Depth-Recursive Tomography Along the Eger Rift Using the S01 Profile Refraction Data: Tested at the KTB Super Drilling Hole,Structural Interpretation Supported by Magnetic,Gravity and Petrophysical Data |
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| Authors: | Miroslav Novotný Zuzana Skácelová Jan Mrlina Bed?ich Ml?och Bohuslav R??ek |
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| Institution: | 1.Institute of Geophysics,Academy of Science of Czech Republic,Prague,Czech Republic;2.Czech Geological Survey,Prague,Czech Republic |
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| Abstract: | The refraction data from the SUDETES 2003 experiment were used for high-resolution tomography along the profile S01. The S01
profile crosses the zone Erbendorf-Vohenstrauss (ZEV) near the KTB site, then follows the SW–NE oriented Eger Rift in the
middle part and continues toward the NE across the Elbe zone and the Sudetic structures as far as the Trans-European Suture
Zone. To get the best resolution in the velocity image only the first arrivals of Pg waves with minimum picking errors were
used. The previous depth-recursive tomographic method, based on Claerbout’s imaging principle, has been adapted to perform
the linearized inversions in iterative mode. This innovative DRTG method (Depth-Recursive Tomography on Grid) uses a regular
system of refraction rays covering uniformly the mapped domain. The DRTG iterations yielded a fine-grid velocity model with
a required level of RMS travel-time fit and the model roughness. The travel-time residuals, assessed at single depth levels,
were used to derive the statistical lateral resolution of “lens-shaped” velocity anomalies. Thus, for the 95% confidence level
and 5% anomalies, one can resolve their lateral sizes from 15 to 40 km at the depths from 0 to 20 km. The DRTG tomography
succeeded in resolving a significant low-velocity zone (LVZ) bound to the Franconian lineament nearby the KTB site. It is
shown that the next optimization of the model best updated during the DRTG iterations tends to a minimum-feature model with
sweeping out any LVZs. The velocities derived by the depth-recursive tomography relate to the horizontal directions of wave
propagation rather than to the vertical. This was proved at the KTB site where pronounced anisotropic behavior of a steeply
tilted metamorphic rock complex of the ZEV unit has been previously determined. Involving a ~7% anisotropy observed for the
“slow” axis of symmetry oriented coincidentally in the horizontal SW–NE direction of the S01 profile, the DRTG velocity model
agrees fairly well with the log velocities at the KTB site. Comparison with the reflectivity map obtained on the reflection
seismic profile KTB8502 confirmed the validity of DRTG velocity model at maximum depths of ~16 km. The DRTG tomography enabled
us to follow the relationship of major geological units of Bohemian Massif as they manifested in the obtained P-wave velocity
image down to 15 km. Although the contact of Saxothuringian and the Teplá-Barrandian Unit (TBU) is collateral with the S01
profile direction, several major tectonic zones are rather perpendicular to the Variscan strike and so fairly imaged in the
S01 cross-section. They exhibit a weak velocity gradient of sub-horizontal directions within the middle crust. In particular,
the Moldanubian and TBU contact beneath the Western Krušné hory/Erzgebirge Pluton, the buried contact of the Lusatia unit
and the TBU within the Elbe fault zone were identified. The maxima on the 6,100 ms−1 isovelocity in the middle crust delimitated the known ultrabasic Erbendorf complex and implied also two next ultrabasic massifs
beneath the Doupovské hory and the České středohoří volcanic complexes. The intermediate mid-crustal P-wave velocity lows
are interpreted as granitic bodies. The presented geological model is suggested in agreement with available gravity, aeromagnetic
and petrophysical data. |
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