Alain Tabbagh

Comptes Rendus Géoscience

C. R. Geoscience 338 (2006) 1131–1132http://france.elsevier.com/direct/CRAS2A/PerspectiveGeophysical prospecting for water resources in arid zonesAlain TabbaghUMR CNRS SISYPHE 7619, université Pierre-et-Marie-Curie (Paris-6), 4, place Jussieu, 75252 Paris cedex 05, FranceReceived 7 September 2006; accepted after revision 7 September 2006Available online 25 October 2006Investigations of underground water-bearing forma-tions in arid zones are rapidly becoming a worldwidepriority with the need for additional water resourcesto satisfy economic development and demographicgrowth. The presence of freshwater in sedimentaryrocks such as sandstone or limestone is characterizedby an average or even high electric resistivity, whereasbrackish water corresponds to conductors with a lowresistivity. It is extremely difficult to distinguish fresh-water from brackish water with any other physicalproperty that can be measured from the surface. It istherefore logical to use vertical electrical soundings, orthe audio-magneto-telluric method, or else the TDEM(Time Domain ElectroMagnetic) sounding to prospectfor aquifers; these methods are all the more attractivebecause they are cheaper than seismic prospecting dueto the short duration of the measurements and to thelight weight of the equipment. This is why the authorsof the three articles [1,3,4] have selected the electricprospecting method for their work. By comparison, pe-troleum engineers mostly use seismic methods as theyare more interested in structural information (structuraloil traps) and less by the presence of freshwater or brine.They try however to relate the physical properties withwhich they are most concerned (permeability and poros-ity) to the properties of the seismic signal.The article [3], by M.T. Hussein and H.S. Awad,deals with vertical electrical soundings (VES) in theKhartoum area of Sudan to locate freshwater aquiferswithin the first 200 m below the ground. The geologyE-mail address: sisyphe@ccr.jussieu.fr (A. Tabbagh).1631-0713/$ – see front matter © 2006 Académie des sciences. Published bdoi:10.1016/j.crte.2006.09.016goes from the Precambrian to the Quaternary, with sand-stone, mudstone, occasional basaltic sills and recent un-consolidated gravel, sand, silt and clay, in the triangulararea between the White Nile and the Blue Nile. The cal-ibration of the VES was made with existing well logs.The outcome of the survey is presented as electric sec-tions with resistivity maps at four different depths (50to 200 m). The major difficulty was to distinguish mud-stones from brackish water aquifers, as they can havethe same resistivity. This was made possible by the goodcalibration of the VES with the well logs. The authorseventually outlined the locations of freshwater aquifersin the area.The article [1], by M. Gouasmia et al., uses VESand audio-magneto-telluric soundings (MTS) to locatea water-bearing reef limestone within the first kilome-tres below the ground. The calibration of both the VESand MTS is made from existing well logs, but the in-terpretation was made independently for each method,as the locations of the soundings were different. TheMTS method is better able to determine the thicknessof the layers at great depth (down to 1 km), which theVES even with 2- or 3-km-long lines on the surfacecannot do. The results are given as cross-sections inthe basin and as isobath and isopach maps of the reeflimestone formation likely to contain hot water. Notethat the temperature here is too low (around 45 ◦C) forany detectable effect on the electric resistivity to occur.The emphasis of the study is on the presence and roleof faults detected by the survey that greatly affect thesystem. Two favourable zones for reconnaissance bore-holes in the reef limestone are indicated.y Elsevier Masson SAS. All rights reserved.1132 A. Tabbagh / C. R. Geoscience 338 (2006) 1131–1132The article [4], by A. Mhamdi et al., deals with thedetection of freshwater versus brackish water aquiferswithin a deep (500 m) basin. The VES method is used,and the soundings are calibrated with existing well logs.The results are presented as cross-sections and isobathand isopach maps. Thick brackish water aquifers areidentified, and the origin of the salinity is discussed. Butdeep freshwater aquifers are also identified, which needto be confirmed by new reconnaissance boreholes. Thepresence and role of faults are also detected.The various techniques used by these authors arewell known and widely used, the test cases are howeveroriginal, and display a wide range of conditions, all inarid zones. The authors have always used a 1-D inter-pretation, i.e. a tabular model, whereas 2-D or even 3-Dinversion is now possible. But their choice in each caseis reasonable, given the geological context, and, as isthe case, when the locations of the measurement pointshave been carefully selected.Although not presented in these three papers, thereare nowadays new developments of electrical meth-ods that help identify fresh groundwater resources atdepth, see, e.g., a recent and comprehensive review byR. Guérin [2]. Among them, the following are worthmentioning:– for shallow aquifers (0 to 50 m), both the electri-cal resistivity tomography and the rapidly movingfrequency domain Slingram multi-depth systems,which allows a full 3D description of the under-ground structure;– TDEM soundings, which can be used in place ofVES for the medium range of depth (from 2 to700 m). First designed for mining prospecting, thismethod can be used with either surface measure-ments, or airborne or helicopter-borne measure-ments.One must also cite the ‘PMR’ method (ProtonMagnetic Resonance, also called Magnetic ResonanceSounding, MRS) with which one can estimate the depthvariations of the total liquid free water content whenrocks have sufficiently weak magnetic properties.References[1] M. Gouasmia, M. Gasmi, A. Mhamdi, S. Bouri, H. Ben Dhia,Prospection géoélectrique pour l’étude de l’aquifère thermaldes calcaires récifaux, Hmeïma–Boujabeur (Centre–Ouest de laTunisie), C. R. Geoscience 338 (2006).[2] R. Guérin, Borehole and surface-based hydrogeophysics, Hydro-geol. J. 13 (2005) 251–254.[3] M.T. Hussein, H.S. Awad, Delineation of groundwater zones us-ing lithology and electric tomography in the Khartoum basin, cen-tral Sudan, C. R. Geoscience 338 (2006).[4] A. Mhamdi, M. Gasmi, M. Gouasmia, S. Bouri, H. Ben Dhia,Évaluation de la qualité de l’eau par application de la méthodegéoélectrique : exemple de la plaine d’el Mida-Gabes nord (Sudtunisien), C. R. Geoscience 338 (2006).

Geophysical prospecting for water resources in arid zones

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Geophysical prospecting for water resources in arid zones

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