The German Continental Deep Drilling Programme

 The drill tower of the main borehole

The drill tower of the main borehole at the German Continental Deep Drilling Programme. Image: W.J.Pilsak

On October 12, 1994, the German Continental Deep Drilling Programme near Windischeschenbach, Bavaria, ended at a depth of 9,101 metres in the Earth’s continental crust, on of the deepest drilling projects in the world.

The German Continental Deep Drilling Programme began in 1987 with an advance borehole, reaching its target depth of 4.000 meters in April 1989. The drilling results were then used as parameters for the main borehole. For instance the temperature gradient the scientists found out that the temperature gradient was higher than originally expected. In 1990, the main drilling started about 200 metres away from the advance borehole for an envisaged depth of 10.000 meters.

During the first 7.500 meters an automatic vertical drill head was used so that the lateral deviation could be limited to 12 metres. It was expected that below 7.500 meters the electronics would fail and a conventional drill head was used below that point. Due to brittleness and ductility the borehole was deforming multiple times, requiring to backfill the hole and start drilling again from a higher place. These operations continued for the last year of the project, unfortunately, the timing and budget constraints put an end to the efforts to go even deeper. In 1994, the drilling ended after 1.468 days at a depth of 9.101 meters and about 300 metres lateral deviation showing a temperature of 265 °C. Until the end of the year, three major experiments were performed.

The scientists hoped to reach the Erbendorfkörper – a mass that reflects seismic waves while the portion that enters it will be transmitted quickly. Even though they did not succeed to do so, the drilling was considered a success. Before the drilling it was expected that the large mass and tectonic pressures would change the nature of the rock at depth, so called metamorphic rock. However, the rock layers were not solid at that depth and instead large amounts of fluid and gas were met pouring into the drillhole. However, the rock layers were not solid at that depth and instead large amounts of fluid and gas were met pouring into the drillhole. Due to the heat and fluids the rock was of a dynamic nature which changes the way that the next superdeep drilling would be planned.

Further testing showed 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 can be interpreted better even when being initiated from the surface. The so called Dipol-Dipol-Experiment was conducted to measure the electric conductibility around the drill hole. It showed lines of graphite spanning through the rock that would also allow the rocks to glide when pressures moves them. Another experiment was titled Integriertes Hydrafrac/Seismik Experiment and in it, high pressure was put into the drillhole so that the rock would start cracking. The resulting seismic activity was measured at multiple stations in the larger area. The third experiment was called the Fluid/Hydraulik-Test and intended to pump large amounts of fluid into the rock which showed it to be generally porous. After the drilling project had ended the GFZ German Research Centre for Geosciences used the borehole to install a seismic observatory at depth that was active from 1996 to 2001.

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