Examination of surface roughness of porous soil via acoustic backscatter

Abstract

Work has been done to determine acoustically the pore properties of soils, such as porosity, permeability, and tortuosity. Early methods used probe microphones to determine the soil pore characteristics by looking at the attenuation and phase change of sound propagating through the soil pores. The attenuation and phase change of the sound were then related to a surface impedance that incorporated the soil pore properties. Seeking noninvasive means of finding these properties, forward scattering or acoustic level difference experiments were performed. It was seen that sound propagated over porous surfaces is reduced by the surface impedance and by the surface roughness. Theories have been developed to take into account the reduction in acoustic signal due to the finite surface impedance. Difficulties have arisen in separating the signal loss due to impedance and the signal loss from roughness scattering. Acoustic backscatter has been used to study the roughness statistics of soil surfaces in an effort to determine the effects of roughness apart from the surface impedance effects. The roughness statistics obtained via acoustic backscatter are then compared with alternate nonacoustic methods for examining surface roughness.