Aplicación de la tomografía eléctrica a la prospección de formaciones evaporíticas

Abstract

The electric imaging permits to obtain a model of the subsurface materials on the basis of the vertical and horizontal resistivity changes along the tomographic profile. The final result, after the processing of the initialy measured apparent resistivity, is a 2D profile that shows the real resistivity of the subsurface; it can be interpreted in geological terms, given that resistivity changes are mainly due to the lithological and water contents variations. Electric imaging has been used in the study of evaporitic formations (gypsum and anhydrite). Differences of resisivity between gypsum (CaSO4·2H2O) and anhydrite (CaSO4) are due to the presence of hydration water in gypsum rocks; the levels of water molecules in the mineral structure operate as conductive levels. Thus, anhydrite is expected to be more resistive than gypsum. The tomographic profiles fit the interval of gypsum resistivities between 80 and 1000 W m, whereas the anhydrite display higher values, from 1000 to 11500m. The ranges of resistivities will depend on the degree of transformation of the anhydrite rock to gypsum. The use of the electric imaging in prospection of evaporite (calcium sulphate) formations with possible economical interest is proposed. This technique is non destructive and comparatively less expensive than classical methods (drilling)