The Effects of Thin, Near-surface Layers on Seismic Signals

Signals generated by moving tracked vehicles can be significantly affected by near-surface materials including pavements, compacted soils, shallow bed rock, and the water table. The ways that seismic signals depend on these surface layers are examined by analyzing impulsive point forces applied at the earth-air interface. Fields are calculated using a plane layered frequency domain wavenumber model. Synthetic time domain seismograms are generated by convolving the layered earth impulse responses over a band of frequencies with an appropriate source force function. The effect of constant Q attenuation is also included. Our results are related to seismic signals from tracked vehicles by noting that the spatially distributed nature of the track-ground forcing at any instant in time, can be obtained by superposition of multiple point source results. In generating earth impulse responses, we use surface layering that is commonly encountered moving vehicle problems. In each layer case considered, we systematically vary layer thickness to demonstrate how thick a particular layer must be to generate a significant alteration in the character of the seismic signals. It is found that surface layers less than about a meter thick do not have a major effect on observed waveforms in the frequency bands of interest. However, for near surface layers greater than 1.5 m, comparisons between high velocity and low velocity materials can have amplitude differences as great as 20 dB at ranges of 100 m. Waveforms from these two models also have substantively different surface waves modal characteristics.