| Abstract: | A second deep Schlumberger sounding has been carried out in South Africa, this time using a newly completed power transmission line, not yet in sevice, as emission line. The maximum current electrode separation attained was 600 km. The sounding was situated on a shield area with a thin cover of sedimentary rocks (Karroo System) overlying the archaean granite. The center was chosen near to the village of Dealesville. It is shown that lateral effects at large AB spacings are unlikely, but due to numerous dolerite intrusions in the sediments surface electrode effects must be guarded against; thus AB positions were chosen by geologists. Because of a low signal/noise ratio at large AB spacings the measurements were treated statistically; in particular, the elimination of non-random events improved the measuring accuracy. Although a thin conducting layer at the base of the sedimentary succession decreased the resolution of the sounding curve, the results, which were interpreted with the aid of a computer, show positively that - (a). the crust is multi-layered with a highly resistant portion having a minimum resistivity of 15000 ohm.m and
- (b). a good, thick conductor appears as the final layer.
The Dealesville results are in complete agreement with those obtained from the first deep Schlumberger sounding at Pofadder (Van Zijl 1969) except that the transverse resistance of the lowermost resistant crust is about 100% larger at Pofadder due probably to a second period of metamorphism. Both electrical studies support the view of a dehydrated lower crust (granulite facies). |
