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.

Seismic reflections from the crystalline crust below the Continental Deep Drilling Site KTB: Modeling and inference on reflector properties

“…[34] The purpose of this appendix is to illustrate the problems of determining a representative quality factor Q p from the VSP measurements in the KTB main borehole. Li [1995] and Pujol et al [1998] have derived Q p values as low as 20 -50 from these data in the frequency range 20-30 Hz which approximately coincides with the dominant frequencies of the ISO-89 3D data studied here. For Figure A1.…”

“…These studies contrast with the results of vertical-seismic profiling (VSP) investigations at KTB. Very low Q p values from 20 to 50 were reported for the main borehole by Li [1995] and Pujol et al [1998]. Appendix A provides a reinterpretation of their VSP seismogram section and shows that these data give evidence both for low and for very high Q p and that, therefore, no representative Q p values can be derived for a larger rock volume.…”

Seismic low‐frequency effects from fluid‐saturated reservoir

“…It is well accepted that Q is frequency-dependent and changed dramatically over liquid saturation and may be less than 10 in sedimentary rocks [9,12]. Fluid may change Q in metamorphic rocks [10] down to 14 and in limestone [4] from 200 (dry) to 20-40 (water saturated). We investigate low-frequency anomalies of seismic reflections from porous layers by physical modeling and analytical solutions by taking into account low frequency-dependent Q values.…”

Seismic scattering attenuation estimates for the German KTB Area derived from well‐log statistics

“…We may assume that other regions of the continental crust exhibit similarly significant scattering attenuation estimates. The evidence that well-log statistics are quite uniform throughout the continental crust [Holliger, 1996] and the reported low Q-values [Pujol et al, 1998 for a review] support this hypothesis. Together with the constraints on absorption from laboratory measurements, these estimates may contribute to the stochastic characterization of seismic heterogeneities in the lithosphere.…”