Seismic wave attenuation in metamorphic rocks from VSP data recorded in Germany’s continental super‐deep borehole

Abstract

Vertical seismic profiling (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. A vibrator source was used to generate the data. In this depth range, the lithology is dominated by a metabasite unit (amphibolite and metagabbro) with various degrees of alteration, intercalated with paragneisses. 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. Inspection of the corresponding attenuation curves showed sharp discontinuities, which correlate with changes in the seismic source (mostly related to changes in location), as recorded by a nearby monitor geophone. To avoid the uncertainties 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 variations in their frequency content, although not much variation in their peak time‐domain amplitudes. To account for these variations, 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 source wavelets extracted from the monitor traces. After these corrections, the values 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 fluids known to exist in the depth range under consideration, although scattering processes may also contribute to the high attenuation. The variation of Q with frequency may be the result of processing artifacts.