[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...

Subsurface investigations in urban areas have to take sealed surfaces and densely built and populated areas into account. The shear-wave reflection seismic system developed at the Leibniz Institute for Applied Geophysics (LIAG) provides a solution for this acquisition technology problem using horizontally polarized shear waves (SH-waves) and noninvasive source and receiver designs.

[1] An active-source seismic experiment at the KTB deep drill hole in southeast Germany reveals seismic P wave anisotropy to exist within a tectono-metamorphic sub-terrane of the crystalline Bohemian massif. The experiment used multi-azimuth vertical seismic profiling whereby downhole sensors recorded surface seismic Vibroseis sources located along six 7.5-km radial profiles emanating from the borehole location. Representative bulk anisotropic P wave velocities of the upper crust were derived from this seismic data and compared with predictions of velocity and anisotropy based on petrophysical laboratory measurements and geological information. We show that azimuth and inclination behavior of the observed anisotropy is consistent with characterization of the geology surrounding the borehole as a coherent regional block containing pervasive highly tilted foliation. This seismic anisotropy can be explained by ''intrinsic'' material properties associated with the well-developed foliation fabrics.

No abstract

No abstract