Geoelectrical characterization of the Hydrogeological System in the part west of the Potiguar Basin applying the Vertical Electric Sounding method

Abstract

This work presents the results of a research project that it had as motivation to obtain a better knowledge of the Hydrogeological System of the Chapada do Apodi, which is an area in the which the federal and state governments are implementing important irrigation projects whose the viability and success are directly related with the considerable volumes exploitation warranty of physicalchemistry good quality groundwater. It has been intensive the increasing of drinking water supply coming from the regional aquifers Alluvium, Jandaira and Acu, This continuous withdraw has resulted in such problems as the decline of the groundwater levels in the regional aquifers. It is known based on previous hydrogeological studies in the Potiguar Basin that the Acu Formation presents outstanding lithologic and hydrologic heterogeneities that should be taken in account in groundwater exploitation for domestic, agricultural or industrial uses. Previous geophysical studies in others areas of the Potiguar Basin used the Electrical Resistivity method, however in the area studied in this work that corresponds to the MedioBaixo Jaguaribe where the Acu Formation recovers an area of 1.100 km they were not still accomplished geophysical studies and consequently one doesn't know your thickness. Due to this fact the research project had as objective to obtain a geoelectrical model of the subsurface that describes in depth the stratigraphic sequence and its lateral variation along some profiles, identifying the Acu and Jandaira Formation’s thickness in the Medio-Baixo Jaguaribe's central portion that is localized in the west part of the Potiguar Basin. As it intended to map in depth the stratigraphic sequence the electrical resistivity survey was carried out using the Schlumberger configuration array. In the study area the crystalline basement is at the depth that varies from -100 m to –300 m in relation to the sea level, consequently the 23 vertical electric soundings (VES) stations were made and interpreted using the array of current electrode up to a maximum spacing AB/2 of 1 km. For the computational processing of the field data it was applied “ridge regression” invers ion outline and used as priore information a lithostratigraphic log profile of a 250 m deep well. The results obtained by the geophysical investigation are analyzed and interpreted in terms of a hydrogeologic model of the deep regional Acu Aquifer. Introduction This project concentrated on the direct contribution of the application of the geophysics for a larger knowledge concerning to the hydrogeology of the west part of the Potiguar Basin specifically in the Medio-Baixo Jaguaribe area. After bibliographical research it was verified that there was a lack of information concerning the behavior and occurrence of groundwater in the studied area denominated Chapada do Apodi. So on the geophysical survey was accomplished on the Chapada do Apodi describing the representative lithostratigraphic sequence of an area of 400 km along a profile of 40 km in the part of the west of the Potiguar Basin denominated Plataforma de Aracati (Figure 1). A fundamental tool for hydrogeological studies is the application of geophysical methods. Souza et al. (1984) using data of formation tests, electric, acoustic, radioactive and lithologic logs profiles of a considerable number of wells in the Potiguar Basin elaborated a model for hydrocarbon secondary migration in the Acu aquifer. These studies evidenced hydraulic gradients of until 5 m/km and lithologic heterogeneities very defined that resulted in a division of the Acu aquifer in four different electrofacies in their capacities to store and to transmit groundwater. They also verified that the Acu Formation shows outstanding lithologic and hydrogeologic heterogeneities that should be take in account to groundwater exploitation for domestic, agricultural or industrial uses. Previous geophysical studies in the Potiguar Basin used the Electrical Resistivity method that was accomplished in the Baixo Jaguaribe area for structural geology studies (Maia et al., 1993). However in the Medio-Baixo Jaguaribe area where the Acu Formation recovers an area of 1.100 km were not still accomplished geophysical studies and consequently one doesn't know its thickness (Feitosa, 1996). The Vertical Electrical Sounding method has been used lonely (Zohdy, 1969) and combined with other electromagnetic methods with success for groundwater prospecting in sedimentary basins (Patra, 1967; Lima, 2000; Carrasquila et al., 1997). As the application of the method became more used it has been trying to develop better techniques for the resistivity data interpretation. After the beginning of the 70 s decade when there was an enormous progress in the processing capacity for computers it was possible to develop new interpretation techniques applying the quasilinear mathematical method of no-linear resistivity inversion problems using different inversion outlines (Inman et al., 1973; Inman, 1975; Bichara and Lakshmanan, 1976; Jupp and Vozoff, 1975; Johansen, 1977; Parker, 1984; Constable et al., 1987; Simms and Morgan, 1992).