Acoustical Properties of Water-Filled Sands

Abstract

Experimental studies were made of the propagation of longitudinal waves in several mixtures of water with quartz sands fractionated as to particle size. The most probable particle size in the several samples ranges from 0.01 to 0.07 cm. The experimental frequencies were approximately 400 to 1000 kc/sec. Additional measurements in the same frequency range cover the reflection and scattering of underwater sound from essentially plane surfaces of these aggregates. The velocity and attenuation data are related satisfactorily to a well‐known analysis for a porous acoustical material having a pliable skeleton. The reflected signal, as a function of angle of incidence, behaves approximately as expected from the conditions for continuity at the plane surface, when the appropriate complex index of refraction is used. The scattering data are compared with the analysis based on a random distribution of scattering amplitude per unit volume, with autocorrelation distance proportional to the particle size. The analysis predicts the limiting behavior of scattering coefficient with respect to particle size and frequency, but predicts a more rapid fall of scattering amplitude beyond the critical angle than was observed. The magnitude and autocorrelation properties of the fluctuations in scattered signal as the apparatus was translated to scan the sand surface were observed, and found to correspond approximately to a model based on a Gaussian distribution of local scattering amplitude.