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
Journal of Geophysical Research, 109 (B9):
B09309+ (Sep 30, 2004)DOI: 10.1029/2004JB002975
Abstract
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 260C 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 and that multipathing
plays an important role in signal formation. The media behaves effectively
smoothly only at wavelength larger than 150 m. It was shown by moving
source profiling that the media is orthorhombic on a regional scale.
The tilt of the symmetry axes of anisotropy varies with depth following
the dip of the geological structure.