Feasibility of Approximating Depth to Shallow Bedrock Directly from the Rayleigh Wave Dispersion Curve

A complementary pair of straightforward, data-driven techniques is presented for use in making an initial estimate of the depth to shallow bedrock for a simple profile consisting of homogeneous soil over bedrock, using only the high-frequency portion of the fundamental-mode Rayleigh wave dispersion curve. These techniques constitute tools that can be useful for developing high-quality starting models for the inversion process that is followed to resolve shear wave velocity profiles from Rayleigh wave dispersion curves and for conducting rapid checks on theoretical dispersion calculations. Synthetic studies addressed a suite of one-dimensional shear wave velocity profiles, each representing a homogeneous soft-sediment layer above the bedrock halfspace. The shapes of the dispersion curves and the wave velocities at the high-frequency limits of the curves were considered in order to develop an empirical relationship to estimate the depth to bedrock. A less subjective, but more time-intensive, estimate of the depth to bedrock is obtained by constructing a suite of master curves against which to compare the experimental data. Both tools were tested upon two experimental datasets collected at shallow bedrock sites. The tools accurately yielded the depth to bedrock in the presence of a homogeneous overburden. In the presence of a layered overburden, the tools were less effective.