Round 1

Reviewer 1 Report

Manuscript ID: remotesensing-794507

Title: GPR and ERT investigations for the smart underground urban management: the case-study of Matera (southern Italy).

This article presents some results of geophysical tests carried out in a historic city.

Although the methods used (GPR and ERT) are not innovative, the equipment, testing techniques and analysis of results, as well as the characteristics of the analyzed structures, are always challenges for researchers involved in this type of studies.

In this work, three distinct zones were analyzed. In one of the situations, the authors used both methods in a complementary way. In each of the other situations, only one method was used. In general, it was possible for the authors to identify transitions between materials of a different lithological nature and buried structures.

The materials and methods used in the study are well explained.

The conclusions are supported by the results presented.

In general, the article is well written and well organized.

The references are comprehensive.

In the opinion of the reviewer some aspects should be reviewed, namely:

• It is not necessary to present figure 3. The aspects are reflected in the following figures.
• The size of some figures must be increased, namely: Figure 5 (a, b, c, d, e, f); Figure 9 (a, b, c, d); Figure 12 (a, b)
• There are errors in the identification of the figures. The current Figure 10 should be Figure 8. After this change, it will be necessary to renumber all subsequent figures. (The current figure 11 should be 10 and the 12 should be 11).
• Section 4 "Discussion", as written, is not necessary. In fact, the authors do not really discuss the results. The main aspects of the study described there can be presented in section 5 "Conclusions".

Author Response

Reply to Reviewer 1

Dear Reviewer,

on behalf of co-authors, I thank you for your positive opinion about the manuscript and the comments that were useful for improving the overall quality of the manuscript.

An accurate revision of the paper was carried out. First of all, we have  again reanalyzed the results of the GPR and ERT surveys results paying great attention to emphasize the key role of the jointly application of these methods in a complex urban environment. Accordingly, we have redone the figures increasing the quality and including more details (spatial scales, orientation, location of the GPR and ERT profiles, etc..).. Finally, we have better specified the implications of our findings with the management of urban underground space of Matera city.

Specific comments

• The Figure 3 was removed.
• The size of the figure 5, 6 and 12 was increased.
• The numbering of the figures was corrected.
• The Section 4 “Discussion” was merged with the “Conclusions” by focusing the attention to the critical analysis of the results and their implications with urban underground management.

Reviewer 2 Report

Overall

Article showing onsite surveys from three case studies using GPR and ERT techniques. Only in one single case study both techniques were used.

Results are poor and do not show anything that can be used by stakeholders or authority under a smart urban management framework.

Integration of techniques does not happen anywhere in the text, despite the paper title: only in one case study both were used, but the analysis was done separately, so, no integration was achieved.

Radargrams were very poor, except for the first case study, which shows very little (when comparing to the high technology and cost of the GPR apparatus).

I propose the following: reject.

Nevertheless, the authors can find below some suggestions and comments in order to improve their paper.

Comments

English should be improved, in general.

Abstract

This section needs a thorough revision, as it lacks a portion stating the methodology and the main results reached.

Title

Nothing was correlated between the data from GPR/ERT and the smart urban management: authors should rephrase the title to consider the contents of the paper data. It seems that this paper is only about the use of GPR/ERT in three case studies.

Section 1

Several statements/sentences are unclear and should be rephrased or erased. For example, in line Line 45: “…tools able to support the City managers…” this part should be reviewed.

Lines 61-62: “Being the knowledge about buried objects often not available or also lost, significant time delay and cost increase occur frequently.” This statement can be erased.

Lines 65-66: “…and allows the achievement of many significant information for the reconstruction of underground setting.” Rephrase.

Lines 77-80: the following statement “…pre-existing cultural assets…” should be better explained by the authors.

Line 81: insert “the” between “of” and most.

Line 91: replace “was” by “were”.

Line 93: replace “till to” by “till a”.

The authors use the impersonal mode when writing, therefore, remove the “us” from line 93.

Section 2

Line 107: remove the “the” in “…richness of the cultural…”.

Line 116: remove the “the” in “…by the Altamura…”.

Section 2.2

The authors must describe how the IDS stream-X device works for the reader to better understand the methodology as well as the results obtained. Specifically, how the measurements were done (direct, reflection, etc.) as well as the frequencies involved (only 200 MHz?).

Line 213: the data inversion explanation should be inserted in a subsection, separated from the general GPR methodology description.

Line 260: when the authors describe the general ERT deployment possibilities, they describe it as “… are the most common devices.” The reviewer suggests replacing “devices” by “ones”.

Section 3

Line 286: why the authors cut the signal after 57ns and made measurements up to 127ns? This should be explained better (and showing it).

Lines 288-289: “The time gating procedure is also applied by forcing to zero the signal portion before 12 ns.” This filter/processing should be briefly explained/justified.

Section 3.1

Line 308: replace “ducts mesh” by “mesh of ducts”.

The authors should review this statement because, in the radargrams, there is no mesh of ducts, or any mesh, in the presented radargrams.

The description of features should be made in the radargrams and not by indicating the coordinates in the text, only. Lines 308, 310/311, 313, etc.

Relatively to the ERT, the authors indicate 10m as the depth reached but figure 6 only depicts 8m. What happened to the other 2m?

Indicate in figure 4 the location (profile) of the ERT probes.

The authors should put a small legend in the inset in figure 4.

Figure 5: indicate targets in radargrams: cisterns and ducts. The authors could briefly explain why no connection, if any, is seen between the duct and the cistern.

Figure 6: indicate in a) the location of b).

Figure 7: what useful information did the authors extract by this image, apart from a simple description of the results.

Section 3.2

Lines 356-357: indicate targets in radargrams rather than only depicting coordinates.

Lines 357-359: indicate targets in radargrams. However, the reviewer thinks that, for the frequency of the antennas used (200 MHz), their resolution is incompatible with such small targets: authors must carefully justify.

Line 359: bad reference to image 8 instead of 10.

Line 362: the authors state “… scattered objects.” This statement must be justified by indicating where/what those objects are.

Lines 369-374: this paragraph explains why ERT was not used in this case study. The reviewer agrees. But, then, because the abstract is about integrating GPR and ERT, this goes against the main objective of the paper. In this way, the abstract/title must be rewritten.

Section 3.3

Line 381: the reviewer does not understand this statement: “… in places.”

Line 382: the reviewer does not understand this statement: “… overly calcarenite deposits…”

Line 387: what the authors mean by “… damaging of the architectural structures…”.

Line 388: authors should refer figure 11 instead of figure 12a. (suggestion) The reference to figure 12a should come after “… software.”.

Figure 12b: are these 3-profile based interpolation representative of the subsoil characteristics?

Lines 395-396: “The occurrence of very conductive layer (2-3 Ωm), probably related to alluvial silt deposits with remarkable water content;” this very conductive layer not only appears at the center but, also, in the outside profiles. Why not a continuation of that layer? The authors must explain this interpolation.

Lines 400-402: “The electrical lateral discontinuity between calcarenitic substratum and the sub-Apennine clays could be associated with the presence of a NE-SW fault.” this statement is related to a single point where the profiles converge. Saying it relates to a fault and not having evidence of its continuity is very hard to believe.

Section 4

Lines 413-418: this paragraph should be put under section 1.

The rest of this section is not a discussion and must erased.

Section 5

In general, there are no conclusions, as per the reviewer, there are no useful information data to derive conclusion from.

Line 446: “… based on the integration of GPR and ERT methods…” this statement is wrong as integration never occurred in any of the case studies.

Author Response

Reply to Reviewer 2

Dear Reviewer,

first of all, on behalf of co-authors I thank you for your criticisms and the useful comments for improving the overall quality of the manuscript.

Reply to the general comments

No matter of discussion, we have not performed any integration of GPR and ERT methods. We have jointly applied these geophysical methods for obtaining information on subsurface with different resolution and spatial scales.

The results are of interest for and was done available to the Municipality of Matera in the framework of a national project on the theme “Smart Cities”. The selection of test sites and the main objectives of the geophysical surveys were preliminary discussed with the technical staff of the Municipality  involved in the smart underground management and risk prevention. Indeed, in the historical center of Sassi, where the surveys were carried, it is quite impossible the planning of direct sounding or other invasive investigations. The use of GPR and ERT methods was very important to make a non-invasive diagnostics of the underground and obtain information about buried environmental and anthropic objects.

The quality of the figures was improved and many of them were completely redone including more details (spatial scale, orientation, etc..). Also, the discussion and the interpretation of the survey results were improved.

The English form was completely revised.

Furthermore, other changes in the revised manuscript are:

• The title has been changed removing the part concerning the smart urban underground management.
• The abstract was reorganized paying more attention to the methods and the main results.

Section 1

• line 45 - done - we have erased the phrase
• lines 61-62 - done - we have erased the phrase
• lines 65-66 - done - the sentence was reformulated
• lines 77-80 - done - the sentence was reformulated
• line 81 - done
• line 91 – done
• line 93 - done

Section 2

• line 107 - done
• line 116 - done

Section 2.2

• Instrument -For the surveys, we used the Radar Stream X 200, which works at the nominal frequency of 200 MHz. As usual in surface penetrating radar, a reflection configuration is adopted and the offset between the transmitting and receiving antennas is negligible in terms of the probing wavelength. Accordingly, a monostatic measurement configuration is adopted. The peculiarity of the system is its capability of collecting data on 15 channels spaced by 12 cm, so that a swath of 1.80 m is acquired. The dimension of the antenna system is 2.4m x 0.92 m and the weight is about 36 Kg. The system can collect data by moving at a speed of 15 km/h-For details, please see the page https://idsgeoradar.com/products/ground-penetrating-radar/stream-x and the related documents
• line 213 We agree with the reviewer's advice. We have separated the data inversion explanation from the general GPR methodology, by inserting in a subsection the description of data inversion.
• line 260 - done

Sections 3

• line 286 – Due to the attenuation of the electromagnetic wave at the investigated test sites, the GPR signal is completely overwhelmed by the noise after 57 ns. On the other hand, cutting the signal at 57 ns corresponds to an investigation depth of about 2m.
• lines 288 - 289 - The time gating corresponds to mute the GPR signal till 12 ns. This procedure permits to erase the direct coupling between the TX and RX antennas, which may overwhelm completely the backscattered signal due to buried targets.

Section 3.1

• The investigation depth of the ERT is 8m. We have corrected the value in the text.
• The location of the ERT profile is reported in the figure.
• Figure 6 - done

Section 3.2

• line 359 - done
• lines 369 - 374 In this case we have not performed the ERT due to the presence of distributed metallic objects in near surface.

Sections 3.3

• line 381 - the term "in places" was removed
• line 382 - rewritten
• line 387 - rewritten
• line 388 – done
• Figure 12b – We have better analyzed the results of these surveys taking into account the information and the constrains of the geological map.
• Lines 395-396 – We have better specified the extension of the conductive layer.
• Lines 400-402 - The comment of the referee is right. It is questionable the association of the resistivity contrast with the presence of a NE-SW fault.  We have cancelled this statement.

Section 4

• This paragraph has been erased and several parts were moved to the introduction and in the conclusions

Section 5

• In general, there are no conclusions, as per the reviewer, there are no useful information data to derive conclusion from.
• Line 446 - We have specified that our approach is based on the jointly application of GPR and ERT methods.

Reviewer 3 Report

The manuscript describes the application of multi-resolution and multi-scale measurements with GPR and ERT techniques for urban monitoring and hazard detection.

The research can be applied to places with high architectural and historical value where identification of cavities is especially important.

However, it seems the novel aspects of the paper are limited or not clearly articulated. The concept is based on a very well known basis. GPR is a well-known technique, there have been recently papers concerning the use of UAV coupled with GPR sensors or the use of artificial intelligence to interpret the results. 

Apart from testing the two techniques in new place/city, I am struggling to see new findings. The processing pipeline seems to be  a standard one. Therefore, novelty is not clear and justified.

I would be interesting to see the higher resolution or larger image of the second test site (Piazza San Giovanni) GPR results with the image of tombs from the previous survey. Are those structures visible with the use of GPR?

The article needs a spelling / grammar check. Some sentences could be rephrased. 

Some of the spell check:

72 Matera was one of two cities designated as 

170 it is necessary

171 techniques

204 instead - is it needed?

207 Born approximation

287 whose average

311 whose length

Author Response

Reply to Reviewer 3

Dear Reviewer,

On behalf of co-authors, I thank you for your comments very useful for improving the quality of the paper.

First of all, we have completely redone the figures improving the quality and including more details. Furthermore, we have better described the implications of our findings with the management of urban underground space.

As it concerns the GPR results obtained in San Giovanni square

We have redone the figures and improved the interpretation of the GPR results, by providing further figures where the detected structures are indicated by means of white dashed lines and named. In particular, we have “reinterpreted” our GPR results by focusing on the consistency of them with the findings of the previous archaeological excavations and analysis, when possible. In this case, a good correspondence appears between the GPR images (constant depth slices) and the location and extent of the buried walls in the shallower layers of the underground (see Figure 11).

Finally the English form was completely revised and some typewriting errors were removed.

• Line 72 done
• Line 170 done
• Line 171 done
• Line 204 – the phrase was reformulated
• Line 207 – done
• Line 287 - done
• Line 311 - done

Round 2

Reviewer 2 Report

The paper is much better and the improvements in the images are significant.

GPR imagery is much better and better presented/explained.

Two points that the authors could include in the conclusions. (i) The interpretation of GPR imagery when no past evidence of the area is available (figures 9 and 10, which interpretation were facilitated by old photographs, which in most cases this is not possible). (ii) What was the real added value from the ERT tests, especially in the first case?

I propose to accept as is.