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

Abstract

The refraction data from the SUDETES 2003 experiment were used for highresolution tomography along the profile S01. The S01 profile crosses the zone ErbendorfVohenstrauss (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 traveltime 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 minimumfeature 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 massi...

Crustal structures beneath the Saxonian Granulite Massif, the České středohoří and the Doupovské hory Mts. based on the depth-recursive tomography

“…Although the comparison of these models just for one x-coordinate may not be representative on the whole, the sweeping out of the LVZ in both previous models can be inferred. For further discussion of low-velocity zones imaging by refraction tomography we refer to Novotný et al (2009).…”

“…Thanks to the results of detailed exploration at and near the KTB site, the S01 velocity model was proved to be consistent with other geophysical and geological evidence, particularly with the log velocities down to ~8 km. We refer to Novotný et al (2009) for the detailed derivation of the S01 velocity model by the DRTG method and its geological interpretation.…”

“…7) and perhaps also by stoping processes, presumably in later phases of magmatic emplacement. The low-velocity host rocks in the crossing S01 profile were interpreted as pre-Variscan granites (Novotný et al 2009). …”

“…Recently, Grad et al (2008) used the S01 wide-angle seismic data to derive a velocity model involving the whole crust. Based only on the distinct first breaks of refraction waves along the S01 line, Novotný et al (2009) applied the DRTG method (Depth Recursive Tomography on Grid) to derive a P-wave velocity image of upper and middle crust yielding here a higher resolution. The DRTG inversion of the refraction first arrivals was performed using a regular network of refraction grid rays that allowed a statistical assessment of the resolution: the lateral sizes of the velocity anomalies to be resolved were derived depending on their depths, velocity excesses and desired confidence level.…”

Gravity response of igneous rocks in the northwestern part of the Bohemian Massif

“…Novotný et al (2009) who applied the method of depth-recursive tomography on the S1 profile (running NE−SW) revealed a high velocity anomaly (6,100-6,250 km/s) beneath the České středohoří volcanic complex at depths of 6-11 km. The source of this seismic velocity anomaly coincides with that of the large České středohoří regional magnetic anomaly.…”