Samuel Pinson

Sound speed profile characterization by the image source method

This paper presents the first results of an imaging technique that measures the geoacoustic structure of a seafloor in shallow water areas. The devices used were a broadband (100 Hz-6 kHz) acoustic source towed by a ship and a vertical array. Among all the acoustic paths existing in the water column, two are used: the direct one and the seabed-reflected one, the latter being composed of the reflections from the seafloors surface as well as that from each buried layer. Due to the good time resolution of the signal and to the short range configuration, the reflected signal can be modeled as a sum of contributions coming from image sources relative to the seabed layers. The seabed geometry and the sound speed profile can then be recovered with the detection and localization of these image sources. The map of the image sources is obtained by a function that combines back-propagation of signals and knowledge of the emitted pulse. The thickness and sound-speed of each layer is finally obtained by a position analysis of the image sources. The results obtained by this data-driven algorithm on both at-sea and synthetic data are satisfactory.

Range dependent sediment sound speed profile measurements using the image source method

This paper presents a range dependent sediment sound speed profile measurement obtained using the image source method. This technique is based on the analysis of the seafloor reflected acoustic wave as a collection of image sources which positions are linked with the thick-nesses and the sound speed of the sediment stack. The data used were acquired by the NURC in 2009 during the Clutter09 experiment. The equipment used was an autonomous undersea vehicle towing a 1600-3500 Hz frequency band source and a 32 m horizontal line array of 32 hydrophones at 12 m above the seabed. Under the assumption of locally range independent seabed properties, the moving horizontal array provides successive range independent sediment sound speed profiles along a track to obtain the range and depth dependent structure of the seafloor. Two key steps include recovery of the time-varying unknown array shape from the data and spatial filtering of the successive sound speed profiles. A comparison of the image source method result and seismic data along nearly the same 14 km track indicates that the seabed stratigraphy is correctly mapped by this method.

Spherical wave reflection in layered media with rough interfaces: Three-dimensional modeling.

In the context of sediment characterization, layer interface roughnesses may be responsible for sound-speed profile measurement uncertainties. To study the roughness influence, a three-dimensional (3D) modeling of a layered seafloor with rough interfaces is necessary. Although roughness scattering has an abundant literature, 3D modeling of spherical wave reflection on rough interfaces is generally limited to a single interface (using Kirchhoff-Helmholtz integral) or computationally expensive techniques (finite difference or finite element method). In this work, it is demonstrated that the wave reflection over a layered medium with irregular interfaces can be modeled as a sum of integrals over each interface. The main approximations of the method are the tangent-plane approximation, the Born approximation (multiple reflection between interfaces are neglected) and flat-interface approximation for the transmitted waves into the sediment. The integration over layer interfaces results in a method with reasonable computation cost.

Seafloor sound-speed profile characterization with non-parallel layering by the image source method: Application to CLUTTER'09 campaign data

The image source method was originally developed to estimate sediment sound speed as a function of depth assuming plane-layered sediments. Recently, the technique was extended to treat dipping, i.e., non-parallel layers and was tested using synthetic data. Here, the technique is applied to measured reflection data with dipping layers and mud volcanoes. The data were collected with an autonomous underwater vehicle towing a source (1600-3500 Hz) and a horizontal array of hydrophones. Data were collected every 3 m criss-crossing an area about 1 km(2). The results provide a combination of two-dimensional sections of the sediment sound-speeds plotted in a three-dimensional picture.

Relative velocity measurement from the spectral phase of a match-filtered linear frequency modulated pulse

Linear frequency modulated signals are commonly used to perform underwater acoustic measurements since they can achieve high signal-to-noise ratios with relatively low source levels. However, such signals present a drawback if the source or receiver or target is moving. The Doppler effect affects signal amplitude, delay, and resolution. To perform a correct match filtering that includes the Doppler shift requires prior knowledge of the relative velocity. In this paper, the relative velocity is extracted directly from the Doppler cross-power spectrum. More precisely, the quadratic coefficient of the Doppler cross-power-spectrum phase is proportional to the relative velocity. The proposed method achieves velocity estimates that compare favorably with Global Positioning System ground truth and the ambiguity method.

Roughness influence on multibeam-subbottom-profiler specular echoes and roughness parameter inversion

Integral solutions for wave scattering over slightly rough surfaces generally include the source and receiver directivity. In this paper, it is shown that integrating the point source, point receiver solution over the source and receiver apertures leads to solutions with a clear physical interpretation. The scintillation, time-of-arrival, and direction-of-arrival spatial covariances of the specular echo are derived for a multibeam-subbottom-profiler configuration and result in surface integrals that can be evaluated numerically. In addition, algebraic expressions are obtained for the variances when the roughness has a Gaussian autocorrelation function and the source and receiver arrays have Gaussian apodization functions. Variances obtained from a numerical evaluation of the surface integrals compare well with estimates from a realistic three-dimensional numerical experiment. A simple inversion scheme is used to extract the roughness parameters from the numerical experiment signals.

Sediment sound speed dispersion inferences from broadband reflection coefficient measurements

The frequency dependence, or dispersion, of sound speed in marine sediments has been a topic of considerable interest and remains a research topic. While experiments on well-sorted sediments (having a narrow range of grain sizes) show promising concordance with theory, the more typical continental shelf sediments exhibit a rather wide range of grain sizes. A major experimental challenge is to measure in-situ sound speed over a sufficiently wide frequency range, such that the underlying mechanisms (e.g., viscous or friction) that control intrinsic dispersion can be isolated. Broadband 1.8-10 kHz seabed reflection measurements in the TREX13 experiment show a critical angle that is very nearly frequency independent. When effects of wavefront curvature, sound speed gradients, layering, and roughness are taken into account, this observation indicates that sediment sound speed must also be nearly independent of frequency. [Research supported by the ONR Ocean Acoustics Program.]

Roughness parameters imaging with a multibeam echosounder

The aim of the study is to perform quantitative imaging of the seafloor random parameters using a multibeam echosounder. More specifically we present in this communication a focus on the interface roughnesses with controlled parameters using a 3D modeling of a layered media with rough interfaces. The modeling consists of a sum of integrals over each interface of the layered medium that implies a reasonable computation cost and the possibility to perform a high number of numerical experiments. Specular reflection and backscattering are analyzed by estimating their means and variances through imaging algorithms. [Research supported by the ONR Ocean Acoustics Program.]

Sound speed profile measurement uncertainties due to rough interfaces: A parametric study using the Langston-Kirchhoff model

In the context of sound-speed profile measurement by the image source method, interface roughnesses are responsible for result uncertainties. The image source method models the reflected wave from a layered media as a collection of images which are the mirror reflections of the real source over the interfaces. From image source positions, one can deduce the sound-speed profile. Interface roughnesses might blur these image sources and reduce the accuracy of their localization. Using the Langston-Kirchhoff model of a 3D layered media with rough interfaces, it is possible to perform a parametric study of these uncertainties as a function of roughness parameters. In the aim of performing roughness parameter inversion, theoretical uncertainties are calculated and compared with estimated uncertainties from numerical experiments.

Spherical wave scattering from rough surfaces and array processing: Application to sound-speed profile measurement uncertainty analysis

A three-dimensional modeling of spherical wave reflection on layered media with rough interfaces is used to study the roughness influence in sediment sound-speed profile measurements. The configuration of the numerical experiment consists of a point source and a horizontal array of hydrophones. In addition, theoretical uncertainties on the angle of arrival and travel time of a spherical wave reflected over a slightly rough interface are derived by including the array processing in the Helmholtz-Kirchhoff integral. The sound-speed profile sensitivity to those parameters is used to compare theory and numerical experiments. The agreement is good and shows a surprisingly strong influence of the roughness on the sound-speed profile measurement.