IMAGING OF CRUSTAL STRUCTURES FROM VERTICAL ARRAY MEASUREMENTS

Walk-away or offset Vertical Seismic Profiling (VSP), a technique wide ly used in oil and gas exploration, was carried out in crystalline roc k 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 bo rehole between 3210 m and 3560 m depth with geophone intervals of 25 m . To identify reflected/refracted seismic energy from crustal structur es the 'Controlled Directional Reception' (CDR) method was used. The k ey element is the vertical geophone array which allows the determinati on of the vertical slowness with great accuracy due to the excellent c oherence of waveforms. Slant stack processing and subsequent direction al migration produced a clear image of mid-crustal reflectors at 10 to 12 km depth. Additionally, a number of dipping faults were detected i n the upper crust, that can be connected to surface geology. As the wa lk-away VSP experiment was part of an integrated seismic measurement p rogram, the results were compared with those from 2D and 3D surface su rveys and borehole measurements. This led us to conclude that hydrauli c fracture zones play an important role in the reflectivity of crystal line rocks at KTB, (C) 1998 Elsevier Science B.V. All rights reserved.