Angle of incidence seismic/acoustic ratios during wet and dry ground conditions

Abstract

Modeling the out-of-plane seismic-to-acoustic (S/A) transfer function shows frequency and angular dependences. The amplitude, frequency, and angle of incidence analysis shows rapid shifts in the frequency locations of the transfer function maxima as the angle of incidence of the sound source becomes greater than critical angles for the deeper soil layers. Outdoor measurements of the transfer function were conducted to verify this critical angle effect. These measurements used a suspended loudspeaker as a source and vertical and horizontal component geophones as receivers to measure the angular dependence of the in- and out-plane S/A coupling ratio during wet ground conditions. The calculated S/A ratio using sound speeds and layer depth collected during dry conditions revealed critical angle effects at the test site that are similar to those in the literature. However, the S/A ratio measurements during wet conditions did not exhibit such effects. Modeling the ground as a low speed wet soil layer over a fast bottom with water table sound speeds agrees with the measured wet ground transfer function data. The wet ground condition properties will be verified and S/A ratio measurements will be made during the dry ground conditions to verify these hypotheses.