Near-Surface Shear-Wave Speed Estimates from P-wave Amplitude Ratios

Focus on local-distance seismic propagation inevitably leads to a consideration of the influence of near-surface (upper few hundred meters) geology on seismic observations. Numerous approaches exist to estimate such site responses, the approach varying with the nature (one component, three-components) and density of the observations. For single-station approaches, site delay estimation, HV ratios, etc. are often employed to image the near-sensor structure. In this work, we apply the P-wave Seismogram Method (commonly used in engineering site characterization analyses) to estimate the near-surface shear-wave speeds beneath the local-distance observations of the Source Physics Experiment (SPE) explosions. The approach is similar to a P-wave receiver function, using only the first arrival on a local distance seismogram and estimating the radial to vertical amplitude ratio of the P-wave. Assuming that the waves horizontal slowness is known, the ratio can be related to the near-surface shear wave speed using the expression for the free-surface receiver function. We use the roughly 2 km long linear seismometer profiles radiated from the SPE explosion epicenter to explore the method. The sensors are spaced about 100 meters apart, and the first arrival times provide reliable estimates of the first-arrivals horizontal slowness. The multiple SPE shots provide up to six measurements at each station and allow consistency checks. Our initial results show spatially consistent values along profiles traversing rock and spatially variable values along profiles crossing alluvium and known faults. We compare our subsurface shear-wave speed estimates with surface geology, travel time delays, and subsurface geology estimates from previous investigations.