Paul C. Hines

Theoretical model of acoustic backscatter from a smooth seabed

Currently available mathematical models of acoustic scattering from the ocean bottom generally fail to predict backscatter levels of sufficient magnitude for two limiting cases: as the bottom becomes increasingly smooth and as the grazing angle becomes small. In this paper a mathematical model of acoustic backscattering that attempts to address these shortcomings for the case of a sediment bottom is derived. In the model, scattering is caused by fluctuations in sediment porosity. The model allows for penetration of the incident wave into the bottom at subcritical grazing, and retransmission of scattered spherical waves through the (planar) interface. The frequency and grazing angle dependence of the acoustic backscatter are determined primarily by the correlation function of the porosity fluctuations, while the magnitude of the backscatter is controlled by the mean‐square value of these fluctuations. Numerical results obtained from the model are tested against backscatter data available in the literature ...

Perception-based automatic classification of impulsive-source active sonar echoes

Impulsive-source active sonar systems are often plagued by false alarm echoes resulting from the presence of naturally occurring clutter objects in the environment. Sonar performance could be improved by a technique for discriminating between echoes from true targets and echoes from clutter. Motivated by anecdotal evidence that target echoes sound very different than clutter echoes when auditioned by a human operator, this paper describes the implementation of an automatic classifier for impulsive-source active sonar echoes that is based on perceptual signal features that have been previously identified in the musical acoustics literature as underlying timbre. Perceptual signal features found in this paper to be particularly useful to the problem of active sonar classification include: the centroid and peak value of the perceptual loudness function, as well as several features based on subband attack and decay times. This paper uses subsets of these perceptual signal features to train and test an automatic classifier capable of discriminating between target and clutter echoes with an equal error rate of roughly 10%; the area under the receiver operating characteristic curve corresponding to this classifier is found to be 0.975.

Boundary characterization experiment series overview

Ocean acoustic propagation and reverberation in continental shelf regions is often controlled by the seabed and sea surface boundaries. A series of three multi-national and multi-disciplinary experiments was conducted between 2000-2002 to identify and measure key ocean boundary characteristics. The frequency range of interest was nominally 500-5000 Hz with the main focus on the seabed, which is generally considered as the boundary of greatest importance and least understood. Two of the experiments were conducted in the Mediterranean in the Strait of Sicily and one experiment in the North Atlantic with sites on the outer New Jersey Shelf (STRATAFORM area) and on the Scotian Shelf. Measurements included seabed reflection, seabed, surface, and biologic scattering, propagation, reverberation, and ambient noise along with supporting oceanographic, geologic, and geophysical data. This paper is primarily intended to provide an overview of the experiments and the strategies that linked the various measurements together, with detailed experiment results contained in various papers in this volume and other sources

Linear and nonlinear measures of ocean acoustic environmental sensitivity

This letter defines linear, linearized, and nonlinear measures of environmental sensitivity for ocean acoustic propagation that account for realistic uncertainties in various environmental parameters (water-column sound-speed profile and seabed geoacoustic properties). Simple interpretations of sensitivity are based on the implicit assumption of a linear relationship between parameter sensitivity and parameter uncertainty. This assumption is examined by comparing the three sensitivity measures over a range of parameter uncertainties about the actual assumed environmental uncertainty. Sensitivity range and depth dependencies are illustrated for realistic geoacoustic uncertainties and oceanographic variability of the sound-speed profile.

Theoretical model of in‐plane scatter from a smooth sediment seabed

The author has previously reported [J. Acoust. Soc. Am. 88, 324–334 (1990)] on a model for acoustic backscatter from the volume beneath a smooth sediment seabed. In the present paper, the model is generalized to an in‐plane scattering model such that the source and receiver need not be colocated but must lie in the same vertical plane. In the model, scattering is caused by fluctuations in sediment porosity. The model allows for penetration of the incident wave into the bottom at subcritical grazing, and retransmission of scattered spherical waves through the (planar) interface. The frequency and grazing angle dependence of the acoustic scatter are determined primarily by the correlation function of the porosity fluctuations, whereas the magnitude of the scatter is controlled by the porosity variance. Results are obtained from the model over the frequency band 5–200 kHz, for a variety of incident and scattered grazing angles, using two sample porosity correlation functions. The first is an exponentially de...

Time-of-Flight Measurements of Acoustic Wave Speed in a Sandy Sediment at 0.6–20 kHz

There is considerable interest within the underwater acoustics community as to whether a fluid model or a poroelastic (Biot) model provides a more accurate representation of sandy sediments. One key metric used to determine this is the acoustic wave speed in the seabed, since the Biot model predicts a sound speed that is frequency dependent whereas the traditional fluid model assumes a sound speed that is constant with frequency. Results obtained during the 1999 Sediment Acoustics Experiment (SAX99) showed some evidence of sound-speed dispersion [IEEE J. Ocean. Eng., vol. 27, no. 3, pp. 413-428, 2002]. The results were consistent with Biot model predictions that employed inputs based on geophysical measurements made at the site. However, only a limited data set was obtained at frequencies from 1 to 10 kHz where the model exhibited its greatest sound-speed variation. Furthermore, these were relative-rather than absolute-measurements of sound-speed dispersion. During the SAX04 sea trial, conducted in autumn 2004 about a kilometer from the location of the SAX99 site, acoustic data were collected on receivers buried in the seabed using a pair of transmitters located within the seabed and a third located in the water column directly above the buried receivers. This source geometry enabled direct time-of-flight (TOF) measurements of acoustic wave speed along all three Cartesian axes. The results are normalized by the acoustic wave speed in the overlying water. Horizontal measurements yielded absolute dispersion estimates but the vertical data were limited to relative estimates due to uncertainty in the depths of the receivers. Results show dispersion within the error limits of the measurement with normalized sediment sound speed increasing from 1.05 at 600 Hz to 1.13 at 20 kHz. The frequency dependence of the measured sound-speed ratios reported on in this paper is in agreement with a simplified poroelastic model [J. Acoust. Soc. Amer., vol. 110, no. 5, pp. 2276-2281, 2001] evaluated using physical parameters measured nearby during SAX99, but the measured sound-speed ratios are about 3% lower than the model predicts; however, some of the vibracores taken at the SAX04 site indicate the presence of small mud inclusions at about 1-m depth, and model results using the oases seismoacoustic model indicate that the lower sound speeds are consistent with the presence of a thin muddy layer. In addition, sound speed along the vertical axis showed substantially greater variability with frequency than did the measurements along the horizontal axes. Results obtained from a simple numerical model indicate that the greater variability in the vertical direction can be explained by interference from reflected arrivals from a low-speed reflector at approximately 1-m depth. Using the porosity β as a free parameter, a best fit of the poroelastic model to the data is obtained for β = 0.425 . Although this is higher than the value of β = 0.385 measured in the sandy sediment during SAX99, heuristic arguments based on the self-consistent model results and the vibracores are presented to support the hypothesis that localized muddy inclusions at the experimental site increased the average porosity over the horizontal propagation paths and resulted in the lower sound-speed ratios.

Evaluation of the endfire response of a superdirective line array in simulated ambient noise environments

Superdirective line arrays can provide high gains whenever the inter-element spacing is much less than half a wavelength. In practice these arrays often fall short for two reasons: First, performance degrades appreciably in the presence of uncorrelated system noise. Second, the optimum array weights are specific to the form of the noise field and the choice of these weights is sensitive to deviations from that noise field. Furthermore, since the process involves computing signal differences between hydrophones, SNR degrades as the steering angle approaches broadside. In spite of these drawbacks, a superdirective array can provide substantial improvement over a conventional array in specific instances. For example, the present application requires an endfire array of limited extent (<1 m in length) and wide bandwidth (1-6 kHz) making a conventional array impractical. Moreover, improvements in array hardware have substantially reduced uncorrelated system noise, and increased computer speed makes it reasonable to process data using several weighting schemes. Given these conditions, a superdirective array provides a useful alternative. In this paper, the superdirective array will be described briefly and its performance will be evaluated using an ambient noise simulation model. The simulation allows one to estimate array performance in a variety of ambient noise environments using realistic values for system noise and number of averages, rather than relying on the results obtained in the theoretical limit. For example, assuming a 3-D isotropic noise field, and system noise of -30 dB relative to the ambient noise background, the simulation indicates that a 6-element array can achieve an array gain of 13 dB at 2.5 kHz and 9 dB at 0.8 kHz.

A wide-band sonar for underwater acoustics measurements in shallow water

Defence Research Establishment Atlantic is developing a free-floating wide-band sonar for collecting acoustic data in the open ocean. The transmitter, a parametric array, offers three advantages: wide bandwidth (1-10 kHz), narrow beamwidth (/spl sim/3/spl deg/), and virtually no sidelobes. These features make the parametric sonar the ideal tool for direct measurements of environmental parameters in shallow water. By direct the authors mean the absence of complications resulting from unwanted interactions of the acoustic pulse with ocean boundaries. To complement the narrow-beam active sonar, a 6-channel superdirective/intensity array is being developed for the receiver. A 900 MHz RF command link is used to steer the array to any combination of azimuth and tilt angle. Along with control over azimuth and tilt angle, the sonar is instrumented to monitor depth, roll and vertical acceleration. Data transmission back to the ship is accomplished via a 2.3 GHz RF data link capable of a data transfer rate of up to 8 Mbits/sec. This paper presents an overview of the system.

Measurement and modeling of seabed particle motion using buried vector sensors

A technique was developed to measure the speed of sound in marine sediments at discrete frequencies from 0.6 to 3 kHz by transmitting pulses from acoustic projectors within the water column and measuring the pressure and acceleration components of the acoustic field on vector sensors buried in the seabed. The burial depth and orientation of the vector sensors was determined by analyzing the amplitude and phase response of the acceleration signals to transmissions from three orthogonal directions, using two acoustic projectors also buried in the seabed and a third in the water column directly above the buried receivers. To determine the properties of the seabed, a sequence of transmitted pulses was repeated from ten different grazing angles, spanning from well above to near the nominal critical angle. Due to the interference of refracted and diffracted (mainly evanescent) components of the acoustic field that penetrate the seabed, the particle motion can be elliptical rather than rectilinear and is not necessarily aligned with the geometric ray path (i.e., according to Snells law). A model was developed to quantify the effect of this interference. It revealed that the parameterization of the seabed as a sand half-space was incapable of explaining the frequency-dependent arrival angles measured by the vector sensors. Further modeling using a computer code for seismoacoustic propagation in horizontally stratified waveguides revealed that the measurement technique is very sensitive to the presence of thin layers with a high-impedance contrast. This modeling suggests that the presence of a thin muddy layer, 0.05-0.2 m thick within the top 1 m of the sediment, is dominating the complicated angular response of the vector sensors.

Acoustic backscatter measurements from littoral seabeds at shallow grazing angles at 4 and 8 kHz

Direct measurement of acoustic scattering from the seabed at shallow grazing angles and low kilohertz frequencies presents a considerable challenge in littoral waters. Specifically, returns from the air-water interface typically contaminate the signals of interest. To address this issue, DRDC Atlantic has developed a sea-going research system for measuring acoustic scatter from the seabed in shallow-water environs. The system, known as the wideband sonar (WBS), consists of a parametric array transmitter and a superdirective receiver. In this paper, backscatter measurements obtained with the WBS at two sandy, shallow-water sites off North Americas Atlantic coast are presented. Data were collected at 4 and 8 kHz at grazing angles from 3 degrees-15 degrees. The backscattering strength is similar at both sites and, below about 8 degrees, it appears to be independent of frequency within the statistical accuracy of the data. The measurements show reasonable agreement with model estimates of backscatter from sandy sediments. A small data set was collected at one of the sites to examine the feasibility of using the WBS to measure the azimuthal variability of acoustic scatter. The data set--although limited--indicates that the parametric arrays narrow beamwidth makes the system well-suited to this task.