John R. Preston

Long range acoustic imaging of the continental shelf environment: The Acoustic Clutter Reconnaissance Experiment 2001

An active sonar system is used to image wide areas of the continental shelf environment by long-range echo sounding at low frequency. The bistatic system, deployed in the STRATAFORM area south of Long Island in April-May of 2001, imaged a large number of prominent clutter events over ranges spanning tens of kilometers in near real time. Roughly 3000 waveforms were transmitted into the water column. Wide-area acoustic images of the ocean environment were generated in near real time for each transmission. Between roughly 10 to more than 100 discrete and localized scatterers were registered for each image. This amounts to a total of at least 30000 scattering events that could be confused with those from submerged vehicles over the period of the experiment. Bathymetric relief in the STRATAFORM area is extremely benign, with slopes typically less than 0.5 degrees according to high resolution (30 m sampled) bathymetric data. Most of the clutter occurs in regions where the bathymetry is locally level and does not coregister with seafloor features. No statistically significant difference is found in the frequency of occurrence per unit area of repeatable clutter inside versus outside of areas occupied by subsurface river channels.

The ONR Five Octave Research Array (FORA) at Penn State

A new towed/vertical array system has been built to support the ONR ocean acoustic programs 6.1 experimental research efforts. The array consists of both a linear section for standard beamforming and a cardioid section giving it port/starboard discrimination capabilities. The linear section is comprised of 4 modules with half-wavelength hydrophone spacing corresponding to cutoff frequencies of 250, 500, 1000, and 2000 Hz. Each linear aperture is made up of 64 hydrophone channels with a total linear aperture length of 189 metres. The cardioid module consists of 78 hydrophone triplets arranged in an equilateral triangle with 38.5 mm spacing between the individual phones. The linear spacing between each triplet set is 0.2 metres for a cutoff frequency of 3750 Hz. In addition to the acoustic sensors, the array contains 3 non-acoustic sensor suites and additional pressure sensor to provide real-time array heading, pitch, roll, and depth along with temperature at various positions along the array. Array control for setting of sampling rates, array gain, and monitoring of both acoustic and non-acoustic data is provided by a standard PC. The array supports sampling rates from 6.25-25 kHz for the acoustic data with 24 bit A/D conversion. Array telemetry is ATM/SONET with a data rate of 155 Mb/s. The acquisition system acquires directly to SCSI ultra 320 disk and is based on a COTS Linux workstation. Since taking delivery in May 2002, the FORA has been deployed during three separate sea trials, including the recent Geoclutter experiment held April-May 2003 on the New Jersey shelf in the Atlantic Ocean. Some preliminary results and data are presented from the different trials illustrating array capabilities along with an assessment of the array data quality for meeting current and future scientific objectives.

Geoacoustic parameter extraction using reverberation data from the 2000 boundary characterization experiment on the Malta plateau

This paper presents some new results from measurements of seafloor reverberation and pulse spreading using horizontal and vertical line arrays. The principal objective of this paper is to extract useful geoacoustic and bottom-scattering parameters that apply over a large ocean area. Analysis is presented on reverberation data from the 2000 Boundary Characterization Experiment performed jointly with North Atlantic Treaty Organization (NATO) Undersea Research Center (NURC), Applied Research Laboratory (ARL) of Pennsylvania State University, Defence Research and Development Canada (DRDC), and Naval Research Laboratory (NRL). Sources were SUS charges and coherent pulses. The receivers were horizontal arrays used monostatically. Data were analyzed in bands from 80 to 4000 Hz. Highlights of the reverberant returns are discussed. The experiment site is the Malta Plateau area south of Sicily, a relatively flat heavily sedimented area, but with a rocky ridge to the east. An original aspect of this paper is the design and implementation of a new automated inverse method using towed-array data to accomplish that goal. For each data set, a multiple-step simulated annealing (SA) algorithm is used together with the Generic Sonar Model (GSM). After automatically adjusting bottom loss and scattering strength, good agreement is achieved between the diffuse reverberation data and model predictions in relatively flat areas. Model/data differences are generally correlated with bottom-scattering features. Since reverberation from SUS charges typically lasts 10-40 s or more, extracted parameters apply over wide areas. Independent acoustic measurements provided a basis for a comparison with extracted values. Local bottom-loss and backscattering measurements were made by Holland in these areas. Additionally, chirp-sonar measurements were analyzed by Turgut. A comparison of geoacoustic models obtained with their methods and with this one was quite good. Comparing transmission loss (TL) predicted with Turguts local inverse method and TL predicted with the method presented here gave answers that were usually within 3 dB of each other. Typical two-way time spreads of 0.25 s were seen at a range of 7.5 km, with normalized peak correlations of 0.5, and which were fairly consistent with predictions made using the inverse results

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

Analysis of backscattering data in the Tyrrhenian Sea

Research has been conducted in the Tyrrhenian Sea basin to measure low‐frequency basin reverberation and relate the measurements to backscattered strength. The receivers were a towed horizontal array and a suspended vertical array. Standard Italian Navy depth (74 kg) and SUS (0.818 kg) charges were used as sources. The unique factor in this broadband basin reverberation experiment was that data were collected simultaneously from the horizontal and vertical array. Towed‐array data were taken on multiple headings; combining different headings permitted removal of the ambiguous directionality information from the towed array. Frequency‐domain beamformed data are presented and reveal that, in the area examined, basin features are the strongest backscatterers. Receiver beam level data are presented primarily for 300 Hz, but some data are given for 100, 450, and 730 Hz as well. Estimates of backscattering strengths of different features are provided and are in the range quoted in other publications. A comparison of the horizontal and vertical array scattering strengths estimated for some selected basin features is also included. Statistics for the dominant backscatterers are given, and the results from one basin feature are presented showing detailed backscatter structure.

Long-range acoustic communication in deep water using a towed array

Underwater acoustic communication typically involves vertical arrays that provide spatial diversity to mitigate channel fading and intersymbol interference when the array elements are well-separated. For closely-spaced arrays (e.g., one-half wavelength), an array gain is achieved by a beamforming process provided that the noise is uncorrelated while the signal is coherent across the array. A recent experiment (LRAC10) showed that coherent long-range acoustic communication (200-300 Hz) is feasible in deep water over ~550-km range between a source and a horizontal towed array (~100 m aperture). This paper demonstrates that beam diversity with a larger horizontal array (~200 m aperture) can be exploited to further improve the communication performance, achieving an error-free data rate of 50 bits/s for BPSK modulation at ~550-km range.

Using Triplet Arrays for Broadband Reverberation Analysis and Inversions

Some directional characteristics of observed clutter and reverberation are presented using line arrays with cardioid elements. It is shown that the cardioid arrays break the left/right ambiguity for reverberation sources above ~600-700 Hz. Data analysis uses a cardioid beamforming algorithm developed by researchers at the NATO Undersea Research Centre (NURC, La Spezia, Italy) but the normalization for this algorithm is derived by the author. It is shown that this normalized algorithm has an upper frequency limitation not previously noted. Reverberation data are taken from two recent experiments. One was the 2004 Boundary Characterization Experiment near the Malta Plateau. The area is rich in clutter objects like wrecks and mud-volcanoes and has some subbottom features that may be important. Sources were monostatic coherent pulses and impulsive sources. The receiver was the NURC cardioid array. The other experiment was the U.S. Office of Naval Researchs 2003 Geoclutter effort to study shallow-water bottom reverberation and clutter in the STRATAFORM area off New Jersey. Sources were bistatic coherent pulses. The receiver was the five-octave research array (FORA). The STRATAFORM has benign surface morphology but strong clutter is observed. Some highlights of the reverberant returns from that area are discussed that include returns from probable fish schools. From these two data sets, it is shown that up to 15-20-dB left/right discrimination is attainable on broadside cardioid beams in the 1200-1750-Hz region for both linear frequency modulation (LFM) pulses and sound-underwater-source (SUS) broadband data. Also, a (previously developed) constrained simulated annealing (SA) inversion technique is applied to the directional reverberation data from the NURC array. A two-layer fluid model of the bottom is assumed. The algorithm is best at estimating compressional speeds, layer thickness, and attenuations in that order. Extracted bottom parameters at the site are compared with independent inversion results previously reported by Holland. Inverting reverberation from directional arrays is shown to give somewhat better quality results since, unlike standard line arrays, the reverberation data apply to only one bearing along the seafloor.

Long-range acoustic scattering from a shallow-water mud-volcano cluster

Analysis of reverberation measurements in the Straits of Sicily shows high intensity, discrete, scattered returns 10-20 dB above background reverberation. These returns are due to scattering from mud volcanoes. The reverberation from the mud volcanoes at ranges of 15-22 km is reasonably consistent over these spatial scales (i.e., kilometers) and temporal scales of several hours; measurements separated by 4 years are also similar. Statistical characterization indicates that the reverberation associated with a mud-volcano cluster is strongly non-Rayleigh and that the reverberation can be characterized by a single (shape) parameter, roughly independent of frequency. The non-Rayleigh statistics, with a concomitant increase in the probability of false alarm, indicate that mud volcanoes are a likely source of clutter. Mean target strengths were estimated at 1-11 dB over 160-1400 Hz and are consistent with target strengths measured during a different year at short (direct-path) ranges. Accumulated evidence points to small (order 10 m diameter and several meters high) carbonate chimneys on the mud-volcano edifice as the scattering mechanism as opposed to the edifice itself or scattering from gas bubbles in the water column. Thus, the results represent acoustic scattering from mud volcanoes in a quiescent state.

Monostatic and bistatic reverberation results using linear frequency‐modulated pulses

Estimates of reverberant scattering strength are presented as a function of range and bearing for selected monostatic and bistatic geometries of a reverberation experiment. Diffuse scattering strengths were found to be comparable (within ∼3 dB) for both monostatic and bistatic geometries. This experiment was performed off the west coast of Sardinia in approximately 2800 m of water during February of 1990. The receiver was a horizontal array of approximately 20 wavelengths. Source and receiver depths ranged from 50 to 120 m. The signals used consisted of linear frequency‐modulated (FM) pulses centered near 1 kHz with durations ranging from 8 to 16 s and bandwidths ranging from 32 to 64 Hz. Monostatic and bistatic scattering strength values are estimated using uncorrelated (i.e., not match‐filtered) pulses. The scattering strengths presented vary from −29 to −48 dB for ranges up to 40 km (but not data near fathometer returns). A method is described that shows different correlator gains against noise and reverberation. Comparisons of measured and modeled reverberation are presented using the generic sonar model (GSM) and curve fits with the decay in monostatic diffuse backscatter show good agreement with Lambert’s rule using a −32 dB constant. Finally, GSM is used to estimate grazing angles from which plots of measured scattering strength versus grazing angle in the 10°–26° interval are presented. Using this method, a Lambert’s rule constant of ∼−30 dB was a better fit to the data.

Bottom parameter extraction from long range reverberation measurements

From 1996 through 2000 the author participated with the SACLANT Undersea Research Centre in four Rapid Environmental Assessment (REA) trials, including experiments for bottom reverberation. SUS charges were used as sources. The receivers were horizontal arrays (four apertures) spanning a wide frequency range. Sets of nearly monostatic recordings were analyzed in frequency bands from 80 to 4000 Hz. Previous results by the author, [Berlin ASA meeting, 1999] showed polar plots of the beam time series superimposed on bathymetric charts, revealing a number of scattering features not on the charts and that directional reverberation measurements are a useful remote-sensing tool. A manual procedure was used in the first 3 sea trials to obtain bottom parameter estimates that could reproduce the reverberation decay for at-sea results. Recently this inversion scheme has been automated using a constrained simulated annealing (SA) algorithm giving more formal inversion results. In May of 2000 a Scientific REA experiment was conducted with SACLANTCEN, ARL/PSU, DREA and NRL near the Malta Plateau. Reverberation data from 100 Hz-4000 Hz was obtained using SUS. The horizontal array data are compared with the Generic Sonar Model (GSM) predictions for both the manual and automated inverse schemes. Results show a definite improvement using the automated technique.