SEISMIC-WAVE ATTENUATION IN METAMORPHIC ROCKS FROM VSP DATA RECORDED IN GERMANY CONTINENTAL SUPERDEEP BOREHOLE

Vertical seismic profiling (VSP) data recorded in the German continent al deep borehole (KTB) between 3 and 6 km have been used to study the attenuation of P-waves. A vibrator source was used to generate the dat a. In this depth range, the lithology is dominated by a metabasite uni t (amphibolite and metagabbro) with various degrees of alteration, int ercalated with paragneisses. The method used to determine attenuation is based on the fit of a straight line to the logarithm of amplitude r atios for variable depth and fixed frequency. Inspection of the corres ponding attenuation curves showed sharp discontinuities, which correla te with changes in the seismic source (mostly related to changes in lo cation), as recorded by a nearby monitor geophone. To avoid the uncert ainties introduced by these changes, the analysis was restricted to a depth interval (3576-4538 m) for which the source was not moved. Even in this case, however, the monitor traces showed significant variation s in their frequency content, although not much variation in their pea k time-domain amplitudes. To account for these variations, the attenua tion coefficients obtained for the actual data were corrected for sour ce variations by subtraction of the attenuation coefficients determine d from the analysis of synthetic VSP data computed using source wavele ts extracted from the monitor traces. After these corrections, the val ues of Q for frequencies between 7.8 and 46.9 Hz are between 14 and 32 . These values are quite low, and may be related to the presence of fl uids known to exist in the depth range under consideration, although s cattering processes may also contribute to the high attenuation. The v ariation of Q with frequency map be the result of processing artifacts .