A New Fast Method To Measure Azimuthal Resistivity Anisotropy

Abstract

Apparent electric anisotropy can be estimated with an electrode configuration formed as the letter X. One electrode is placed at the end of each branch of the X and one is placed at about 1/3 of the distance from the center. Together with one electrode at the center, by the instrument, this makes a total of nine electrodes. By combining different electrodes it is now possible to measure the potential difference to current ratio in a large number of ways. Wenner arrays can e.g. be formed along the two diagonals of the X and square array measurements can be made with the four outermost electrodes. It is convenient to use multi-channel equipment since many electrode combinations can be measured quickly. For single reading methods we usually normalize the data by calculating an apparent resistivity. The X-configuration measurements are also normalized, but in this case we use a homogeneous anisotropic half-space for the normalization. There is of course no explicit formula for this conversion. The data are fitted to the model in a least-squares sense by Monte-Carlo inversion. The output of the inversion is apparent resistivity, apparent anisotropy direction and apparent anisotropy coefficient. The apparent resistivity is defined as the geometric mean of the horizontal resistivities and it is usually a good estimate of the bulk resistivity of the investigated rock volume. The influence of any local heterogeneity is small due to the large number of measurements with different geometry and the rather large electrode separations. Examples are shown where X-configuration measurements have been performed on Precambrian rocks in Sweden. The apparent anisotropy direction correlates well with direction of bedding in folded supracrustal rocks and with the direction of foliation in metamorphic rocks.