COMPARISON OF SHEAR-WAVE VELOCITY PROFILES FROM SASW AND DOWNHOLE SEISMIC TESTS AT A STRONG-MOTION SITE

摘要

The spectral-analysis-of-surface-waves (SASW) method is a relatively new in-situ method for determining shear-wave velocity profiles. Testing is performed on the ground surface, making it less costly than other methods that require boreholes. The basis of the SASW method is the dispersive characteristic of Rayleigh waves traveling through a layered medium. SASW testing consists of measuring surface-wave velocity at different frequencies to generate the dispersion curve for the site and interpreting it to obtain the corresponding shear-wave velocity profile. To establish the reliability of this less direct technique, a "blind" evaluation of the SASW method was conducted by researchers at the University of Texas at Austin and the United States Geological Survey. SASW testing was performed at nine strong-motion stations at which borehole seismic measurements were previously made. The SASW data were interpreted independently of the borehole measurements and then systematically compared with the borehole results. The results for Rinaldi Receiving Station are shown as a typical and uncomplicated example of the sites in the study. Generally, the shear-wave velocity profiles from SASW and downhole testing compare very well, which shows that the method works. Differences are due, in part, to lateral variability in the subsurface, the difference between borehole "point" and SASW "global" measurements, and values of Poisson's ratio used in SASW modeling. The difference in predicted ground-motion amplification between the V_s profiles from SASW and downhole testing are less than about 5% for most frequencies, which is an inconsequential difference. This study demonstrates that in many situations the SASW method can provide V_s profiles suitable for site response predictions.