Jb. Stoll et al., Electrical double-dipole experiment in the German Continental Deep Drilling Program (KTB), J GEO R-SOL, 105(B9), 2000, pp. 21319-21331
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.