Shear Wave Velocity Estimation Using PS Converted-Wave

Shear (S-) wave velocity structure over the basement is one of the key elements for the estimation of strong ground motion generated by earthquakes. PS converted-wave reflection survey is superior to S-wave reflection survey in imaging the deeper structure, thus give us the deeper S-wave velocity information. But in the large Vp/Vs area, conventional velocity analysis and RMS velocity assumption fails to give us a good estimate of the S-wave velocity. The P-wave velocity derived from the velocity analysis is relatively reliable. When we get good images in both P-wave and PS converted-wave sections, correlation of the reflectors in the two sections gives us good estimates for Vp/Vs. Using these Vp/Vs values and P-wave velocities derived from P-wave velocity analysis, we can calculate S-wave velocities. We applied this methodology to a 3-component survey at Fuchu, Tokyo in Japan. The survey line was along the river and there is a well near by which is deep enough to reach the basement, about 2000m. The source was two vibrators. Three-component geophones are set in 10m interval. Geophones are fixed in 192 locations. The source-line was 3080m long so that we can get long offset data to use in PS converted-wave processing. From the P-wave sections, we can see the strata are inclining towards east. P-S converted-wave section has a good agreement with P-wave section. By correlation of the two sections, we derived Vp/Vs map of the section. Then we calculated the S-wave velocity using Vp/Vs value and P-wave velocity. We compared the velocity structure with the well log data. The derived S-wave velocities, ranging from 570m/s to 1780m/s, are in good agreement with the well data.