Geoacoustic Properties

Abstract

The geoacoustic properties to be discussed in this chapter include speed and attenuation of both compressional and shear waves, acoustic impedance (expressed as the “index of impedance”), and bulk density (which is included in both the physical and geoacoustic categories in this monograph). The speed and attenuation of waves propagating through seafloor sediments (body waves) or at the sediment-water interface (interface waves) have been measured and modeled for over half a century. The types of measurements and models are as varied as the motivations of the investigators (Exploratory Geophysics, Sedimentology, Soil Engineering, Acoustic Modeling, Sediment Dynamics). Measurements using widely different techniques make comparisons difficult, especially for wave attenuation. The frequency dependence of wave speed (dispersion) and attenuation makes prediction of wave speed or attenuation difficult when the measured and required acoustic frequencies are substantially different. In spite of these problems, some generalizations can be made, and values of wave speed and attenuation as well as their gradients can be measured or predicted using physical and empirical models. This chapter will concentrate on measurement and prediction of near-surface (upper few meters), high-frequency wave speed and attenuation. Three types of body waves will be considered: compressional, shear, and the Biot “slow wave.” Interface waves (Love waves and Stonely waves) will only be discussed where these techniques provide data on speed and gradients of shear waves. Following common practice, the compressional wave speed in sediments will often be referred to as the “sound speed.”