The acoustic properties of gassy soil
Compression wave sound speed and attenuation measurements are made in the laboratory over frequencies in the range from 10 kHz to 1 MHz on samples of estuarine silty clay that have been provided with varying amounts of uniformly distributed gas bubbles. The soil can be idealised as a two phase material of spherical gas pockets within a matrix of saturated soil, where the pocket sizes are in the range from 100 to 2000 μm diameter. It is shown that the acoustic response of this soil is dominated by the gas fraction and bubble size of the gas phase, and by the elastic moduli of the matrix phase, within the frequency range that causes bubbles to pulsate radially (resonate). Below this range, it is shown that the sound speed response is not significantly affected by bubble size, except close to the saturated soil condition. The gas fraction and bubble size distribution of each sample is measured and used, with values of the appropriate elastic moduli from the literature, to provide curves from current theory. The theory is evaluated in comparison to the experiment results, and proposals are made, which may provide the basis for the prediction of the gas fraction and bubble sizes of sea-bed sediments that are suspected of containing gas. Such a process would involve the use of an acoustic probe to obtain in situ sound speed and attenuation measurements using the signal analysis techniques developed in this study.