Towed Array Research Articles

Observation of the Relationship between Ocean Bathymetry and Acoustic Bearing-Time Record Patterns Acquired during a Reverberation Experiment in the Southwestern Continental Margin of the Ulleung Basin, Korea

We observed a distinct drop-off region in the bearing-time record of acoustic reverberation data acquired from the south-western continental margin of the Ulleung Basin, East Sea, in the summer of 2015. 3 kHz continuous waves with pulse lengths of 0.1, 0.3, and 1.0 s were used as source pulses, with an R/V Cheonghae vessel towing a variable depth source and a triplet towed array toward the deep sea from shallow water. The observed pattern changed as the R/V Cheonghae moved across the continental slope further into the sea. This pattern arises as a result of the downward-refracted beams in the 1/2 convergence zone interacting with the soft bottom. In addition, the boundary of the drop-off region was modeled with the two-way maximum travel time of the first bottom-reflected rays using the bathymetry model of the General Bathymetric Chart of the Oceans, 2020. Some discrepancies were observed when comparing the modeled curve to the measured results, and the inaccuracy of the bathymetry model on the continental slope could be the main cause of these discrepancies. This pattern could be useful for bathymetry mapping, as well as estimations of source and receiver configurations.

Geoacoustic inversion for a 14-km autonomous underwater vehicle survey on the Malta Plateau

We consider signal processing and inversion of 1487 source instances recorded on a towed array along a 14-km seabed survey on the Malta Plateau. The data were acquired by autonomous underwater vehicle (AUV) and processed as reflection coefficients versus grazing angle and frequency. Data acquisition caused several artifacts that are studied by various data representations. These representations expose periodic fluctuations of reflection coefficients with angle and frequency that do not depend on seabed location. Averaging over source instances, frequencies, and angles reduces the artifacts. The Bayesian inversion assumes a one-dimensional seabed structure for each data set and is parametrized by an unknown number of homogeneous layers, sound velocities, densities, and attenuations. Results from individual inversions and sequential Monte Carlo sampling are compared. We demonstrate that removing data artifacts reduces over-fitting problems from previous considerations of the same data. Comparisons to piston and gravity core estimates, and separate wide-angle data show good agreement with the AUV results for two locations along the track. However, results at greater depths exhibit high uncertainty and strong influence of chosen prior boundaries. When considering results for all 1487 data sets, dipping and terminating layers are found along the track with high resolution (∼10 cm).

Target localization in range and depth from a passive linear towed array

Stealthily estimating the range and depth of a sound source from a submarine passive sonar is of capital tactical importance: it allows to adopt the appropriate escape or attack strategy towards the threat. The main difficulty in passive localization is that the transmitted waveform is unknown. Nevertheless, array processing can reveal some features of the acoustic field allowing localization, such as the arrival angles of the multiple paths, and the time delays between them. In this paper, we have compared two methods. The first one is a direct method, which simulates the field features from a set of possible source locations, and compares them to the measured features. The second one, an indirect method, uses the measured features to backpropagate the acoustic field. The former method has already been tested on simulated and real data in (Bennaceur et al., in OCEANS’18 MTS/IEEE, Charleston) and (Bennaceur et al., in OCEANS 2021, Singapore-U.S. Gulf Coast). Here, both methods are applied on real signals from a linear towed array, using bearings only and both bearings and time delays. The main advantage of these methods is that they are automatic in the sense that no sonar operator is required to estimate the source location.

Numerical investigation of the effect of convex transverse curvature and concave grooves on the turbulent boundary layer along a cylinder in axial flow

Axisymmetric turbulent boundary layers that develop around streamwise oriented long cylinder-like objects can be found in many applications, such as towed array sonars or marine seismic streamers. In many of these applications, turbulent fluctuations within the boundary layer flow can have a negative impact compared with laminar flow conditions. The aim of the present work is to design a surface modification that influences the turbulent boundary layer around a cylinder in axial flow in order to reduce turbulent fluctuations. To design the surface we consider recent findings regarding the turbulence damping effects of groove-like surface structures and combine these insights with the effect of convex transverse curvature on turbulence. We use large-eddy simulations to investigate the flow around a cylinder of modified design and around a reference circular cylinder. Both flows have a radius-based Reynolds number of Rea≈1.23⋅104. The modified design leads to a 20 % decrease in the average wall shear stress and results in local reductions in the turbulent intensities, Reynolds stress, the temporal velocity spectrum, and the turbulent dissipation rate. The analysis within the anisotropy-invariant space reveals a tendency towards flow relaminarization. However, the new design has no effect on turbulent pressure fluctuations. We provide suggestions on how to further improve the surface design to achieve even greater flow stabilization.

Optimal subarray selection strategy for long towed array based on target range

The larger the aperture of the towed array, the higher the array gain. However, when the aperture dimension is very large, the far field assumption may no longer hold. As such, simply using the plane wave beamforming method with the whole array to obtain the target signal will inevitably cause signal distortion. This article first proposes that in order to obtain less distorted signal, the coherence bandwidth of the beam-domain signal should be larger than the bandwidth of the source signal. Following, this paper analyzes the locating performance of the towed array. It is shown that the bearing estimation is more accurate than the range estimation. However, when the shape of the towed array is almost colinear, the range of the target can be approximately estimated. Hence, this paper puts forward a dynamic aperture choosing algorithm according to the target range, which dynamically selects the optimal subarray to perform appropriate beamforming method by trading-off between the array gain and the coherence bandwidth. The sea trial data verifies the feasibility of estimating the target range by long towed array when the tow ship moves slowly in a straight line. Simulation results show that this algorithm has better performance than previous methods when the targets are located at any range.

Model-based localization of deep-diving cetaceans using towed line array acoustic data.

Passive acoustic monitoring using a towed line array of hydrophones is a standard method for localizing cetaceans during line-transect cetacean abundance surveys. Perpendicular distances estimated between localized whales and the trackline are essential for abundance estimation using acoustic data. Uncertainties in the acoustic data from hydrophone movement, sound propagation effects, errors in the time of arrival differences, and whale depth are not accounted for by most two-dimensional localization methods. Consequently, location and distance estimates for deep-diving cetaceans may be biased, creating uncertainty in abundance estimates. Here, a model-based localization approach is applied to towed line array acoustic data that incorporates sound propagation effects, accounts for sources of error, and localizes in three dimensions. The whale's true distance, ship trajectory, and whale movement greatly affected localization results in simulations. The localization method was applied to real acoustic data from two separate sperm whales, resulting in three-dimensional distance and depth estimates with position bounds for each whale. By incorporating sources of error, this three-dimensional model-based approach provides a method to address and integrate the inherent uncertainties in towed array acoustic data for more robust localization.

TMA from Cosines of Conical Angles Acquired by a Towed Array

This paper deals with the estimation of the trajectory of a target in constant velocity motion at an unknown constant depth, from measurements of conical angles supplied by a linear array. Sound emitted by the target does not necessarily navigate along a direct path toward the antenna, but can bounce off the sea bottom and/or off the surface. Observability is thoroughly analyzed to identify the ghost targets before proposing an efficient way to estimate the trajectory of the target of interest and of the ghost targets when they exist.

The Fiber Optic Reel System: A Compact Deployment Solution for Tethered Live-Telemetry Deep-Sea Robots and Sensors.

Tethered deep-sea robots and instrument platforms, such as Remotely Operated Vehicles (ROVs) and vertical-profiling or towed instrument arrays, commonly rely on fiber optics for real-time data transmission. Fiber optic tethers used for these applications are either heavily reinforced load-bearing cables used to support lifting and pulling, or bare optical fibers used in non-load bearing applications. Load-bearing tethers directly scale operations for deep-sea robots as the cable diameter, mass, and length typically require heavy winches and large surface support vessels to operate, and also guide the design of the deep-sea robot itself. In an effort to dramatically reduce the physical scale and operational overhead of tethered live-telemetry deep-sea robots and sensors, we have developed the Fiber Optic Reel System (FOReelS). FOReelS utilizes a customized electric fishing reel outfitted with a proprietary hollow-core braided fiber optic fishing line and mechanical termination assembly (FOFL), which offers an extremely small diameter (750 μm) load-bearing (90 lb/400 N breaking strength) tether to support live high-bandwidth data transmission as well as fiber optic sensing applications. The system incorporates a novel epoxy potted data payload system (DPS) that includes high-definition video, integrated lighting, rechargeable battery power, and gigabit ethernet fiber optic telemetry. In this paper we present the complete FOReelS design and field demonstrations to depths exceeding 780 m using small coastal support vessels of opportunity. FOReelS is likely the smallest form factor live-telemetry deep-sea exploration tool currently in existence, with a broad range of future applications envisioned for oceanographic sensing and communication.

Tom and Thode’s excellent adventure: Practical guidance on surviving seedy bars, preparing for a DJ career, dismantling the U.S. Panama Canal zone, and other useful tips when working with towed acoustic arrays in the Eastern Tropical Pacific

Tom Norris used many acoustic tools in his research career, but it is safe to say that towed passive acoustic arrays were one of his specialties. In 1998 the National Marine Fisheries Service conducted the Stenella Population and Abundance Monitoring (SPAM) cruise. The NOAA ship Endeavor was to survey enormous dolphin populations in the Eastern Tropical Pacific (ETP), starting from Lima, Peru, and ending in Panama City, Panama. Jay Barlow, one of my PhD thesis committee members, permitted me to participate in the cruise as a member of his towed array passive acoustic monitoring team. The first time I ever met Tom, the acoustics team leader, was when he was carrying a surfboard into his cabin, and I wondered how in the world he had managed to ship a surfboard into Peru. It was the first of many times I have enjoyed both working with and wondering about Tom, and our first trip together taught me practical things they forgot to teach in graduate school. Here, I will review Tom’s tips on Mackie mixer boards, buying drinks in dangerous bars, tracking dolphin whistles, not antagonizing visual observers, and liberating furniture from the U.S. Panama canal zone.

Autonomous-underwater-vehicle-based marine multicomponent self-potential method: observation scheme and navigational correction

Abstract. Marine self-potential (SP) investigation is an effective method to study deep-sea hydrothermal vents and seafloor sulfide deposits. At present, one of the commonly used marine self-potential systems is a towed array of electrodes. Large noises are recorded when great changes in electrode distance and array attitude occur due to the complex seafloor topography. In this paper, a new multicomponent electrical field observation system based on an autonomous underwater vehicle (AUV) was introduced for the measurement of seafloor self-potential signals. The system was tested in a lake, and the multicomponent self-potential data were collected from there. Observed data involve the navigational information of the AUV, which could be corrected using a rotation transform. After navigational correction, measured data can recover the location of the artificial source using self-potential tomography. The experimental results showed that the new SP system can be applied to marine SP observations, providing an efficient and low-noise SP acquisition method for marine resources and environmental investigations.

Robust Wideband DOA Estimation Based on Element-Space Data Reconstruction in a Multi-Source Environment

In the presence of multiple sources, the performance of direction-of-arrival (DOA) estimation based on beam power maximization is susceptible to the energy leaking from the interference beams to the target beam, especially in the case that the signal of interest (SOI) is quite weak. To address this issue, a robust wideband DOA estimation method is proposed in this paper. Unlike those conventional high-resolution methods which mitigate the influence of energy leakage by reducing beamwidths and sidelobe levels, the proposed method achieves this by directly reconstructing the element-space data to approach the received hydrophone data of the single-source scenario containing only the SOI. Element-space data reconstruction (ESDR) for the SOI is achieved by removing the element-space waveforms of all the interference signals from the received hydrophone data. Moreover, an iterative algorithm is developed to adaptively extract the element-space waveform of each interference signal, without requiring the prior information of the array amplitude response coefficients and hydrophone coordinates of a distorted towed array. Simulation results show that the proposed method outperforms its counterparts in terms of estimation accuracy for a multi-source scenario. Meanwhile, the DOA estimation performance of the proposed method in a multi-source environment is close to that obtained by the beam power maximization method in the single-source scenario, even if the signal-to-interference ratio (SIR) is as low as −25 dB. At-sea experimental results prove that, even though the number of signal sources is as many as seven and the SOI is contaminated by real ocean ambient noise, the proposed method still achieves a better DOA estimation performance compared to existing state-of-the-art methods.

Conditioned measurement fused estimate Unscented Kalman filter for underwater target tracking using acoustic signals captured by Towed array

In the paper, an algorithm named Robust Unscented Kalman filter (R-UKF) is proposed to handle the popular ocean issue called underwater passive object/target tracking in a more efficient manner than the conventional algorithms. This R-UKF algorithm using acoustic signals supplied by a Towed array is developed when two novel Techniques named as Estimate Fusion (EF) Method and Measurement Conditioning (MC) Method are applied simultaneously to the existing Unscented kalman filter (UKF). Estimate Fusion method and Measurement Conditioning methods operate on the principles of weighted averaging of measurements in space and time respectively. EF Method contributed to the improvement by believing that the collective opinion about state estimation is much better than the individual opinions. This is accomplished by relying on Multiple sensors of TA and Multiple Intermediate estimators instead of single one. On the other hand MC method contributed to the improvements by application of the soothed measurements instead of traditional ones. The soothing is possible by weighted averaging of the current and track of past sensor data. Montecarlo simulations in Matlab(R2009a) shows that, R-UKF display better performance that its base algorithm UKF. Moreover, Optimal Performance (produce low estimation errors without demanding complex processors) of R-UKF makes it even more attractive.

Hybrid Unscented Kalman Filter with Rare features for Underwater Target tracking using Passive Sonar Measurements

A Novel estimator called as Hybrid Unscented Kalman Filter(HUKF) is developed in the paper to tackle the issue of passive target tracking in underwater scenarios using bearing-only measurements (captured by a towed array). As the name indicates the algorithm is a hybrid one obtained by combining three existing algorithms namely UKF, Integration technique and Pre-processing mechanism to yield much better performance than any of them individually. The sensor noise reduction in spatial measurements caused by integration technique together with noise reduction in time measurements caused by Pre-processing mechanism in successive time iterations cumulatively contribute to the improvement in performance. Moreover the montecarlo simulations in Matlab(R2009a) provide evidence that, HUKF also display attractive features like Optimal Performance(produce less estimation errors at less computational complexity), Efficient Tracking in critical conditions(long ranges/severe noise) and Non-Divergence even with nasty initialization. Lastly the algorithm can be helpful in promoting TA usage on regular basis despite of the associated maneuvering issues.

Numerical prediction of flow noise levels on towed sonar array

Flow noise originating in the turbulent boundary layer (TBL) often severely limits the performance of towed sonar array. Therefore, it is necessary to predict this noise for the design of an efficient towed array. This paper presents large eddy simulation methodology to establish the TBL properties and wall pressure fluctuations on a 12 m long towed array with length to diameter ratio of 1200 in the operating tow speed range of 2 to 5 knots in water. The computed flow noise levels are compared with experimental measurements available in the literature successfully. The effectiveness of scaling the flow noise spectra with the diameter and tow speed is discussed, and non-dimensional wall pressure spectra presented with respect to non-dimensional frequency. The overall sound pressure levels are also compared with experimental data that show good accuracy achieved by the proposed numerical methodology.

On Robust Beamforming for Distorted Towed Linear Array Using Product Theorem

To solve the beamforming performance degradation problem of the towed linear array caused by the shape distortion, we propose a robust beamforming way for distorted towed linear array. The proposed method basically uses the segmented processing, which can be divided into 3 steps. Firstly, towed linear array is divided into sub-arrays. Then, the weights of sub-arrays are obtained and outputs of beamforming of sub-arrays are obtained. In the end, according to the principle of the product theorem, the correlation calculations are performed on the output of the sub-array. Due to the adoption of sub-array correlation calculations, the proposed method can upgrade beamforming performance. Compared with the traditional distortion array processing method, the proposed method can improve the local signal-to-interference plus noise ratio (SINR) by about 2–3 dB. Simulation and sea trial data verify the effectiveness of the proposed method.

Identification and Removal of Non-acoustic Noise in Towed Array Sonar Using F-Κ Transform for Enhanced Torpedo Detection

Low frequency passive towed array sonar is an essential component in a torpedo detection system for surface ships. Compact towed arrays are used for torpedo detection and they will be towed at higher towing speeds compared to conventional towed array sonars used for surveillance. Presence of non-acoustic noise in towed array sensors at higher towing speeds degrades torpedo detection capability at lower frequencies. High wavenumber mechanical vibrations are induced in the array by vortex shedding associated with hydrodynamic flow over the array body and cable scope. These vibrations are known to couple into the hydrophone array as nonacoustic noise sources and can impair acoustic detection performance, particularly in the forward end fire direction. Lengthy mechanical vibration isolation modules can isolate vibration induced noise in towed arrays, but this is not recommended in a towed array which is towed at high speeds as it will increase the drag and system complexity. An algorithm for decomposing acoustic and non-acoustic components of signals received at sensor level using well known frequency-wavenumber transform (F-K transform) is presented here. Frequency-wavenumber diagrams can be used for differentiating between acoustic and non-acoustic signals. An area of V shape is identified within the F-K spectrum where acoustic energy is confined. Energy outside this V will highlight non-acoustic energy. Enhanced simultaneous spatio-temporal and spatio-amplitude detection is possible with this algorithm. Performance of this algorithm is validated through simulation and experimental data.

Three-dimensional dislocations in a uniform linear array's isotropic sensors-Direction finding's hybrid Cramér-Rao bound.

The linear array's one-dimensional spatial geometry is simple but suffices for univariate direction finding, i.e., is adequate for the estimation of an incident source's direction-of-arrival relative to the linear array axis. However, this nominal one-dimensional ideality could be often physically compromised in the real world, as the constituent sensors may dislocate three-dimensionally from their nominal positions. For example, a towed array is subject to ocean-surface waves and to oceanic currents [Tichavsky and Wong (2004). IEEE Trans. Sign. Process. 52(1), 36-47]. This paper analyzes how a nominally linear array's one-dimensional direction-finding accuracy would be degraded by the three-dimensional random dislocation of the constituent sensors. This analysis derives the hybrid Cramér-Rao bound (HCRB) of the arrival-angle estimate in a closed form expressed in terms of the sensors' dislocation statistics. Surprisingly, the sensors' dislocation could improve and not necessarily degrade the HCRB, depending on the dislocation variances but also on the incident source's arrival angle and the signal-to-noise power ratio.

A Depth-Bistatic Bottom Reverberation Model and Comparison with Data from an Active Triplet Towed Array Experiment

In this paper, a depth-bistatic bottom reverberation model that employs the ray theory is presented. The model can be applied to an active towed array in the ocean. The reverberation time series are modeled under the depth-bistatic assumption and their Doppler shift is calculated based on the actual source–receiver geometry. This model can handle N × 2D range-dependent bathymetry, the geometry of a triplet array, and the Doppler motion of the source, targets, and receiver. The model predictions are compared with the mid-frequency reverberation data measured by an active triplet towed array during August 2015 in the East Sea, Korea. These data are collected with a variable depth source at mid-frequency and the triplet line array in a deep-water environment. Model predictions of the beam time series and its spectrogram are in good agreement with the measurement. In particular, we discuss the effects of the source and receiver depths on the reverberation in deep water observed in both the measured and modeled results.

Design of an adaptive noise canceller for improving performance of an autonomous underwater vehicle-towed linear array

Noise radiated from an autonomous underwater vehicle (AUV) is one of the major sources of interference that impact the acoustic performance of its towed array sensor, especially during passive detection under low ambient noise conditions. In this paper, an adaptive noise canceller (ANC) based on the partitioned fast block least-mean-square adaptive algorithm is designed and applied to mitigate the AUV-radiated noise to improve the detection performance of a linear array towed by an AUV platform. The gain, defined as the ratio of the output signal-to-noise ratio (SNR) to the input SNR, is taken as the metric for our ANC. Results obtained from processing the experimental data indicate that the gain of the ANC is up to 20 dB.

Joint towed array shape and direction of arrivals estimation using sparse Bayesian learning during maneuvering.

Large aperture towed arrays are widely used underwater to detect weak targets. During maneuvering, the beamformer performance degrades significantly if a wrong array configuration is assumed. Currently, engineering sensors and/or (augmented) acoustic sources are used to estimate the array element positions. The results are often inadequate depending on the number of measurements available. In this paper, an adaptive bow (AB) sparse Bayesian learning (SBL) algorithm is proposed, called ABSBL. Assuming the towed array follows a parabola shape during slow turns and treating the array bow as a hyperparameter in SBL, the bow and directions of arrival (DOAs) of the signals can be jointly estimated from the received acoustic data. Simulations show that ABSBL yields accurate estimates of the bow and target DOAs if the turning direction is known. ABSBL is applied to the MAPEX2000 data. The estimated array bow and DOA agrees with that estimated from relative time delays measured from acoustic pings and SBL, better than that estimated from the GPS data using the water-pulley model. The method can potentially be applied without engineering sensors.