Broadband Sonar Signals Research Articles

Waveguide Invariant-based mitigation of reverberation and interference in active and passive sonar

Multipath propagation poses many challenges for broadband sonar signal processing in underwater acoustic channels. The waveguide invariant (WI) provides a useful relation between the temporal and frequency fading characteristics in shallow-water channels that can been exploited to improve broadband target detection in both active and passive sonars. In active sonar, the WI motivates an alternative to conventional constant-false-alarm rate (CFAR) normalization of the background by averaging over observed time-frequency striations to estimate the reverberation level. A WI-based generalized likelihood ratio test is reviewed and shown to outperform conventional cell-averaged and frequency-invariant CFAR detectors in both simulation and with real Mediterranean data. In passive towed-array sonar, targets near endfire are often masked by angularly spread broadband multipath interference whose statistics must be estimated with limited training data for effective suppression. A WI relationship characterizing the frequency-dependence of wavenumber differences permits averaging of narrowband array snapshots to form a low-rank broadband “focused” covariance matrix. Simulation experiments in a realistic ocean environment are reviewed showing that adaptive beamforming using WI focused covariance matrices can provide a significant array gain improvement over conventional adaptive methods with limited observation time. Finally, the potential for waveguide invariant-based blind source separation is discussed. [Work supported by ONR.]

Analysis of the temporal structure of fish echoes using the dolphin broadband sonar signal

Behavioral experiments indicate that dolphins detect and discriminate prey targets through echolocating broadband sonar signals. The fish echo contains components from multiple reflections, including those from the swim bladder and other organs, and can be used for the identification of fish species and the estimation of fish abundance. In this paper, temporal structures were extracted from fish echoes using the cross-correlation function and the lowpass filter. First, the echo was measured from an anesthetized fish in a water tank. The number, reflector intensity, and echo duration were shown to be dependent on the species, individual, and orientation of the fish. In particular, the echo duration provided useful information on the fish body height and for species identification. Second, the echo was measured from the live fish suspended by nylon monofilament lines in the open sea. It was shown that this duration could be estimated regardless of whether or not the fish were moving.

Measuring the target strength spectra of fish using dolphin-like short broadband sonar signals

Dolphins identify their prey using broadband sonar signals. The broadband spectrum of the target strength (TS) of fish is believed to be a key factor in target discrimination. In this study, the TS spectrum was measured using sonar signals generated by two different dolphin species: finless porpoise and bottlenose dolphin. First, the broadband form functions of a tungsten carbide sphere and a copper sphere were measured in a water tank, and a close agreement between measurements and theoretical values was confirmed. Second, the TS spectra of anesthetized fish from three species were measured in a water tank. Although the results showed characteristics similar to previous measurements, they varied among species, individuals, and tilt angles. Third, the TS spectra of live fish suspended and tethered by nylon monofilament lines were measured at sea. The dolphin-like sonar signals were effective in obtaining the broadband TS spectra of the fish. Cross-correlation processing of the echo from a tungsten carbide sphere showed a further advantage of using the dolphin-like sonar signals: the signal-to-noise ratio increased by more than 10 dB. The variation of TS spectra with fish behavior provides useful information for target identification.

Estimation of the temporal structure of the echo from the fish by using the broadband sonar signal of the dolphin.

Dolphins can identify prey by using the broadband sonar signals. Behavioral experiments indicate that dolphins can detect and discriminate targets in highly cluttered and noisy environments (Au, (1993)]. The sonar system can be improved by clarifying the ability of the dolphin and this improved sonar system is useful for fish species identification. The echo from the fish contains components resulting from multiple reflections, for example, the swim-bladder and body surface of the fish. Therefore, it is necessary to determine the distance and intensity, corresponding to the reflectivity, of these reflectors. We analyzed the echoes from fishes in seas close to Japan by using the Hilbert transform. First, we analyzed echoes which were measured from three species of the anaesthetized fish in a water tank. It was clarified that both the numbers and intensity of reflectors and the duration of the echo were varied dependent on the species, the individual, and the tilt angle. Second, we analyzed the echo which was measured from the fish suspended by nylon monofilament line in the open sea. It was clarified that the numbers and intensity of reflectors were varied dependent on the species and movement of the fish.

Measurements of target strength spectra of metal spheres and fish by using broadband sonar signals of dolphin

Dolphins identify prey by using their broadband sonar signals. Considering that the spectrum of fish target strength (TS) leads to identification of objects, we measured the spectrum using dolphin’s sonar signals with a broadband system. The TS spectrum is obtained by dividing the echo spectrum by the incident spectrum while setting the projector and hydrophone face to face. First, we measured the broadband form function of a tungsten carbide and copper sphere in a water tank and confirmed good agreements with theoretical value. Second, we measured the spectra of TS of three species of anaesthetized fish in a water tank. Although the spectra showed nearly similar tendencies of past measurements, they changed according to the species, the individual, and the tilt angle. Third, we measured TS spectra of fish suspended by nylon monofilament line in the open sea in semi-in-situ. We tried to remove the directivity of transducers by a tungsten carbide sphere suspended near the fish. The change of the TS spectra should be important information for the fish identification.

Target parameter estimation using resonance scattering analysis applied to air-filled, cylindrical shells in water

Current research on classification of submerged objects is concerned with using broadband sonar signals to insonify the targets, and applying signal-processing techniques to the backscattered signals. One characteristic of target echoes which may provide classification clues is the so-called resonance scattering response, the characteristics of which depend on target elastic properties. This paper presents algorithms for extracting resonance information based on autoregressive (AR) spectral estimation techniques. The AR-based representation is useful for detecting and accurately localizing resonances in the frequency domain. The extracted resonance frequencies are grouped into identified wave families, and processed in order to characterize the scatterer in terms of elastic and geometrical parameters on the basis of equations derived from resonance scattering theory. The targets considered are water-loaded elastic, cylindrical, thin-walled shells immersed in salt water under free-field conditions. Analysis was performed on data collected at sea at low-intermediate frequencies [ka∈(0,50)] and provided good results. The proposed approach is a first step towards the characterization of more complex targets either lying proud on the seabed or buried in bottom sediments.

A method of target characterization based on the analysis of Scholte–Stoneley and Lamb-type waves scattered by submerged fluid-filled thin-walled shells

The scattering inverse problem is addressed as a powerful aid to target classification. Broadband sonar signals are used to insonify submerged elastic objects, and signal processing techniques are applied to the backscattered response. Cylindrical fluid-filled thin-walled shells are considered. The ka range is (2,50). One characteristic of target echoes playing a major role at low-intermediate frequencies is the resonance contribution, which depends on the target’s elastic properties. Here resonance information is extracted by algorithms based on autoregressive (AR) spectral estimation techniques. Assuming a target as a linear acoustic system, AR modeling is used for estimating its poles and, among them, resonance modes which are identified on the basis of resonance scattering theory. Previous investigations on scattering by thin plates and shells demonstrated the existence of fluid-born Scholte–Stoneley waves and shell-born Lamb-type waves. Here, from the study of these waves and internal reflections, a set of equations are formulated which correlate resonance characteristics with geometrical and geophysical target parameters. The algorithms are shown to succeed in estimating parameters such as the shell outer radius, material, and wall thickness. Simulated and real data backscattered by cylinders at broadside are considered. Analyzing simple-shaped objects is seen as preliminary for characterizing more complicated targets.

Broadband sonar signal processor and target recognition system

Target discrimination involves processing of sonar echoes in the time-dom and using features of the targets that the human auditory system uses (time separation and amplitude of echo highlights). Known targets are placed in the water and ensonified by multiple broadband transient signals. Echoes from each of the known targets have time-domain feature data extracted and stored to construct a library of known target features. Unknown targets echoes of transmitted multiple broadband transient signals are received and time-domain feature data is extracted from these. A comparison of the unknown target echo features with the known target echo features provides a discrimination of the unknown target echoes.

Digital equalization of underwater transducers for the projection of broadband acoustic signals

There is a need to project undistorted broadband sonar signals in certain approaches to the development of a bionic sonar and other broadband sonar systems. Most transducers by themselves cannot project undistorted broadband pulse signals having 3-dB bandwidths on the order of an octave and higher. A digital technique which considered both the magnitude and phase of the transfer function of a transducer was used to derive a digital equalization filter. The digital equalization filter was used to predistort the original signals to compensate for the nonuniform transmission response of a transducer. A bionic signal with an octave bandwidth could be projected with a 0.99 cross correlation coefficient between the projected and desired signal using an Edo Western 6122 transducer. The fidelity of the reproduced sounds was determined as a function of the center frequency and the bandwidth of the signal. For a 1-octave bandwidth signal, it was found that the center frequency could also be varied over an octave and still achieve cross correlation coefficients of 0.97 and greater.