German Continental Deep Drilling Programme

The drill tower of the main borehole

The German Continental Deep Drilling Program (in German: Kontinentales Tiefbohrprogramm der Bundesrepublik Deutschland), abbreviated as the KTB borehole, was a scientific drilling project carried out from 1987 to 1995 near Windischeschenbach, Bavaria. The main superdeep borehole reached a depth of 9,101 meters in the Earth's continental crust.

The German Federal Ministry of Research funded the project with 528 million DM (270 million Euros). The Lower Saxony LBEG mining office (State Office for Mining, Energy and Geology (DE)) took over the project lead. After the drilling project ended, GFZ (German Research Centre for Geosciences) used the borehole to install a seismic deep observatory (Tiefenobservatorium) that was active from 1996 to 2001. The derrick - one of the largest in the world - became a tourist attraction site in the region. The two boreholes were kept open for further scientific research.

History

In October 1986 Dr. Heinz Riesenhuber (German Minister for Research and Technology) officially announced that the super-deep borehole of the Continental Deep Drilling Program of the Federal Republic of Germany (KTB) would be drilled in the Oberpfalz area of Northern Bavaria.

The site of the drilling project is in the ZEV, Zone von Erbendorf-Vohenstrauß. Just south of the Erbendorflinie is the contact zone where Laurasia and Gondwana merged into Pangaea. Due to the geological genesis, the Earth's crust consists of a number of layers exhibiting a number of phenomenon such as strong magnetic areas and the body of Erbendorf, a large mass drifting at about 11 kilometers below the surface with a relatively high speed and seismic activity.

The project began in September 1987 with an advance borehole using state-of-the-art technology. The borehole reached its target depth of 4,000 meters in April 1989. The results of this drilling were used as parameters for the main borehole - for example, the temperature gradient was higher than originally expected. The main drilling started on September 8, 1990 about 200 meters away from the advance borehole for an envisaged depth of 10,000 meters. The first 7,500 meters used an automatic vertical drill head so that the lateral deviation could be limited to 12 meters. Due to the expected failure of the electronics below that depth, a conventional drill head was used below that point. As a result of brittleness and ductility, the borehole became deformed multiple times, prompting back-filling of the hole and re-drilling from a higher point. These operations continued to the last year of the project; the timing and budget constraints put an end to the efforts of going deeper. The drilling ended on October 12, 1994 after 1,468 days at a depth of 9,101 meters and about 300 meters lateral deviation, showing a temperature of 265°C. The three main experiments planned were performed until December 31, 1994. The facilities were partially dismantled until December 31, 1995.

Results

The KTB project utilized several innovations to drill design - the Kola drilling experiment had shown problems with high friction that was increased by the vertical instability when drilling to such deep depths. German scientists designed a new drill head that would keep the direction with minimal lateral difference. The drill head was also designed to withstand temperatures between 250 and 300 °C (482 and 572 °F). The original expectations had been that this temperature would be reached at a depth about 10 to 14 kilometers. This is also the reason why the ZEV place was chosen. There was another location in the Black Forest that had been considered equal to the scientific interest level but it was expected to have a higher temperature gradient. At the ZEV location, however, it was hoped to possibly reach the Erbendorfkörper - a mass that reflected seismic waves while the portion that entered it would be transmitted quickly.

Even before the Erbendorfkörper was reached, the KTB drilling was widely considered a success. For one thing, the temperature rose much more quickly than expected which caused discussion and a reform on the theories about the temperature gradient of very deep drill holes. Other theory changes were also required - it had been expected that the large mass and tectonic pressures would change the nature of the rock at a depth so-called metamorphic rock. Quite unexpectedly, however, the rock layers were not solid at that depth and, instead, large amounts of fluid and gas were met pouring into the drill hole. Due to the heat and fluids, the rock was of a dynamic nature which changed how the next superdeep drilling would be planned.

The experiments of the KTB did also show new results. The initial seismic tests showed very different recordings compared to those near the surface so that the theories on the source of seismic reflections needed to change. Using the data, the reflections from the deep could be interpreted better even when initiated from the surface.

The first experiment, the "Dipol-Dipol-Experiment" was about measuring the electric conductibility around the drill hole. This showed lines of graphite spanning through the rocks that allowed the rocks to glide when pressures moved them. The second experiment was to exert high pressure in the drillhole such that the rock would start cracking, the "Integriertes Hydrafrac/Seismik Experiment". The resulting seismic activity was measured at multiple stations in the larger area - the result was that the overall pressure came from the South, the African tectonic plate at work. The third experiment, the "Fluid/Hydraulik-Test" pumped large amounts of fluid into the rock which proved it to be generally porous.

The experiences were the foundation of the follow-up project, the International Continental Scientific Drilling Program (ICDP) founded in 1996. The German scientists of the KTB were also called to help with the San Andreas Fault Observatory at Depth drillhole.

External links

• geological surface map around the drilling site (picture)
• geological cross section northwest-southeast (picture)
• geological cross section southwest-northeast (picture)
• GFZ Potsdam - ICDP successor project (English)
• GFZ Potsdam - photo material on the KTB
• additional information on the KTB (German)
• Main coordination centre of the KTB