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

Abstract

S U M M A R YA 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 inversion of 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 Q, of 40 is found between 1 and 5 Hz, a frequency band where classical methods failed. This is in close agreement with Q, at frequencies larger than 15 Hz obtained from the classical method based on the slope of spectral ratios.

Seismic‐Wave Propagation in Shallow Layers at the GONAF‐Tuzla Site, Istanbul, Turkey

“…However, as discussed later in this article, the high seismic velocities and heterogeneities at the Tuzla site lead to strong interference effects between upgoing and downgoing waves, the latter ones being reflected at the surface and at reflectors between the borehole seismometers. These interference effects cause complicated spectra, which makes a stable determination of Q based on spectral ratios difficult (Trampert et al, 1993;Bethmann et al, 2012). As a result, our application of them failed to give meaningful results.…”

“…This approach also allowed us to determine the velocity structure between the borehole sensors. Several algorithms exist for interferometry, but here we focus on the deconvolution technique (e.g., Trampert et al, 1993;van Vossen et al, 2004;Mehta et al, 2007;Parolai et al, 2009;Nakata and Snieder, 2012).…”

“…For example, Snieder and Şafak (2006) determined Q S in a building by fitting the slopes of the logarithm of envelopes of deconvolved seismograms. Trampert et al (1993) estimated Q S between a 500-m-deep borehole geophone and a surface seismometer by a time domain SHpropagator inversion. The theory of the propagator inversion technique was extended by Mehta et al (2007) for the P-SV case for attenuating media.…”

“…The deconvolution was also checked in the reverse sense, using the deepest sensor as reference. Instead of performing an inversion as proposed by Trampert et al (1993) or analyzing the deconvolved wavefield in the frequency domain as proposed by Parolai et al (2010Parolai et al ( , 2012, we forward modeled the deconvolved seismograms in the time domain. With a grid search we found the seismic velocities for the P and S waves, V P and V S , and quality factors Q P and Q S that best fit the data in a least-squares sense.…”

Seismic response of an 8-story RC-building from ambient vibration analysis

“…This technique has been applied in many seismological contexts and, in particular, to downhole seismology, where the interferometry has been used to determine the velocity and attenuation structures in correspondence of instrumented boreholes (e.g. Trampert et al, 1993;Metha et al, 2007;Assimaki, 2008;. The interferometric approach has been widely used to evaluate the shear wave velocity and attenuation inside the building using earthquake data (e.g.…”