Sandrine Rakotonarivo

Forward modeling for marine sediment characterization using chirp sonars

This paper investigates the forward modeling of chirp-sonar data for the quantitative characterization of marine subbottom sediment between 1 and 10 kHz. The forward modeling, based on a transfer function approach, included impacts of layering or impedance mismatch, attenuation, roughness, and transitional layers, i.e., continuous impedance variations. The presented approach provided the best compromise between the number of available geoacoustic parameters from chirp-sonar data and the subbottom modeling accuracy. The forward model was tested on deep-sea chirp-sonar data acquired at a central frequency of 3.5 kHz. Comparisons between synthetic and experimental seismograms showed good agreement for the first 15 m of buried layers. Performance of the inversion using this forward model was also examined through sensitivity analysis. The results suggested that estimations of layer thickness, impedance, and transitional layer thickness were robust, whereas roughness and attenuation estimations were subject to wavelength and layer thickness conditions.

Model-independent range localization of a moving source in shallow water

A method for range localization with a single sensor in an ocean waveguide is derived. Range localization typically requires an accurate environmental acoustics model used for processing acoustic data on a multi-element array. Recently, an alternative method for estimating range has emerged based on the waveguide invariant which still requires either an array of sufficient horizontal extent or data from a moving source for which range rate is known. In analogy to the waveguide invariant derivation, it is shown that the magnitude of the square of the difference between the acoustic field at two different ranges contains information about the range interval, Δr. Since the range interval is manifest in the time interval, Δt between field measurements, range rate can be ascertained. Experimental results confirm this single sensor localization method.

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.

Prediction of a body’s structural impedance and scattering properties using correlation of random noise

This paper derives a method to estimate the structural or surface impedance matrix (or equivalently the inverse of the structural Greens function) for an elastic body by placing it in an encompassing and spatially random noise field and cross-correlating pressure and normal velocity measurements taken on its surface. A numerical experiment is presented that utilizes a cross-correlation method to determine the structural impedance matrix for an infinite cylindrical shell excited by a spatially random noise field. It is shown that the correlation method produces the exact analytic form of the structural impedance matrix. Furthermore, using standard impedance formulations of the scattered and incident pressure fields at the object surface that are based on the equivalent source method and using this estimated structural impedance, a prediction of the scattered acoustic field at any position outside of the object can be made for any given incident field. An example is presented for a point (line) source near a cylindrical shell and when compared with the analytical result, excellent agreement is found between the scattered fields at a radius close to the shell.

Localization of a small change in a multiple scattering environment without modeling of the actual medium

A method to actively localize a small perturbation in a multiple scattering medium using a collection of remote acoustic sensors is presented. The approach requires only minimal modeling and no knowledge of the scatterer distribution and properties of the scattering medium and the perturbation. The medium is ensonified before and after a perturbation is introduced. The coherent difference between the measured signals then reveals all field components that have interacted with the perturbation. A simple single scatter filter (that ignores the presence of the medium scatterers) is matched to the earliest change of the coherent difference to localize the perturbation. Using a multi-source/receiver laboratory setup in air, the technique has been successfully tested with experimental data at frequencies varying from 30 to 60 kHz (wavelength ranging from 0.5 to 1 cm) for cm-scale scatterers in a scattering medium with a size two to five times bigger than its transport mean free path.

Experimental estimation of in vacuo structural admittance using random sources in a non-anechoic room

Identification of unexploded ordinance buried in the sediment in the littoral waters throughout the world is a problem of great concern. When illuminated by low-frequency sonar some of these targets exhibit an elastic response that can be used to identify them. This elastic behavior is embodied and identified by a quantity called the in vacuo structural admittance matrix Ys, a relationship between the sonar-induced forces and resulting vibration on its surface. When it is known it can be combined with surface impedances to predict the three-dimensional bistatic scattering in any fluid-like media and for any burial state (depth and orientation). At the heart of this is the measurement of Ys and it is demonstrated in this paper that this can be accomplished by studying the target in a simple (acoustically unaltered) in-air laboratory environment. The target chosen in this study is a thick spherical shell that was illuminated by a nearly spatially isotropic array of remote loudspeakers. Ys is constructed from ensemble averages of the cross-correlations of eight collocated accelerometers and microphones placed on the surface of the object. The structural admittance determined from the data showed excellent agreement with theory.

Measurement of the structural admittance matrix using external random noise sources

The structural admittance Ys also called the structural Green’s function characterizes the vibration, radiation, and scattering physics of a vibrator. When it is known, the vibration of the radiating surface, and hence the acoustic radiation from the surface, is also known for any specified surface load. We demonstrate in this talk that Ys can be constructed when the object is placed in an isotropic random noise field. This construction consists of ensemble averages of cross-correlations of the resulting total normal velocity and total pressure (incident plus scattered) measured over the complete surface of the object. However, the measurement of the surface fields could be prohibitive as the temporal frequency of interest increases. Instead of a surface measurement, one can use a dual conformal surface of microphones placed near to the object together with near-field acoustical holography (NAH) to determine the fields on the surface via the back projection capability of NAH. Although a large number of se...

Data-based sensitivity kernel in a highly reverberating cavity

Our goal is to acoustically localize medium inhomogeneities, (i.e., scatterers) in a very complex medium without having to resort to constructing an accurate propagation model. Instead, we use a data-based sensitivity kernel approach to characterize medium changes in this complex medium which, in this study, is a highly reverberating cavity. The efficacy of the method is confirmed in an experiment with a moving aggregate of ping-pong balls inside a fish tank of 5.6m-diameter and water depth of 1.05m in the ~10KHz frequency regime; acoustic sources and receivers are on the periphery of the tank. Using a sensitivity kernel constructed from field data for scatterers at a sparse set of known positions, we demonstrate that we can localize other scatterers at unknown positions.

Remote localization of a moving target in a random scattering medium based on the Born approximation.

This paper presents a technique to remotely localize a moving target embedded in a fixed multiple scattering medium. The method, based on the Born approximation, does not require full modeling or even a moderately detailed knowledge of the multiple‐scattering background. Using a multi‐source/receiver laboratory setup in air, the technique was tested with experimental data at frequencies varying from 30 to 60 kHz (wavelength ranging from 0.5 to 1 cm) for centimeter‐scale scatterers in a multiple scattering medium with 3% volume fraction. The method shows very good localization of the target with a precision of a few wavelengths.