Efeitos do strike das estruturas geológicas de duas dimensões nas pseudos-seções de IP resistividade

Abstract

The IP-resistivity surveys carried out in the Carajás mineral district were not orthogonal to the geological structures because they were executed on lines already opened by the geochemistry crews. This fact motivated us to study theoretically the influence of the direction of the IP-resistivity survey lines with respect to the strike of the structure. We used Rijo's finite element program (1977), developed for surveys perpendicular to the structures, with the necessary adaptations. The main modification was in the inverse Fourier transform routine. For the simple case of the perpendicular surveys, the inverse transform is an easy discrete integral with seven points. However, for oblique surveys the integrand is oscillatory and therefore the integral to be evaluated is more complex. We adapted a method presented by Ting and Luke (1981) using eighteen points in each integration. It was found that the effect of the direction of the line with respect to the strike is negligible for angles greater than 60 degrees. For smaller angles the effect is to "open" the anomaly with minor changes in its center. There is no obvious way to compensate this effect with changes in the model parameters.