DETERMINATION OF ELECTRICAL-RESISTIVITY, ITS ANISOTROPY AND HETEROGENEITY ON DRILL CORES - A NEW METHOD

This paper presents a new method for the measurement on core samples o f their electrical resistivity, its anisotropy and heterogeneity. The equipment used has been developed in the field laboratory of the Germa n Continental Deep Drilling program KTB in the north-east of Bavaria o n the western rim of the Bohemian Massif. The apparatus measures the r esistivity at a fixed frequency as a function of the drill core azimut h and along the core by moving point electrode configurations. From th ese azimuth and depth dependences, mean values of resistivity and addi tional information about its anisotropy and heterogeneity are determin ed. Geometrical averaging is used, because the resistivity data follow s a log normal distribution. The quantitative parameters 'azimuth fact or', corresponding to horizontal anisotropy, and 'heterogeneity factor ' are introduced. The depth logs of resistivity, azimuth factor and he terogeneity factor, measured on cores obtained from the KTB main drill hole (gneisses and amphibolites) at depths between 4150 m and 8080 m are presented. The geometrically averaged mean values of resistivity o f gneisses and amphibolites are in the same range (approximate to 10(3 ) Omega m). The resistivities tend to decrease with depth. The stress release of the drill cores during recovery produces microcracks which may partially account for this effect. Reduced resistivities (down to 150 Omega m) within an amphibolite core correlate with an alteration z one. One sample of this core displays alteration from fresh to complet ely altered. This sample is also electrically heterogeneous (heterogen eity factor approximate to 2). Other samples with uniform low alterati on are more homogeneous (heterogeneity factor approximate to 1.4). In general, higher anisotropies are observed in gneisses (mean azimuth fa ctor 2.8), lower anisotropies in amphibolites (mean azimuth factor 1.3 ). Examples of isotropic and homogeneous samples, as well as anisotrop ic and heterogeneous samples are also presented.