Sound speed and attenuation measurements in unconsolidated glass-bead sediments saturated with viscous pore fluids

As part of a recent ocean sediment acoustics experiment, a number of independent sound speed and attenuation measurements were made in a well-characterized sandy sediment. These measurements covered a broad frequency range and were used to test both Biot-Stoll theory and Buckingham’s more recent grain-to-grain shearing model. While Biot theory was able to model the sound speed well, it was unable to predict the attenuation measured above 50kHz. This paper presents a series of measurements made in the laboratory on a simple glass-bead sediment. One goal of these measurements was to test the hypothesis that the attenuation measured at-sea was a result of scattering from shells within the sediment. The laboratory sediments used were saturated with fluids with different viscosities in order (assuming that Biot-Stoll theory is correct) to shift the dispersion into the frequency range of the measurement system. The measured attenuation in the glass-bead sediments exhibited the same frequency dependence as observed in the ocean experiment even though no shells were present. The laboratory results motivated development of a sediment model which incorporates both fluid viscosity and grain-to-grain interactions as embodied in a simple frequency-dependent, imaginary frame modulus first suggested by Biot.