ALBERT

Description: The high reflectivity of the upper and middle crust of the Oberpfalz, both for near vertical and wide-angle reflections, was one reason for selecting this area as site for the German Continental Deep Drilling Program KTB. A peculiarity in the middle crust beneath the KTB site is the so-called Erbendorf-Body (EB) giving rise to extremely strong wide-angle reflections. It deserves special interest because its position right on the border between the Saxothuringian and Moldanubian zones of the Vari scan fold belt, the unusual high p-wave velocities (over 7.0 km/s) in its lower part at 11 to 14 km depth, and associated dipping reflectors suggest a possible lower crustal origin of the EB. A specially designed and so far unique wide-angle 3D-survey was carried out as part of the program "Integrated Seismics Oberpfalz 89" for investigating the spatial extent, the velocity distribution and the internal structure of the EB. The first results show clearly that the EB ist not a local phenomenon beneath the DEKORP4 line, but that it exists, yet with remarkable complexities, beneath the whole covered area between the Franconian Line and the Falkenberg granite complex. The wide-angle shots were also recorded by four 3-component geophones in the KTB pilot hole at 3195 to 3295 m depth and provided convincing evidence for s-wave splitting in the upper crust SE of the KTB location. This anisotropy effect seems to be related to the overall strike and dip of rock foliation in the zone of Erbendorf-VohenstrauB (ZEV).

Description: The moving source profiling (MSP) measurements are done by moving seismic sources along a surface line crossing the well site while a chain of geophones, placed within the well bore at a certain depth, records the seismic response. A multifold coverage of the subsurface can be obtained by repeating the source profile for a number of different geophone depths. This idea of altering the conventional vertical seismic profiling (VSP) geometry to allow illumination of subsurface structure away from the well is an attractive idea because it is designed to better locate horizons below the drill bit. If these target horizons can be correlated with reflectors in usual seismic profiles, recorded on the surface, the latter can be calibrated by the MSP results. Two MSP experiments were realized in the KTB pilot borehole. The first (MSP 1) included two N-S and E-W orientated source profiles of 10 km length and a single threecomponent geophone at 3585 m depth. Later a full MSP experiment (MSP 2) was run for one NE-SW orientated source profile and 20 different geophone depths. The source line was extended 7 km to the Northeast and 3 km to the Southwest of the well. A vibrator source produced seismic signals every 50 m. These shots were recorded by three-component geophones at depths from 3210 m to 3685 m with 25 m intervals resulting in a 20-fold coverage of the illuminated subsurface. Due to the difficulties encountered in crystalline environments, different processing techniques were combined for interpretation of the MSP data set. Aside from comparing measured first-break times with theoretical ones to determine seismic velocities of the overburden an MSP-CDP transformation for migration were applied. The steeply dipping boundary of the Falkenberg granitic intrusion was mapped as a distinct velocity contrast east of the KTB well. On the other hand, some remarkable seismic reflectors at depths between 4000 m and 10000 m are predicted to be hit by the future KTB main borehole.

Description: For several reasons the IS0'89 3D-seismic data are at hand not as conventional, phase-consistent stack so far but as 'envelope stack' only. Therefore, the common wave-field migration procedures cannot be applied to attain information on the spatially correct position of the particular reflector elements. To cope with this problem a simplified 3D-horizon migration program was developed at the DEKORP Processing Center Clausthal. It allows to migrate not the sejsmic traces of the IS0'89-3D survey themselves but the traveltime values for specific horizons derived by interactive picking with a 3D-seismic interpretation system on the basis of an envelope stack. The motivation, the basic assumptions and the principles of this procedure are outlined as well as the mathematical background and the particular processing steps. The migration results of some selected key horizons in the area of the KTB are depicted by isochrone maps and 3D-views. By this, a preliminary geological interpretation based upon the true position and shape of several detected seismic surfaces is enabled although the final processing, i. e. a phase-consistent stack of the entire JO-dataset (and its subsequent wave-field migration), has not been finished yet.