Conceiving an electrical karstification index

Abstract

The research focuses on the electric behavior of karst aquifers when pervaded by a steady electric current. In professional practice, the difficulties of applying the electrical resistivity method in these aquifers have led to the search for a more appropriate field approach and efficient criteria for the interpretation of electrical measurements aiming at water well site locations. Between 1999 and 2004, six resistivity studies were performed in the Jandaira Aquifer which develops itself in Cretaceous carbonate sediments of the Potiguar Basin (RN). Extensive 1D horizontal profiling was carried out by means of the linear symmetric Schlumberger array where the current electrode separation was chosen from previous vertical electrical soundings data. It was perceived that zones exhibiting noticeable karstification are electrically resistive showing a marked electrical variability. This behavior, ascertained by comparing electrical measurements to wells data, led to the conception of an Electrical Karstification Index (EKI) which emphasizes and quantifies the electrical variability, making it possible to select favorable areas to exploit groundwater in limestones. The EKI of a measurement station i, in an electrical profile, is calculated as the mean value of the summation of electrical variations with regard to (i+1) and (i-1) stations. Four ways for calculating the IEC may be adopted depending on the geologic and hydrochemical behavior of the area under study. Results may be presented as linear profiles or, depending on the distance among profiles, as contour maps. Examples of the approach under discussion are presented regarding the Jandaira Aquifer in the Potiguar Basin (RN and CE), the Salitre Aquifer in the Irecê Basin (BA), and the Undifferentiated Paraopeba Subroup/MG. The application of the EKI proved to be useful in characterizing locally and regionally potential aquifer zones.