Distributed Fiber Optic Strain Sensing for Geomechanical Monitoring: Insights from Field Measurements of Ground Surface Deformation

Round 1

Reviewer 1 Report

This paper reports interesting results from DFOSS during field measurements of ground surface changes. Please see below some detailed comments.

Line 80: It is arguable to state “In all cases, the optimum monitoring direction should be perpendicular to the deformation direction”. Could the authors justify or clarify this statement a bit more?

Line 100: Please describe the instrument and optical fiber cables used in this field test. Brand, model/product numbers. Please include measurements details, such as when the reference was taken? How the temperature interference was compensated?

Line 144: What are the temperature measurement results?

Line 152: Please clarify why negative frequency shifts indicate subsidence and positive indicates uplift.

How does the soil moisture play a role on strain changes? It was found by some studies that the polymer jacket of some SM fibers can absorb water and swell.

Will some deformation exceed the high strain limit of the DSS sensor?

Author Response

Thank you for your very careful review of our manuscript, and for the comments, corrections and suggestions. We have provided our answers  to your comments and also revised the manuscript as you advised. Please see the attachment.   

Author Response File: Author Response.docx

Reviewer 2 Report

This manuscript presents an important quantitative test on using DFOSS to measure ground deformation. The authors demonstrate different installation methods responding to various types of deformation. I would recommend the publication of this manuscript after some moderate revisions based on the comments I list as follows. 

The authors should include a section of mechanical modeling or discussion that connects the DFOSS measurements with ground movement. DFOSS measures strain change along the fiber axial direction, which is related to but not a direct measurement of ground uplifting and subsidence. I found it confusing that the manuscript directly describes the compression and extension strain measurements as uplifting and subsidence in most cases. 

The manuscript emphasizes potential CO2 sequestration monitoring in many places, including abstract and conclusion. However, little is addressed and discussed on how this localized surface strain monitoring is connected to CO2 sequestration. The surface response of deep subsurface injection is in general very long wavelength and is usually better to use technologies like InSAR for better spatial coverage. I fail to understand how a CO2 injection operation can generate such localized (in terms of meters) strain response on the surface. 

Some additional comments:

Line 44: surface uplift/subsidence may not necessarily be caused by pressure migration to the surface. Subsurface pressure inflation/depletion can also cause surface deformation. For example, the O&G production in the North Sea has caused meter-level surface subsidence, but little evidence there is pressure communication between surface and reservoir except at the wells. 

Line 108: typo, “carefully”.

Line 124: the manuscript mentioned 2.9 km fiber length and 50 cm installation depth three times. There is no need to duplicate this information so many times. 

Figure 7: this figure is very confusing. I don’t believe the deformation and airbag pressure should behave like the step function plotted. It reads like the deformation maintains 0 when pressure increases from 0 kpa to 25 kpa, and then the deformation suddenly increases to 0.4 mm without any pressure changes. 

Figures 8 and 10: it seems that the strain builds up with time after placing the iron plate instead of instantaneous elastic response. Some strong response shows before placing the plate, especially the 7.2t one in figure 10. The authors should discuss the cause. 

Line 351: “the deformation in the dynamic water test started later and weaker and grew faster”. I don’t think this is a meaningful comparison. The loading processes of water tank test and iron plate test are very different.

Author Response

Thank you for your very careful review of our manuscript, and for the comments, corrections and suggestions. We have responded to your review comments and also have revised the manuscript as you advised. Please see the attachment.

Author Response File: Author Response.docx

Reviewer 3 Report

Please see the attached PDF for detailed comments.

Comments for author File: Comments.pdf

Author Response

Thank you for your very careful review of our manuscript, and for the comments, corrections and suggestions. We have responded to your review comments and please see the attachment. We have revised the manuscript as you advised and the manuscript has been reviewed for English language and style.  

Author Response File: Author Response.docx

Round 2

Reviewer 3 Report

The reveiwer still thinks the contents on Lines 21 to 25 (and the related through the manuscript) are not so convincing or promoting fiber optic application. The reviewer hoped that the authors could provide more reasonable elaboration on this, which was not done so far. It is recommended strongly that the authors should consider about this aspect in their furture research and publication.