Groundwater Investigation through Electrical Resistivity Tomography in the Galhareri District, Galgaduud Region, Somalia: Insights into Hydrogeological Properties
Round 1
Reviewer 1 Report
The manuscript deals with application of Electrical Resistivity Tomography (ERT) to recognition of water-bearing zones in the Galhareri District, Galgadud Region in central Somalia.
The manuscript is a “report” from field ERT profiling in the area of scarcity of water supply resources. The authors show results of ERT profiling in the vicinity of 3 villages in form of resistivity distribution 2D profiles correlated adequately to hydrogeological conditions. The manuscript shows the usefulness of ERT method to recognition of hydrogeological conditions in complex geological settings where groundwater-bearing zones are difficult to identification.
In my opinion the manuscript is interesting and worth to publication because it shows the way of practical application of geoelectrical methods for recognition of hyderogeological settings in the zones of groundwater scarcity. The manuscript could be a valuable example for the readers dealing with hydrogeology and groundwater exploration.
However, in actual form the manuscript contains many shortcomings, inaccuracies and missing information in data presentation, which need to be improved. I recommend manuscript to publication after major revision.
My remarks are included in pdf file.
Comments for author File: 
Comments.pdf
English language needs minor correction, mainly spelling.
Author Response
Reviewer #1:
The manuscript deals with application of Electrical Resistivity Tomography (ERT) to recognition of water-bearing zones in the Galhareri District, Galgadud Region in central Somalia.
The manuscript is a kind of “report” from field ERT profiling in the area of scarcity of water supply resources. The authors show results of ERT profiling in the vicinity of 3 villages in form of resistivity distribution 2D profiles correlated adequately to hydrogeological conditions. The manuscript shows the usefulness of ERT method to recognition of hydrogeological conditions in complex geological settings where groundwater-bearing zones are difficult to identification.
In my opinion the manuscript is interesting and worth to publication because it shows the way of practical application of geoelectrical methods for recognition of hyderogeological settings in the zones of groundwater scarcity. The manuscript could be a valuable example for the readers dealing with hydrogeology and groundwater exploration.
However, in actual form the manuscript contains many shortcomings, inaccuracies and missing information in data presentation, which need to be improved. I recommend manuscript to publication after major revision.
Response: we are extremely grateful for your feedback as it will only strengthen our paper further. We appreciate the effort and time you invested in reading our work. We valued your suggestions, and the manuscript is extensively revised following your comments and suggestions.
My remarks are following:
- The study area is not adequately presented. There is not sufficiently clear where the study area is located: on both figures 1 and 2 study area is shown as a “point”. What does mean? – is it too small to the map scale? – It was mentioned that 4 ERT surveys was performed in the vicinity of three different villages: “The survey focused on three areas of interest, namely Xaji Iman, El Jiqow and Daba-Duleel villages .” (i) Why this villages are not shown on the map? (ii) check the names of this villages – because they are written differently in the different parts of the text; (iii) please add topographical map of adequate detail scale to show position of villages and profile lines.
Response: Thank you for the comments. We added a new figure (Fig. 3), which shows locations of the ERT profiles, villages, and topography of the survey area. We checked and corrected the names of the villages.
- Description of geological settings needs improvement to show the real lithology and stratigraphy in the study area where ERT profiles were conducted.
Actually the description concerns only the general geological settings of huge area to greater depth of 4 km. The ERT profiling is made for distance less than 2 km and depth not exceeding 435 m. This general description of geological settings is good, however it does not show the more detailed situation in the study area. The description of geological settings should present the real situation in the study area which corresponds with resistivity profiles in Fig. 3
Response: Thank you for the suggestion. We added a new table (Table 1) that shows the shallow stratigraphy penetrated by nearby wells. Moreover, we added text (refer to Line 164-166) explaining the shallow stratigraphy compatible with the ERT profiles.
- Line (section): 92-102: there is description of lithologic strata together with stratigraphy with reference to fig.1. However, the stratigraphy is not presented in the fig.1. Please add the stratigraphy to fig.1 or include additional figure presenting vertical profile of typical stratigraphic and geological units exclusively in the studied area.
Response: Thank you for the suggestion. Table 1 summarizes the shallow stratigraphy encountered in the surrounding area. Figure 2 provides an overall vertical cross-section of the region.
- It is not clear how the cross section in fig.2, showing the geological settings in the whole region to greater depths of 4 km, refers to geological settings in the surveyed areas? Please add the typical lithological-stratigraphical profile encountered in the surveyed area – in form of figure or scheme.
Response: Thank you. Please refer to Table 1.
- Cross section presented in Fig.2 is not shown on the figures 1 or 2? Please show the cross section line on the figure; additionally show the location of the studied area on the Fig. 2.
Response: Thank you. We added the location of the cross-section in Fig. 1. The location of the study area is also added in Fig. 2.
- 2: actually this figure does not show stratigraphic units sufficiently clear; according to figure caption there is only geological formations showing rock types. Not all fields in the Fig. 2 are described in relation to stratigraphy. This fig. 2 should be improved.
Response: Thank you. Fig. 2 is revised
- Line 98: “The oldest exposed sediments in the region belong to the Cretaceous Yesomma Formation.” - According to fig. 2 the Yesomma Formation is not exposed on the surface. Please remove this inconsistency or improve the geological cross section which will refer better to the study area.
Response: Thank you. The text is revised
- Line 107: “…..the Taleh Formation, is widespread across central Somalia. This evaporitic formation…,” - the evaporites are not mentioned in the Fig.2 captions. The fig.2 should be improved.
Response: Thank you. Fig. 2 is revised
- Karkar formation - is missing on fig. 2
Response: Thank you. The Karkar Formation is eroded in the region. The text is revised.
- Ogaden plateau – is missing on figures.
Response: Thank you. The text is revised. We could not label it in the figures as it is located outside of the maps.
- Line 98: “The oldest exposed sediments in the region belong to the Cretaceous Yesomma Formation.” - According to fig. 2 the Yesomma Formation is not exposed on the surface. Please remove this inconsistency or improve the geological cross section which will refer better to the study area.
Response: see above (point e)
- Line 107: “…..the Taleh Formation, is widespread across central Somalia. This evaporitic formation…,” - the evaporites are not mentioned in the Fig.2 captions. The fig.2 should be improved.
Response: see above (point f)
- Karkar formation - is missing on fig. 2
Response: see above (point g)
- Ogaden plateau – is missing on figures.
Response: see above (point h)
- Description of hydrogeological settings needs to be more developed. There is missing information about actually existing wells (if any) which supply mentioned villages; about existing wells (drilled or dug) which were eventually taken to geological (hydrogeological) recognition in the area of study in the context of resistivity model correlation.
Response: Thank you for the suggestions. There are no existing wells in the study area. However, we included stratigraphy and water quality of nearby wells (see Table 1).
- The study area is not adequately presented. There is not sufficiently clear where the study area is located: on both figures 1 and 2 study area is shown as a “point”. What does mean? – is it too small to the map scale? – It was mentioned that 4 ERT surveys was performed in the vicinity of three different villages: “The survey focused on three areas of interest, namely Xaji Iman, El Jiqow and Daba 72 D ule e l villages .” Why this villages are not shown on the map?
Please add topographical map of adequate detail scale to show position of villages and profile lines.
Response: Thank you for the suggestions. We added a new figure (Fig. X), which shows the locations of the ERT profiles, villages, and topography.
- Line 167 – 168: description of applied equipment and instruments, electrodes, software, etc. should contain more details: full name, brand, model, company, country of origin.
Response: Thank you for the suggestion. We added description of the equipment and software used for the study (see Lines 214-229)
- Line 210-212: there is 3 or 4 layers distinguished by the resistivity distribution in the model?
Response: Thank you. The text is corrected (see Lines XX)
- Line 257-258: it is not clear; seems like not finished sentence… Needs improvement.
Response: Thank you. The text is corrected (see Lines XX)
- Caption to Fig.3 is not clear:
- the resistivity sections here rather do not show “hydro-stratigraphic layers” but hydrogeological layers, because in the text there is description concerning lithology, hydrological properties and eventual water bearing zones. Please reconsider using these terms.
Response: Thank you. The text is corrected (see captions of Figs. 4-7)
- there is mentioned about “well data” used to interpretation and correlation the resistivity models: however there is missing information in the text what king of wells were taken to such interpretation, how many wells, its location and depts.? Please add this missing information – see remark in point 2m.
Response: Thank you. The borehole information (Table 1) is added.
- In the section “Results and Discussion” there is missing important information concerning the real verification of the resistivity modeling: is there any other field drillings or any hydrogeological survey which confirms the conclusions from the ERT profiling regarding existence of water-bearing zone, and chemical composition (or mineralization) of groundwater. Please add information regarding independent verification of the resistivity profiling performed in this study.
Response: Thank you. The text is revised (see Lines 323-325). We verified our interpretation using wells drilled in the surrounding region, since there are no wells drilled in the survey area.
Yours sincerely,
Mahad Hussein
Author Response File: Author Response.pdf
Reviewer 2 Report
Dear Authors,
the manuscript entitled "Groundwater Investigation through Electrical Resistivity Tomography in the Galhareri District, Galgadud Region, Somalia: Insights into Hydrogeological Properties" presents and discusses the results of DEEP 2D Geoelectrical resistivity tomography survey finalized to identify aquifer geometry and to through more insight on the quality of the groundwater.
The manuscripts describes the results of a conventional ERT survey that is being used, perhaps, for the first time as a tool to explore the subsurface for possible groundwater exploitation.
I include annotated copy of the manuscript. The reported annotations regard the elimination of repeated sentences and the correction of some technical terms (Lines: 192-193; 194).
other issues to address:
Q1: authors do not mention the apparent resistivity data quality? no resistivity pseudosections are included in the manuscript? why?
Q2: the RMS of the inverted resistivity models is not mentioned? pl. add
Q3: no map is included, in the text, to show the location of the ERT profiles!
Q4: the ERT profiles were acquired on flat topography? according to Fig. 1 it seems no; if topography is not flat, authors have to run the inversion with the correct topography. If flat, add the elevation instead of depth.
2.2. section
Lines 151-
the aquifers with their EC values should be indicated on the geological
section (Fig. 2).
3 Methodology:
what was the maximum current dipole distance (i.e. report the number?
of electrodes for each segment acquired along the profile),
4. Results and Discussion
in the survey area do you have shallow wells?
what are the EC values of the groundwater?
you should add some labels and lines on the ERT sections
to help readers better understand the significance of these
models
Lines: 225-227
the resistivity values of 30-40 Ohm.m are interpreted as sand dunes;
these values, according to my experience, can easily be attributed to Silty Sand
sediments or to lime. pl. comment and explain and if possible add hard data such as
grain size distribution or so. If know data are available, the world 'confidently' cannot
be used.
Line 234: when referring to a figure add (a, b,...)
Line: 240-241
'weathered limestone...'
these words should be considered as conservative interpretation as
no direct information are presented in the manuscript. Pl. modify this sentence.
In case no subsurface information are available, data calibration
can be achieved by conducting few ERT profiles in areas where
subsurface information is known (my suggestion). However, if authors are
sure about the subsurface geology (provided section) they should compare
the ERT section with the geologic section (Fig. 2).
Lines: 237-241
resistivity values are also compatible with marl or marly limestone. pl. comment
Lines: 243-247
the resistivity values are also compatible with sediments saturated with brackish water? pl. comment
In Fig. 2 no information about fresh-salt water interface are shown, why?
4. Conclusion
Line 274:
..... exhibited very low apparent resistivity,...
change to real resistivity as authors are describing inverted
resistivity sections and not experimental apparent resistivity data.
pl. pay attention and check for this issue the manuscript.
Line 274 and Line 79
two interpretations are given for the fourth geoelectric layer; are
both interpretations correct? if yes provide more insights.
Why not suggesting the use of Time Domain EM methods which can
imcrease the depth of investigation to at least 500/600 meters.
Minor issues:
use ERT acronym instead of repeating the meaning in the text.
The manuscript needs to be deeply restuctured in order to be published. I encourage authors to follow the suggested recommendations and submit a new revised version.
Regards,
Reviewer
Comments for author File: Comments.pdf
Author Response
22nd August 2023
Dear Editors and reviewers,
Please find enclosed a revised copy of the manuscript ‘water-2524662 Groundwater Investigation through Electrical Resistivity Tomography in the Galhareri District, Galgadud Region, Somalia: Insights into Hydrogeological Properties’
The following is a detailed response to all the points raised during the review process. In this revision, we have addressed the comments and suggestions of the reviewers, and we have carefully revised the manuscript. Moreover, we have expanded the manuscript. It is now more than 4000 words.
Please note that all changes to the manuscript can be readily seen in the marked-up version that accompanies the updated submission. Below, the reviewers’ comments are in black colour and our responses are in blue colour.
Author Response File: Author Response.pdf
