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

Abstract

Among the most important rationales to drill the German Continental Deep Drilling Program (KTB) borehole was the necessity to calibrate geophysical methods. Deep and hitherto inaccessible seismic reflectors, high‐conductivity layers, and temperature belong to this group of deep crustal properties which can be predicted from surface measurements, but whose depth and nature are a matter of dispute. One problem is the unknown influence of inhomogeneous superficial layers on the determination and resolution of the model parameters. In the case of electrical resistivity a number of presite experiments had detected a high‐conductivity layer of regional extent at a mean depth of ∼10 km. Distorting superficial layers were expected to cause severe ambiguity in the interpretation of the specific properties of this layer, even feigning its existence at all. The drilling yielded direct evidence of high‐conductivity material within the range of 8 km depth. After completion of the KTB a large‐scale dipole‐dipole experiment was carried out using a vertical electric receiver dipole with one of the electrodes in the 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 close 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 conductive fracture zones are almost overlapping, even though these fracture zones are the cause of a strong structural anisotropy of the apparent resistivity measured with magnetotellurics. Such a strong anisotropy should also show up 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 model, As a result, the interpretation of this experiment suggests a NE dipping electrically conductive fault system soling out into a high‐conductivity horizontal layer at 7–8 km depth. The conductivity is increased due to graphite and high‐salinity fluids, in a depth near the fossil Cretaceous brittle‐ductile transition zone for quartz‐rich rocks.