Nonlinear acoustics of water-saturated marine sediments

Abstract

Interest in the acoustic qualities of water-saturated marine sediments has increased considerably during recent years. The use of sources of high-intensity sound in oil propsecting, in geophysical and geological studies of bottom and subbottom materials and profiles and recently in marine archaeology has emphasized the need of information about the nonlinear acoustic qualities of water-saturated marine sediments. While the acoustic experiments and theoretical investigations hitherto performed have concentrated on a determination of the linear acoustic qualities of water-saturated marine sediments, their parameters of nonlinear acoustics are still unexplored. The strong absorption, increasing about linearly with frequency, found in most marine sediments and the occurrence of velocity dispersion by some marine sediments restrict the number of nonlinear acoustic test methods traditionally available for fluids to a few being used for a determination of the nonlinear acoustic parameters of water-saturated marine sediments. These test methods, comprising static, thermodynamic and finite-amplitude wave distortion and absorption methods, aiming at a determination of B/A for marine sediments, are given a critical evaluation with emphasis put on limitations in their applicability. B/A-values of some marine sediments—quartz medium and fine sand, silt, and clay—characterizing the seabed of the Danish archipelago and measured in our laboratory by means of the test methods discussed are given and are compared with nonlinear acoustic qualities determined through small-scale explosion tests performed in the sediments. Sources leading to possible deviations between test results arising from prospective in situ measurements and from measurements carried out under laboratory conditions are discussed.