservoir modeling (Section 3.3).
Therefore, comparison studies were implemented to achieve more findings. It should
be noted that all the parameters in the numerical models were from the existing
publications, including the carbonate reservoir. The sandstone reservoir model was
simplified from the Sleipner project.
(3) Factors including pressure and temperature were still ignored in both the
numerical analyses of rock samples and reservoir models since the objective of our
simulations is to quantitatively observe any seismic wave responses caused by CO2
flooding. Additionally, since we focus on the long-term monitoring CCS, not for the
injection process, it is not irrational to assume that the CO2 reservoir is in a stable
condition in the terms of pressure and temperature in such a long run storage
scenario. The findings cannot be nonsense since they support the geophysical theory
with scientific data, which is expected to help the readers understand some seismic
wave philosophies.
(4) A Discussion section was added to estimate conditions at which the Rayleigh wave
becomes sensitive and more interesting findings were presented. Also, additional
research works, such as coupled seismic/geomechanics modeling and microseismicity
studies were acknowledged in this section as recommended by the reviewer.
I have also few minor comments, which are listed below.
1.Explain the reasoning for the model(s) used. Why you've decided to consider such a
tiny reservoir at a reasonably shallow depth?
-The carbonate reservoir model we used in the manuscript is from a publication
(Khatiwada et al. 2009), in which the researchers from the Boise State University used
this layered basalt to investigate the feasibility of monitoring CO2 sequestration
through analyzing coda waves. In the revised manuscript, we presented typical survey
data we collected about the CCS projects in the real world (Appendix table) and
included an additional model based on the Sleipner project. We expected in this way,
our simulations and findings are of more practical values for the readers.
2. CO2 state providing the depth (e.g. not quite random P/T conditions) will also
likely to be not quite random. What is the most probable situation?
-We acknowledge that CO2 state changes under different environment. Using the
Clausius-Clapeyron equation, CO2 phase diagram can be obtained. In our
simulations, three special states (gaseous, liquid and supercritical fluid) were selected.
Since pressure and temperature were not included in our study, environment was
unable to be described exactly. But the three particular states we involved, each can
be within certain P/T range in the CO2 phase diagram that are presented in many
publications (Carcione et al. 2006, Eiken, Ringrose et al. 2011), can qualitatively
show the effects of injected CO2 on reservoir rocks.

3. Increasing the depth of the reservoir by changing thicknesses of all of the layers
above you're changing elastic properties of the section much more significantly
compared to changes induced by putting CO2 into the reservoir. I do not think it is
valid (you need to compare model with CO2 to exactly the same geometrically model
without CO2).
- Thanks for the comment. We agree that the approach we took to change the layer
thicknesses above the CO2 reservoir did cause confusion if improper comparison were
made. Therefore, we did not include these modification cases in the revised
manuscript, instead, considering the factor of the thickness-depth ratio.
4. Changing thickness of the reservoir by 10% does not make any sense if we're
talking about MASW. I would consider changes by 100%-1000% or more?
- In the Discussion section we added in the revised manuscript, we expanded our
discussion by continually increasing the thickness-depth ratio and the shear wave
velocity (Lines 332-352; Figures 11-12). It is very interested to find that at a certain
level, the Rayleigh wave velocity becomes insensitive to the increase of CO2 reservoir
thickness. The shear wave velocity change in the CO2 layer, as it reaches enough
amplitude, does induce detectable Rayleigh wave responses.
5. Somewhere you need to discuss actual sensitivity of the MASW method. Can you
realistically discuss detectability of phase velocity variations by first precents? What
kind of MASW technique do you have in mind? Passive MASW where you have a
principal possibility to stack data for days/months/years, or active time-lapse MASW?
- Thanks. What we suggested for further investigation is the passive MASW or
microtremor, which does require stack data for reliable analysis. For instance, if,
through an efficient time lapse passive seismic survey on the ground, a geological site
can be determined whether it contains an underground CO2 reservoir, the CO2 flow
perimeter at the horizontal plane may be estimated. We added a general discussion on
the resolution or sensitivity of passive MASW/microtremor in the revised manuscript
(Lines 302-320).
References
Carcione, J., Picotti, S., Gei, D. and Rossi, G. (2006). "Physics and Seismic Modeling
for Monitoring CO2 Storage." pure and applied geophysics 163(1): 175-207.
Eiken, O., Ringrose, P., Hermanrud, C., Nazarian, B., Torp, T. A. and Høier, L. (2011).
"Lessons learned from 14 years of CCS operations: Sleipner, In Salah and Snøhvit."
Energy Procedia 4(0): 5541-5548.
Khatiwada, M., van Wijk, K., Adam, L. and Haney, M. (2009). "A numerical sensitivity

analysis to monitor CO2 sequestration in layered basalt with coda waves." SEG
Technical Program Expanded Abstracts 28(1): 3865-3869.

Review # 2
Originality
- The microtremor survey method (MSM) has been well reported in the geophysical
literature for mining, petroleum and geothermal exploration as well as civil
engineering applications.
- What appears absent (to the reviewers knowledge) in the available literature is the
assessment or application of MSM technology for time-lapse monitoring of fluid flow
in the subsurface, or specifically, for monitoring long-term CO2 storage.
- The MSM technology offers itself as a low cost and low impact technology to
monitor CO2 storage in the subsurface for extremely long periods of time in
comparison to other more acquisition intensive time-lapse monitoring methods.
Previous Research
- The manuscript builds on previous research, of which, a reasonable list of relevant
references has been provided.
 Thank you.
Structure
- The manuscript is well laid out, but will require further editing to improve grammar
and correct minor spelling errors.
- The title may be somewhat misleading since the reservoir scenario described in the
manuscript is analogous to a thin carbonate reservoir with an extremely stiff rock
frame.
- The abstract is a reasonable summary of the main results in their current state.
- The table and figures are appropriate and complement the text.
 Thanks for the comments. We have conducted an editing work in the revision (red
words in the revised manuscript). In the revised manuscript, we also expanded
our simulations by involving a soft rock formation case with the Sleipner project
as a background. This additional numerical simulation work was selected from a
comprehensive survey of more than 155 CCS projects (typical cases are listed in
Appendix table of the revision). Comparison studies were implemented to achieve
more findings with both carbonate and sandstone rock samples and reservoir
cases. Moreover, a Discussion section was added to estimate conditions when the
Rayleigh wave becomes sensitive. With all these revision works, we kept the title
unchanged.
Results
- The reference-case reservoir scenario described in the manuscript is analogous to a
relatively thin (41m) carbonate reservoir with an extremely stiff rock frame. This
reservoir scenario may not necessarily be analogous for large-scale CO2 injection
projects which will more likely target very thick (100s m) siliciclastic reservoirs with