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Inversion of Q_S of Deep Sediments from Surface-to-Borehole Spectral Ratios Considering Obliquely Incident SH and SV Waves

¹ Ohsaki Research Institute
2-2-2, Uchisaiwai-cho
Chiyoda-ku, Tokyo 100-0011
Japan

Previous studies (e.g., Satoh et al., 1995a) on inversion of quality factor for S waves, Q_S, of sediments using surface-to-borehole spectral ratios of seismograms, are mostly based on one-dimensional wave propagation of vertically incident SH waves. In this study a method is proposed for inverting frequency-dependent Q_S, S-wave velocity V_S, P-wave velocity V_P and incidence angle by considering obliquely incident SH and SV waves at the bedrock. The method accounts for S to P conversion waves as well as direct and reflected SH and SV waves in the multilayered structure. This method is tested using seismograms recorded on ground surface and deep (GL- 1206m) boreholes in the Sendai basin, Japan. Surface-to-borehole spectral ratios of transverse, radial, and vertical components for S-wave windows in the frequency range from 0.5 to 10 Hz are used in the inversion. The peak frequencies of observed spectral ratios are typically different between radial and transverse components, and this difference is reproduced reasonably well by the computed spectral ratios as a result of considering obliquely incident SH and SV waves with the inverted incidence angle. In addition, three components of observed seismograms are also matched well by the synthetics in the time domain. The inverted Q_S is modeled as Q_S = V_S f ^0.63/ 28 (V_S in m/sec and frequency f in Hz) for soft soil with V_S of about 200 m/sec above 50 m and as Q_S = V_S f ^0.63/54 for hard soil and sedimentary rock with V_S between 500 m/sec and 3,000 m/sec.

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