Kontinentale Tiefbohrung Research Articles

A borehole investigation of zircon radiation damage annealing

AbstractThis contribution reports Raman radiation damage measurements of zircons from the Kontinentale Tiefbohrung, on the western border of the Bohemian Massif. The mean wavenumbers (ω3) and widths (Γ3) of the ν3(SiO4) Raman band are constant down to 3 km, decrease (ω3) resp. increase (Γ3) between 3 and 5 km, and are again constant between 5 and 7 km. Uniform high Γ3 values associated with ω3 values close to those of undamaged zircon between 5 and 7 km are interpreted as due to residual damage predating the exhumation of the Bohemian Massif. A superimposed post‐exhumation signal indicates full damage retention down to 3 km depth, partial annealing between 3 and 5 km, and zero retention at greater depth. An attempt to calculate radiation damage ages gives results of a meaningful order of magnitude but also exposes difficulties associated with dating basement samples with complex damage accumulation and annealing histories.

The installation campaign of 9 seismic stations around the KTB site to test anisotropy detection by the Receiver Function Technique

Abstract. The project at hand is a field test around the KTB (Kontinentale Tiefbohrung) site in the Oberpfalz, Southeastern Germany, at the northwestern edge of the Bohemian Massif. The region has been extensively studied through the analysis of several seismic reflection lines deployed around the drilling site. The deep borehole had been placed into gneiss rocks of the Zone Erbendorf-Vohenstrauss. Drilling activity lasted from 1987 to 1994, and it descended down to a depth of 9101 m. In our experiment, we aim to recover structural information as well as anisotropy of the upper crust using the receiver function technique. This retrieved information is the basis for comparing the out-coming anisotropy amount and orientation with information of rock samples from up to 9 km depth, and with high-frequency seismic experiments around the drill site. For that purpose, we installed 9 seismic stations, and recorded seismicity continuously for two years from June 2012 to July 2014.

3D fracture networks in the crystalline upper crust ‐ A new seismic model of the Continental Deep Drilling Site (South Germany)

ABSTRACTBased on three‐dimensional seismic reflection data, we present the first comprehensive three‐dimensional model of the fault and fracture inventory of the crystalline upper crust penetrated by the Continental Superdeep Drillhole (Kontinentale Tiefborung, Oberpfalz, Southeast Germany). The investigated volume spans ∼19×19 km2 down to 16‐km depth. It can be regarded as a typical example of metamorphic crust that has undergone numerous phases of ductile and brittle deformation since the start of the Variscan orogeny. We developed an automated workflow for identifying and quantifying the crustal fracture network in order to prepare a basis for an assessment of fluid pathways and geothermal potential of crystalline crust. The workflow comprises the following steps: determination of (i) a three‐dimensional model of major faults by structural tensor analysis and (ii) a three‐dimensional model of middle‐scale fractures by log‐Gabor filtering and image processing; (iii) validation of results by comparison with geophysical borehole data, with independent seismic and seismicity data; and (iv) definition of a three‐dimensional fracture density function serving for a statistical assessment of fracture connectivity and “relative permeability.” This assessment is based on probabilistic fractionation and percolation theories. The derived three‐dimensional distribution of “relative permeability” may be used as a kernel function for inverting hydraulic permeability from hydrothermal field experiments. By comparing borehole and three‐dimensional seismic data, we could confirm that the faults and fractures of the Kontinentale Tiefbohrung area follow a fractal law that is consistent with Turcotte's fractionation model. An important conclusion from this is that upscaling and downscaling between one‐dimensional borehole and three‐dimensional seismic data appear possible. The corresponding one‐, two‐, and three‐dimensional fractal dimension are about 0.8, 1.9, and 2.8, respectively.

The KTB apatite fission-track profiles: Building on a firm foundation?

Deep boreholes serve as natural laboratories for testing thermochronometers under geological conditions. The Kontinentale Tiefbohrung (KTB) is an interesting candidate because the geological evidence suggests that approximate isothermal holding since the last documented exhumation in the Late Cretaceous to Palaeocene is a reasonable assumption for the thermal histories of the KTB samples. We report 30 new apatite fission-track ages and 50 new mean confined track lengths determined on cores from the 4km deep pilot hole. The ϕ- and ζ-external detector ages are consistent with the population ages from earlier studies and together define a clear age profile. The mean track lengths from this and earlier studies reveal the effects of experimental factors. The measured age and length profiles are compared with the predictions of 24 annealing models for isothermal holding. There are clear discrepancies between the measured and calculated profiles. Down to 1.5km depth, the measured mean track lengths are shorter than the predicted. The balance of methodological evidence indicates that this is due to seasoning, i.e., a shortening of the fossil confined tracks without attendant age reduction. From 2.5 to 4.0km depth, the mean track lengths are longer than the predictions. This suggests that the bias model that weights the probabilities of observing tracks of different length and which is based on experiments relating surface track densities to mean track lengths is not appropriate for confined tracks. Experimental and methodological factors are sometimes difficult to disentangle, but present a sufficient margin for there to be no need to go against the independent geological evidence. Unknown geological events cannot be ruled out but their existence cannot be inferred from the fission-track data alone, much less can the nature or magnitude of such events be specified.

Influence of models on seismic-event localization

Source localization is a fundamental problem in seismology. Current localization techniques often rely on homogeneous isotropic models, even if a survey region is known to be geologically complex or anisotropic. We investigated a model’s influence on localization, using data from a hydraulic injection experiment at the Continental Deep Drilling (Kontinentale Tiefbohrung, or KTB) site in Germany. We performed the localization with a grid-search algorithm and two additional methods for verification. From previous work, a homogeneous isotropic model based on a borehole check shot and a heterogeneous 3D isotropic model were available. Vertical seismic profiling (VSP) and borehole data as well as laboratory data from rock samples provided homogeneous anisotropic models. Although localization with the isotropic and anisotropic models led to the same magnitude of residuals and therefore to a comparable quality of fit, the distribution of the event cloud differed significantly for isotropic and anisotropic media. The isotropic model leads to a southward lateral shift of the event cloud of about [Formula: see text] from the injection point; the cloud appears almost centered at the injection point for the anisotropic models. This implies the classical diffusion concept for event generation, whereas isotropic models suggest the existence of a fault system providing a migration path for injected fluid, generating seismicity at an offset from the injection point. These results indicate that meticulous model building is essential for seismic-event localization and subsequent interpretation.

Earthquake Cluster: What Can We Learn from Waveform Similarity?

Abstract The seismograms of earthquakes, which have closely spaced hypocenters, tend to be similar due to the similarity of the Green’s functions characterizing the source-receiver paths. Based on the λ /4 criterion, it is frequently argued that similar earthquakes may represent repeated slip of the same patch of a fault. Because of the phenomenological nature of waveform similarity, such interpretations are strongly dependent on seismic signal characteristics and on the way, the waveform similarity is defined. In this article, we use two-dimensional synthetic wave-field simulations in lateral heterogeneous media to investigate how the waveform similarity of closely spaced hypocenters changes with interevent separation. We analyze the influence of correlation window length, signal frequency bandwidth, and source-receiver geometry on the waveform similarity and discuss under which conditions the λ /4 criterion can be applied to the synthetic data set. With the correlation window length defined as 2.8 times the travel-time difference between the S - and P -phase onsets, we find a correlation threshold value of 0.95 independent of the signal frequency bandwidth. We use the same threshold value for two field data examples that are similar to the synthetic data in frequency content and waveform complexity, and we discuss the implications of the λ /4 criterion. For three microearthquakes occurring during a fluid-injection experiment at the German deep drilling site (Kontinentale Tiefbohrung [KTB]), the interevent separation constrained by the λ /4 criterion is sufficient to identify these events as a sequence of repeating earthquakes in the sense that at least a fraction of the source area experienced repeated slip. For a second data example of four natural (micro-) earthquakes occurring near the island of Crete, the λ /4 criterion does not sufficiently constrain the hypocenter location to identify these events as repeating earthquakes due to the lack of high-frequency information.

Deformation, seismicity, and fluids: Results of the 2004/2005 water injection experiment at the KTB/Germany

During the past decades, the research into fluid‐controlled geodynamic processes in the upper crust of the Earth is put foreward regarding the stress accumulation, deformation, and seismicity. In a large‐scale injection experiment at the deep borehole site KTB (Kontinentale Tiefbohrung der Bundesrepublik Deutschland) in Germany, more than 84.000 m3 fresh water was injected in 4000 m depth over 10 months, and a lot of geoscientific investigations were connected to this injection test. The pore‐pressure change of more than 10 MPa yielded an induced deformation, which was detected by a tiltmeter array. The used five borehole tiltmeters of the ASKANIA type with a resolution of better than 1 nrad belong to the most sensitive tiltmeters worldwide. The poro‐elastic finite‐element modeling used for the interpretation of the observed tilts revealed an uplift of 3.1 mm above the injection point. Furthermore, it was shown that the induced stress and deformation fields depend mainly on the fluids inside the fault zones, particularly the SE2‐reflector which is the Franconian fault zone, and the local Nottersdorf fault: While the maximum pore pressure is concentrated close to the injection point at the SE2 zone, the maximum deformation of about 3 cm is located at the intersection line of both faults in 4 km depth. The area of the modeled maximum deformation is highly correlated with the region of the observed induced seismicity, thus linking seismicity and deformation. The connection of tilt observation and modeling provides a useful tool for the investigation of fluid coupled geoprocesses.

A model for fluid-injection-induced seismicity at the KTB, Germany

SUMMARY The 9.1 km deep KTB (Kontinentale Tiefbohrung, Germany) drilling hole is one of the best investigated deep-drilling sites in the world. Among other parameters, in situ measurements revealed continuous profiles of principal stresses, pore fluid pressure and fracture geometry in the vicinity of the borehole. The present study combines these parameters with hydraulic and seismicity data obtained during fluid-injection experiments conducted at the KTB to derive a conceptual model for fluid-injection-induced seismicity at the KTB. This model rests on the well constrained assumptions that (1) the crust is highly fractured with a permeable fracture network between 9 km depth and the Earth's surface and (2) the crust is in near-failure equilibrium, whereby a large number of fracture planes are under near-critical condition. During the injection experiment, the elevated pore fluid pressure remained well below the least principal stress and thus was too small to cause hydraulic opening of existing fractures. Consequently, the geometry of the fracture network was assumed to have not changed during fluid injection with induced seismicity occurring solely as a result of lowering of the effective normal stress, consistent with observed source mechanisms. The key parameter in the present model is the fracture permeability, which exhibits large spatial and directional variations. These variations are proposed to primarily control fluid migration paths and associated propagation of elevated fluid pressure during fluid injection. In contrast with common models based on isotropic fluid diffusion or spatially averaged permeability, highly permeable branches of the fracture network strongly affect the propagation of fluid pressure and prohibit the concept of a smooth ‘pressure front’. We find evidence that major fluid flow exists at comparatively low fluid pressure (below the critical pressure required to cause seismic failure) without being detected seismically. This might also explain the difference between 1011 J of hydraulic energy inserted into the system during fluid injection and ∼108 J of seismic energy: a major part of the hydraulic energy might be converted to potential energy of the ground water level caused by upward migrating fluid. From the fluid level response to changes of injection rate observed in a second borehole we estimate fluid signal velocities to be as large as 300 m d−1. Importantly, the suggested model also accounts for the occurrence of repeating earthquakes (multiplets), a large number of which were observed during the injection experiment. The present model also suggests that coseismic changes of the stress field caused by tectonic shear stress release are very local and of small magnitude. This is consistent with the observation that none of the larger induced events is followed by aftershock series that would be expected if coseismic processes had noticeably perturbed the local stress field.

Probing the Crust to 9-km Depth: Fluid-Injection Experiments and Induced Seismicity at the KTB Superdeep Drilling Hole, Germany

A 60-day, long-term fluid-injection experiment was performed at the 9.1-km-deep Kontinentale Tiefbohrung, Germany (KTB), borehole. About 4000 m3 of water were injected into the well head to induce seismicity near the open-hole section at 9-km depth. Because of several leaks in the borehole casing (unknown before), seismicity occurred at distinct depth levels between 3-km and 9-km depth. Two events occurred at 10-km and 15-km depth. The combination of a temporary, 40-element, three-component surface network of seismometers and a three-component downhole sonde at 3.8-km depth in the nearby pilot hole enabled us to determine absolute hypocenter locations by using a velocity model that was calibrated from several downhole shots at depths of 5.4 km and 8.5 km. Of a total of 2799 induced events, hypocenter locations were obtained for 237 events having good signal-to-noise ratio at surface stations. The spatiotemporal distribution of hypocenters at each depth level exhibits complex structures extending several hundred meters from the injection points, with strong spatial and temporal clustering. Regions that were seismically active at a certain time often showed reduced or no activity at later times, indicating local shear-stress relaxation. A similar “memory” effect (Kaiser effect) is observed by comparing hypocenter locations of the present experiment with those obtained for a previous injection experiment at KTB. The limitation of hypocentral depths to 9.1 km for events near the borehole suggests changes in rheological properties of the upper crust and thus supports a transition from the regime of brittle failure to ductile deformation at this depth. Large fluid-level changes observed in the nearby pilot hole demonstrate that fluid flow occurs over distances greater than 1.5 km and that major flow zones are not mapped by the induced seismicity. This might also explain the occurence of isolated events at greater distances and depths. Brittle failure at depths greater than 10 km indicates the existence of critically stressed fractures even at temperature over 300°C.

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.

Positioning the titanite fission-track partial annealing zone

As part of the KTB (Kontinentale Tiefbohrung) programme in Germany, titanite (sphene) fission-track data were acquired for samples to a depth of 9000 m, where the host rock temperature is ∼265°C. At this depth, age and confined track length information suggest that the partial annealing zone for the fission tracks in titanite was not reached. By reconstructing the thermal history and `backstacking' of an uplifted partial annealing zone, it is suggested that the in situ partial annealing zone is likely to be found in the region 265–310°C. This value is more precise than previous estimates of the titanite partial annealing zone, and it is also higher than previous estimates. Microprobe analysis indicates that the chemical composition of the titanites is rather homogeneous throughout the whole drilled section.

Magnetotelluric and deep geomagnetic induction data in the Bohemian Massif

Manifestations of the increased tectonic activity (seismic activity, remnants of Tertiary volcanism and riftogenesis) in the Bohemian Massif are bound to geologically extremely complicated regions with a system of discordant structures and a mosaic of fault zones. The presented results concern the deep geoelectrical features of the tectonic transition between the Saxothuringian and Moldanubian tectonoblocks on the territory of West Bohemia, in close vicinity to the German deep drilling experiment KTB (Kontinentale Tiefbohrung der BRD). Three first-order tectonic lines demarcate the region under study í the Litomerice deep fault to the north, the West Bohemian deep fault zone to the east, and the Central Bohemian fault to the south. As a whole, the region involved is characterized by a regionally increased seismic activity with the most active zone just beyond the northern end-point of the profile investigated. The contribution can be considered an example of possible interpretation of MT/MV/AMT data in geologically extremely complicated conditions with evidently discordant structures affecting the geoelectrical data.

Late to post-Variscan deformation phases and palaeostresses in the KTB pilot research well (Bohemian Massif, Germany)

Brittle faults and veins are very widespread in the crystalline rocks of the northern Oberpfalz, particularly in the 4000 m deep Kontinentale Tiefbohrung (KTB) pilot well. In the late Variscan (Late Carboniferous), subvertical tension gashes were formed under NE-SW extension. A genetic relationship between the vein development and late-phase magmatic activity of the late Variscan granites is obvious. Subsequently, but still during the Late Carboniferous, graphite-enriched reverse faults developed under E-W and NE-SW compression. In the deeper part of the KTB pilot hole these faults were formed within the brittle-ductile transition regime of the paragneisses. Since, during the late Variscan, the geothermal gradient was remarkably elevated, the brittle-ductile regime was situated at a relatively high crustal level where cataclasis, crystal plasticity and diffusion-controlled deformation mechanisms were active simultaneously. In post-Variscan time, probably during the Cretaceous, a further generation of reverse faults developed in addition to subhorizontal tension gashes and minor strike-slip faults. During these events the temperature was much lower compared with the conditions during the late Variscan deformations. The reverse faults evolved primarily under N-S compression, which is probably related to the Cretaceous subduction and collision events of the Alpine orogeny further to the south. The youngest, normal, faults are probably connected to the subsidence of the nearby Eger Graben which evolved during the Neogene uplift of the investigated area. The stress data, derived by using faults and striations from surface outcrops and from several oriented core intervals from the KTB pilot well, display weak depth-dependent variations. Although the deformation was polyphase, the “inversion method” has proved to be a suitable mode for determining reliable palaeostress data. However, if the brittle-ductile transition regime or broad shear zones are reached, the data become more and more invalid.

The use of mica track microfilters and CR-39 SSNTD in the quantitative determination of the Rn-222 content in the rinse water of a drill-hole for the KTB (kontinentale tiefbohrung)

A method for quantitative determination of the Rn-222 content in water has been developed, which combines measurements of the alpha- and gamma-activity of Rn-222 and its daughters. Drill-hole water from the Kontinentale Tiefbohrung (KTB) is measured simultaneously with a Ge(Li)-detector and with CR-39 SSNTD. The track density of CR-39 is counted and can be correlated with the alpha-activity of the sample.