Using Time-Lapse Resistivity Imaging Methods to Quantitatively Evaluate the Potential of Groundwater Reservoirs

Round 1

Reviewer 1 Report

The authors based their analysis on Archie's law taken as a resistivity equation. For soils, it is know that in addition to the bulk conductivity term associated with the pore space, there is surface conductivity term (related to the interface characterized by the CEC or the specific surface area). Since the analysis presented in the manuscript is petrophysically-controlled and since the basic petrophysical relationship used is not appropriate, then it should be said that the complete analysis is wrong. There is an easy patch but implying that the full study needs to be reconducted : this is to do an induced polarization time lapse survey (i.E.e, adding normalized chargeability to the conductivity because the normalized chargeability is directly proportional to the surface conductivity). In this case for each pixel, there are two observations and two unknowns and the authors will NOT have to rely on unrealistic assumptions. 

Best regards

Author Response

The Archie’s Law (1942) is mainly the empirical law that applies for clay-free sandstone or sediments. It can be modified to account for the surface conduction of clays though various models, such as the Simandoux (1963) equation or the Waxman & Smits (1968) equation. The Simandoux equation takes into account the volume portion of the clayey matrix and its resistivity, and the Waxman-Smits equation consider the cation exchange capacity per unit volume and the clay’s surface equivalent conductance. Yet from the engineering and geological borehole records, we discovered the sediments in our study area, the Minzu Basin area, are mainly consisted of sand and gravels. With no obvious clay layers within 100-m deep, we found that the simple Archie’s law should be used for our estimation without the need to re-conduct the correction for the surface conduction effects from the clays. To clarify our point of view, we have added a new Figure 2 to show the drilling and logging records of the Xinming well up to 120-m deep. And we also modified relative description and add the sentence:” Since the deposits are mainly consisted of sand and gravel within 100-m deep in our study area according to the borehole records, we are able to use Equation (1) for the estimation of the hydrological parameters without applying the correction for clay surface-conduction effects.”, in order to notify the readers that our approaches can only be valid where the clay contents are few or absent in the study area.

Reviewer 2 Report

This paper presents the novel approach of estimation of the groundwater levels and the specific yields of an unconfined aquifer using Time-lapse Resistivity Imaging Methods. The goal of the work is well defined and authors provided a detailed explanation of the methodology used. The literature review is thorough and well documented. Conclusions are clear and are in accordance with the results of research.

I have only one remark: delete the part of the sentence in the line 353: „Authors should discuss the results and how they can be interpreted from the“.

Author Response

Thanks for pointing it out. It is actually our mistake in editing the manuscript into the journal template. We have deleted the odd sentence accordingly and go through the proofreading check once more to make sure no such mistakes.

Reviewer 3 Report

Although the research content of this article is relatively consistent with the topic, I can also understand from the article that the author has done a lot of sampling work and data analysis work, but the overall structure of the article is relatively chaotic, and the meaning expressed by the figures and tables is not clear. The logic of the article needs to be strengthened. I do not understand what is the purpose and focus of the author's discussion section, and I suggest that the article should be major revised before it can be reviewed again.

Specific comments:

1. Page 2, Line 73: “2.1 2.1 The Survey Area and the Design of the Electrical Resistivity Imaging “.

I suggest that the authors add relevant descriptions of climate change in the study area.

2. Please explain the content and meaning of Figure 2 in detail in the text. And, I also recommend that the author add a note to Figure 2 to better understand what SP, 16”normal (Ω-m) and 64” normal (Ω-m) means.
3. Table 1 also needs to be supplemented with notes, indicating the meaning of each letter.
4. Line 341, obviously Fig. 9 here.
5. I feel that the structure of the article needs to be changed. For example, Equation 7 and Equation 8 should be placed in the “Materials and Methods”.
6. The discussion needs to be stepped up, I didn't see what I was expecting to see, and I suggest the author rewrite this section.
7. Conclusion. There is too much text in the conclusion section, the author should reduce the word count and highlight the important points that need to be expressed.

Author Response

Comment#1: Page 2, Line 73: “2.1 2.1 The Survey Area and the Design of the Electrical Resistivity Imaging “.

I suggest that the authors add relevant descriptions of climate change in the study area.

Reply: We added the relative description from Line78 to Line86 in section 2.1 of our revised manuscript, to describe the issue and trends of regional climate change according to reviewer’s suggestion.

Comment#2: Please explain the content and meaning of Figure 2 in detail in the text. And, I also recommend that the author add a note to Figure 2 to better understand what SP, 16” normal (Ω-m) and 64” normal (Ω-m) means.

Reply: We deleted the SP curve, since this value would not directly related to our findings. And we also added the description regarding the meaning of 16” and 64” normal (Ω-m) resistivity logging in the caption of Figure2 accordingly.

Comment#3: Table 1 also needs to be supplemented with notes, indicating the meaning of each letter.

Reply: We have addressed the meaning of parameter letters in the caption of table 1.

Comment#4: Line 341, obviously Fig. 9 here.

Reply: We have modified the Figure number in Line 352(originally Line 341) in the revised manuscript.

Comment#5: I feel that the structure of the article needs to be changed. For example, Equation 7 and Equation 8 should be placed in the “Materials and Methods”.

Reply: Equation 7 and 8 is two important equations in our discussion for the evaluation of the groundwater reservoirs in the discussion. Therefore, we think they may be better to be in the same discussion section. The readers will be easy to find them and go through the discussion.   

Comment#6: The discussion needs to be stepped up, I didn't see what I was expecting to see, and I suggest the author rewrite this section.

Reply: We have re-written the entire discussion and try to re-organize the sentences and descriptions, in order to make our point of view clear and have the discussion better understood by the readers.

Comment#7: Conclusion. There is too much text in the conclusion section, the author should reduce the word count and highlight the important points that need to be expressed.

Reply: We also rewrote the conclusion and try to make it short, deleted the redundant descriptions, and had the descriptions straight to the points as possible.

Round 2

Reviewer 1 Report

The authors have done only a cosmetic change to their manuscript. Even clay-free siliciclastic materials can present surface conductivity effects (see Revil, A., P. Kessouri, and C. Torres-Verdín, Electrical conductivity, induced polarization, and permeability of the Fontainebleau sandstone, Geophysics, 79(5), D301–D318, doi: 10.1190/GEO2014-0036.1, 2014.). The problem is that the choice of the parameters entering the conductivity equation is not supported by ANY petrophysical study from core samples from the site. This makes the whole analysis extremely weak (no measurements of the Archie’s parameters, CEC or surface conductivity). I should remind the authors that ANY impedance meter used in the field for ERT can be used for induced polarization, which can be in turn used to assess surface conductivity (see for soils e.g., Revil, A., A. Coperey, Z. Shao, N. Florsch, I. L. Fabricius, Y. Deng, J.R Delsman, P.S. Pauw, M. Karaoulis, P. G. B. de Louw, E. S. van Baaren, W. Dabekaussen, A. Menkovic, and J. L. Gunnink, Complex conductivity of soils, Water Resour. Res., 53, 8, 7121-7147, doi: 10.1002/2017WR020655, 2017). Therefore it would be pretty easy to remedy to the weakness of the current study and to do a much better work without the unrealistic assumptions used by the authors. People have been abusing Archie’s law for decades in hydrogeophysics. It is likely the time to remedy to this behavior.

Author Response

We have provide the word file that contains the point-to-point response. Please see the attached file.

Author Response File: Author Response.docx

Reviewer 3 Report

The article has been revised very well and can be considered for publication.