Sonar In Shallow Water Research Articles

A Scaled Tank Experiment for the Stochastic Rough Interface Scattering

Bottom reverberation is an important limitation that affects the detection performance of active sonar in shallow water. The stochastic rough seafloor surface scattering is one of the main scattering sources of the bottom reverberation. Since there are much uncertainties, such as propagation fluctuation, unstable station of transmit and receive equipment, it is difficult to investigate the quantitative relationship between the random rough interface parameters and the statistical property by processing the experimental reverberation data collected in the ocean In this paper, the statistical characteristics of the irregular 3D printed interface scattering field are studied by a scaled tank experiment, and the detailed results are presented by the comparison between the COMSOL simulations and the recorded data, the feasibility of the random rough interfaces experiment is verified well. Because the relationship between parameters is precisely controllable, it lays a foundation for the subsequent experimental research of target extraction under the background of interface scattering.

Laboratory measurements and simulations of reflections from a water/clay interface during the diffusion of salt.

Estuarine, riverine, and certain continental shelf environments experience significant temperature and salinity variability near the ocean bottom that can produce significant changes in how sound interacts with fine-grained sediments, presenting challenges in applications including shallow water sonar and bottom surveys. To begin to understand the effects of this variability on acoustic reflection, reflection measurements in the laboratory near 1 MHz were obtained from a water-clay interface while varying the salinity of the bottom water. At certain angles of incidence, salinity variations caused changes in bottom loss up to 15 dB and 180-degree phase shifts in the reflected signal, and induced changes in the reflectivity of the clay through the diffusion process, thereby leading to complicated, coupled interactions at the water-clay interface. By modeling the reflectivity of clay during molecular diffusion of salt, the diffusion coefficients were experimentally inferred and simulations at lower frequencies and longer timescales were performed. Derived characteristic length scales associated with the molecular diffusion of salt are compared with acoustic wavelengths to identify frequency regimes that are sensitive to salinity fluctuations. Results indicate that the dynamic nature of the bottom water can cause measurable and significant effects in reflectivity at and below frequencies applicable to sonar.

Vector intensity properties from direct and reverberant field from a mid-frequency sonar in shallow water

The Intensity Vector Autonomous Recorder (IVAR) is a benthic lander system that coherently measures 3-axis acoustic particle velocity and pressure (combined sensor). Results using IVAR in a mid-frequency propagation and reverberation study conducted near Geoje Island, Republic of Korea, are presented. The measurements were made within shallow semi-circular bay (depth ~25 m) with bay opening to deeper waters, and with directional wave measurements made within the bay waters. This presentation will focus on measurements made at a fixed range (100–1000 m, depending on test) from the Autonomous Reverberation Measurement System (ARMS), a benthic lander with a source and receive array mounted on a rotation stage in order to collect reverberation data as a function of bearing angle. Several vector and scaler metrics emerge based on different combinations of second-order acoustic fields are discussed, such as rate of energy transport and active and reactive intensity that lend insight into the process of shallow water reverberation. For example, in one continuous measurement made over a 12 hour period, there is high correlation between wave slope and horizontal beam width as determined with horizontal active intensity. This and other effects related to variability in shallow water reverberation will be discussed.

Transient reflection from mud during molecular diffusion of salt

Harbor basins and estuarine environments experience drastic salinity fluctuations in the water near the water-sediment interface, which can significantly affect how sound interacts with a low-velocity mud bottom. This presents challenges in applications including mine detection, port protection and shallow water sonar. In a previous investigation of this system, a mud sample that was saturated with fresh water was instantaneously exposed to salt water. Laboratory measurements of plane wave reflection from the water-mud interface were obtained using a time-gated broadband impulse as time evolved. Results suggested molecular diffusion of salt into the sample had altered the reflected pulse’s amplitude and caused a depth-dependent impedance profile in the mud. The sediment was discretized into layers much thinner than a wavelength and a multi-layer steady-state model was used to predict the reflection from the diffusing mud. As the effective diffusion length reached a critical value, predictions of the steady-state model began to deviate from the measurements, indicating that a transient solution was required. A model of the transient reflection of a finite-length pulse from a half space with an arbitrary impedance will be presented and model results compared with laboratory measurements. [Work supported by ONR.]

Laboratory measurements of a phase shift in the reflection from a water-mud interface due to water column variability

Harbor basins and certain continental shelf environments experience significant variability due to tides, surface heating, currents, and other oceanographic processes. Dynamic processes such as these can cause substantial sound speed and density fluctuations in the water column over short time periods, which are often unknown to sonar users. Such variations are not critical over high-velocity bottoms such as sand, but can produce significant changes in how sound interacts with low-velocity fine-grained sediments. High levels of variability and a low-velocity bottom, such as mud, can therefore present challenges in applications including mine detection, port protection and shallow water sonar. At certain penetration angles, temporal variations in the water-to-sediment sound speed ratio can cause 180 degree phase shifts in the reflected field, which in turn produces waveguide propagation and target scattering field variability. To begin to understand these processes, laboratory measurements of plane wave reflection were obtained from a water-mud interface while varying salinity and temperature of the water. Results indicate that the dynamic nature of sound speed ratio at the ocean bottom can cause significant effects in shallow water environments with muddy bottoms. [Work supported by ONR.]

A theoretical model of linearly filtered reverberation for pulsed active sonar in shallow water.

This paper presents a statistical model useful for characterizing pulsed active sonar reverberation in shallow water. The model is based on the fundamental assumption that reverberation consists of echoes from point scatterers having random positions, strengths, and Doppler dilations. Receive array beam patterns, simple propagation losses, and planar bistatic geometry are included. The probability distribution of uniformly dense scatterers as a function of echo delay and bearing is explicitly related to the change in the area from which scatterer echoes contribute to the reverberation, and is presented in closed form. The cross Q-function of the transmitted waveform and the linear filter applied to the received signal arises naturally from the development. This function, along with environmental spreading, determines the shape of the reverberation along the Doppler axis. The assumptions and simplifications under which the reverberation decouples into independent spatial (delay and bearing) and Doppler terms are presented.

Simulation for Range Prediction of Active Sonar in Shallow Water

In shallow water the acoustic wave from ambient noise sources carries a large number of environment information based on the complicated reflection both on the surface and seabed interface. The sound speed profile is one of the influencing factors of sound propagation characteristic, while for a long distance the sound absorption coefficient of water medium has an important significance to propagation range. The simulation results show that in shallow water sound absorption of seabed, sound speed profile and sound absorption of water were taken into account, then range prediction of active sonar can be exactly obtained using normal-mode propagation.

Investigation of low-frequency acoustic tissue properties of seagrass

Understanding the acoustic properties of seagrass is important for applications in mine hunting, shallow water sonar performance, and acoustic remote sensing for ecological surveys. Previous laboratory and field investigations have shown that the plant biomass and tissue structure of seagrass, rather than just the overall gas content, play a determinant role in its acoustic behavior. Hence, effective medium models of propagation through seagrass meadows have been ruled out, and a complete description of both tissue structure and tissue elastic properties is required to describe the acoustic response of seagrass meadows. To begin to address these deficiencies, a resonance tube experiment was set up to determine the low-frequency acoustic response of multiple species of seagrass in relation to leaf biomass and tissue acoustic compliance independent of tissue structure. Responses to frequency-modulated signals in the range from 0.5 to 10 kHz were obtained for Thalassia testudinum (turtle grass) and Halodule wrightii (shoal grass), two species with well-differentiated morphological features. An elastic waveguide model was used to account for the minor effect of the tube walls on the resonance characteristics. Initial measurements of tissue compliance will be presented. [Work supported by ONR and ARL:UT.]

Time–frequency approach to radar, sonar and seismic wave propagation with dispersion and attenuation

In many physical situations that involve wave propagation there is dispersion and attenuation. Examples include sonar in shallow water, underground radar, seismic wave propagation, fibre optics, among many others. The author shows that phase-space methods are particularly suited to study propagation with dispersion since in such situations the velocity of propagation is frequency dependent. Depending on the situation the phase space may be time-frequency or position-wavenumber. The author derive explicit expressions for the Wigner distribution for both cases and how it evolves with time are derived. The application to the propagation of noise fields is also discussed.

Laboratory investigation of the acoustic response of seagrass tissue in the frequency band 0.5–2.5 kHz

Previous in situ investigations of seagrass have revealed acoustic phenomena that depend on plant density, tissue gas content, and free bubbles produced by photosynthetic activity, but corresponding predictive models that could be used to optimize acoustic remote sensing, shallow water sonar, and mine hunting applications have not appeared. To begin to address this deficiency, low frequency (0.5-2.5 kHz) acoustic laboratory experiments were conducted on three freshly collected Texas Gulf Coast seagrass species. A one-dimensional acoustic resonator technique was used to assess the biomass and effective acoustic properties of the leaves and rhizomes of Thalassia testudinum (turtle grass), Syringodium filiforme (manatee grass), and Halodule wrightii (shoal grass). Independent biomass and gas content estimates were obtained via microscopic cross-section imagery. The acoustic results were compared to model predictions based on Wood's equation for a two-phase medium. The effective sound speed in the plant-filled resonator was strongly dependent on plant biomass, but the Wood's equation model (based on tissue gas content alone) could not predict the effective sound speed for the low irradiance conditions of the experiment, in which no free bubbles were generated by photosynthesis. The results corroborate previously published results obtained in situ for another seagrass species, Posidonia oceanica.

Demonstration of coherent gain from synthetic aperture for a midfrequency sonar in shallow water

The Clutter 07 experiment was carried out this May in the Straits of Sicily on board the R/Vs Alliance and Oceanus with the participation of NATO Undersea Research Centre, NRL, Applied Research Lab-Pennsylvania State University, and Defense Research and Development Canada. High repetition rate monostatic active sonar data collected off the alliance on the Malta Plateau during this experiment is processed to show significant inter-ping correlation indicative of the utility of performing coherent synthetic aperture sonar (SAS). Strip SAS images of scattering from a rocky outcrop on the bottom known as the Ragusa ridge show that the lateral extent of returns from compact objects can be reduced through SAS techniques for real shallow water environments using autofocusing techniques. [Work supported by ONR.]

Study of reverberation pattern and its cancellation method in shallow water

In shallow water, the primary limitation of the performance of active sonar is the reverberation that originates from volume and boundaries scattering as well as multi-path propagation. There?fore, reverberation cancelation is an important research topic for increasing the performance of active sonar in shallow water. In this research, the reverberation pattern is simulated using MAT?LAB software. The simulated frequency is 30-kHz in the research. There are two main aims of this work. The first is to create the signals that include the reverberation and the target. The second is to perform the reverberation cancelation for the active sonar in shallow water. The analysis of the reverberation for the spherical target is based on the propagation theory of image source, surface scattering of Rayleigh criterion of roughness, bottom scattering of Lambert?s Law, and multiple scattering. The signal containing the reverberation and the target is then compressed or enhanced by AGC (Automatic Gain Control). The echo of the target is then distinguished through the method of cross correlation. The follow?ing phenomena can be found: (a) AGC can compress the signal in a speci?c dynamic range. (b) cross correlation can be used to locate and distinguish the echoes of the target in a high reverberation environment.

Underwater Noise Modeling and Direction-Finding Based on Heteroscedastic Time Series

We propose a new method for practical non-Gaussian and nonstationary underwater noise modeling. This model is very useful for passive sonar in shallow waters. In this application, measurement of additive noise in natural environment and exhibits shows that noise can sometimes be significantly non-Gaussian and a time-varying feature especially in the variance. Therefore, signal processing algorithms such as direction-finding that is optimized for Gaussian noise may degrade significantly in this environment. Generalized autoregressive conditional heteroscedasticity (GARCH) models are suitable for heavy tailed PDFs and time-varying variances of stochastic process. We use a more realistic GARCH-based noise model in the maximum-likelihood approach for the estimation of direction-of-arrivals (DOAs) of impinging sources onto a linear array, and demonstrate using measured noise that this approach is feasible for the additive noise and direction finding in an underwater environment.

A method to predict active sonar detection range in uncertain shallow water environments

Knowledge of maximum detection range is important for the appropriate operation of search sonar. For example, effective spacing of multistatic sonar sources and receivers will be influenced by good estimates of achievable detection range. But predicting detection range for low-frequency active sonar in shallow water can be problematic in the face of inadequate environmental characterization. Thus, incomplete knowledge of bottom sediment geoacoustics and bottom scattering strength, and absence of a good understanding of bottom reverberation mechanisms, will greatly hamper the accurate prediction of dependable detection ranges. Given a bistatic sonar configuration in shallow water, a method is proposed and described that exploits the observed transition time from reverberation to noise-limited conditions to simply and robustly predict maximum detection range for specified target strength. Modeling and analysis suggest that estimates of maximum detection range within 25% accuracy can be readily achieved.

Relating the distribution of bathymetry to clutter distributions

One source of clutter for active sonar in shallow water is backscatter from the rough ocean bottom. Backscatter may often be dominated by Bragg scale roughness. Bathymetry measurements are not generally made at sufficient resolution to determine the Bragg scale roughness distribution directly. We examine the distribution of ocean bottom slopes and determine power law relationships. These relationships can be used to define fractals which allow extrapolations of bathymetry to finer scale. The fine-scale bathymetry is then related to the distributions of sonar clutter. The generalized gamma distribution is particularly useful to describe clutter. The upper tail of this distribution approaches a power law related to the power laws in the bathymetry. [This work is sponsored by the Office of Naval Research (ONR).]

Detection in shallow water using broadband—DORT

The decomposition of the time-reversal operator (DORT) [Prada et al., J. Acoust. Soc. Am. 99, 2067–2076 (1996)] has been extended into a coherent, broadband method. Broadband DORT has also been shown to isolate resolvable scatterers at various depths and ranges in a bistatic, active sonar in shallow water. Results are shown from the application of DORT to sea data taken in an area south of Hudson Canyon off the New Jersey coast during Geoclutter II. The vertical source/receiver array with 56 hydrophones spanning the water column was operated between 3.0 and 3.5 kHz. The elements were divided into four groups, with each group acting as a coherent, broadside source. Two methods were used for exciting the separate channels. One method was the use of subsequent LFMs and the other was the use of simultaneous transmission of four pseudorandom-noise signals. The target was a midwater column echo-repeater. Results are compared with modeling based on in situ environmental measurements during the experiment. [The authors acknowledge signal-processing expertise from Dr. Ning Xiang, University of Mississippi, and ENS Alan Meyer, LLNL, support from Dr. Jeff Simmen, ONR, and assistance from Dr. Charles Holland, ARL/PSU. Work supported by ONR.]

Multistatic broadband sonar (MSBA): Principles and measurements

Shallow-water (SW) coastal areas with depths between 40 and 200 m with bottom sediment layers (<25 m) formed by deposition and sea level variations are known to have frequency-dependent sound transmission and reverberation characteristics especially under the adverse conditions of downward refraction. Coastal ocean-dynamics complicates these environments by coupling the surface, volume, and bottom characteristics and can dramatically affect the temporal and spatial coherency of the signal. The broad-band signal and processing smear these frequency-dependent effects as well as signal excess fluctuations. Furthermore, recent experiments have shown that despite these complexities, explosive sources coupled with directional receivers have the capability to detect, localize, and classify sonar targets in the presence of clutter most likely due to bottom features, fish schools, and sunken objects. Active search sonar operations in SW are typically reverberation limited and experiments have shown that MSBA sonar when employed in bistatic source-receiver geometries can be effective. This paper reviews the principles underlying the MSBA sonar in shallow water and presents results from several experiments that demonstrated the utility of MSBA. The results are discussed in terms of a classical sonar model with basic statistical considerations to define limits on the detection, classification, and localization with multiple receivers.

Broadband-DORT applied to bistatic active sonar

The decomposition of the time reversal operator (DORT) is a single frequency method of spatially isolating scatterers with a multiple-source/multiple-receiver system [C. Prada, S. Manneville, D. Spoliansky, and M. Fink, J. Acoust. Soc. Am. 99, 2067–2076 (1996)]. This technique has 8 been extended into a coherent, broadband method. Broadband DORT has also been shown to isolate resolvable scatterers at various depths and ranges in a bistatic, active sonar in shallow water. Several limitations of the method are demonstrated through the use of numerical simulations and backpropagations. One limitation is the resolvability of a system in shallow water. The dependence of resolution on environmental parameters, e.g., signal-to-noise ratio, water depth, bottom parameters and sound speed profile—and system parameters—e.g., frequency content and source/receiver configuration—is shown by forming images of a single scatterer, two scatterers and many scatterers. The limits of continuity over frequency of the broadband singular vectors and values caused by frequency fading are also shown through data visualization techniques. [Work supported by ONR.]

Reviews Of Acoustical Patents

The purpose of these acoustical patent reviews is to provide enough information for a Journal reader to decide whether to seek more information from the patent itself. Any opinions expressed here are those of the reviewers as individuals and are not legal opinions. Printed copies of United States Patents may be ordered at $3.00 each from the Commissioner of Patents and Trademarks, Washington, DC 20231. Patents are available via the Internet at http://www.uspto.gov.

Active sonar detection in shallow water using the Page test

The use of active sonar in shallow water results in received echoes that may be considerably spread in time compared to the resolution of the transmitted waveform. The duration and structure of the spreading and the time of occurrence of the received echo are unknown without accurate knowledge of the environment and a priori information on the location and reflection properties of the target. A sequential detector based on the Page test is proposed for the detection of time-spread active sonar echoes. The detector also provides estimates of the starting and stopping times of the received echo. This signal segmentation is crucial to allow further processing such as more accurate range and bearing localization, depth localization, or classification. The detector is designed to exploit the time spreading of the received echo and is tuned as a function of range to the expected signal-to-noise ratio (SNR) as determined by the transmitted signal power, transmission loss, approximate target strength, and the estimated noise background level. The theoretical false alarm and detection performance of the proposed detector, the standard Page test, and the conventional thresholded matched filter detector are compared as a function of range, echo duration, SNR, and the mismatch between the actual and assumed SNR. The proposed detector and the standard Page test are seen to perform better than the conventional thresholded matched filter detector as soon as the received echo is minimally spread in time. The use of the proposed detector and the standard Page test in active sonar is illustrated with reverberation data containing target-like echoes from geological features, where it was seen that the proposed detector was able to suppress reverberation generated false alarms that were detected by the standard Page test.