Towed Array Research Articles

A note on observations of cetaceans in the Indian Ocean Sanctuary, Australia to Israel, April 1995

A forty-day voyage from Hobart (Australia) to Haifa (Israel) included a visual and acoustic census for cetaceans in the Indian Ocean Sanctuary. One hundred and sixty-three sightings were made, 156 occurring within the Sanctuary. Twelve species were identified. Sperm whales (Physeter macrocephalus) were encountered most frequently (51% of identified encounters), whilst spinner dolphins (Stenella longirostris) were numerically dominant. Other species identified included bottlenose dolphins (Tursiops spp.), short-beaked common dolphin (Delphinus delphis), pan-tropical spotted dolphin (Stenella attenuata), rough-toothed dolphin (Steno bredanensis), Risso’s dolphin (Grampus griseus), killer whale (Orcinus orca), short-finned pilot whale (Globicephala macrorhynchus), false killer whale (Pseudorca crassidens), Bryde’s whale (Balaenoptera edeni) and Cuvier’s beaked whale (Ziphius cavirostris). The most sightings occurred WNW of the Seychelles, east of Somalia, and in the Gulf of Aden and Red Sea. Half of the sperm whale sightings were to the east of Ras Hafun (Somalia), and included adults, subadults and at least one calf. Thirteen hours were spent listening for cetaceans using a towed array whilst the ship was underway. Cetaceans were detected at 78% of the listening stations, with a possible four species recorded (sperm whale, spinner dolphin, pilot whale, bottlenose dolphin). The survey shows the value of platforms of opportunity for studying the pelagic communities of cetaceans in the Indian Ocean Sanctuary. It highlights the need for further research in the northwestern sector where anthropogenic threats are varied and increasing.

An end-capped lead zirconate titanate broadband hydrophone theoretical calculation and electroacoustic measurement for towed array applications

Hydrophone transducers for towed array applications require low self-noise, small form factor and a linear frequency response below 50 kHz for general purpose long-range ocean sensing. The design specification for this hydrophone application has several aspects including acoustic sensitivity, frequency response, dissipation factor, and physical size. A comprehensive noise model informs filter design for pre-amplifiers after the piezoelectric element. Theoretical calculation of pressure to voltage conversion is made here by estimating the Free-Field Voltage Sensitivity (FFVS) using methods from R.A. Langevin and G.W. McMahon. Using these methods to achieve the desired design specification, we use two cylindrical ceramics with a split electrode poled in the radial direction with series stacked ceramics using end caps with air backing to increase sensitivity and reduce overall size. An experiment is carried out to measure the resonant modes using a simple impedance measuring circuit which then can be used to estimate FFVS linearity. These results are compared with theoretical calculation for hydrophone response of resonant modes and results from measurement circuit.

Fiber Optic Hydrophones for towed array applications

In this paper, we report the design and development of a fiber optic hydrophone for underwater applications. The sensing configuration is based on a Michelson interferometer where a fiber coil is wrapped around a compliant mandrel acting as a sensitive element. The acoustic wave leads to a deformation of the mandrel that is transferred into strain in the optical fiber. Two Fiber Bragg gratings are used at the terminal ends of the fiber coil to enable the fiber elongation measurement. A similar “dummy” hydrophone is used as reference. The sensor was designed to be sensitive to sea state zero. The fabricated hydrophones were characterized in an instrumented tank. An acoustic source was used to generate acoustic tones in the frequency range 3 ÷ 25 kHz. A reference piezoelectric hydrophone was used to record the underwater acoustic waves in the tank. The hydrophone exhibited a responsivity of about 19 nm/Pa in the frequency range 3 ÷ 10 kHz allowing a resolution of 0.3 mPa/Hz1/2. The linearity and directivity of the sensors were carefully investigated. The fiber optic hydrophone was used as a basic building block to conceive and fabricated a towed array with five elements. The multiplexed hydrophones in the towed array were simultaneously interrogated and the responsivity of the hydrophones were retrieved. Beyond the good repeatability in terms of hydrophone responsivity, the tests in the tank confirm the capability of the fiber optic technology to offer technological solutions for underwater operation with potential advantages in terms of lightness and small-size for use in unmanned underwater vehicles.

A wave glider-based, towed hydrophone array system for autonomous, real-time, passive acoustic marine mammal monitoring.

An autonomous surface vehicle known as a wave glider, instrumented with a low-power towed hydrophone array and embedded digital signal processor, is demonstrated as a viable low-noise system for the passive acoustic monitoring of marine mammals. Other key design elements include high spatial resolution beamforming on a 32-channel towed hydrophone array, deep array deployment depth, vertical motion isolation, and bandwidth-efficient real-time acoustic data transmission. Using at-sea data collected during a simultaneous deployment of three wave glider-based acoustic detection systems near Stellwagen Bank National Marine Sanctuary in September 2019, the capability of a low-frequency towed hydrophone array to spatially reject noise and to resolve baleen whale vocalizations from anthropogenic acoustic clutter is demonstrated. In particular, mean measured array gain of 15.3 dB at the aperture design frequency results in a post-beamformer signal-to-noise ratio that significantly exceeds that of a single hydrophone. Further, it is shown that with overlapping detections on multiple collaborating systems, precise localization of vocalizing individuals is achievable at long ranges. Last, model predictions showing a 4× detection range, or 16× area coverage, advantage of a 32-channel towed array over a single hydrophone against the North Atlantic right whale upcall are presented for the continental shelf environment south of Martha's Vineyard.

Applied Geophysics for Managed Aquifer Recharge.

Increasing water stress and decreasing supplies caused by growth and climate variability have expanded demand for managed aquifer recharge (MAR) projects to provide water supply resilience. Some of the most important factors in determining the performance of a MAR project include site selection, subsurface hydrogeologic characteristics and associated properties of the storage zone. Costs for invasive subsurface investigations to address these factors have slowly increased over the past two decades, with drilling costs increasing dramatically by as much as 30% or more since COVID-19 hit, a result of supply chain issues, steel prices, and manpower challenges. This paper provides a high-level review of major geophysical methods that have become more mainstream over the past decade or two to supplement invasive subsurface investigations and are very cost effective when compared to drilling boreholes and installing wells, which provide only point data. The more commonly used surface geophysical methods include ground-based and airborne time-domain electromagnetic methods (TEM), electrical resistivity, and seismic reflection. Airborne TEM methods (AEM) collect data very quickly, avoiding ground-based access constraints, and land-based methods are especially efficient using towed arrays. Electrical resistivity measurements provide resolution comparable to TEM but require more time than towed methods. Seismic reflection surveys are more expensive than other methods but typically have a much greater depth of penetration and can provide high resolution information on aquifer geometry, geology, and faults. Borehole geophysics is one of the more common methods used in MAR, providing near hole formation data and ground truths surface geophysics.

Observability Criteron Guidance for Passive Towed Array Sonar Tracking

Nonlinear sonar contact tracking always presents a significant observability challenge. Observability as it relates to sonar tracking is the metric used to determine if a state estimate is uniquely identifiable given a history of measurements. Poor observability is troublesome to all nonlinear tracking filters to varying degrees. Observability criteria are used to assess the degree of observability and many observability criteria require, in some way, an observer maneuver. But when observability criteria are evaluated with real data they are evaluated in the presence of “micromaneuvers” due to unmodeled forces. The issue associated with these micromaneuvers is that they satisfy, mathematically, observability criteria but hold little practical value for obtaining trustworthy state estimates. This article chooses a recent observability criterion and examines its utility for passive sonar tracking with a towed array. The analysis performed provides guidance to all similar observability criteria when applied to the passive sonar tracking with a towed array and implies such criteria will be most valuable in applications with minimal navigation perturbations.

Fault Detection and Prognostic Health monitoring of Towed array sonars

Sonars are used to detect underwater targets and are especially important in maintaining naval superiority. Towed array sonars can operate at very low frequencies thus giving larger ranges and can be deployed to any desired depth of operation. Towed array sonars offer long range surveillance capability and is the sensor of choice for sustained surveillance operations. Reliable operation and maintenance of towed array sonars need effective methods of health monitoring and reliability prediction. For any prognostic health monitoring to be done we need to identify certain parameters which can be observed and will give system health status in the present condition. This paper proposes some metrics which are easily measurable in-situ and which offer insights into the health of the sonar system. These metrics give direct measureable impact for each of the failure modes and offer insights into the current health of an operational towed array sonar. Simulation results are shown to demonstrate the effectiveness of the proposed metrics and detailed trial data results from different towed array trials are analysed to validate them in operational scenarios.

Spatial Spectral Enhancement of Broadband Signals in a Towed Array Using Deconvolved Subband Peak Energy Detection

Conventional energy detection is a robust method that is usually applied to underwater broadband acoustic signal processing for towed arrays. Due to its low resolution, the weak target detection performance of conventional energy detection is severely degraded in shallow sea environments with strong acoustical reverberation. Subband peak energy detection is an effective method to improve the display resolution of conventional energy detection. However, subband peak energy detection produces false alarms due to the presence of high sidelobe levels. In order to improve the underwater target detection performance, a deconvolved subband peak energy detection method for towed arrays is proposed in this paper. Compared with conventional beamforming, minimum-variance distortionless response with forward–backward averaging and diagonal loading algorithm and subband peak energy detection, the proposed method could robustly provide higher-resolution results and suppress the fake peaks induced by subband peak energy detection. The performance of the proposed method was evaluated with simulation results, and the sea experimental data processing results show that the proposed method is effective in engineering applications.

Tow ship interference mitigation in shallow ocean using correlated mode decomposition technique

Passive towed arrays assume great importance in shallow ocean surveillance, owing to their capability to detect long range underwater acoustic targets. The detection and classification performance is severely impacted by the tow ship interference manifestation in the pressure sensor array towed behind the ship. In this work, a robust tow ship multipath interference mitigation algorithm intended for shallow ocean operation is developed, based on the dynamic mode decomposition (DMD) technique, which in turn exploits the coherent structure of the multimode acoustic wave propagation inherent in the shallow ocean scenario. The proposed algorithm effectively identifies the coherent normal mode interference subspace and eliminates its components from the received array data vector. Further, this approach compliments the shallow ocean normal mode acoustic field propagation and ensures simultaneous mitigation of all the multipath generated interferences, making it a superior method to reduce the azimuth spread of the interference and tow ship spectral leakage into the panoramic detection beams. This greatly enhances the detection and classification capability of sonar, especially for distant stealthy targets. The efficacy of the algorithm is validated using Monte Carlo simulations and further ratified by experimental data captured during the field trials in the Arabian Sea.

The effects of array design on acoustic data collected during marine seismic reflection surveys

Marine seismic reflection surveys provide an abundance of acoustic data that are potentially useful for an array of analyses within and in addition to geoacoustic studies. A single cruise typically produces thousands to tens of thousands of acoustic events, with hundreds of hydrophones recording each event over towed arrays that can span up to 15 km or more. However, the structure of the airgun source arrays and the receiver arrays is typically not obvious from the data alone and the pre-processing of the data may yield misleading results if not properly accounted for. Generally, the acoustic source consists of an array of airguns configured such that the acoustic energy is focused towards the ocean floor, significantly impacting the relative intensity of the direct and reflected paths. Additionally, each array channel output typically consists of multiple hydrophone outputs, which are not individually available, averaged to generate a single channel output. Understanding the impact of these constructions is crucial for many analyses, such as mitigation of airgun pulse impacts on marine mammals. Here, we quantify the impact that these factors have on the output data and provide an analysis of this influence for an experimental case. [Work supported by ONR.]

Left–Right Ambiguity Resolution Methods for Closely Spaced Arrays

One linear array with omnidirectional sensors cannot discriminate between the azimuth angle and its complementary angle due to axisymmetry about the array axis. This article examines solutions to the bearing ambiguity problem known as left–right (LR) ambiguity resolution using two closely spaced parallel arrays. We design a second-order nulling (SoN) beamformer to produce a cardioid-type spatial response with a null electronically steered to the complementary angle of the direction of interest. Three methods—namely, SoN, null constrained beamformer (NCB), and 2-D direct beamformer (2-DDB), are studied. The performance is evaluated in correlated noise conditions modeled by a first-order auto-regressive process. This work establishes analytical results for the bound on the detection gain (DG) of the two parallel arrays in terms of the Rayleigh quotient of the circulant noise correlation matrix. A closed-form expression is developed for estimating the loss due to cardioid processing at closer inter-array spacing. The expressions are validated based on the deflection criterion. Twin-array processing using the 2-DDB method results in a higher DG, while cardioid-based beamformers give a higher rejection ratio (RR). Data from an at-sea experiment using a towed array are analyzed. Field results show that the deeper null achieved by the cardioid method results in a higher RR compared to 2-D direct beamformer. From the field data, approximately 19-dB LR discrimination is demonstrated at 1650 Hz for the broadside. Experimental results from a towed array corroborate theory and simulations.

Use of towed array reverberation data for rapid environmental assessment

Towed array reverberation beam time series can provide information about the underwater acoustic environment by providing a snapshot of the scattering in both range and bearing. By mapping time into range and beam angle into azimuth, John Preston pioneered the use of polar plots to survey an area [Preston et al., J. Acoust. Soc. Am., 87,119–134 (1991)]. When the polar plot is superimposed on the bathymetry of an area, a scattering map can be made. High scattering is generally associated with bottom features. With additional analysis and modeling, bottom loss and scattering strengths can be obtained [Preston and Ellis, J. Marine Systems 78, S359–S371 (2009)]. This talk emphasizes results from the rapid environmental assessment (REA) Rapid Response exercises 1996–1998, a multi-nation collaboration organized by NATO MILOC (military oceanography). While transmission loss experiments along a single radial could take hours, a single charge dropped near the towed array gave information on all radials in just minutes. Anomalies could be immediately identified, for investigation by more precise techniques. The REA exercises attracted interest and led to the Boundary Characterization and Clutter Joint Research Projects between CMRE (the NATO Centre for Maritime Research and Experimentation), Canada, and US. [Work supported by ONR.]

Pacific Echo: A deep ocean collaborative experiment

This paper summarizes experiments carried out using towed line arrays in the Pacific Echo sea trials. Although John Preston was not a participant, the experiments are examples consistent with his efforts in promoting collaborative research at sea with horizontal arrays. The overall goal of Pacific Echo was to study the impact of thin sediment ocean bottom environments on sound propagation in deep water. The objective of the experiments described here was to study the evolution of young oceanic crust. The hypothesis was that the sound speed of young basalt increased with age of the crust. Sound speed was inferred from low frequency measurements of the reflection coefficient versus grazing angle at sites of increasing distance from the deep ocean spreading center. The experiments introduced a novel design for measuring the reflection coefficients using two ships and small explosive charges. The signals at the array were spatially filtered to resolve the specular reflections as the ships opened range on set courses. The data provided estimates of both the compressional and shear wave speeds of the basalt. Results from the broadside reflection technique showed that sound speed increased with crustal age and were consistent with measurements obtained from conventional seismic reflection surveys.

The legacy of the five octave research array and its impact on future towed array development

The five octave research array (FORA) was developed and implemented in support of the Office of Naval Research (ONR) Ocean Acoustics program’s experimental efforts. Dr. John R. Preston spearheaded planning, development, and experimental operations of this well-known acquisition system. Planning and development of the system took place between 1997 and 2002, while engineering assessment and array enhancements continued through 2004. From initial delivery to the Penn State Applied Research Laboratory to the present, the FORA made possible over 20 very successful sea trials. These trials were primarily in the interest of oceanographic and underwater acoustics, as well as signal processing research. The data collected, with John at the helm, fostered hundreds (if not thousands) of scholarly works, while John’s collaborative nature allowed an enormous number of researchers and students to benefit from the data and experience in at-sea experimentation. One of John’s final contributions was to renew planning and discussions for a replacement research array to continue in place of the aging FORA, which was becoming less dependable and increasingly expensive to maintain. This talk will discuss the development and history of the FORA, its involvement in a selection of collaborative ONR trials, and the FORA replacement acquisition system.

Environmentally constrained modeling of mid-frequency transmission loss and reverberation measured during the Target and Reverberation Experiment 2013

The goal of the Target and Reverberation Experiment (TREX13) was to make contemporaneous measurements of mid-frequency (1.5–4 kHz) transmission loss and reverberation with extensive environmental measurements so detailed model/data comparison can be achieved and important environmental factors can be identified for different applications. APL-UW collaborated with ARL-Penn State, led by John R. Preston, to deploy the Five Octave Research Array (FORA) on a “clothesline” about 2.1 m above the seafloor. This fixed-source/receiver configuration helped eliminate uncertainties from the motion of a towed array and allow reverberation measurement along a narrow, 7-km-long section of seafloor. The experiment site had a fairly complex spatial distribution of both sediment type and sediment scattering properties and keeping the FORA in a fixed position was instrumental in understanding and modeling the reverberation. While soft sediments make up only 27% of the sediments by area at the site, it is necessary to account for this spatial dependence so that both transmission loss and reverberation can be modeled using a consistent set of environmental inputs. These models and their implications for our understanding of the environmental factors which most impact mid-frequency reverberation will be discussed. [Work supported by the U.S. Office of Naval Research.]

Left-Right ambiguity resolution for coprime sparse arrays.

Coprime Sparse Arrays (CSA) with two collocated uniform linear arrays are used to estimate O(MN) direction of arrivals for a narrow band source with only O(M+N) sensors. The CSA cannot discriminate the azimuth and its complementary angles due to the symmetry along the array-axis. The bearing ambiguity, also referred to as a Left-Right (LR) ambiguity, is addressed in this paper using CSA. We design Left-Right resolved Coprime Sparse Arrays (LRCSA) to produce a null that is electronically steered to the complementary angle of the desired direction. LRCSA twin array achieves higher spatial resolution than the conventional fully populated LR array with same number of sensors, much like the collocated CSA. Three methods, namely, Null Constrained Beamformer (NCB), two-dimensional Direct Beamformer (2DDB), and LRCSA are studied. The bound on the detection gain is analytically established for the LRCSA under correlated noise scenarios modeled by the first-order autoregressive process. Twin array processing using the LRCSA method results in narrow mainlobe beam width along with a higher rejection ratio (RR) compared to 2DDB and NCB. Data obtained from a sea experiment using a towed array is analyzed. The results confirm that the CSA based cardioid processing resolved LR ambiguity along with a higher RR and narrower beams.

Underwater target tracking in three-dimensional environment using intelligent sensor technique

PurposeThis paper aims to target tracking in the marine environment is typically obtained by considering the measurement parameters like frequency, elevation and bearing. Marine environmental surveillance provides critical information and assistance for the exploitation and maintenance of marine resources.Design/methodology/approachWith the use of intelligent sensor techniques like Hull-mounted and towed array sensors, convenient, precise and dependable three-dimensional (3D) underwater target tracking is introduced.FindingsThis research investigates a method to develop a reliable Unscented Kalman Filter (UKF) algorithm for enhanced underwater target tracking in a 3D scenario by using bearing, frequency and elevation measurements. In applications for underwater target tracking, uncertainty and inaccuracies are typically described by using Gaussian additive noise.Originality/valueThe proposed UKF algorithm is tested and analyzed using 100 Monte Carlo simulations with the Gaussian generated noise.

A Nonlinear Data-Driven Towed Array Shape Estimation Method Using Passive Underwater Acoustic Data

Large-aperture towed linear hydrophone array has been widely used for beamforming-based signal enhancement in passive sonar systems; however, its performance can drastically decrease due to the array distortion caused by rapid tactical maneuvers of the towed platform, oceanic currents, hydrodynamic effects, etc. In this paper, an enhanced data-driven shape array estimation scheme is provided in the passive underwater acoustic data, and a novel nonlinear outlier-robust particle filter (ORPF) method is proposed to acquire enhanced estimates of time delays in the presence of distorted hydrophone array. A conventional beamforming technique based on a hypothetical array is first used, and the detection of the narrow-band components is sequentially carried out so that the corresponding amplitudes and phases at these narrow-band components can be acquired. We convert the towed array estimation problem into a nonlinear discrete-time filtering problem with the joint estimates of amplitudes and time-delay differences, and then propose the ORPF method to acquire enhanced estimates of the time delays by exploiting the underlying properties of slowly changing time-delay differences across sensors. The proposed scheme fully exploits directional radiated noise targets as sources of opportunity for online array shape estimation, and thus it requires neither the number nor direction of sources to be known in advance. Both simulations and real experimental data show the effectiveness of the proposed method.

Are dolphins modulating whistles in interspecific group contexts?

ABSTRACT Cetaceans depend highly on acoustic signals, such as whistles, for social communication. Mixed-species associations (MSAs), temporary or long-term encounters between different species, may play an important role on the acoustic dynamics of the species involved. Because of whistles’ communication function, we used these sounds to investigate the acoustic relationships of Tursiops truncatus when involved in different group contexts: in MSAs with two other delphinid species and in single species group. Acoustic recordings of T. truncatus single species groups and in associations with Globicephala melas and Grampus griseus were collected in the Western South Atlantic Ocean through a towed array of hydrophones. A total of 1267 whistles were analysed and acoustical parameters were extracted. To verify the differences among whistles produced in such contexts a support vector machine and random forest analysis were implemented. Both analyses revealed a clear separation of whistles from the single species versus the MSA as well as between both MSAs. The results indicate that interspecific associations may influence the whistle structure and suggests that T. truncatus whistles can be modified during interspecific interactions. These findings are useful to elucidate the possible factors underlying behavioural plasticity and interspecific associations and to understand dolphins’ acoustic communication.

Surface Target Tracking Using Towed Array Sonars With Direct and Bottom Bounce Underwater Sound Signals

Surface Target Tracking Using Towed Array Sonars With Direct and Bottom Bounce Underwater Sound Signals