Tenth International Congress of the Brazilian Geophysical Society Joint Evaluation of Electromagnetic and Electric Methods for Groundwater Exploration Emin Ulugergerli, Department of Geophysics Engineering/ Onsekiz Mart University, Turkey José Pedro Rebes Lima, DHIMA/FUNCEME, Brazil Nelson Raulino, DHIMA/FUNCEME, Brazil Copyright 2007, SBGf - Sociedade Brasileira de Geofísica This paper was prepared for presentation at the 10 th International Congress of The Brazilian Geophysical Society held in Rio de Janeiro, Brazil, 19-22 November 2007. Contents of this paper were reviewed by the Technical Committee of the 10 th International Congress of the Brazilian Geophysical Society and do not necessarily represent any position of the SBGf, its officers or members. Electronic reproduction, or storage of any part of this paper for commercial purposes without the written consent of the Brazilian Geophysical Society is prohibited. ____________________________________________________________________ Abstract Electrical and electromagnetic methods jointly used and evaluated for hydro-geological investigation. Geoelectrical sections and porosity approximation helps to identify possible well locations. A survey conducted in Itapiúna (Ceará) using direct current resistivity method and two-loop electromagnetic method. Jointly evaluation of result from two dimensional modelling of direct current resistivity data and the result of empirical one-dimensional modelling of two-loop electromagnetic data maps lateral variations and provides valuable depth information to locate drillings. Results also indicate that fracture zones with and without water can be traced. As a result of experimental works, a number of drilling locations were proposed. Some of the locations already had wells in use. Introduction The Two-loop EM method known as “Ground Conductivity Meter” (GCM) that uses the system EM34, which is a variant of the Swedish Slingram EM induction method and direct current resistivity (DCR) method are common methods to explore the aquifer systems in crystalline areas (e.g. Slater et al., 1998; Monteiro Santos et al, 2002; Carrasquilla et al., 1997). The working area is in Itapiúna in Ceara State, NE Brazil (Fig. 1) and is covered with the Precambrian rocks, granite and alteration zone and/or eroded and flowed units. In terms of the geophysical parameters, crystalline rocks have larger electrical resistivity values while clay or water filled fault zones have lower value. The survey area was selected according to aerial photos and previous works. Both methods applied to locate possible well locations over fracture zones. Two computer codes were developed in the frame of this project first one is a 2D modelling code based on the Dey and Morisson (1979)’s finite differences formula. Damped least square inversion technique was used to recover geoelectrical model from observed DCR data. Second code was developed to evaluate two-loop EM (TLEM) data by means of one-dimensional approach (e.g. Monteiro Santos, 2004) based on the formulas given by McNeill (1980). Jointly evaluation of both results of independent interpretations suggested that both methods are capable to detect the fractures in crystalline environment. Regional Geology The study area (Folha de Itapiúna) inserted in Ceará State (northeastern Brazil) (Fig. 1) that is located in the northwestern part of a neoproterozoic erogenic belt know as the Borborema Province (BP) (Almeida et al.,1981) and is known as else as Borborema Province’s Northern Tectonic Domain (Van Schmus et al., 1997). The Northern Tectonic Domain is subdivided by two continental-scale shear zones, the Transbrasiliano and Senador Pompeu lineaments, into three major crustal blocks: the Northern Ceará Domain, the Central Ceará Domain and the Rio Grande do Norte Domain (Fig. 1). The Folha de Itapiúna Area includes at the most part of it the Central Ceará Domain and some of the western part of the Rio Grande do Norte Domain. The basement rocks in these blocks consist of variably metamorphosed and migmatized orthogneisses assemblages (Fetter et al., 2000). Overlying parts of the basement complex are a series of supracrustal sequences ranging from Middle Paleoproterozoic to Late Neoproterozoic in age (Van Schumus et al., 1995, 1997; Fetter, 1999). Both basement complex and supracrustal rocks are intruded by abundant alkaline and calcalkaline plutons and plutonic complexes that were generated during Brasiliano orogenesis (Almeida et al., 1981). At the Central Ceará Domain the basement complex of the Central Ceará domain is dominated by high-grade felsic orthogneisses and migmatites that are primarily tonalitic to granodioritic in composition (Fetter et al., 2000). At the Rio Grande do Norte Domain the basement complex of the Rio Grande do Norte domain in Ceará is dominated by paragneisses and schists, but some tonalitic to granodioritic orthogneisses are present locally. Most of these basement gneisses show variable degrees of migmatization, from small degrees of partial melting to almost complete remelting in some locales (Fetter et al., 2000). Local Geology The Lithologie is represented in the Folha de Itapiúna as geological units belonging to Precambrian, Palaeozoic, Mesozoic and Cenozoic Ages. The Precambrian crystalline rocks recover most of the study area mapped surface, embracing entities of Inferior, Medium and Superior Proterozoic Ages. There are too Jurassic basic dikes, tertialy/quartenary sandy/muddy colluviums sediments and recent alluvium deposits (Fig. 2).