Seismic Acoustic Ratio Estimates Using a Moving Vehicle Source

Seismic and acoustic noise from moving tracked vehicles was used to determine the seismic to acoustic signal coupling ratio (SAR) in the ground. The seismic signal received on a geophone contains some energy that has propagated as seismic waves and some energy that couples from acoustic waves to seismic waves in the vicinity of the geophone. We use the frequency domain coherence between the microphone and geophone signals to determine when the seismic signal is predominantly due to acoustic to seismic wave coupling. In frequency bands where the microphone and geophone coherence is above 0.8 the ratio of the seismic ground particle velocity to sound pressure, SAR, can be determined with less than 2 dB of error. The method is applied to data from a summer experiment with grass ground cover and at two winter experiments with snow covered ground. At 100 Hz, the summer analysis yields a SAR of 1x10 [(m/s)/Pa]. Also at 100 Hz, the two winter analyses yield SAR values of 0.1 and 1.0x10 [(m/s)/Pa]. In the later result the SAR using blank pistol shots gives very close values. These results can be used to remove acoustic contamination from seismic signals. We also show a loose correlation between the SAR values and the shear strength of the ground