German Continental Deep Drilling Research Articles

A non-extensive view of the Pressure Stimulated Current relaxation during repeated abrupt uniaxial load-unload in rock samples

Laboratory experiments have confirmed that the application of uniaxial stress on rock samples is accompanied by the production of weak electric currents, named Pressure Stimulated Currents (PSCs). In the present work the PSC emissions in marble and amphibolite (from the German Continental Deep drilling) samples are studied in the frame of non-extensive statistical mechanics, after having applied sequential loading-unloading cycles. It was ascertained that by increasing the sequential order of the loading-unloading cycle, the emitted PSC exhibits q-exponential behavior with a non-additive entropy parameter q which decreased in each applied uniaxial load-unload cycle. The behavior of the Tsallis q-parameter leads to the conclusion that fracturing is a subadditive process with hierarchically constrained dynamics. Furthermore, assuming an assembly of relaxed subdomains created due to microfracture, a non-extensive behavior of the observed macroscopically PSC relaxation is discussed in the frame of a superstatistical approach.

Tidal and non-tidal signals in groundwater boreholes in the KTB area, Germany

Tides and barometric pressure variations cause pore pressure changes in the solid earth. In boreholes which are hydraulically connected to confined aquifers these pore pressure changes can be observed as water level variations. In case of confined aquifers boreholes can be regarded as volumetric strainmeters. From June 2004 until May 2005 a large scale injection experiment was realised in the pilot borehole (4000 m) at the German Continental Deep Drilling (KTB) site in Bavaria. During this test the injection induced deformation was observed by a tiltmeter array around KTB consisting of five tiltmeter stations at horizontal distances from 1.6 to 3 km from the injection borehole. At each station the local groundwater level was observed in depths between 30 and 50 m. The pore pressure was recorded in the boreholes. The time series were checked for tidal signals and injection influence. The injection was not verified presumably due to the fact that the penetration of the injected water was controlled by local geology and tectonic faults zones. Although the boreholes extending only to shallow depth tidal signals are clearly observable in at least two of five stations. On December the 26th 2004 the Sumatra–Andaman earthquake occurred which is clearly detectable in the pore pressure variations at all five stations.

Electronically conducting brittle-ductile shear zones in the crystalline basement of Rittsteig (Bohemian Massif, Germany): Evidence from self potential and hole-to-surface electrical measurements

Crustal-scale shear zones may act as prominent electrical conductors given that sufficient amounts of graphite or fluids are present. There are several graphite-enriched shear zones within the 583-m-deep Rittsteig drilling (Bavaria, Germany), two of which are of major importance. One shear zone cuts through Moldanubian biotite-muscovite schists at 320 m depth. The other shear zone separates Tepla-Barrandian amphibolites from Moldanubian biotite-muscovite schists at 460 m depth. To detect these shear zones adjacent to the drilling, the self-potential method and the non-linear impedance spectroscopy have been applied. From the new data, obtained from hole-to-surface and surface measurements, we conclude that (1) graphite is pervasively distributed within the Rittsteig shear zones resulting in powerful electronic conductors, (2) the graphite-bearing shear zones are dipping moderately towards the S, and (3) the shear zone drilled at 320 m depth extends to the Earth’s surface where it has been detected ca. 400 m to the N of the drilling. As the graphite-bearing shear zones of both the Rittsteig drilling and the German Continental Deep Drilling (KTB) developed in the brittle-ductile regime of quartzofeldspathic rocks, we argue that the brittle-ductile boundary layer may act as a significant graphite attractor in the continental crust.

Pyrrhotite varieties from the 9.1 km deep borehole of the KTB project

We used transmission electron microscopy (TEM) to study pyrrhotite from the German Continental Deep Drilling (Kontinentale Tiefbohrung, KTB) project. Our goals were to determine the distribution of structure types with depth and to establish relationships between the bulk thermomagnetic behavior and the microstructures of pyrrhotite. In samples from the deep section of the borehole (9080 m below surface, which is equivalent to an in-situ temperature of similar to 260 degrees C), the dominant variety of pyrrhotite is 1C that has a mostly disordered vacancy distribution. Faint, diffuse superstructure reflections in the selected-area electron diffraction (SAED) patterns indicate some nC-like ordering occurs, probably in small domains. In addition to the disordered pyrrhotite, a 5C type is also present. According to bulk thermomagnetic measurements, pyrrhotite grains from 9080 m are antiferromagnetic at room temperature; we attribute this behavior to the dominance of the disordered 1C type. In the upper section of the hole (at 564 and 2325 m) several pyrrhotite varieties occur, but the 4C type is most common, in agreement with the ferrimagnetic character of most pyrrhotite grains. Optical microscopy of pyrrhotite grains that are covered with a magnetic colloid reveal intergrown ferrimagnetic and antiferromagnetic lamellae. TEM images show that these grains are intergrowths of 4C, 5C, and nC types. We interpret these microstructural variations to be responsible for the variation in magnetic properties within single pyrrhotite grains.

Fluid-Deposited Graphitic Inclusions in Quartz: Comparison Between KTB (German Continental Deep-Drilling) Core Samples and Artificially Reequilibrated Natural Inclusions

We used Raman microsampling spectroscopy (RMS) to determine the degree of crystallinity of minute (2–15 μm) graphite inclusions in quartz in two sets of samples: experimentally reequilibrated fluid inclusions in a natural quartz grain and biotite-bearing paragneisses from the KTB deep drillhole in SE Germany. Our sequential reequilibration experiments at 725°C on initially pure CO 2 inclusions in a quartz wafer and the J. Krautheim 1993experiments at 900–1100°C on organic compounds heated in gold or platinum capsules suggest that, at a given temperature, (1) fluid-deposited graphite will have a lower crystallinity than metamorphosed organic matter and (2) that the crystallinity of fluid-deposited graphite is affected by the composition of the fluid from which it was deposited. We determined that the precipitation of more-crystalline graphite is favored by lower f H 2 (higher f O 2), and that the crystallinity of graphite is established by the conditions (including gas fugacities) that pertain as the fluid first reaches graphite saturation. Graphite inclusions within quartz grains in the KTB rocks show a wide range in crystallinity index, reflecting three episodes of carbon entrapment under different metamorphic conditions. Isolated graphite inclusions have the spectral properties of totally ordered, completely crystalline graphite. Such crystallinity suggests that the graphite was incorporated from the surrounding metasedimentary rocks, which underwent metamorphism at upper amphibolite-facies conditions. Much of the fluid-deposited graphite in fluid inclusions, however, shows some spectral disorder. The properties of that graphite resemble those of experimental precipitates at temperatures in excess of 700°C and at elevated pressures, suggesting that the inclusions represent precipitates from C-O-H fluids trapped under conditions near those of peak metamorphism at the KTB site. In contrast, graphite that is intimately associated with chlorite and other (presumably low-temperature) silicates in inclusions is highly disordered and spectrally resembles kerogens. This graphite probably was deposited during later greenschist-facies retrograde metamorphism at about 400–500°C. The degree of crystallinity of fluid-deposited graphite is shown to be a much more complex function of temperature than is the crystallinity of metamorphic graphite. To some extent, experiments can provide temperature-calibration of the crystallinity index. However, the difference in time scales between experimental runs and geologic processes makes it difficult to infer specific temperatures for naturally precipitated graphite.