ACOUSTIC WAVES IN OCEAN SEDIMENTS

Abstract

An acoustic model for unconsolidated sediments is used to study velocity, attenuation, and reflection in ocean sediments. The model predicts attenuation and wave velocity on the basis of physical parameters such as porosity, grain size, permeability, and effective stress. Two mechanisms for energy loss are included in the model; one accounts for intergranular losses in the skeletal frame and the other for viscous losses in the porewater as it moves relative to the frame. As a result, in certain sediments such as sands and silts, attenuation is found to vary in a manner quite different from the usual dependency on the first power of frequency that is almost universally assumed. Furthermore, the amplitudes of reflected and refracted waves at boundaries between water and sediment or between sediment layers become frequency dependent. In the immediate vicinity of such boundaries, a significant amount of energy may be lost owing to the generation of a second kind of dilatational wave with extremely high attenuation. The model is able to handle variations in a variety of different physical parameters such as overburden stress, fluid compressibility, and stiffness of the sediment frame owing to lithification. For this reason it is well suited for use in predicting changes in velocity and attenuation with depth in real sediments where nonhomogeneous changing conditions are the rule and simple extrapolation of experimental data is not possible.