DETERMINATION OF 1-D SHEAR WAVE VELOCITIES USING THE REFRACTION MICROTREMOR METHOD

Current commonly used techniques of estimating shallow shear velocities for assessment of earthquake site response are too costly for use in most urban areas. They require large sources to be effective in noisy urban settings, or specialized independent recorders laid out in an extensive array. The refraction microtremor (ReMi) method (Louie, 2001) overcomes these problems by using standard P-wave recording equipment and ambient noise to produce average one-dimensional shear-wave profiles down to 100m depths. The combination of commonly available equipment, simple recording with no source, a wavefield transformation data processing technique, and an interactive Rayleigh-wave dispersion modeling tool exploits the most effective aspects of the microtremor, spectral analysis of surface wave (SASW), and multichannel analysis of surface wave (MASW) techniques. The slowness-frequency wavefield transformation is particularly effective in allowing accurate picking of Rayleigh-wave phase-velocity dispersion curves despite the presence of waves propagating across the linear array at high apparent velocities, higher-mode Rayleigh waves, body waves, air waves, and incoherent noise. It has been very effective for quickly and cheaply determining 30-m average shear wave-velocity (V30) and thus the NEHRP (National Earthquake Hazard Reduction Program) soil classification. In addition, it has also been used for liquefaction analysis, soil profile determination, mapping the subsurface and estimating the strength of subsurface materials, and finding buried cultural features, such as dumps and piers. In this paper we briefly discuss the method, data acquisition and interpretation of the data to yield one-dimensional shear-wave velocities