Estimating the compressional and shear wave speeds of a shallow water seabed from the vertical coherence of ambient noise in the water column

Abstract

Due to the multiple bottom reflections encountered in shallow water environments, the spatial structure of the ambient noise field depends strongly on the geoacoustic properties of the seabed, which are invariant over time scales associated with most measurements. The vertical directionality and coherence are relatively stable features of the noise that are determined primarily by the seabed, rather than temporal variations in the surface source distribution. In this paper, estimates of the compressional and shear wave speeds are determined from ambient noise measurements over shear supporting seabeds. Using a model of wind-generated noise over an elastic seabed, it is shown that the noise is sensitive to the compressional and shear wave speeds in the upper few meters of seabed. An inversion procedure is developed based on a matched field of the complex, broadband coherence from a single hydrophone pair. Using ambient noise data from two shear supporting sites, compressional and shear wave estimates are obtained that are in good agreement with independent surveys. For one site where the bedrock is exposed, a half-space model of the seabed yields reasonable estimates of the seabed parameters. For the other site, the presence of a thin sedimentary layer results in a low estimate from the half-space model. However, when the layer is included in the model, the estimates of the underlying bedrock are in good agreement with a seismic survey.

Interface Waves

“…Another and entirely different approach is based on using ocean ambient noise recorded by ocean bottom cable to extract information on the ocean subsurface. This approach has attracted much attention as being both economical and environmental friendly (Carbone et al, 1998;Shapiro et al, 2005;Bensen et al, 2007;Gerstoft et al, 2008;Bussat & Kugler, 2009;.…”

Effects of sediment properties on surface-generated ambient noise in a shallow ocean

“…However, the ambient noise may also treated as a useful sound in that it contains valuable information about the environment itself due to its strong interaction with the supporting media and the boundaries, in turn allowing inference of the envionmental characteristics through the inverse processing-a problem referring to as ambient noise inversion which is of current interest in the underwater acoustics community (e.g. Buckingham, 2004;Harrison and Simons, 2002;Carbone et al, 1998). In such applications, it is important to first understand how each of the environmental factors affects the ambient noise field.…”

Vertical coherence of ambient noise in shallow water

“…Noise characteristics such as power spectral density vary over time as it is strongly dependent on sea state. However, noise coherence which is normalized with respect to power spectral density is expected to be stable over a wide range of environmental conditions [2]. Since spatial structure of time averaged ambient noise is largely dependent on seabed properties, coherence can be used to estimate geoacoustic properties of the sea bed [1][2][3][4].…”