The role of porosity fluctuations in scattering from sand sediments and in propagation losses within the sediment

Acoustic backscattering from a diver-smoothed sand sediment was measured at frequencies from 200 to 500 kHz as a function of grazing angle. The residual roughness of this smoothed surface was measured using a laser line scanning system capable of measuring sub-millimeter heights over a 4-meter track. Using the measured sediment roughness, perturbation theory underestimates the scattering strength at all frequencies for angles greater than the critical grazing angle. The absence of large, discrete scatterers in the sediment suggests that scattering from fluctuations in the sediment properties may be the dominant scattering mechanism for these angles. The sound speed and attenuation were also measured in this sediment and the attenuation was found to exhibit a linear frequency dependence similar to that observed for other sand sediments. To account for this linear attenuation, a theory that incorporated scattering losses due to porosity fluctuations into the effective density fluid model has been developed. This theory suggests that the propagation losses may be connected by the same physical mechanism to the scattering of sound from the sediment. This connection is explored in the context of these scattering and propagation measurements.