Seismic Wave Attenuation in Germany's Deep Borehole Area from VSP Data

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9219208 Pujol This project involves the determination of seismic wave attenuation using VSP (Vertical Seismic Profiling) data recorded in a 4-km-deep pilot borehole next to the super-deep borehole currently being drilled in Germany (expected to reach between 10 and 12 km). In spite of extensive research, attenuation mechanisms are not well understood, and even its measurement is plagued with uncertainty, as wave amplitudes are also modified by wave propagation effects generally only approximately known. For this reason, VSP data offer the best chance to observe the actual change in the amplitude and shape of seismic waves with the minimum amount of interference. The importance of the German deep- drilling program for this proposal is two-fold. (1) The borehole is one of the very few drilled in crystalline rocks. Very little information about attenuation in this type of rocks is available. (2) Several zero-offset VSP data sets for different seismic sources have been collected. Explosive, vibrator, and impact sources have been used to generate P and S waves. This is an exceptional data set that will be used to study the influence of the type of source and the frequency content of the waves on the measurement of attenuation, a matter on which little or no research has been carried out so far because of the unavailability of data. Although the measurement of attenuation (Q) using VSP data is the most reliable technique and getting a number for Q is a rather straightforward procedure, there are a number of factors that affect the measurements. In fact, different methods of measurement (e.g., spectral ratio, rise time) and even the choice of a tapering window may affect the results. In addition, wave propagation in the real earth affects wave amplitudes and frequency content because of scattering and the filtering effect of thin layers. For this reason, the main goal of this research will be the computation of reliable intrinsic Q values and a study of the factors that affect it. This will be the most detailed information on attenuation in crystalline rocks and will be extremely useful to calibrate the attenuation that may be determined from the analysis of high-quality reflection seismic data collected in the borehole area. ***