Acoustic pulse propagation over laterally inhomogeneous ground

Abstract

Measurements were conducted outdoors over paths having areas with vastly different ground properties to investigate the effect of acoustic wave interaction with inhomogeneous ground conditions. Acoustic pulses with a bandwidth of approximately 10 to 500 Hz were produced by blank pistol shots, and were digitally recorded after propagating horizontal distances of 30 to 150 m. A natural snow cover was used to control the ground properties. The first measurement was conducted over an undisturbed snow cover, a highly porous material with low acoustic impedance. Then, portions of the snow cover were removed by plowing and the measurements repeated. By removing the snow, a less porous, higher impedance frozen ground surface was introduced into the propagation path. The snow cover was removed in stages so that several different inhomogeneous ground conditions were sampled, and a final measurement was made with the snow cover entirely removed. Changes in the pulse waveforms were observed when only a small proportion of the propagation path was modified by plowing. The measurement results will be compared with existing theories. [Work supported by U.S. Army.]