Advances in near-surface seismology

Determining how a mechanical wave propagates through very shallow unconsolidated soils is important for optimizing near-surface seismic techniques, locating and identifying UXO/IEDs, and for improving understanding of related processes such as land sliding. The intent of this paper is to present the result of isolating and altering mechanical wave variables in three soils: sand, clay loam, and silt loam. A controlled-frequency acoustic seismic source (CASS) is used to modulate source signal frequency, amplitude, and load time. Frequency was modulated between 60-1000 Hz, amplitudes of 5 and 10 Vpp, and load times of 0, 5, and 10 s were applied respectively. Results show that certain specific applied frequencies yield the greatest received maximum amplitudes from geophones (preferred frequencies). Preferred frequencies for sand, clay loam, and silt loam at three test sites are 300-330, 100-140, and 140-260 Hz respectively. Results also show that for sand the frequency applied is equal to the frequency recorded with the maximum amplitude over a wide band from 60-1000 Hz. For clay loam and silt loam the frequency-equivalence band is more narrow, 60-360 Hz and 60-550 Hz respectively. Observed preferred frequency shifts for each soil are dependant upon applied amplitude and load time.