High Resolution Monitoring of Seawater Intrusion in a Multi-Aquifer System-Implementation of a New Downhole Geophysical Tool

Round 1

Reviewer 1 Report

I wrote some suggestions (check the attached file)

Best regards

Comments for author File: Comments.pdf

Author Response

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Author Response File: Author Response.docx

Reviewer 2 Report

Brief summary 

This manuscript describes the application of a down-hole geophysical tool that allows high temporal and spatial resolution measurements in the contest of monitoring seawater intrusion. The paper describes an interesting practical application of the technique in a Mediterranean coastal area.

The authors applied the method to monitor the change in fresh/salt water interface in high time resolution trying to minimize any borehole-fluid effect.

Broad comments 

Although the case study, the used technique and results are interesting, some paragraphs need to be improved in order to allow the reader to better understand the paper results.

In particular, the Introduction, Hydrogeological background and Methodology sections lack some essential elements for the correct evaluation of the results.  

Introduction:

- Why the used method is more efficient than others?

- There are several applications of cross-hole electrical resistivity measurements for saline intrusion monitoring with high spatial and temporal resolution. Please, underline better the advantages of the used method compared to cross-hole electrical resistivity measurements.

- It could be useful to show other successful application of the same proposed geophysical tool in different contests.

Hydrogeological background:

- This section lacks of fundamental hydrogeological information concerning porosity, hydraulic conductivity, the depth of the fresh/salt water interface in the Unit B in correspondence of T71-2 and T72 wells. Reading the text, some other hydrogeological elements are introduced. However, it would be appropriate if they were expanded and collected in the hydrogeological background for a greater clarity.

Methodology:

- The readers would benefit from more detail explaining the used geophysical system concerning the characteristics of the cable and electrodes, the energizing and receiving systems, the current intensity injected, the electrodes array used. Please, justify your choices.  

- Doetsch et al. (2010) underline the evidence of a borehole-fluid effect in electrical resistivity measurements causing apparent resistivity underestimation or overestimation, for different electrodes configurations.  Please, explain why you can consider this effect to be negligible with shorter bipole spacing.

[Doetsch et al. (2010), The borehole-fluid effect in electrical resistivity imaging, GEOPHYSICS, 75, F107–F114.]

- The authors do not show any measured electrical resistivity profile and do not provide information on acquired data quality, measurement errors evaluation and data processing. Please, add it.

Furthermore, did you record any degradation of the acquisition system (electrodes corrosion phenomena) that could get worse the acquired data quality?

- Electrical conductivity data are normalized to temperature? Do you take into account the tide effects?

Results and discussion:

- All wells depth should be normalized to the see level in order to allow a better comparison of results.

Specific comments 

Line 43: change ‘an order’ with ‘one order’

Line 78: change ‘surveys (CHIRP)’ with ‘surveys CHIRP (Compressed High Intensity Radar Pulse)’

Line 103: change ‘remote-controlled geophysical tool’ with ‘remote-controlled multi sensor geophysical tool’

Lines 110-114: Please, justify why you decided to use the Waxman-Smits eq. in sandstones instead of the classic Archie law formulation.

Furthermore, explain better how you estimated surface conductivity and formation factor and clearly indicate what Cs and F values you used for the water EC estimation.

Line 117: change ‘5.5”’ with ‘5.5” (about 14 cm)’

Line 145: change ‘(Neyens, 2017)’ with ‘[25]’

Lines 164-165: In figure 5, it is possible to see two steps in the EC curves. Please, explain the meaning of the two steps and the depth of fresh/salt water interface.

Lines 187-190: it would be useful if the authors provided a schematic log of the well 71-2 indicating the depth of the most coarse-grained zone.

Lines 208-209: did the CTD observe the same sharp fluctuation in well 71-2? Did the authors correcte the measured CTD curves considering the tide effect and temperature variations?

Figure 2: Please, add UNIT A, UNIT B, UNIT C labels in the figure and correct the scale of ‘the distance from shoreline’.  

Figure 3: Please, translate in English the equation.

Figure 4: Please, give a range of values for high and low power.

Author Response

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Author Response File: Author Response.docx