SH Propagator Matrix and Qs Estimates From Borehole- and Surface-Recorded Earthquake Data

Abstract

Summary A new method is proposed for the solving of elastic and anelastic parameters of a medium between two seismometers, one in a borehole and one at the surface. This allows us to retrieve equivalent quality factors of the materials and, at the same time, gives wave velocities as well as reflection and transmission coefficients in a layered structure. the study of local site effects shows that the quality factor of shallow materials can be completely masked by resonance effects and near-surface amplification. Our method is based on the SH propagator matrix in the time domain. This separates the effects of resonance and amplification in the elastic structure from those of attenuation due either to scattering or physical attenuation, and we find that the elastic problem is decoupled from the anelastic one. After a synthetic test, we applied the method to seismic records made with a borehole-surface instrument operated in the Southern Rhine Graben valley, on the French-German border. Due to high seismic noise at the surface, we were only able to resolve a one-layered mean 500 m deep structure between the borehole and surface instrument. an apparent Qs of 40 is found between 1 and 5 Hz, a frequency band where classical methods failed. This is in close agreement with Qs at frequencies larger than 15 Hz obtained from the classical method based on the slope of spectral ratios.