Refraction Microtremor And Optimization Methods As Alternatives To Boreholes For Site Strength And Earthquake Hazard Assessments

A thorough assessment of shallow shear velocity is important to both earthquake-hazard<br>assessment and efficient foundation design. The only standard procedure for determining shear velocity,<br>crosshole seismic (ASTM D4428), is not much used as it requires two boreholes with high-precision<br>positional logs. Downhole shear-wave profiles in a single hole are adequately accurate, but still too<br>expensive for many projects. We tested two methods for inexpensively estimating shallow shear velocities<br>with seismic refraction equipment, at the sites of several boreholes in California and Nevada. The sites<br>ranged from hard to soft (NEHRP hazard classes A to D). The first method, refraction microtremor,<br>records ambient ground noise on simple seismic refraction equipment (as in ASTM D5777). Wavefield<br>analysis of the noise allows picking of Rayleigh-wave phase velocities. It works well in dense urban areas<br>and transportation corridors. The second method, SeisOpt®@2D™ by Optim LLC, takes standard (ASTM<br>D5777) seismic refraction arrival picks and finds an optimized 2-d P-velocity model. The shear velocities<br>estimated from both methods are just as effective as borehole velocities for two purposes: estimating 30-<br>meter depth-averaged shear velocity for foundation design; and estimating the seismic spectrum input for<br>earthquake-hazard evaluations of sites.