Gravity and Magnetic Investigations at the KTB Locations Schwarzwald and Oberpfalz

[1] The lowermost section of the continental superdeep drill hole German Continental Deep Drilling Program (KTB) (south Germany) has been investigated for the first time by vertical seismic profiling (VSP). The new VSP samples the still accessible range of 6-8.5 km depth. Between 7 and 8.5 km depth, the drill hole intersects a major cataclastic fault zone which can be traced back to the Earth's surface where it forms a lineament of regional importance, the Franconian line. To determine the seismic properties of the crust in situ, in particular within and around this deep fault zone, was one of the major goals of the VSP. For the measurements a newly developed high-pressure/high-temperature borehole geophone was used that was capable of withstanding temperatures and pressures up to 260°C and 140 MPa, respectively. The velocity-depth profiles and reflection images resulting from the VSP are of high spatial resolution due to a small geophone spacing of 12.5 m and a broad seismic signal spectrum. Compared to the upper part of the borehole, we found more than 10% decrease of the P wave velocity in the deep, fractured metamorphic rock formations. P wave velocity is $5.5 km/s at 8.5 km depth compared to 6.0-6.5 km/s at more shallow levels above 7 km. In addition, seismic anisotropy was observed to increase significantly within the deep fracture zone showing more than 10% shear wave splitting and azimuthal variation of S wave polarization. In order to quantify the effect of fractures on the seismic velocity in situ we compared lithologically identical rock units at shallow and large depths: Combining seismic velocity and structural logs, we could determine the elastic tensors for three gneiss sections. The analysis of these tensors showed that we need fracture porosity in the percent range in order to explain seismic velocity and anisotropy observed within the fault zone. The opening of significant pore space around 8 km depth can only be maintained by differential tectonic stress combined with intense macroscopic fracturing. VSP reflection imaging based on PP and PS converted reflected waves showed that the major fault system at the KTB site is wider and more complex than previously known. The so-called SE1 reflection previously found in two-and three-dimensional surface seismic surveys corresponds to the top of an $1 km wide fault system. Its lower portion was not illuminated by surface seismic acquisition geometry. VSP imaging shows that the fault zone comprises two major and a number of smaller SE dipping fault planes and several conjugate fracture planes. The previously recognized upper fault plane is not associated with a strong velocity anomaly but indicates the depth below which the dramatic velocity decrease starts. Regarding the complexly faulted crustal section of the KTB site as a whole, we found that fluctuation spectra of rock composition and seismic velocity show similar patterns. We could verify that a significant amount of P wave energy is continuously converted into shear energy by forward scattering a...

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“…The boundary between ZEV and granite was assumed to dip $45°radially from the KTB site [cf. Soffel et al, 1989;Tillmanns et al, 1996].…”

Section: Regional Anisotropic P Wave Velocity Fieldmentioning

confidence: 99%

Superdeep vertical seismic profiling at the KTB deep drill hole (Germany): Seismic close‐up view of a major thrust zone down to 8.5 km depth

et al. 2004

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“…The German Continental Deep Drilling Project KTB (Kontinentales Tiefbohrprogramm der Bundesrepublik Deutschland) offered the unique chance to investigate the sources of magnetic anomalies in unweathered samples down to a depth of 9.1 km. The KTB drill site is located at the western margin of the Bohemian Massif in an area with a prominent magnetic anomaly (Soffel et al 1989; Bosum et al 1997). The KTB pilot drill hole (KTB‐VB) was completed in 1989 and reached a depth of 4.0 km.…”

Section: Introductionmentioning

confidence: 99%

Statistical analysis and origin of the magnetic susceptibility of drill cuttings from the 9.1-km-deep KTB drill hole

2000

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Magnetic susceptibility was measured at 2 m depth intervals on drill cuttings from the main drill hole of the German Deep Drilling Project KTB. Metamorphic rocks (metabasites and gneisses) were the rock types most frequently found down to a depth of 9101 m. Petrophysical (susceptibility, density), geochemical (element concentrations), lithological and petrological data (ore mineral concentrations, lithological components, alteration index) were used for a statistical analysis. The histograms of magnetic susceptibility show nearly log‐normal distributions with two distinct peaks depending on the lithology. The most frequent susceptibility values are 0.266 × 10−3 SI for gneissic rocks and 0.847 × 10−3 SI for metabasic rocks (mainly amphibolites). The higher level of metabasite susceptibility is caused by higher contents of paramagnetic silicates such as hornblende. A theoretical paramagnetic susceptibility was calculated from the iron and manganese contents derived from X‐ray fluorescence (XRF) measurements. The ferrimagnetic susceptibility was determined by subtracting the theoretical para‐magnetic susceptibility from the measured susceptibility. Cross‐plots of the ferrimagnetic susceptibility versus density are used to discriminate between samples with predominantly magnetite or pyrrhotite as the main ferrimagnetic mineral. Samples containing mostly pyrrhotite show susceptibilities not exceeding 6 × 10−3 SI, whereas the highest measured susceptibilities of 66.5 × 10−3 SI correspond to zones exclusively with magnetite. A factor analysis was applied to investigate the background factors representing the data variabilities. The factor analysis reduces 13 original variables from the complete depth section to five independent initial factors. These explain in total 66.2 per cent of the total data variance. The most significant factor, 1, correlates with metabasite content, density and paramagnetic susceptibility and it anticorrelates with gneiss content. The next significant factor, 2, correlates with ferrimagnetic susceptibility and magnetite content. Factor 3 correlates with the amount of cataclastic rocks, factor 4 with hornblende gneiss and factor 5 with pyrrhotite

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“…The drilling site has been placed into the gravity high of Erbendorf known from pre-site surveys (Bu¨cker 1986;Plaumann 1986;Soffel et al 1989). This anomaly correlates partly with the Erbendorf anomaly mapped by aeromagnetic (Pucher 1986) and helicopter magnetic measurements (Bosum et al 1997).…”

Section: Introductionmentioning

confidence: 99%

Gravity anomalies in the KTB area and their structural interpretation with special regard to the granites of the northern Oberpfalz (Germany)

et al. 1997

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By fieldwork within the frame of the KTB project a new set of 1800 gravity data has become available. Prominent Bouguer anomalies are produced by the ZEV (high with !6 mGal), the Grafenwo¨hr block (low with !24 mGal) and the granites (low with !46 mGal). The vertical gradient map gives information about the near-surface structure. In addition, spectral analysis and upward continuation of the field data give information about depth extension of the granites and other units. The density structure of the crust is outlined by 3D interactive forward modeling. Regional anomalies are caused by the Erbendorf body and the dipping Moho. Local and dominating anomalies have different sources: (a) the foreland sediments with an underlying deep structure (possibly of ZEV material); (b) the ZEV metamorphic units (gneisses and metabasites); and (c) the granites. The ZEV units are controled by the steeply inclined SE1 reflector as a tectonic element. The Falkenberg granite provides two thirds of the total granitic volume and its depth reaches 6.5 km, whereas other granites reach depths of 2-3 km. The nature of the crustal material below the granites and of the Erbendorf body remains unknown.

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Gravity and Magnetic Investigations at the KTB Locations Schwarzwald and Oberpfalz

Cited by 7 publications

References 1 publication

Superdeep vertical seismic profiling at the KTB deep drill hole (Germany): Seismic close‐up view of a major thrust zone down to 8.5 km depth

Superdeep vertical seismic profiling at the KTB deep drill hole (Germany): Seismic close‐up view of a major thrust zone down to 8.5 km depth

Statistical analysis and origin of the magnetic susceptibility of drill cuttings from the 9.1-km-deep KTB drill hole

Gravity anomalies in the KTB area and their structural interpretation with special regard to the granites of the northern Oberpfalz (Germany)

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