Locating Potential Groundwater Pathways in a Fringing Reef Using Continuous Electrical Resistivity Profiling

Round 1

Reviewer 1 Report

The hypothesis that groundwater runoff from high volcanic islands extends below fringing reef systems is tested by:

1.      Identifying ‘confined groundwater confined beneath the reef over greater distances from shore’;

2.      if identified, characterising the spatial distribution of groundwater using a geophysical method to profile the reef surrounding the high volcanic island of Mo’orea, French Polynesia; and

3.      discussing the potential movement of groundwater below the reef in terms of what is currently understood about shallow reef structure and 1 permeability.

The study found that boat-towed continuous resistivity profiling (CRP) revealed resistive features at about 10-15 m depth, ranging in width from 30-200 m. The features could represent the occurrence of freshened porewater confined within the reef, but a change in porosity due to secondary cementation cannot be ruled out. Groundwater-freshened reef pore water has been observed near-shore on Mo’orea and suggested elsewhere using similar geophysical surveys, but synthetic models conducted as part of this study demonstrate that CRP alone is insufficient to draw these conclusions. These CRP surveys suggest reefs surrounding high islands may harbor pathways for terrestrial groundwater flow, but invasive sampling is warranted to demonstrate their roles in terrestrial runoff.

The paper is well-written and have no major comments on how to improve the paper. My main concern is that it should be totally predictable that geophysics should generate ambiguous results without invasive sampling. The complexity and cost of sampling can be prohibitive, and the absence of alternative verification approaches mean validation or otherwise of the hypothesis remains unresolved. Other work, such as the synthetic profiles and the development of porosity in such atolls have been conducted, but fail to rule out alternative causes. The authors have not tried to hide this and have couched the results correctly. However, it begs the question as to whether the paper is publishable without such invasive sampling being done. One previous published study did include invasive sampling; and another did not, hence leaving the question open; and some have overstated the ability of geophysics to detect fresher water. On balance, and having re-read the conclusions, I believe the paper should be published, in current form, as a warning not to overstate the value of geophysics without invasive sampling.

Author Response

We appreciate the reviewer’s comments and her/his understanding of our position.  The reviewer has exactly captured the situation with which we were faced in preparing this article. The interpretation is incomplete due to a lack of in-situ sampling, but we felt there was some value in providing the data and our preliminary interpretation as it currently stands. 

Reviewer 2 Report

Dear All,

My comments are available in the manuscript that is attached.

Best Wishes

Comments for author File: Comments.pdf

Author Response

This reviewer provided all comments as markup in an attached pdf.  We respond point-by-point to the substantive comments here:

Abstract:  Please improve your abstract with some for your findings.

We thank the reviewer for pointing out the lack of major findings in the abstract and have added the following to clarify:

“Boat-towed continuous resistivity profiling (CRP) revealed electrically resistive features at about 10-15 m depth, ranging in width from 30-200 m. These features were repeatable in dupli-cate survey lines but resolution was limited by current channeling through the seawater column.  Anomalous resistivity could represent the occurrence of freshened porewater confined within the reef, but a change in porosity due to secondary cementation cannot be ruled out.“

Introduction:  The reviewer suggested we improve the introduction by citing the following articles. 

Mohamed, Ahmed, Abdullah Othman, Wael F. Galal, and Ahmed Abdelrady. "Integrated geophysical approach of groundwater potential in Wadi Ranyah, Saudi Arabia, using gravity, electrical resistivity, and remote-sensing techniques." Remote Sensing 15, no. 7 (2023): 1808.

Raju, N. J., & Reddy, T. V. K. (1998). Fracture pattern and electrical resistivity studies for groundwater exploration. Environmental Geology, 34, 175-182.

Although these are very interesting papers, we do not understand why they would support the introduction that we have written.  Although they provide an example application of electrical resistivity to groundwater applications, these applications do not involve coastal environments, modern carbonate deposits, or volcanic rocks.  We could not find a way to include these citations in our introduction.

86. Please clarify your objectives.

We have added the following statement that we hope clarify our objectives:

The purpose of the current investigation was to identify confined groundwater confined beneath the reef over greater distances from shore and, if identified, to characterize the spatial distribution of groundwater.  Our objective was to use geophysical surveying to provide a spatially heterogeneous indication of the sub-reef structure and salinity. 

Figure 1.  Provide coordinates

We have added the lat-long to the caption and referred the reader to supplementary material where the exact coordinates of each survey line are provided.

Figure 1. Map of CRP surveys collected from the reefs of Mo'orea, French Polynesia (lat. -17.5, long -149.8), 2021 Boat-towed electrical resistivity continuous resistivity profiling (CRP) surveys are shown in yellow.  Profiles and their geographic coordinates are provided in supplementary material.

 

Section 2.4  Do you have ground-based data for validation?

We do not understand this question as all data are ground-based.  Perhaps the reviewer is referring to independent resistivity measurements?  As noted in the following section, these data are not available.

 

Section 2.5  No Need

We interpret this comment to mean that Archie’s Law section is not needed.  We disagree with this comment as without an understanding of archie’s law, the article is not interpretable.  However, we have shortened this section so as to reduce its focus in the paper and removed Figure 3, which perhaps was more detail than is required. 

 

Figure 4  Please improve this figure

We do not know what the reviewer believes needs improving in this figure so we cannot adjustments.

 

4.1  This section needs to be revised. 

Again, without specific comments, we do not know how to revise this section.

 

Conclusions: Could you please add a section about the implication of your work. future research and comparison with other previous studies  and  Please shorten it

The conclusions have been tightened and some discussion of future research and implications have been added in the revised section.