Seafloor sound-speed profile and interface dip angle characterization by the image source method

Abstract

The image source method is an efficient way to perform a sound-speed tomography for seafloor characterization. To date, however, it has been limited by a locally range-independent approximation. In other words, the layer boundary had to be parallel and flat within the Fresnel zone of the measurement system. Here the method is extended to take into account realistic variations of interface dip angles. To do so, the elliptical wavefront shape approximation of the reflected waves is used. This permits a fairly simple equation relating travel time to the sine of the dip angle, and consequently to an equation for the equivalent medium sound speed. The Radon transform is exploited to extract the dip angle. Simulations with varying layer dip angles and curvature provide insight into the strengths and limitations of the method.

The image‐source method: a tool for geoacoustic inversion

“…Here the equations to obtain the SSPs with non‐parallel interfaces are presented (Pinson and Holland ). Instead of the recursive algorithm presented in Section 2.4.1, only top and bottom interface reflections of a given layer are used to deduce its sound speed.…”

“…In grey is the ray diagram for the Δ horizontally shifted measurement system. Source : Reproduced from Pinson and Holland (), with the permission of the Acoustical Society of America.…”

“…This term is obtained by analysing the image‐source position evolution when the source and array are moving horizontally. More precisely, Pinson and Holland () used a linear Radon transform of the image‐source squared travel times as a function of the source and array horizontal coordinate. Thus the value of was extracted from the Radon transform by picking up local maxima.…”

“…From this configuration, SSPs were obtained for many successive measurement over a 14 km track showing a slightly range dependence of the seafloor sound‐speed structure. Due to this simple representation of interface reflections with image sources, it has been possible to extend the ISM to include layer‐interface dip angles (Pinson and Holland , ) and to estimate the SSP measurement uncertainties due to noise and interface roughnesses (Pinson, Guillon and Cervenka ; Pinson ). Also, the image‐source concept can be exploited in another way, that is, to separate the two effects of wave reflection (from interfaces) and scattering (from heterogeneities) by subtracting the image‐source echoes to image seafloor heterogeneities or buried scatterers (Pinson and Holland ).…”

Seabed Geoacoustic Structure at the Meso-Scale

“…One technique (Ref [15]), exploits the high information content of the reflection coefficient versus angle and frequency, estimating all the geoacoustic parameters and their uncertainties with depth and along the track, i.e.. a 2D model with a lateral resolution of about 7 m (with Peter Nielsen, CMRE, Jan Dettmer, U. Victoria). The other technique Is several orders of magnitude faster computationally and estimates the 2D sound speed profile using the image source method with a lateral resolution of-50 m. See Ref [4], [8]). (with Samuel Pinson, post-doc).…”