| Popis: |
The results of an experiment designed to measure the shear‐wave velocity structure of the shallow‐water sediments at three locations in the East China Sea, in water depths ranging from 40 m to 150 m, are reported. Both vertically polarized and horizontally polarized shear‐wave velocities were measured by placing both source and receiver on the seafloor. The recorded data consist of interface waves of Love wave and Stoneley/Scholte wave type, and both horizontally polarized (SH) and vertically polarized (SV) body waves. With the aid of synthetic seismogram analysis, the travel times and amplitudes of these phases were used to constrain the velocity structure of the seabed to depths of ∼50 m. These results show that velocities range from less than 50 m/s at the seafloor to greater than 400 m/s at subseafloor depths of only 50 m. Vertical velocity gradients of up to 200 and 15 s−1 are required in the upper 1 and 10 m of the seabed, respectively. Comparison of profiles carried out at the same approximate location shows that the velocity structure of the upper 10 m varies by up to 80% over lateral distances of less than a few kilometers. In the data bandwidth of 5–30 Hz, shear Q values range from 10–20 (attenuation factor of 0.07–0.14 m−1 at 20 Hz) at the seafloor to 40 (attenuation factor of 0.004 m−1 at 20 Hz) at depth. The shear velocities immediately below the seafloor differ by less than a factor of 2 from the values predicted by applying the Biot theory to the geotechnical parameters measured on cores taken at the experiment locations. In two of three cases the differences between the predicted and measured values are less than 8%. |
