Transducer Design Experiments for Ground-Penetrating Acoustic Systems

摘要

The transmission of impulsive acoustic signals into a homogeneous soil medium was investigated. Measurements performed using a piezoelectric driver and a hydrophone as the acoustic sources demonstrate that both types of transmitters (positioned at the air soil interface) are capable of producing acoustic waves in soil. The relative strength of the acoustic signals produced by each source was found to be approximately equal. The angular distribution of acoustic energy transmitted into the soil was found to be omnidirectional. Accelerometers buried within a 1.2 m deep soil box were used to measure the attenuation of acoustic signals as a function of frequency. The attenuation was observed to increase with increasing frequency from a value of approximately 8 dB/ft at 1 kHz to 35 dB/ft at 8 kHz. Measurements of the wave motion at the air soil interface induced by a surface mounted transmitter show that large amplitude surface waves are produced by both types of transmitters. The amplitudes of the observed surface waves were large in comparison to the estimated amplitude of signals produced by subsurface scattering of acoustic waves. A longitudinal wave receiver designed to reduce the amplitude of surface waves in favor of acoustic waves scattered from subsurface targets was successfully tested.