Towed Array Research Articles

Method and apparatus for a non-oil-filled towed array with a novel hydrophone design and uniform buoyancy technique

A structure and method for constructing a non-oil-filled towed array providing a single cable entry point for each channel irregardless of the number of hydrophones used to make up a hydrophone group or array is described. The method and apparatus enables uniform buoyancy of the hydrophone array and the primary cable around which the hydrophone array is wound. Uniform buoyancy is achieved through the addition of hollow micro-spheres into a Reaction Injection Molded (RIM) polyurethane material used to mold the hydrophones. Additional buoyancy may be desired adjacent heavier cable sections where connectors and telemetry modules are located. An embodiment enables precise adjustment of hydrophone cable buoyancy by providing precise adjustment of the concentration of hollow glass micro-spheres in areas where more or less buoyancy is desired.

Supporting structure of hydrophones for towed array sonar system

A support structure of hydrophones for a towed array sonar system includes elastic members arranged in each sensor package and adapted to support a hydrophone received in the sensor package while attenuating vibrations transmitted to the sensor package. External vibrations, such as vibrations generated due to the running of a towing vessel or vibrations induced due to motions of a towing cable, which are irrespective of acoustic waves reflected by a target moving underwater, are attenuated by the hydrophone support structure during the transmission thereof from the sensor package to the hydrophones, so that the hydrophones are suppressed from vibrating due to the external vibrations. Accordingly, it is possible to reduce noise included in output signals of the hydrophones, thereby achieving an improvement in the sensing performance of the towed array sonar system.

Adaptive beamforming for nonstationary arrays.

Adaptive beamformers automatically adjust to the input data on the sensors in an attempt to maximize the bearing resolution of a signal and minimize the effects of noise or interfering signals. To the author's knowledge, all adaptive beamformers currently available in the literature assume that the sensors are stationary over the time required to collect the data. This assumption is invalid on arrays of sensors mounted on nonstationary platforms, and results in poor beamforming performance. In this paper we present an adaptive beamformer that has been designed to operate on nonstationary arrays. The beamformer directly incorporates any changes in array shape or heading that may occur during the time required to collect the data. The output of the adaptive beamformer is shown for both synthetic data and for real data collected on a towed array. Results show that signal detection, bearing accuracy, bearing resolution, and interference suppression all increase when the array shape and track are incorporated into the beamformer if the sensor platform is not stationary.

Automated two-dimensional passive tracking of free-ranging dolphins using two towed arrays and frequency-domain beamforming

Dolphin whistles form time-frequency contours that are complex, overlapping, and unpredictable. These features make automatic detection and localization of whistles difficult and labor-intensive whenever standard cross-correlation methods are used. A technique is presented for automatically extracting ranges and bearings of multiple free-ranging dolphins by post-processing acoustic data from two towed arrays. Each array had two elements spaced 2 m apart, and the distance between towed arrays was 350 m. The data are processed in the frequency domain, a natural framework for automation that also permits individual bearings to be extracted from whistles that overlap in the time domain. The frequency contours of the whistles are also automatically obtained as a byproduct of the analysis. Data from a February 2002 survey cruise off the Atlantic seaboard are used to illustrate the process. [Work supported by SE Fisheries, National Marine Fisheries Service.]

Passive three-dimensional tracking of sperm whales using two towed arrays during the 2001 SWAMP cruise

The capability to passively track multiple sperm whales simultaneously in three dimensions can provide useful information about the animals’ acoustic interactions, sound production mechanisms, and potential responses to anthropogenic noise. In 2000 this capability was demonstrated using a single towed array in the Gulf of Mexico, by exploiting the multipath information from surface and bottom returns. The relative paucity of bottom returns, and the difficulties involved in automatically detecting these returns, limited the utility of that method. In this presentation data collected during the 2001 SWAMP cruise in the Gulf of Mexico are used to explore how two arrays deployed serially behind a survey vessel can provide a means for automatically extracting all the relevent information required for 3-D tracking. Using the signal-processing program Ishmael, and segments of time where two whales are vocalizing simultaneously, it is shown that enough information can be extracted to track the animals. However, the problem complexity is increased by sound-speed refraction effects and the lack of information about the array depths. [Work supported by SE Fisheries, National Marine Fisheries Service, Minerals Management Service, and the ONR.]

1998–99 blue whale call abundance off the central California coast using a bottom-mounted hydrophone

Blue whale vocalizations were monitored off the central California coast during 1998–99 using the Naval Postgraduate Schools (NPS) Ocean Acoustic Observatory (OAO), a former US SOSUS array. The density of B calls were estimated using a matched filter detector based on a deverberated blue whale B call recorded from a towed array [T. C. Moore, Naval Postgraduate School, thesis (1999)]. The detectors’ performance characteristic was factored into the estimation to come up with the associated errors. Diurnal, seasonal, and inter-annular trends in call density were investigated. Also, the inter-call interval of the B call was measured. An unexpected large call density was observed during the first week of January in 1999. Historically, blue whales are sighted in Monterey Bay from approximately late July to October. Results of the blue whale call density estimation are discussed.

Geoacoustic inversion of tow-ship noise via near-field matched-field processing

The estimation of geoacoustic parameters of marine sediments and sub-sediment layers involves the measurement of acoustic fields in the ocean waveguide for comparison with accurate forward propagation models. This paper discusses self-noise geoacoustic inversion of tow-ship noise data acquired via a horizontal towed array from the standpoint of near-field matched-field processing (MFP). In particular, the sensitivity of the reconstructed source power to perturbations in the parameters of a short-range propagation model are quantified. Through simulations and experimental results, it is shown that even very quiet ships radiate sufficient noise power to enable self-noise inversion of basic geoacoustic parameters such as effective bottom velocity. The experimental results presented are particularly encouraging in view of the high level of interference shown to be tolerated from nearby shipping.

Range-dependent seabed characterization by inversion of acoustic data from a towed receiver array.

The MAPEX2000 experiments were conducted in the Mediterranean Sea in March, 2000 to determine seabed properties using a towed acoustic source and receiver array. Towed systems are advantageous because they are easy to deploy from a ship and the moving platform offers the possibility for estimating spatially variable (range-dependent) seabed properties. In this paper, seabed parameters are determined using a matched-field geoacoustic inversion approach with measured, towed array data. Previous research has successfully applied matched-field geoacoustic inversion techniques to measured acoustic data. However, in nearly all cases the inverted data were collected on moored, vertical receiver arrays. Results here show that seabed parameters can also be extracted by inverting acoustic measurements from a towed array of receivers, and these agree with those inverted using data received simultaneously on a vertical array. These findings imply that a practical technique could be developed to map range-dependent seabed parameters over large areas using a towed acoustic system. An example of such a range-dependent inversion is given using measurements from the MAPEX2000 experiments.

Turbulence and dispersion modeling near highways

Traffic-induced turbulence plays a dominant role in the dispersion of pollutants near highways. The formulations for velocity deficit and turbulence in vehicle wakes, developed from theoretical and physical modeling studies of Eskridge and his colleagues at US EPA about 20 years ago, are discussed. The vehicle wake parameterizations incorporated in ROADWAY-2, a near-highway pollutant dispersion model, and its evaluation results are described. The first field measurements of velocities and turbulence in the vehicle wake, using a towed array of 3-D sonic anemometers, are analyzed, and the results are presented and discussed. Specific recommendations are made for additional work in field measurements, laboratory studies, and mathematical model development and evaluation.

Subarray partitions of large aperture planar towed arrays

The current focus of passive detection and localization is in littoral regions where acoustic propagation becomes complicated by severe bottom interaction. The resultant high-transmission loss motivates the need for high-array gain for effective performance over long ranges. Large planar seismic arrays, with aperture dimensions upwards of 3 km×0.5 km, have potential to achieve high gain and good resolution when using matched field processing. In realistic environments, however, large arrays are suceptible to signal gain degradation mechanisms, particularly due to spatial decorrelation of the signal and non-stationary environments. One approach to overcoming this is partitioning the array. Subarray processing reduces stationarity requirements and extracts optimum coherent gain, thus achieving higher gain than that of smaller arrays. This work examines criteria for partitioning planar arrays to perform localization using MFP. Trade-offs between spatial resolution, array gain, and resilience to motion will be quantified and discussed. Performance of different subarray geometries will be presented using adaptive and conventional MFP. [Work sponsored by DARPA under Air Force Contract No. F1962800-00-C-0002. Opinions, interpretations, conclusions, and recommendations are those of the authors and are not necessarily endorsed by the Department of Defense. AB’s work was supported by ONR through the SECNAV CNO Chair.]

Adaptive beamforming of a towed array during a turn

During maneuvering, towed-array beamforming will degrade if a straight array is assumed. This is especially true for high-resolution adaptive beamforming. This problem can be reduced if a proper curved array is assumed. The tow vessel’s Global Positioning System (GPS) can be used to estimate this curvature, and this reduces the need for instrumentation in the array. Based on estimated array shape from GPS, both the conventional beamformer and the white-noise-constrained (WNC) adaptive beamformer are shown to track the source well. For calculating the steering vector in the WNC approach, a matrix inversion of the cross-spectral density matrix is involved. This matrix inversion can be stabilized by averaging the cross-spectral density matrix over neighboring frequencies.

Multi-platform sonar concepts for buried target detection and classification

Buried target classification is of paramount importance in mine countermeasures (MCM) applications. The Generic Oceanographic Array Technology Sonar (GOATS) project approaches this fundamental problem by exploiting the assets of autonomous underwater vehicles (AUVs). Recent developments in unmanned vehicle technology have opened the door for new sonar design concepts that may be applied to create alternative target detection and classification methods in complex environments. Adaptive vehicle behaviors provide the potential for the sonar system to adjust its geometry for optimal performance. The vehicles can also access very shallow water environments, which allows for more complete multi-aspect views of targets of interest than are possible with remote towed array systems. In this paper, it is demonstrated that the multi-platform system can provide improved detection and classification capability, while at the same time reducing the computational requirements. Data from prior GOATS experiments (1998 and 2000) are used to illustrate the effectiveness of the multi-platform sonar concepts, and real-time simulations are used to show the planned implementation of these techniques on-board the AUVs for GOATS’02.

Temporal domain processing for a synthetic aperture array

A method for the synthesis of an aperture with improved bearing resolution and signal gain is described. The proposed method temporally synthesizes data from an overlap correlator, which is obtained by aperture domain averaging of phase differences. Previous studies, such as extended towed array measurements (ETAM), had a restriction in that the overlapped hydrophones between successive measurements of a towed array were required to have identical positions in space. In this paper, however, it is shown that the phase correction factors can be estimated without restriction on the positions of the overlapped hydrophones. This implies that the proposed method is able to utilize more snapshots to extend the towed array. Simulation results showed that the proposed method resulted in higher estimation accuracy than ETAM. In addition, the effects of coherency and other systematic errors on the proposed method were examined.

ESTIMATED SOURCE LEVELS OF FIN WHALE (BALAENOPTERA PHYSALUS) VOCALIZATIONS: ADJUSTMENTS FOR SURFACE INTERFERENCE

AbstractWe recorded a series of 34 fin whale vocalizations using four calibrated hydrophones in a towed array. The whale's location was determined using arrival‐time differences of each call at the hydrophones. The recordings showed evidence of interference between signals propagating along direct and surface‐reflected paths (the “Lloyd Mirror”). We made preliminary estimates of source level (SL) using a spherical spreading model of transmission loss. We then applied a conservative correction for the effect of interference by subtracting the maximum possible signal enhancement from the preliminary SL estimate. These adjusted spherical spreading estimates had a median of 171 dB re 1 μPa at 1 m, with a range of 159–184 dB. These are the first SL estimates that explicitly attempt to correct for possible interference effects. We also propose a method that simultaneously estimates SL and source depth by fitting a Lloyd Mirror transmission loss curve to the measured received level, known receiver depth, and source‐to‐receiver range. Attempts to apply this method to the present data set yielded inconsistent results, probably because of the small number of hydrophones available. With more hydrophones in the interference field, this new method should produce reliable estimates of SL and depth of vocalizing whales.

Towed array history, 1917–2001

Towed array research and developments over 84 years is examined from the inventions of Dr. Harvey C. Hayes during World War I to multiple arrays towed by oil exploration ships of today. A time line of the effort over the 80 years is presented with a discussion of the driving technical and military reasons for system evolutions. Military surface ship towed arrays from Hayes electric eel to the AN/SQR-14, SURTASS, ETAS, and APL/Johns Hopkins dual towed arrays are reviewed. Submarine array programs are described from the early 1960s through today. A prediction of future towed array developments is presented.

Interpreting research results: Government regulation of anthropogenic noise sources

The Precautionary Principle, which advocates taking anticipatory management action in the absence of complete proof of harm, is embedded in several U.S. environmental laws, including the Marine Mammal Protection Act and the Endangered Species Act. However, in practice it is often ignored. Recent U.S. regulatory actions addressing the following anthropogenic noise sources are evaluated as cases in point: the North Pacific Acoustic Laboratory (NPAL), the U.S.S. Winston S. Churchill ship shock trials, and the Surveillance Towed Array Sensor System (SURTASS) Low Frequency Active (LFA) sonar. The impact of anthropogenic noise on marine mammals is poorly understood. Therefore, establishing adequate mitigation measures for noise sources is difficult. In an effort to increase knowledge and build toward certainty regarding the impacts of anthropogenic noise on the marine environment, recent research on low-frequency noise has focused on behavioral responses of large whales. These studies address limited hypotheses, but their results have been broadly interpreted to justify regulatory decisions. The pursuit of carefully designed research on the impacts of noise on the marine environment and the refinement of noise source mitigation is important, but interpreting research results is subject to political and economic pressures, casting doubt on the research conservation value.

Resolution of left–right ambiguity in towed array sonars

A method is proposed for resolving the left–right ambiguity in passive sonar systems, a problem that arises in source localization when the array is straight. In practice, the array is not straight and a statistical analysis within the Neyman–Pearson framework is developed for a monochromatic signal in the presence of noise, assuming that the exact array shape is known. For any given array shape, an expression for the probability of correct resolution (PCR) is derived as a function of two parameters: the SNR and an array-lateral-displacement parameter. The first parameter measures the strength of the signal relative to the noise and the second the curvature of the array relative to the acoustic wavelength. The PCR is calculated numerically for a variety of array shapes of practical importance. The model results are found to agree with intuition; the PCR is 0.5 when the array is straight and is increasing as the signal is becoming louder and the array more curved. It is explained why the method is useful in practice and the effects of correlation between beams are discussed. [Research partially supported by Thomson Marconi Sonar Ltd.]

ASSESSMENT OF 3-D EFFECTS IN THE KEY WEST LWAD EXPERIMENT

The role of 3-D effects has long been debated in the ocean acoustics community. Typically, 3-D problems are handled in an approximation that uses a fan of bearing lines to build up a 3-D image of the acoustic field with many 2-D slices. This is the so-called N by 2-D approach and it is probably safe to say it is adequate for simple calculations of the SONAR range-of-the-day. On the other hand, acoustic models are now finding an important role in matched-field processing (MFP) where they are integrated with the signal processing to correct for the channel effects. These applications demand much more from the models but, somewhat surprisingly, recent broadband experiments have revealed that the models are up to the task. The main features of the echo patterns are well-predicted by such models and can be reliably exploited, at least in areas where the ocean bottom is fairly flat. The challenge now is to push MFP to more realistic scenarios with complicated topography and potential 3-D effects. One phase of the December 1997 Key West LWAD (Littoral Warfare Advance Development) experiment was designed to address exactly this issue. We use this Key West scenario as a backdrop to discuss modeling approaches combining beams and modes and the impact of the resulting 3-D variation on MFP for towed arrays.

Model-based characterization of electromagnetic induction signatures obtained with the MTADS electromagnetic array

A sensor response model based on empirically determined orthogonal response coefficients /spl beta/ is presented for the analysis of electromagnetic induction (EMI) sensor data. The model is applied to high-quality survey data from the multi-sensor towed array detection system (MTADS) program. Data from both a prepared test range and a live-site survey are analyzed. At the test field, the authors show that the model successfully reproduces the measured signatures of ordnance, ordnance simulants, flat plates, and clutter. At the live site, fit statistics were collected showing that for the most abundant ordnance target, 81-mm mortars, the three beta response parameters are distributed log-normally. This allows for a simple method of discrimination. A receiver operating characteristic (ROC) analysis based on this discrimination method is performed and the results discussed.

Multisensor towed array detection system for UXO detection

The multisensor towed array detection system (MTADS) was designed to be an efficient, sensitive tool for the detection and characterization of buried unexploded ordnance. It comprises arrays of total-field magnetometers and time-domain electromagnetic induction (EMI) sensors, associated navigation and data acquisition hardware, and a custom data analysis system. The MTADS has conducted eleven demonstrations and surveys. The system has shown the ability to detect ordnance at its likely self-penetration depths with a probability of detection of 0.95 or better The model-derived positions and depths of the detected ordnance items are generally well within the physical size of the targets, making remediation much quicker and less costly than with standard techniques. Data sets corresponding to many of the MTADS surveys are available to others in the field.