INVESTIGATION OF ELECTRICAL ANISOTROPY IN THE DEEP BOREHOLE KTB

Information about electrical anisotropy in and around the deep borehol e of the German Continental Deep Drilling Program (KTB) has been acqui red using three different methods: (1) Under surface temperature and p ressure conditions, the azimuthal dependence of resistivity has been m easured in horizontal planes perpendicular to the axes of drill cores. (2) Simulated in-situ conditions (high temperature and pressure) have been applied to mini cores drilled at different orientations to struc tural features such as foliation. The resistivity was then measured in the direction of the mini cores' long axes. (3) Under quasi-in-situ ( downhole) conditions, the azimuthal dependence of resistivity has been measured in horizontal planes using the Azimuthal Resistivity Imager (ARI), a down hole logging tool. These investigations have shown that electrical anisotropy is strongly related to structural features. The anisotropy of gneisses is the highest due to the strong foliation (ave raged anisotropy factor 2.8). Lamprophyres are most isotropic (factor 1.2). Most rocks from KTB are amphibolites with intermediate anisotrop ies (factor 1.3). The anisotropies measured under surface temperature and pressure conditions and quasi-in-situ conditions on amphibolites a re approximately the same, whereas anisotropies under simulated in-sit u conditions are partially higher. Furthermore, the anisotropies measu red on the cm to m scale using the various methods outlined above shal l be compared with large scale anisotropies of the km scale measured a round KTB using Magneto Tellurics.