Alain Maguer

Mechanisms for subcritical penetration into a sandy bottom: Experimental and modeling results

This paper presents preliminary results of a recent study whose overall objectives are to determine the mechanisms contributing significantly to subcritical acoustic penetration into ocean sediments, and to quantify the results for use in sonar performance prediction for the detection of buried objects. In situ acoustic measurements were performed on a sandy bottom whose geoacoustical and geomorphological properties were also measured. A parametric array mounted on a tower moving on a rail was used to insonify hydrophones located above and below the sediment interface. Data covering grazing angles both above and below the nominal critical angle and in the frequency range 2-15 kHz were acquired and processed. The results are compared to two models that account for scattering of sound at the rough water-sediment interface into the sediment. Although all possible mechanisms for subcritical penetration are not modeled, the levels predicted by both models are consistent with the levels observed in the experimental data. For the specific seafloor and experimental conditions examined, the analysis suggests that for frequencies below 5-7 kHz sound penetration into the sediment at subcritical insonification is dominated by the evanescent field, while scattering due to surface roughness is the dominant mechanism at higher frequencies.

In situ estimation of sediment sound speed and critical angle

Understanding the basic physics of sound penetration into ocean sediments is essential for the design of sonar systems that can detect, localize, classify, and identify buried objects. In this regard the sound speed of the sediment is a crucial parameter as the ratio of sound speed at the water-sediment interface determines the critical angle. Sediment sound speed is typically measured from core samples using high frequency (100s of kHz) pulsed travel time measurements. Earlier experimental work on subcritical penetration into sandy sediments has suggested that the effective sound speed in the 2-20 kHz range is significantly lower than the core measurement results. Simulations using Biot theory for propagation in porous media confirmed that sandy sediments may be highly dispersive in the range 1-100 kHz for the type of sand in which the experiments were performed. Here it is shown that a direct and robust estimate of the critical angle, and therefore the sediment sound speed, at the lower frequencies can be achieved by analyzing the grazing angle dependence of the phase delays observed on a buried array. A parametric source with secondary frequencies in the 2-16 kHz range was directed toward a sandy bottom similar to the one investigated in the earlier study. An array of 14 hydrophones was used to measure penetrated field. The critical angle was estimated by analyzing the variations of signal arrival times versus frequency, burial depth, and grazing angle. Matching the results with classical transmission theory yielded a sound speed estimate in the sand of 1626 m/s in the frequency range 2-5 kHz, again significantly lower the 1720 m/s estimated from the cores at 200 kHz. However, as described here, this dispersion is consistent with the predictions of the Biot theory for this type of sand.

Target parameter estimation using resonance scattering analysis applied to air-filled, cylindrical shells in water

Current research on classification of submerged objects is concerned with using broadband sonar signals to insonify the targets, and applying signal-processing techniques to the backscattered signals. One characteristic of target echoes which may provide classification clues is the so-called resonance scattering response, the characteristics of which depend on target elastic properties. This paper presents algorithms for extracting resonance information based on autoregressive (AR) spectral estimation techniques. The AR-based representation is useful for detecting and accurately localizing resonances in the frequency domain. The extracted resonance frequencies are grouped into identified wave families, and processed in order to characterize the scatterer in terms of elastic and geometrical parameters on the basis of equations derived from resonance scattering theory. The targets considered are water-loaded elastic, cylindrical, thin-walled shells immersed in salt water under free-field conditions. Analysi...

Experimental results for detection of buried objects at low grazing angles

Extension of buried object detection sonar system concepts to include low grazing angle geometries (i.e., beyond critical angle) would provide useful gains in coverage rate, provided acceptably high detection probabilities can be maintained for sufficiently low probabilities of false alarm. Several experiments have recently been conducted at shallow water sites with sandy seabottoms near Elba Island, Italy, in order to study the acoustic backscattering characteristics of the seabed and buried targets in a range of grazing angles both above and below the nominal critical angle of the seabed. A parametric sonar with significant secondary frequency source levels in the range 2–16 kHz was used (the size of the targets yielded ka values also in the approximate range 2–16). Modeling results for buried objects have shown that subcritical insonification can produce relatively high monostatic echo levels, especially at lower frequencies where the evanescent field is strongest. Some results of the experiments are d...

A synthetic aperture parametric sonar for buried object detection

This paper describes an experimental comparison between a parametric sonar and a conventional sonar for the detection of small objects on the seabed. First, a quick description of the parametric sonar used is given. Second, rail and sea experiments are described. The parametric array was mounted on a rail which was able to move at a constant speed (0.25 m/s). As a first experiment the sonar was kept fixed on the rail. A comparison of the detection of the object put on the seabed is performed between conventional sonar and the parametric one. The obtained results show a great improvement brought on by the parametric sonar which is less affected by bottom reverberation and which allows very precise identification of the detected object. As a second experiment, the sonar was moving on the rail in order to be able to perform synthetic aperture sonar processing with the parametric array. Significant improvements in resolution and signal‐to‐noise ratio were obtained when integrating together successive pings tr...

Measurements of acoustic scattering from partially and completely buried thin‐shelled targets

Conventional high‐frequency imaging sonars become less effective in detecting and classifying bottom‐laid targets when the targets become buried into seabottom sediments. Some recent work at the NATO SACLANT Undersea Research Centre has concentrated on investigating the use of lower‐frequency sonars (2–16 kHz) in order to better exploit scattering features of buried targets that can aid in their detection and classification. Part of the recent GOATS’98 experiment performed near Elba Island, Italy, involved carefully controlled monostatic measurements of scattering by thin‐shelled objects (spheres and cylinders) that were partially and completely buried in sand, as well as in free‐field. Preliminary results are presented which show promising comparisons of the data to existing models. In addition to examining peak levels of the returns for detection purposes, special attention is paid to resonance effects (examined in the time and frequency domain), and the effect of burial on this part of the target respo...

Measurement of 3‐D scattering from buried targets in very shallow water

The objective of the GOATS’98 experiment was to determine the three‐dimensional, spatial characteristics of seabed target scattering and reverberation in coastal environments with water depth less than 15 m. The experiment was carried out at Marciana Marina, Elba Island, in May 1998 in a joint effort between SACLANTCEN and MIT. Artificial targets were buried at various depths into the sandy bottom and insonified by a parametric source mounted on a 10‐m tall tower which could be repositioned on a bottom‐mounted rail under remote control. This arrangement allowed for insonifying the targets at a wide range of incident angles, including both the sub‐ and supercritical regimes. The scattering and reverberation were measured using a suite of fixed and mobile arrays. Specifically, an eight‐element line array was mounted on an MIT Odyssey‐II autonomous underwater vehicle (AUV) in a ‘‘swordfish’’ configuration. The AUV was launched from R/V Alliance, anchored off‐shore, to perform survey patterns in the target ar...

Sediment critical angle estimation via in situ acoustic measurements

The basic physics of sound penetration into ocean sediments is a current area of active research since a good understanding of this phenomenon is essential for designing sonar systems that can detect, localize, classify, and identify buried objects. The sound speed of the sediment is a crucial parameter since the ratio of sound speeds at the water–sediment interface determines the critical angle. Sediment sound speed is typically measured with core samples using high‐frequency (100’s of kHz) pulsed travel time measurements. Simulations using Biot theory and taking into account sediment permeability have shown that there may be a significant frequency dependence to sediment sound speed in the range 1–100 kHz. Here, sediment sound speed and critical angle estimation are investigated through the analysis of in situ acoustic measurements. Acoustic energy in the frequency range 2–16 kHz was directed toward a sandy seabottom, and a buried 14‐element hydrophone array measured the incoming signals. The critical a...

An inversion approach based on multiple‐aspect resonance analysis of finite cylindrical shells in water

Inverse scattering of fluid‐loaded, elastic, thin‐walled cylindrical shells is addressed by multiple‐aspect resonance analysis. A set of aspect‐dependent acoustic phenomena is selected from membrane wave and resonance scattering theories, which are expected to be backscattered by fluid‐filled or empty thin‐walled shells and to give rise to resonance phenomena in the ka range (1,50). The features selected are from spatial axial modes, and Lamb‐type, Scholte–Stoneley, and shear helical waves. The last three wave families are significant only over a small range of target aspects near broadside, with the width of this range depending on shell properties. Approximate equations are formulated relating resonance behavior in the aspect‐frequency domain, to target parameters such as shell outer radius, thickness, length, and material. The inverse methodology is validated on experimental data from a steel cylindrical shell with flat end‐caps, filled with air or water and suspended in the water column. The target wa...

Comparison between subcritical penetration models and in situ data

This paper presents the recent results of a study whose overall objectives are to determine the mechanism(s) contributing to anomalous high‐frequency sound penetration into sediments, and to quantify the results for use in sonar performance prediction for the detection of buried objects. To date, several mechanisms have been hypothesized in order to explain this phenomenon, the most frequently mentioned being: (a) the existence of a Biot slow wave in the sediment, (b) surface roughness, and (c) scattering of the evanescent wave by volume inhomogeneities within the sediment. In situ acoustic measurements were performed on a sandy bottom whose geoacoustic properties were carefully identified. A parametric array mounted on a tower moving on a rail was used to insonify hydrophones located above and below the sediment interface. An extensive data set covering a wide range of grazing angles (both above and below the estimated critical angle) and frequencies (2–50 kHz) was acquired and processed. The results are...