Finite Difference Modeling of Surface-~Wave Scattering for Shallow Cavity Detection

Data collection and analysis of scattering of Rayleigh-type surface waves are investigated for locating shallowly buried cavities. Surface-based seismic experiments conducted by others at the University of Nevada, Las Vegas (UNLV) Engineering Geophysics Test Site (EGTS) demonstrate scattering of Rayleigh waves caused by buried empty barrels (essentially air-filled cavities). The interpretation of the data is complicated by factors such as the presence of a high-velocity geologic layer just below the cavity. This research uses a finite-difference seismic wave propagation code to compute time histories for a model that represents the Engineering Geophysics Test Site. By adding cavities to a background model, the time histories show scattering of surface waves due to the cavity. Complications in the wave train are caused by the high velocity layer. Starting by modeling a through-going cavity in an otherwise homogeneous soil model allows for identification of essential characteristics of time histories that reveal the presence of the cavity. The same homogeneous model without a cavity serves as a baseline to compare time histories and their frequency content. Layers are added and cavity length is reduced to create a more realistic model for comparing synthetic data with experimental data. This research compares experimental data to synthetic data in the forms of time histories, overtone images, and stacks of overtone images. The stacks of overtone images succeed for complex situations where the other approaches fail. Comparing the synthetic data from the model that represents the EGTS, using a convolved real source, to the experimental data from the EGTS, the stacks of overtone images demonstrate similarities. Variations in amplitude mark the locations of the cavities in both cases. Power spectrums and a cavity characterization parameter proposed by others are also studied for their capacity to indicate the presence and location of the cavities