During several student excursions of the FU Berlin, geoelectric investigations were carried out in the SW of the German Continental Deep Drilling (KTB) site across the Franconian Lineament (Fränkische Linie), which separates the sedimentary (Mesozoic) foreland from the crystalline nappe of the Zone Erbendorf-Vohenstrauss (ZEV). A strong negative self- potential anomaly was found to run parallel to the fault zone, reaching values of up to -800mV. A DC-geoelectric sounding was conducted along profile I, displaying a very good shallow conductor. Graphite, perhaps associated with pyrite, is the most plausible explanation for this anomaly (which is also expressed in VLF and Slingram soundings).

 

Self-potential anomaly in the vicinity of the village of Glasern in the Oberpfalz. FL is the trace of the Franconian Lineament, the blue arrow marks a part of the geoelectric profile, conducted along profile I.

SP anomaly along line I.

 

2-D model of geoelectric profiling across the SP anomaly. The low resistivities correspond to the SP anomaly; an electronic conductor provides a shortcut between oxidizing and reducing environments.

 

VLF data (measured as part of a diploma thesis by T. Martin with an OMNI instrument) across the FL. Anomaly A is due to a buried pipe, B displays the rise of resistivities towards the basement and C is the location of the SP/geoelectric anomaly. The highly variable readings on the ZEV hint at a deeply fractured, inhomogeneous basement. Transmitter FUO is located perpendicular to strike direction (TM mode), while GBZ and NAA are almost in strike direction and give almost identical readings (TE mode). This assignment of modes is, however, only approximately correct, and correspondingly, a mode mixing is observed, in particular in the In-Phase and tilt values. It shows the inherent weakness of the method to obtain true TE and TM data even if the anomalous body is 2-D due to the limited availability of transmitters. Other prominent sources observable in the spectrum (lower left) are even less suitable.