Geoacoustic inversion of stratified seafloor with elastic layers by the image source method

Abstract

A new geoacoustic method was recently proposed : the image source method (S. Pinson & L. Guillon, JASA, 128, pp 1685-1693). Under the Born approximation, a broadband signal reflected by a layered seafloor can be represented as a sum of signals emitted by image sources whose positions are directly linked to the sound speed of the layers and to their thicknesses. Therefore, a localization of these sources, done with array processing, provides a direct access to these geoacoustic parameters. The inversion algorithm uses the time-of-flight of the signals, their arriving angle, and the Snell-Descartes law. This method has been applied to data recorded by vertical and horizontal array and the obtained results were successfully compared to ground truth. However, this method is based on the assumption of fluid layers. For layers deeply buried in the seafloor, this hypothesis is probably too restrictive. This paper analyses the effect of the presence of elastic layers inside the sediment stack. Existence of shear waves leads to two effects: modification of the reflection coefficient of each layers and presence of additional image sources which may lead to bad sound speed or thicknesses. These two effects are studied on numerical simulations.