Eric Pouliquen

Backscattering from bioturbated sediments at very high frequency

Recent backscattering measurements made in the Gulf of La Spezia, Italy, using a sonar operating at 140 kHz combined with thorough characterization of seabed interface and volume properties illustrate the importance of seabed volume scattering. Three-dimensional fluctuation statistics of density variability and vertical density gradients, both of which are attributed to the level of bioturbation (e.g., sea shell fragments, burrows, pockets of water) have been quantified using X-Ray computed tomography. Two-dimensional interface roughness spectra have also been determined using a digital stereo photogrammetry system. The combined ground truth has allowed a backscattering model to be fully constrained. Measured backscattering strength versus angle is compared to a model that includes the effects of varying density and sound speed. Data-model comparisons show that scattering from the volume of strongly inhomogeneous sediments can often be a primary contributor to seafloor scattering away from normal incidence.

Time-evolution modeling of seafloor scatter. I. Concept

A time-series model for acoustic seafloor backscattering is described. The method analytically expresses the elementary time-backscattered response of every seafloor surface and every seafloor volume infinitesimal element. For chosen geometric, acoustical, and acquisition parameters, they are summed to produce in the time domain a realization of the reverberation time-pressure field received at the source. The approach is based on the Kirchhoff approximation for the seafloor interface backscattering and on the Small Perturbation theory for the seafloor volume. It only accounts for single backscattering mechanisms of the compressional wave with the seafloor. The model is implemented using calculated height fields for the water-sediment interface and distributed seafloor volume inhomogeneities. The analytical description of the model and its limitations is described in this paper.

Penetration of acoustic waves into rippled sandy seafloors

The Helmholtz-Kirchhoff integral and the Kirchhoff approximation are applied to model the penetration of sound waves into rough sandy seafloors at grazing angles above and below the critical angle. As the seafloor of interest is anisotropic, emphasis is placed on simulating the response from a two-dimensional interface. The analytical development of the method is first presented, followed by numerical examples. Simulations and data acquired at sea are in very good agreement in the 2-15 kHz band [Maguer et al., J. Acoust. Soc. Am. 107, 1215-1225 (2000)]. The model predicts, in agreement with the 2-15 kHz acoustic data, the contributions due to roughness effects that dominate the evanescent wave component over most of this frequency band. Secondary effects such as coherent (Bragg) influence patterns and the loss of signal coherence with grazing angle or depth are correctly predicted. The model simulations strongly suggest that roughness of the sediment interface is most likely the cause of anomalous sound penetration into the seabed.

Time-evolution modeling of seafloor scatter. II. Numerical and experimental evaluation

A time-evolution model of seafloor scatter is numerically implemented and experimentally evaluated. The model is based on analytically expressing the elementary time-backscattered response of every seafloor surface and every seafloor volume infinitesimal element. The implementation of the model is based on a statistical realization of the seabed interface and volume inhomogeneities, from which the time series are computed by coherent summation of the scatter from small elements over the insonified area and volume. The analytical expressions and the implementation are evaluated for the image solution case, for which an almost perfect agreement is found. Examples are shown of how the beam width and seabed roughness affect the time-series return from both the surface and from the volume. The results of the model are compared with data from two different bottom types recorded with a parametric sonar. Reasonable accordance is found between the model and the data.

Advances in high-resolution seafloor characterization in support of high-frequency underwater acoustics studies: techniques and examples

Studies designed to examine high-frequency seafloor scattering or penetration mechanisms have been limited by the difficulty of accurately characterizing seabed properties at millimetre scales. Both the two-dimensional (2D) interface controlling roughness scattering and the three-dimensional (3D) internal structure of the top tens of centimetres of seafloor sediments controlling volume scattering require accurate measurement tools that are non-destructive and that have high spatial resolution. Subcritical penetration mechanisms have not been satisfactorily explained to date due in part to the lack of two-dimensional seafloor roughness measurements. Recently, tools for addressing these experimental shortfalls have been developed to support seafloor acoustics measurements, and information obtained from these systems is being used to address many open questions in high-frequency seafloor acoustics. This paper provides a review with examples of the development and use of several non-traditional technologies in seafloor acoustics studies including x-ray computed tomography and digital photogrammetry.

Predicting Scattered Envelope Statistics of Patchy Seafloors

Local hydrodynamic or biological influences often produce seafloors in shallow water that consist of differing types of material. The scattering properties from the components of these kinds of seafloors may have a complicated relationship in terms of their frequency dependence and/or angular response. Consequently, this relationship directly influences the angular and frequency response of the scattered envelope distributions. The probability distribution function (PDF) for a scattering scenario such as this is not easy to obtain analytically. However, a recently developed model for a patchy seafloor with a single dominating component [1] allows for numerical analysis of the envelope PDF for more complicated seafloors through the use of Hankel transforms of the joint characteristic function (JCF) of the complex envelope. The JCF is straightforward to construct for complicated patchy seafloors. In this study, a direct link between environmental parameters and the envelope distributions of backscatter is developed. The influence of the relative scattering properties of the seafloor patches on the scattered envelope statistics will be examined in detail.

Measurements of high‐frequency acoustic scattering from seabed vegetation

Knowledge of the acoustic properties of seabed vegetation is required for accurate seafloor characterization in many shallow‐water sites. As the study and modeling of acoustic scattering from seabed vegetation have been very limited to date, characteristics such as the dependence of scattering strength on grazing angle or frequency are still largely unknown. In order to quantify these properties, acoustic scattering experiments were conducted in beds of Posidonia Oceanica at several sites near the islands of Elba and Sardinia, Italy and in Saros Bay, Turkey with side‐scan, single beam, and parametric transducers. Normal incidence broadband measurements were made at both the primary (40 kHz) and secondary (8 kHz) frequencies of a parametric sonar. Oblique incidence measurements were made with linear sources at frequencies from 29 to 385 kHz and with coverage over a large range of grazing angles. Results of these measurements are presented in terms of scattering strength versus grazing angle as well as scat...

Comparison of Seafloor Roughness and Scattered Acoustic Temporal Decorrelation

Connecting changes in acoustic scattering from the seafloor with changes in seafloor topography is essential for modeling the time dependence of the scattering and the development of acoustics as a tool for the remote sensing of benthic activity. An equation is derived that links the decorrelation of scattered acoustic power with the decorrelation of seafloor roughness spectral estimates. The result is assessed through a comparison of decorrelation values generated by processing topographical data recorded by a digital photogrammetry system and backscattering data acquired with a translating source/receiver assembly. Both data sets were collected off the western coast of Florida as part of the U.S. Office of Naval Research (ONR)-sponsored sediment acoustics experiment (SAX04), during which the primary mechanism of topographical change at the frequencies of interest appeared to be fish feeding. Although decorrelation curves proved to be both space and time dependent, and the collection of data sets was neither collocated nor synchronized, the agreement between averaged topographical and acoustic decorrelation values was reasonable. Both types exhibited a strong frequency dependence, which should prove beneficial in classifying and quantifying sources of seabed transformation if it is mechanism specific.

The Helmholtz–Kirchhoff approach to modeling penetration of acoustic waves into rough seabeds

The Helmholtz–Kirchhoff integral is applied to model the penetration of sound waves into sandy seafloors at grazing angles above and below the critical angle. Although the conditions for the validity of the Kirchhoff approximation can be limiting, this approximation should be valid at high frequency for gently undulating seafloor surfaces even at moderate to low grazing angles, providing that the self‐shadowing effect is carefully removed. The analytical development of the method is first presented, followed by numerical examples and comparisons with the experimental data of Maguer et al. [SACLANTCEN report, SR‐287, April 1998]. The model predicts, in agreement with the 2‐ to 15‐kHz acoustic data, the frequency where the contributions due to roughness effects begin to dominate those due to the evanescent wave. Secondary effects such as Bragg interference patterns and the loss of signal coherence with grazing angle or depth are correctly predicted. The model simulations strongly suggest that roughness of t...

Model‐based seafloor identification with parametric sonar data: Experimental results

A method is proposed to estimate the geoacoustic parameters of the uppermost seafloor strata using normal‐incidence backscattered acoustic data obtained with a parametric sonar in the 4–12 kHz frequency range. A model‐based identification approach has been pursued. The time‐domain backscattering model BORIS [Bergem et al., SACLANTCEN Memorandum SM‐328 (1997)] has been selected as a forward model. A specific wavelet transform has been defined in order to obtain a significant measure of the discrepancy between data and model predictions in the wavelet‐transformed domain. This discrepancy is minimized as a function of the bottom parameters. The technique has been tested on experimental data gathered in different locations in order to assess its perfomance on different bottom types. Identification results have been compared with independent ground truth collected by core analysis and video camera inspection. The comparison shows excellent agreement between acoustically determined parameters and ground truth. ...