(Fig.18,19)
Well, the drill site is situated (this is a view from the Alps) at the boundary of two major geological units in central Europe - the Moldanubian and Saxothuringian units. This boundary is regarded as a suture ?, formed by the closure of a Paleozoic ocean, and as was expected from surface geology, two main rock types were encountered：
* metabasic rock types derived from former basaltic ocean floor, and
* a variety of paragneisses derived from graywackes and pelitic rocks, deposited in an active continental margin environment.
(Fig.20,21)
These rocks were complexly folded and deformed. This Figure depicts the model： the rocks were deposited about 500 million years ago. They experienced a very complex double subduction cycle. They were subducted very fast to a depth of over 45 kilometers, exhumated, and then, in a second cycle, again overprinted under medium pressure conditions.
(Fig.22)
Understanding the nature of seismic reflectors has been the top priority scientific target of our program. Therefore, already before we started drilling, a number of geophysical seismic surveys have been carried out.
(Fig.23)
This shows the deep seismic reflection profile of the DEKORP program which was done over a distance of 180 km along the western margin of the Bohemian Massif in 1986. This is a migrated line drawing of this experiment, which shows that the crust which has a thickness of about 30 km in that area, is distinguished by a high seismic reflectivity, and the borehole was placed above a crustal portion where a large number of seismic reflectors were detected.
(Fig.24,25)
A broad spectrum of petrophysical studies were conducted with very different scale lengths from kilometers to centimeters. These include 3D, 2D, wide-angle and near-vertical seismic surveys, VSP, MSP measurements, surface-to-borehole experiments, and they were complemented by a variety of logging measurements, drill-core analysis and petrophysical studies under in-situ conditions.
(Fig.26,27)
For the first time a 3-D seismic survey covering an area of 19 times 19 kilometers was carried out in continental basement rocks. This is a scheme for the VSP and MSP measurements which were conducted in the pilot hole before the main borehole was drilled. And this figure depicts the 3-D geophysical experiment. I will show you a 2-dimensional section across this 3-D experiment. This is the extraordinary informativ seismic line called KTB 8501.
(Fig.28)
With this 3-D experiment a number of pronounced seismic reflectors could be identified and the most prominent reflector element was predicted to occur at a depth of around 7 km. It was actually penetrated (at a depth) between 6,800 and 7,200 meters, and turned out to be a major fault system which can be traced back to the surface. All in all, most of the reflector elements in the upper-most crust are clearly related to fractures. Impedance contrasts produced by fractures are much greater than impedance contrasts due to lithologic or structural differences between the different rock types.
