IMAGING OF CRUSTAL STRUCTURES FROM VERTICAL ARRAY MEASUREMENTS

Abstract Walk-away or offset Vertical Seismic Profiling (VSP), a technique widely used in oil and gas exploration, was carried out in crystalline rock at the KTB (Kontinentales Tief-Bohrprogramm) drill site in Germany. The walk-away VSP experiment comprised a shot profile of 10 km length at the surface across the well and a vertical receiver array in the borehole between 3210 m and 3560 m depth with geophone intervals of 25 m. To identify reflected/refracted seismic energy from crustal structures the ‘Controlled Directional Reception’ (CDR) method was used. The key element is the vertical geophone array which allows the determination of the vertical slowness with great accuracy due to the excellent coherence of waveforms. Slant stack processing and subsequent directional migration produced a clear image of mid-crustal reflectors at 10 to 12 km depth. Additionally, a number of dipping faults were detected in the upper crust, that can be connected to surface geology. As the walk-away VSP experiment was part of an integrated seismic measurement program, the results were compared with those from 2D and 3D surface surveys and borehole measurements. This led us to conclude that hydraulic fracture zones play an important role in the reflectivity of crystalline rocks at KTB.