Target localization through a data-based sensitivity kernel: a perturbation approach applied to a multistatic configuration.

A method to isolate the forward scattered field from the incident field on an object in a complex environment is developed for the purpose of localization. The method is based on a finite-frequency perturbation approach, through the measurement of a data-based sensitivity kernel. Experimental confirmation of the method is obtained using a cylindrical tank and an aggregate of ping-pong balls as targets surrounded by acoustic sources and receivers in a multistatic configuration. The spatial structure of the sensitivity kernel is constructed from field data for the target at a sparse set of positions, and compared with the expected theoretical structure. The localization of one or a few targets is demonstrated using the direct-path only. The experimental observations also show that the method benefits from including later arrivals from the tank wall and the bottom/surface reverberation, which indeed enhance the localization.

Experimental measurement of the acoustic sensitivity kernel.

In the Born approximation, the acoustic scattering from a spherical obstacle of a size comparable to the acoustic wavelength can be evaluated in the framework of the sensitivity kernel approach, which describes the relationship between the pressure-field fluctuation and the position of a local change in the propagation medium. The spatial structure of the sensitivity kernel is here investigated through experimental observations made in a water tank at the ultrasonic scale and compared to an analytical model. The pattern of the sensitivity kernel is discussed in the case of a source-to-receiver wave field that includes a direct path and one surface reflection.

Sensitivity kernel for surface scattering in a waveguide.

Using the Born approximation, a linearized sensitivity kernel is derived to describe the relationship between a local change at the free surface and its effect on the acoustic propagation in the water column. The structure of the surface scattering kernel is investigated numerically and experimentally for the case of a waveguide at the ultrasonic scale. To better demonstrate the sensitivity of the multipath propagation to the introduction of a localized perturbation at the air-water interface, the kernel is formulated both in terms of point-to-point and beam-to-beam representations. Agreement between theory and experiment suggests applications to sensitivity analysis of the wavefield for sea surface perturbations.

Target detection and localization in shallow water: an experimental demonstration of the acoustic barrier problem at the laboratory scale.

This study demonstrates experimentally at the laboratory scale the detection and localization of a wavelength-sized target in a shallow ultrasonic waveguide between two source-receiver arrays at 3 MHz. In the framework of the acoustic barrier problem, at the 1/1000 scale, the waveguide represents a 1.1-km-long, 52-m-deep ocean acoustic channel in the kilohertz frequency range. The two coplanar arrays record in the time-domain the transfer matrix of the waveguide between each pair of source-receiver transducers. Invoking the reciprocity principle, a time-domain double-beamforming algorithm is simultaneously performed on the source and receiver arrays. This array processing projects the multireverberated acoustic echoes into an equivalent set of eigenrays, which are defined by their launch and arrival angles. Comparison is made between the intensity of each eigenray without and with a target for detection in the waveguide. Localization is performed through tomography inversion of the acoustic impedance of the target, using all of the eigenrays extracted from double beamforming. The use of the diffraction-based sensitivity kernel for each eigenray provides both the localization and the signature of the target. Experimental results are shown in the presence of surface waves, and methodological issues are discussed for detection and localization.

Application of acoustic feedback to target detection in a waveguide: experimental demonstration at the ultrasonic scale.

People are familiar with the acoustic feedback phenomenon, which results in a loud sound that is heard when a musician plays an electric instrument directly into a speaker. Acoustic feedback occurs when a source and a receiver are connected both acoustically through the propagation medium and electrically through an amplifier, such that the amplified received signal is continuously re-emitted by the source. The acoustic feedback can be initiated from a continuous sine wave. When the emitter and the receiver are in phase, resonance is obtained, which appears to be highly sensitive to any fluctuation of the propagation medium. Another procedure consists in initiating the acoustic feedback from a continuous loop of ambient noise. It then generates an unstable self-sustained feedback oscillator (SFO) that is tested here as a method for monitoring temperature fluctuations of a shallow-water oceanic environment. The goal of the present study is to reproduce and study the SFO at the laboratory scale in an ultrasonic waveguide. The experimental results demonstrate the potential applications of the SFO for the detection of a target in the framework of the acoustic-barrier problem in shallow-water acoustics.