Determination of the aquifers geometry in arid zones by using geoelectrical method

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ORIGINAL PAPER

Determination of the aquifers geometry in arid zones by using geoelectrical method Samia Hadj-Said & Aziz Zeddouri & Larbi Djabri & Belhadj Hamdi-Aissa

Received: 24 February 2011 / Accepted: 22 September 2011 / Published online: 15 October 2011 # Saudi Society for Geosciences 2011

Abstract Water resources in the Algerian South are rare and difficult to reach because they are often too deep. This is the case of Guerrara which is characterized by an annual precipitation average of less than 60 mm. The water supply is warranted from groundwater, frequently too deep and badly known. The main purpose of the present study is to determine the geometry of aquifer from geophysical data. Fourteen vertical electrical soundings covering the total surface area were carried out by using an arrangement of electrodes called “Schlumberger array.” The length of the selected transmission line (AB) was 1,000 m, which allowed a vertical investigation reaching up to 160 m of depth. The analysis of the results shows that the prospected zone is characterized by the succession of layers with different electrical resistivities. A sandstone aquifer characterized by resistivities near 100 Ω m overcoming a limestone aquifer stronger with values that exceed 1,000 Ω m, separated by a conductive layer of clay with average resistivity of 15 Ω m. Distribution map of sandstones thickness shows the structural variations of this horizon allowing an estimation of its hydraulic potential. Keywords Algerian South . Guerrara . Electrical prospecting . Arid zone

Introduction Water reserves in the Sahara are mainly formed by the northern Sahara aquifers. This vast basin contains a S. Hadj-Said (*) : A. Zeddouri : B. Hamdi-Aissa Biogeochemistry laboratory of desert environments, University Kasdi Merbah, BP511, Ouargla 30000, Algeria e-mail: [email protected] L. Djabri Department of Geology, University Badji Mokhtar, BP12, Annaba 23000, Algeria

groundwater system composed by two main superposed aquifers: the Intercalary Continental and Terminal Complex (OSS (Observatoire du Sahara et du Sahel) 2002). In the region of Mzab, the boundaries of these two aquifers are badly known and are associated with other aquifer levels. Noting also that the old oases were founded by the exploitation of water from wadis that crossed the region and water routing was done through seguias. These hydraulic structures, traditional and ingenious, allow recharging superficial aquifer under the oasis and irrigation is done as well. These last years have been marked by an overexploitation of the resource, which could jeopardize the existence of Seguias. To enable sustainability for this system, we proceeded a first step to the identification of different aquifer horizons present in the oasis of Guerrara. Geophysical studies have attempted not only to relate resistivity to the hydraulic properties of the aquifer (Kelly 1977; Mazac et al. 1990; Cassiani and Medina 1997) but also as a means of assessing water quality (Mhamdi et al. 2006; Boughriba et al. 2006; Al-ahma