Seismic wave attenuation studies using VSP data recorded in Germany`s continental ultradeep borehole
- Univ. of Memphis, TN (United States)
- GeoForschungsZentrum, Potsdam (Germany)
VSP data recorded in the German continental deep borehole (KTB) between 3 and 6 km have been used to study the attenuation of P-waves. The main lithologic unit in this depth range is amphibolite (a metamorphic rock formed at high temperatures and pressures) with various degrees of alteration, intercalated with gneisses. The method used to determine attenuation is based on the fit of a straight line to the logarithm of amplitude ratios for variable depth and fixed frequency, but inspection of the corresponding plots showed sharp discontinuities, which correlate with changes in the location of the seismic source. These changes were recorded by a monitor geophone buried near the source. To avoid the uncertainties introduced by changes in the source location, the analysis was restricted to the longest depth interval (3,576 to 4,538 m) for which the source was not moved. Even in this case, however, the monitor traces showed pronounced variations in their frequency content, although not much variation in their time-domain amplitudes. Therefore, the attenuation coefficients obtained for the actual data were corrected for source variations by subtraction of the attenuation coefficients determined from the analysis of synthetic VSP data computed using the monitor traces. After these corrections, the values of Q for P-waves for frequencies between 7.8 and 46.9 Hz are between 14 and 32. These values are quite low, but are probably related to the presence of fluids known to exist in the depth range under consideration. The variation of Q with frequency may be the result of processing artifacts.
- OSTI ID:
- 542872
- Report Number(s):
- CONF-951013-; TRN: 97:004463-0060
- Resource Relation:
- Conference: SEG `95: 65. annual meeting and international exposition of the Society of Exploration Geophysicists (SEG), Houston, TX (United States), 8-13 Oct 1995; Other Information: PBD: 1995; Related Information: Is Part Of SEG/Houston `95 - technical program; PB: 1623 p.
- Country of Publication:
- United States
- Language:
- English
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