Jean-Pierre Sessarego

Two Scholte–Stoneley waves on doubly fluid-loaded plates and shells

Previous theoretical and experimental studies of sound scattering from plates and from evacuated cylindrical or spherical shells with one-sided water loading have demonstrated the existence of a water-borne Scholte–Stoneley wave, and its acoustic excitation, in addition to that of the Lamb-type plate or shell wave modes. For two-sided water loading two Scholte–Stoneley waves, of symmetric (S) and antisymmetric (A) nature, were predicted on plates, with only the A wave surviving for the case of one-sided loading, while for loading with two different fluids, again two such waves have been demonstrated theoretically. In the present investigation, these two Scholte–Stoneley waves are studied experimentally via short-pulse scattering from water-immersed, thin-walled cylindrical shells filled alternatingly with air, water, and alcohol, and a theoretical analysis of their dispersion curves is presented.

Scattering by an elastic sphere embedded in an elastic isotropic medium

The scattering of acoustic waves by an elastic sphere embedded in an elastic isotropic medium is investigated. Expressions for the scattered waves are given in terms of monostatic and bistatic scattering cross sections. The resonances of the solid sphere were determined numerically in the individual normal mode amplitudes; dispersion curves for the phase velocities of the circumferential waves were also obtained. Computations and experimental results for an aluminum sphere embedded in Plexiglas were in good agreement.

Frequency Dependence of Phase Speed, Group Speed, and Attenuation in Water-Saturated Sand: Laboratory Experiments

Sound propagation in water-saturated sandy sediments was studied under controlled laboratory conditions in the 0.1-1.3 MHz frequency range. In the ¿low-frequency domain¿ (100-200 kHz), the results obtained were consistent with classical Biot theory, which predicts a positive phase-speed dispersion, but at higher frequencies (f > 500 kHz), a strong negative dispersion of the phase speed was observed, which is in apparent contradiction with the Biot theory predictions. In addition, the attenuation coefficient was found to increase nonlinearly with frequency from 0.5 to 1.3 MHz, whereas Biot theory predicted an almost linear increase of this coefficient. The unexpected behavior of the phase speed and the attenuation coefficient observed in these sandy sediments in the very high-frequency domain may be attributable to scattering by the sand particles, which becomes the predominant mechanism involved when the size of particles is comparable to the wavelength.

Time-frequency analysis of the bistatic acoustic scattering from a spherical elastic shell

The development of low-frequency sonar systems, using, for instance, a network of autonomous systems in unmanned vehicles, provides a practical means for bistatic measurements (i.e., when the source and receiver are widely separated) allowing for multiple viewpoints of the target of interest. Time-frequency analysis, in particular, Wigner-Ville analysis, takes advantage of the evolution time dependent aspect of the echo spectrum to differentiate a man-made target, such as an elastic spherical shell, from a natural object of the similar shape. A key energetic feature of fluid-loaded and thin spherical shell is the coincidence pattern, also referred to as the mid-frequency enhancement (MFE), that results from antisymmetric Lamb-waves propagating around the circumference of the shell. This article investigates numerically the bistatic variations of the MFE with respect to the monostatic configuration using the Wigner-Ville analysis. The observed time-frequency shifts of the MFE are modeled using a previously derived quantitative ray theory by Zhang et al. [J. Acoust. Soc. Am. 91, 1862-1874 (1993)] for spherical shells scattering. Additionally, the advantage of an optimal array beamformer, based on joint time delays and frequency shifts is illustrated for enhancing the detection of the MFE recorded across a bistatic receiver array when compared to a conventional time-delay beamformer.

High frequency broad band scattering from water-saturated granular sediments: Scaling effects

Sound backscattering from water-saturated granular sediments at frequencies from 150 kHz to 8 MHz at oblique incidence was studied in controlled laboratory conditions. Two kinds of sediments, medium and coarse sands, were degassed, and their surface was flattened. In these conditions, the sediment granular structure can be considered as a controlling mechanism of backscattering. Comparison of frequency dependencies of backscatter for the two sediments with different mean grain size shows the existence of a persistent scaling effect that allows description of the backscattering strength as a function of one parameter, the mean grain size/wavelength ratio.

Time-Frequency Analysis of Signals Related to Scattering Problems in Acoustics Part I: Wigner-Ville Analysis of Echoes Scattered by a Spherical Shell

Numerous studies (both theoretical and experimental) have been devoted to the problem of acoustical scattering by targets of simple shapes. This paper will show some of the main results which have been obtained. It will point out the encountered problems and the classical signal analysis tools which are available.

Propagator methods for finding wideband source parameters

Abstract In this paper, an investigation of the passive angular location of sources using an array of sensors is presented. First, the propagator method, which is an algebraic and linear operator extracted from the cross-spectral matrix of the received data, is extended to the wideband signals through the transformation matrices. This development reduces the rather expensive eigendecomposition of cross-spectral matrices at each frequency of the analysis bandwidth used in the well known eigenstructure high-resolution methods. Then, in the presence of white noise with different powers along the array, a new partitioned propagator method is developed which improves the localisation of the sources. This improvement is based on a particular choice of the partition of the matrix corresponding to the propagation operator. The developed method is also used to estimate the antenna shape from the received wideband signals. It is well known, in the narrowband case, that the interest of knowing of the antenna shape is illustrated by the improvement of the signal to noise ratio and the antenna directivity. The results of simulations are provided to illustrate the theoretical predictions.

Scaled model experiment of long-range across- slope pulse propagation in a penetrable wedge

In this paper, laboratory scale measurements of long-range across-slope propagation of broadband pulses in a shallow-water wedge-shaped environment with a sandy bottom are reported. The scaled model was designed to study the three-dimensional (3D) acoustic field in the presence of only a few propagating modes. The recorded time series exhibit prominent 3D effects such as mode shadow zones and multiple mode arrivals. Inspection of the spectral content of the time signals gives evidence of intra-mode interference and frequency dependence of the mode cut-off range in the across-slope direction.

Inversion of synthetic and experimental acoustical scattering data for the comparison of two reconstruction methods employing the Born approximation.

This work is concerned with the reconstruction, from measured (synthetic or real) data, of a 2D penetrable fluid-like object of arbitrary cross-section embedded in a fluid of infinite extent and insonified by a plane acoustic wave. Greens theorem is used to provide a domain integral representation of the scattered field. The introduction therein of the Born approximation gives rise to a linearized form of the inverse problem. The actual inversion is carried out by two methods. The first diffraction tomography (DT), exhibits the contrast function very conveniently and explicitly in the form of a wave number/incident angle Fourier transform of the far backscattered field and thus requires measurements of this field for incident waves all around the object and at all frequencies. The second discretized domain integral equation with Born approximation method, is numerically more intensive, but enables a wider choice of configurations and requires less measurements (one or several frequencies, one or several incident waves, choice of measurement points) than the DT method. A comparison of the two methods is carried out by inversion of both simulated and experimental scattered field data.

Imaging an object buried in the sediment bottom of a deep sea by linearized inversion of synthetic and experimental scattered acoustic wavefields

This paper is concerned with the reconstruction, from measured (synthetic and experimental) data, of a 2D penetrable fluid-like cylindrical object of arbitrary cross-section imbedded in a fluid-like (sediment) half-space separated by a plane interface from another fluid half-space (deep water) wherein propagates a plane acoustic interrogating wave. The Green theorem is used to provide (1) a domain integral representation (DIR) of the scattered field and (2) a domain integral equation (DIE) for the pressure field in a test region containing the object. Both the DIE and DIR are discretized by collocation, thereby leading to a linear system of equations for the discretized pressure in the test region and a linear transform for the discretized pressure outside the test region. This is the means adopted herein for generating synthetic scattered field data. The inverse problem is linearized by replacing the (unknown) field in the test region by the (known) field which is established in the water/sediment system in the absence of the object. Using this Born approximation and minimizing the discrepancy between the measured and model scattered fields gives rise to a linear system of equations for the (unknown) discretized index-of-refraction contrast function in the test region. Due to its ill conditioned nature, the linear system is solved by a singular value decomposition technique. Images of the index-of-refraction contrast representation of the object obtained by inversion of both simulated and experimentally measured scattered field data are presented and compared.