Seismic attenuation and velocities of P- and S-waves in the German KTB area

Abstract

Compressional- and shear-wave velocities ( V p and V s) and attenuation ( Q p −1 and Q s −1) are investigated from VSP data set recorded between 3 and 6 km in the main borehole of the German KTB (Continental Deep Drilling Program): (a) V p and V s are measured as functions of depth by picking the P-wave and the converted S-wave first arrivals. They are compared with sonic measurements made in the same depth range. The surprising negative time drifts between the VSP and the sonic measurements of both the P- and S-waves are explained by the refraction of the VSP-raypath in the presence of three different dipping reflectors in this depth range of 3–6 km, and by the seismic anisotropy observed in this area. The negative time drift of the S-waves is much larger than that of the P-waves. (b) Attenuation of the P- and S-waves ( Q p −1 and Q s −1) from the same data set is estimated as functions of depth and frequency using modified spectral ratio methods. Frequency-dependent Q p and Q s are observed. In general, the estimated quality factors Q p and Q s are much lower than those one would desire for the crystalline rocks in this depth range. However, they are consistent with observations made around the world from independent investigations.