Seepage-Induced Pore Pressure Variations Beneath an Earthen Levee Measured with a Novel Seismic Tool

Round 1

Reviewer 1 Report

Seepage-induced Pore Pressure Variations Beneath an Earthen Levee

Locci-Lopez

Review by Warren Wood

 

Abstract: Excellent. Clear, contains some details, conclusions and implications.

This is a nicely formed manuscript, that for the most part is already in good shape. The analyses are performed well, and support the conclusions, which are useful.

 

That said I have the following comments/suggestions

 

Throughout; be sure to use the words “stress” and “pressure” precisely and consistently. I know there is a difference in their usage among the different communities (civil engineering vs. seismic). I think of pressure as a scalar and stress as a vector so “vertical effective stress” is the stress felt by the grain contacts.

 

The arguments could be significantly strengthened by either displaying or at least discussing the uncertainties in the velocity analysis, and how they would propagate to the graphs in Figure 15 (or perhaps more pertinently, figures 10 and 11). That is, how significant is the Vint (and vertical effective stress) difference between the leaky and confined aquifer?

 

Does it really matter that some data were analyzed as CMP and others as PWA? The distinction is maintained throughout the paper but it was not explained why the reader should care.

 

Discussion.

It’s not really the kriging interpolation that provides the improved lateral continuity over piezometers – the piezometers could be interpolated using kriging as well. The improvement comes from the actual, closely spaced CMP samples.

 

The authors did not perform a surface wave analysis (MASW, e.g. http://www.masw.com/WhatisMASW.html) for comparison. A little discussion on why not would help put this study in context.

 

Conclusion

The authors did not really present strong evidence that the sediment grain composition is heterogeneous at the meter scale (although that is not difficult to believe). Their analysis does suggest that the shallow vertical hydraulic conductivity is significantly different between their West and East sites. At this particular pair of sites, this difference might have been guessed at due to the visible cracks in the ground on the east side parking lot. Perhaps a more effective use of this technique would be in the analysis of a yet undeveloped area, to inform future building or remediation sites.

 

Minor comments and typos

line 106

By using a power regression of laboratory data [2] we can convert VINT to 106 effective pressure.

Why use an empirical model? Are deterministic models available?

 

168

This upper blanket thins and rupture towards the landside, thus weakening the lower- 168

(ruptures?)

 

134

Geological and geotechnical setting of the study area 134

This section probably OK as is, but might be trimmed to focus just on the hydrology of the environment which is the point of the paper.

 

212

their respective river in the Vet. School grounds.

(river level?)

 

Fig 14 "stress distribution" missspelled in legend

Author Response

Dear Dr. Warren Wood,

 

Thank you for your review of our manuscript.

 

We appreciate the time and effort that you dedicated to providing feedback on our manuscript and are grateful for the insightful comments on and valuable improvements to our paper. We have incorporated most of the suggestions. Please see below for a point-by-point response to your comments.

 

 

1) Throughout; be sure to use the words “stress” and “pressure” precisely and consistently. I know there is a difference in their usage among the different communities (civil engineering vs. seismic). I think of pressure as a scalar and stress as a vector so “vertical effective stress” is the stress felt by the grain contacts.

 

Thank you for the feedback, the appropriate changes were made throughout the manuscript.

 

2) The arguments could be significantly strengthened by either displaying or at least discussing the uncertainties in the velocity analysis, and how they would propagate to the graphs in Figure 15 (or perhaps more pertinently, figures 10 and 11). That is, how significant is the Vint (and vertical effective stress) difference between the leaky and confined aquifer?

 

Please see the changes in figure 6 (256), figure 7 (260), figure 10 (342), and figure 11(346). Also, the discussion of the uncertainty in the velocity and effective stress analysis (560).

 

 

3) Does it really matter that some data were analyzed as CMP and others as PWA? The distinction is maintained throughout the paper but it was not explained why the reader should care.

 

The PWA was an efficient and quicker method employed during intermediate river stage to confirm that the velocity values are also intermediate between high and low river stages (201). The CMP has higher fold and S/N in comparison with the single fold PWA but still, there is a continuous trend between the surveys acquired during low, intermediate, and high river stages (Figure 15).

 

Discussion.

It’s not really the kriging interpolation that provides the improved lateral continuity over piezometers – the piezometers could be interpolated using kriging as well. The improvement comes from the actual, closely spaced CMP samples.

 

The manuscript was updated (475).

 

The authors did not perform a surface wave analysis (MASW, e.g. http://www.masw.com/WhatisMASW.html) for comparison. A little discussion on why not would help put this study in context.

 

A section in the discussion was added to address the comparison between methods and highlight the benefits (483).

 

We incorporated the following references:

 

91. Park, C.B. MASW horizontal resolution in 2D shear-velocity (Vs) mapping. Open-File Report, Lawrence: Kansas Geologic Survey 2005, 36.
92. Ivanov, J.; Miller, R.D.; Feigenbaum, D.; Morton, S.L.; Peterie, S.L.; Dunbar, J.B. Revisiting levees in southern Texas using Love-wave multichannel analysis of surface waves with the high-resolution linear Radon transform. Interpretation 2017, 5, T287-T298.

 

 

Conclusion

The authors did not really present strong evidence that the sediment grain composition is heterogeneous at the meter scale (although that is not difficult to believe). Their analysis does suggest that the shallow vertical hydraulic conductivity is significantly different between their West and East sites. At this particular pair of sites, this difference might have been guessed at due to the visible cracks in the ground on the east side parking lot. Perhaps a more effective use of this technique would be in the analysis of a yet undeveloped area, to inform future building or remediation sites.

 

We appreciate the reviewer’s comments and observations. The borings logs with soil descriptions (153) with a combination of previous geological and geotechnical studies in the Duncan Point bar area (167, 190), and visual inspection of visible leaking cracks in the ground of the east side parking lot served us a reference for understanding the geological characteristics of the area.

 

Minor comments and typos

 

line 106

By using a power regression of laboratory data [2] we can convert VINT to 106 effective pressure.

Why use an empirical model? Are deterministic models available?

 

 

Please see the changes incorporated in the manuscript starting at line 309 to 335.

 

 

168

This upper blanket thins and rupture towards the landside, thus weakening the lower- 168

(ruptures?)

 The correction was updated in the manuscript.

134

Geological and geotechnical setting of the study area 134

This section probably OK as is, but might be trimmed to focus just on the hydrology of the environment which is the point of the paper.

 Thank you for the feedback.

212

their respective river in the Vet. School grounds.

(river level?)

 

The correction was updated in the manuscript.

 

Fig 14 "stress distribution" missspelled in legend

 

The correction was updated in the manuscript.

Reviewer 2 Report

This paper uses time-lapse Sh velocity analysis to interpret the pore pressure variations near a levee. The analyzing results are convincing and the explanations linking the pore pressure variations with the river stage and formation permeability are reasonable. Overall, this paper is  well organized and its science and methods are scholarly developed. I think it can be accepted after minor review. I have some comments below. 

 

1.The predicted pressure results highly depend on the obtained velocities. How much do you trust the velocities, what about the uncertainties?

 

2. The authors use kriging interpolation algorithm to make the data continuous laterally. Why do you use kriging interpolation, not the others, e.g., linear, cubic, spline interpolation?

 

3. The power-law trend is derived based on the data from Prasad (2002) and Zimmer (2003). How much  their data can represent the soil velocities in you study area? is there any laboratory data of the soils in your study area?

 

4. On line 345, do you develop the fluid flow model or you just use the model from the reference #8, because you indicate a citation of reference #8 on equation 4 (line 367).

 

5. On line 230-231, change "more vertical resolution" to "higher vertical resolution"

 

6. On line 232, change "S-wave" to "S-waves"

 

7. On line 182, delete "it"

 

8. On line 189, change "perpendicularly" to "perpendicular"

 

9. In Table 2, why A-A' is east and B-B' is west, while they seem to be very close in Figure 2?

 

10. On line 262, change "enhance" to "enhancing"

 

11. On line 298, change "predict directly" to "directly predict"

 

12. On line 413, change "result" to "results"

 

13. On line 434, change "a changed from" to "a change from"

 

14. On line 453, "which is representative"

 

15. On line 532, delete "a"

Author Response

 

Dear Reviewer 2,

 

Thank you for your constructive review of our manuscript.

 

All comments have been considered carefully and the response to each comment is provided below.

 

 

Comments and Suggestions for Authors

This paper uses time-lapse Sh velocity analysis to interpret the pore pressure variations near a levee. The analyzing results are convincing and the explanations linking the pore pressure variations with the river stage and formation permeability are reasonable. Overall, this paper is well organized and its science and methods are scholarly developed. I think it can be accepted after minor review. I have some comments below. 

 

1.The predicted pressure results highly depend on the obtained velocities. How much do you trust the velocities, what about the uncertainties?

            The method used for estimating the shear-wave velocities is semblance analysis, which is a measure of coherency in the CMP gathers. The survey used the shoot-through/split-spread acquisition geometry to maximize fold and signal-to-noise ratio. By having higher fold means that the probability of having the same hyperbolic events velocity-corrected to be flat at equal time is higher. Therefore, in Figure 8 the higher semblance values represent the highest confidence points.

The uncertainty range of the contoured high-value velocity spectra (VRMS) in the semblance analysis have a mean is 28 m/s and a standard deviation = ± 7 m/s.

The manuscript was updated with this information (282), its corresponding discussion (495).

2. The authors use kriging interpolation algorithm to make the data continuous laterally. Why do you use kriging interpolation, not the others, e.g., linear, cubic, spline interpolation?

The kriging interpolation is one of the most flexible and accurate gridding methods when the sample points are not constant intervals, such as the semblance analysis velocity picking that depends on the resolvability of the seismic reflectors. The kriging interpolation compensates for clustered data by giving less weight to the cluster in the overall prediction. The nodes in the grid are based on the known data points neighboring the node, therefore each data point is weighted by its distance away from the node (Golden Software, I., 2018, Surfer 11 User’s Manual Online).

The manuscript was updated with this information (295).

3. The power-law trend is derived based on the data from Prasad (2002) and Zimmer (2003). How much their data can represent the soil velocities in you study area? is there any laboratory data of the soils in your study area?

The manuscript was updated addressing the question above (572).

4. On line 345, do you develop the fluid flow model or you just use the model from the reference #8, because you indicate a citation of reference #8 on equation 4 (line 367).

By using equation 4 from reference 8 (Ozkan et. al, 2008) we created the transient fluid flow model. We incorporated the equation in Wolfram Mathematica and used the parameters extracted from the borings (visual classifications and grain size laboratory analysis of sediment samples drilled, unit weight (γ), and hydraulic conductivity values) and the river stages from the Mississippi River (figure 4) to adapt the model to the study area conditions.

The manuscript was updated addressing the question above (365, 376).

5. On line 230-231, change "more vertical resolution" to "higher vertical resolution"

The correction was updated in the manuscript.

6. On line 232, change "S-wave" to "S-waves"

The correction was updated in the manuscript.

7. On line 182, delete "it"

The correction was updated in the manuscript.

8. On line 189, change "perpendicularly" to "perpendicular"

The correction was updated in the manuscript.

9. In Table 2, why A-A' is east and B-B' is west, while they seem to be very close in Figure 2?

Surveys A-A’ and B-B’ were acquired on the west side of the LSU School of Veterinary Medicine building. Surveys C-C’ and D-D’ were acquired on the east side of the building. There was a typo in Table 2. Corrections were updated in the manuscript (125, 209).

10. On line 262, change "enhance" to "enhancing"

The correction was updated in the manuscript.

11. On line 298, change "predict directly" to "directly predict"

The correction was updated in the manuscript.

12. On line 413, change "result" to "results"

The correction was updated in the manuscript.

13. On line 434, change "a changed from" to "a change from"

The correction was updated in the manuscript.

14. On line 453, "which is representative"

The correction was updated in the manuscript.

15. On line 532, delete "a"

The correction was updated in the manuscript.

Reviewer 3 Report

The topic is of interest to monitors seepage-induced pore pressure variations by seismic reflection method. The authors have results which are worth publishing. There are some points in this manuscript that need to be further clarified.

 Specific comments:

(1)    In time-lapse seismic application, the same acquisition system and S/N seismic data are required to detect the difference. From Fig 8(a) and Fig8(c), we can see different S/N data, how do you assess the S/N of seismic data and the resulting errors.

(2)    It is indirect method to derive shear velocity by the semblance analysis and dix equation inversion. There are big uncertainties, especially for deep reflection target, What quality control process do you apply?

(3)    For pressure prediction, the big difference between the results for drilling data and that of the shear velocity (Fig. 14). Why? The authors should explain which boring data are used for pressure model. If you make two models for boring data respectively from West (1,5,7) and East(4,6,10), what results are shown.

(4)    In Fig.6, a big change is shown on lateral location 30m. But from Fig 5(a), there are laterally continuous events and no evident cracks. Do you explain this change?

Manuscript should be checked?

(1) In Fig.2, A-A’ is shown as seismic west. But in Table 2, the A-A’ is marked seismic east.  

(2) Figure 5, The section range is 0-48m and the fold count range is 0-30m

(3) In Line 277. The Vrms versus –depth?  In general, the Vrms versus-time value is derived by the semblance velocity analysis.

(4) In line 469, 10 cm and 100m in clay?  You check this unit.

(5) In Reference 21, the page number is missing

(6) In reference 27, Journal name is missing.

Author Response

Dear Reviewer 3,

 

Thank you for your constructive review of our manuscript.

 

All comments have been considered carefully and the response to each comment is provided below.

 

 

Comments and Suggestions for Authors

The topic is of interest to monitors seepage-induced pore pressure variations by seismic reflection method. The authors have results which are worth publishing. There are some points in this manuscript that need to be further clarified.

 

 Specific comments:

(1)    In time-lapse seismic application, the same acquisition system and S/N seismic data are required to detect the difference. From Fig 8(a) and Fig8(c), we can see different S/N data, how do you assess the S/N of seismic data and the resulting errors.

Identifying the seismic reflectors in a single shot-gather is difficult with the noise that can overlap or obscure the reflection hyperbolas. This noise is from the surface waves at shallower intervals or contamination from external sources in the urban areas of the LSU campus (cars, people walking, etc.). A way to increase S/N is by increasing the fold with a shoot-through/split-spread acquisition geometry. The highest fold is at the middle of the survey and decreases towards the edges. Having a higher fold means that the probability of having the same hyperbolic events velocity-corrected to be flat at an equal time is higher. Therefore, in Figure 8 the higher semblance values represent the highest confidence points. After flattening the CMP gathers with the correct RMS velocity, the stacking or sum process of the traces makes sure that we reduce the uncertainty of external unwanted noise. The stacking procedure of gathers with high fold is one of the biggest noise suppressants. As a rule of thumb, we picked the highest bull’s eye points in the semblance plot with increasing velocity to avoid possible multiples being mixed in the analysis. Therefore, the fold can be seen as a measure to reduce uncertainty in the data, the higher the fold the less the uncertainty.

The manuscript was updated with this information (495).

(2)    It is indirect method to derive shear velocity by the semblance analysis and dix equation inversion. There are big uncertainties, especially for deep reflection target, What quality control process do you apply?

The first quality control is the use of the higher fold data in the semblance analysis to increase the probability of picking the seismic reflectors at the correct time. This reduces the uncertainty of picking unwanted noise or multiples that are not repeatable within the shoot-through/split-spread acquisition geometry. The second quality control is the high lateral resolution (0.5m) of the CMP gathers interpolated with the kriging method that compensates for clustered data by giving less weight to the cluster in the overall prediction. The nodes in the grid are based on the known data points neighboring the node, therefore each data point is weighted by its distance away from the node. This results in laterally smoother shear-wave profiles (Figures 6 and 7) that minimize the strong lateral variations caused by noise and compensate for the lower vertical resolution (10 m).

The manuscript was updated with this information (295).

(3)    For pressure prediction, the big difference between the results for drilling data and that of the shear velocity (Fig. 14). Why? The authors should explain which boring data are used for pressure model. If you make two models for boring data respectively from West (1,5,7) and East(4,6,10), what results are shown.

                  The available boring data have visual classifications and grain size laboratory analysis of sediment samples drilled, unit weight (γ), and hydraulic conductivity values. The borings do not have pressure measurements with varying water levels on the Mississippi River. The transient fluid flow model developed is based on the laboratory data (Figure 12a) and the river stages from the Mississippi River (figure 4). The borings indicate a range from 5 to 15 meters of thickness of the upper low-permeability layer in the area (Figure 12a). We choose 10 meters conservatively.

The manuscript was updated addressing the question above (365, 376).

(4)    In Fig.6, a big change is shown on lateral location 30m. But from Fig 5(a), there are laterally continuous events and no evident cracks. Do you explain this change?

The manuscript was updated addressing the question above (526).

Manuscript should be checked?

• In Fig.2, A-A’ is shown as seismic west. But in Table 2, the A-A’ is marked seismic east.  

The correction was updated in the table of the manuscript (209).

• Figure 5, The section range is 0-48m and the fold count range is 0-30m

The survey range is 48 m but there is a total of 32 CMP gathers (sorted by CMP and offset) evenly space within the 48 m.

• In Line 277. The Vrms versus –depth?  In general, the Vrms versus-time value is derived by the semblance velocity analysis.

 The correction was updated in the manuscript (279).

• In line 469, 10 cm and 100m in clay?  You check this unit.

The correction was updated in the manuscript (537).

• In Reference 21, the page number is missing

The correction was updated in the manuscript

• In reference 27, Journal name is missing.

The correction was updated in the manuscript