Electrical double-dipole experiment in the German Continental Deep Drilling Program (KTB)

Among the most important rationales to drill the German Continental Deep Dr illing Program (KTB) borehole was the necessity to calibrate geophysical me thods. Deep and hitherto inaccessible seismic reflectors, high-conductivity layers, and temperature belong to this group of deep crustal properties wh ich can be predicted from surface measurements, but whose depth and nature are a matter of dispute, One problem is the unknown influence of inhomogene ous superficial layers on the determination and resolution of the model par ameters. In the case of electrical resistivity a number of presite experime nts had detected a high-conductivity layer of regional extent at a mean dep th of similar to 10 km. Distorting superficial layers were expected to caus e severe ambiguity in the interpretation of the specific properties of this layer, even feigning its existence at all, The drilling yielded direct evi dence of high-conductivity material within the range of 8 km depth. After c ompletion of the KTB a large-scale dipole-dipole experiment was carried out using a vertical electric receiver dipole with one of the electrodes in th e main drill hole at 9065 m depth and a second in the earlier drill hole at 4000 m depth. The idea was to find out whether the buried electrode was cl ose to a high-conductivity layer of regional extent. The surprising result was that the two apparent resistivity curves measured with the transmitter spread perpendicular and parallel to the NNW striking very highly conductiv e fracture zones are almost overlapping, even though these fracture zones a re the cause of a strong structural anisotropy of the apparent resistivity measured with magnetotellurics. Such a strong anisotropy should also show u p in the buried electrode experiment except when a high-conductivity layer close but above the buried electrode at 9000 m depth is introduced in the m odel, As a result, the interpretation of this experiment suggests a NE dipp ing electrically conductive fault system soling out into a high-conductivit y horizontal layer at 7-8 km depth. The conductivity is increased due to gr aphite and high-salinity fluids, in a depth near the fossil Cretaceous brit tle-ductile transition zone for quartz-rich rocks.