New VSP to 8.5 km Depth at the KTB Super-deep Drillhole - Seismic In Situ Investigation of Deformed Crystalline Crust
Abstract
The deepest part of the superdeep borehole of the Continental Deep Drilling Program (KTB), Germany, from 6000 m to 8500 m, has been sampled for the first time by Vertical Seismic Profiling. The VSPs show P-, S- and PS-converted waves. We observed S-wave splitting over the whole depth range. The azimuth of the polarization of the faster S-wave correlates with the azimuth of rock foliation. The velocity-depth profiles are of high spatial resolution due to the narrow geophone spacing of only 12.5 m. High accuracy was achieved by using a reference geophone at 3.8 km depth in the KTB pilot hole. Amphibolite units show an average P-wave velocity of 6500 m/s with 10% variation caused by composition, fractures and anisotropy. In a Gneiss sequence at 8.5 km depth low velocity of only 5500 m/s is observed correlating with changes in the relative orientation of foliation and fractures. To quantify the effect of intrinsic anisotropy and fractures, reference velocities were computed based on laboratory measurements and on the depth function of mineral composition. By analysing the differences between real and simulated velocity depth profiles we could determine that the maximum P-wave anisotropy of the amphibolite rock matrix in situ is about 9%. Also, we could identify borehole sections with increased fracture density in particular at largest depths. The complexly faulted and folded metamorphic rocks show P-wave traveltime fluctuations with scale lengths between 140 and 220 m in terms of a random media approach. The transition from scattering to effective anisotropic media could be observed for shear waves at different frequency windows. Computer simulations show that a large part of the wavefield is affected or even generated by scattering. Wavefronts are no more circular but strongly distorted depending on the local velocity contrasts. Forward scattering occurs as conversion from P- to S-wave energy generating complex waveforms of simulated and real data which show high similarity with another. Depth profiles of the energy PS-converted waves correlate with geological structure and fracture zones. The importance of these effects for deep crustal reflection studies is illustrated by records of remote VSP shotpoints showing a pronounced coda of scattered arrivals for subhorizontal raypaths through the mid crustal brittle-ductile transition zone. The VSP-records reveal high-resolution images of a major fault zone drilled at ca. 7 km (SE1 zone). PP- and PS-reflections show that the SE1 zone is a bundle of structural elements only one of which is directed towards the previously known intersection with the borehole at ca. 7 km depth. Other reflection elements correlate with the low velocity zone found at the bottom end of the velocity-depth profile where the largest differences between computed reference and in situ velocities were found. This suggests that the strongest reflections are caused by fractures rather than by lithology.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2001
- Bibcode:
- 2001AGUFM.S31A0594R
- Keywords:
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- 0915 Downhole methods;
- 0935 Seismic methods (3025);
- 7205 Continental crust (1242)