Johannes Stoll

Why is the electrical resistivity around the KTB hole so low

The low-resistivity anomaly close to the KTB borehole coincides with both a self-potential and a static magnetic anomaly. If this coincidence is not accidental, it may yield information about the conditions required for the existence of low-resistivity anomalies in the deeper crust. A possible answer to the question in the title of this paper is that the oxygen fugacity in the crust around the KTB hole must be low enough to stabilize graphite on a grain boundary scale. This could partially explain the extremely low resistivities of the graphitized cataclastic zones, the formation of magnetic pyrrhotite from ∼ 250 to 4000 m deep and the large self-potential anomaly. The latter requires a large vertical gradient of the oxygen fugacity and a continuous graphitic conductor through this gradient zone.

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

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.

Towards an Integrative Inversion and Interpretation of Airborne and Terrestrial Data

The aim of the joint research project is to generate information from airborne geophysical measurements that are properly transferred from physically quantitative descriptions of the subsurface (electrical conductivities, densities, susceptibilities) into spatial structures and information matching the understanding of end-users: geologists, hydrogeologists, engineers and others. We suggest new types of inversion, which are integrated in the interactive workflow to support typical trial and error approaches of inverse and forward EM and gravity/magnetic field modelling for 1D and 3D cases. Subsequently, we combine resistivity and density models with geological 3D subsurface models. The integrated workflow minimizes uncertainties in the interpretation of geophysical data and allows a significantly improved and fast interpretation and imaging of the 3D subsurface architecture. The results of the AIDA project demonstrate that combined 3D geological and geophysical models enable a much better reconstruction of the subterraneous space. AIDA stands for “From Airborne Data Inversion to In-Depth Analysis” and is part of the R&D program: Tomography of the Earth’s Crust—From Geophysical Sounding to Real-Time Monitoring.

Optimization of signal‐to‐noise ratio in dc soundings

Geoelectric (dc) sounding methods are usually limited to electrode spacings smaller than 1 km, which restrict the depth of investigation to only several hundreds of meters. Greater depths of investigation require both a larger electrode spacing and motor-generator driven transmitters. In order to increase the depth of investigation into the middle crust, the distance between transmitter and receiver dipole must be increased up to 100 km as well. Instead of a Schlumberger electrode configuration, we use a dipole-dipole electrode arrangement, which avoids cable connections of several tens of kilometers. It takes less logistics in the field and requires less precautions to control the cable circuits. However, the electric field of a grounded dipole decays by the power of 3. Even if a strong current source with a power of 30 kW is employed, at distances beyond 10 km the amplitude of the transmitted dc signal becomes considerably smaller than the naturally inductive field and cultural noise. Here, we present a technique which resolves the transmitted dc signal at sites up to 60 km apart using an electric dipole source. The suggested technique removes the inductive part in the time series and, therefore, reduces the noise level. It requires applying the magnetotelluric technique before and/or after the geoelectrical sounding. Both the magnetic and electric variational fields are recorded during geoelectrical sounding. If the magnetotelluric transfer function is obtained at a site, then the natural inductive electric field can be predicted, which in turn is used to remove the inductive electrical part from the receiver dipole record.