Single Ping Research Articles

Studies on geometrical backscattering models of marine bodies

The target strength of marine bodies depends on two components—one of them is easy to measure and a good relationship can be established with the target strength value, while the other presents a higher variability that no method can help to reduce, which include the orientation of the fish and a range of environmental conditions and a set of biological facts. Studies on physical models and aspects are expected to provide a clear insight into the issues relating to the target strength variability. Such physical models are developed by converting the physiological shape of the fish into standard and simple geometrical shapes. Data obtained from some of the commercially important species, individually positioned at the center of the acoustic beam, 3 m from the transducer in a test facility were used for the computation of target strength. The target strength value obtained from these reference targets is an indicator of the model performance. Mathematical description of the scattering by some of the species and subsequent comparison with laboratory data have demonstrated that the scattered level by an individual due to a single ping, strongly depends upon size, shape, frequency, material properties, and orientation. Perhaps one of the most notable peculiarities of this work is the simplicity of the approximation and the close agreement between the real world value and the model solution.

Inferring fish orientation from broadband-acoustic echoes

Abstract A new method has been developed for inferring the orientation of fish through the use of broadband-acoustic signals. The method takes advantage of the high range resolution of these signals, once temporally compressed through cross-correlation. The temporal resolution of these compressed signals is inversely proportional to the bandwidth, thus the greater the bandwidth the higher the resolution. This process has been applied to broadband-chirp signals spanning the frequency range 40–95 kHz to obtain a range resolution of approximately 2 cm from the original, unprocessed resolution of about 50 cm. With such high resolution, individual scattering features along the fish have been resolved, especially for angles well off normal incidence. The overall duration of the compressed echo from live, individual alewife, as measured in a laboratory tank, is shown to increase monotonically with orientation angle relative to normal incidence. The increase is due to the greater range separation relative to the transducer between the echoes from the head and tail of the fish. The results of this study show that with a priori knowledge of the length of the fish, the orientation could be estimated from the duration of a single, compressed broadband echo. This method applies to individual, acoustically resolved fish. It has advantages over previous approaches because it derives the orientation from a single ping and it does not use a formal, mathematical scattering model. Design parameters for applications in the ocean are given for a range of conditions and fish size.

Fisheries applications of Doppler sonar

Signal processing inherent to broadband Doppler current profiling systems has great potential for applications in fisheries monitoring. The large bandwidth (25%–50% of the carrier frequency) contains information that can be used in target characterization, the system determines current profiles and there is the potential to make direct measurements of fish swimming speed. The capabilities of these systems are explored: field data collected with an RD Instruments 307-kHz Workhorse Acoustic Doppler Current Profiler (ADCP) demonstrate observations of large herring schools, performance is also evaluated using observations made in a tow-tank facility. For large concentrations of fish, speed and direction can be determined but for lower concentrations of fish new data processing techniques are required. For the system configuration as tested, single ping estimates have a standard deviation as low as 10 cm<th>s−1. This accuracy is substantially better than the accuracy of ≂40 cm<th>s−1 expected and suggests that for isolated targets the approach is equivalent to fully coherent Doppler processing. There is a difficulty discriminating discrete targets because of the long pulse code sequences transmitted. [This research was funded by the Natural Sciences and Engineering Council of Canada, and by Fisheries and Oceans Canada.]

Localization analysis based on subband processing of broadband LFM signals

The increased utilization of broadband signals in underwater research and development is related to sonar performance enhancement in littoral waters, optimum operational frequency selection, and interplatform, multisensor experimental settings requiring frequency compatibility. Preliminary wide-band experiments indicate potential performance improvement in reverberation-limited environments. Here the frequency dependence of active broadband detection/localization is examined based on 1200-Hz LFM signals (2300–3500 Hz). A subband matched filter scheme is devised according to which a replica of the transmitted pulse is segmented into ten 120-Hz subbands. Although comparison of source localization indicates comparable performance of all subbands, ping-to-ping variability of the ten correlator outputs suggests that the detection performance maybe improved by employing incoherent processing schemes. Doppler effects and subband detection synchronization problems that lead to performance degradation in large time-bandwidth signal scenarios are addressed. A method to estimate range rate (relative velocity between source and receiver) based on single ping differential time delay between subband matched filter outputs is developed. This intraping technique is an alternative to the standard interping method which requires multiping detection history.

Wideband fisheries sounder. From individual echoes analysis to classification of schools at sea

Several works have been carried out in the last decade concerning wideband fisheries systems and their ability to discriminate fish species. The paper will first review previous work showing the relevance of wideband systems for fisheries acoustics (analysis of individual echoes). A short description of a wideband prototype (20 to 80 kHz) will then be given showing the technological issues. An emphasis will be made on the analysis of fish schools either in a controlled situation (lakes) or at sea. Wideband echo processing was based on autoregressive modeling of individual pings (modeling of spectrum resonances). This modeling concentrates the information contained in the wideband echo in a reduced set of parameters that could be used for classification. A neural network was trained to recognize several fish species. Ground truth was achieved by species identification, at sea, using associated trawling. For the construction of an echo data base, only monospecific schools were considered. Classification results at sea using only the spectral signature of the echoes were as high as 75% from a single ping. Lake tests have shown that the use of several pings (ten and above) could increase the recognition rate to more than 90%.

Sound scattering by several zooplankton groups. II. Scattering models.

Mathematical scattering models are derived and compared with data from zooplankton from several gross anatomical groups--fluidlike, elastic shelled, and gas bearing. The models are based upon the acoustically inferred boundary conditions determined from laboratory backscattering data presented in part I of this series [Stanton et al., J. Acoust. Soc. Am. 103, 225-235 (1998)]. The models use a combination of ray theory, modal-series solution, and distorted wave Born approximation (DWBA). The formulations, which are inherently approximate, are designed to include only the dominant scattering mechanisms as determined from the experiments. The models for the fluidlike animals (euphausiids in this case) ranged from the simplest case involving two rays, which could qualitatively describe the structure of target strength versus frequency for single pings, to the most complex case involving a rough inhomogeneous asymmetrically tapered bent cylinder using the DWBA-based formulation which could predict echo levels over all angles of incidence (including the difficult region of end-on incidence). The model for the elastic shelled body (gastropods in this case) involved development of an analytical model which takes into account irregularities and discontinuities of the shell. The model for gas-bearing animals (siphonophores) is a hybrid model which is composed of the summation of the exact solution to the gas sphere and the approximate DWBA-based formulation for arbitrarily shaped fluidlike bodies. There is also a simplified ray-based model for the siphonophore. The models are applied to data involving single pings, ping-to-ping variability, and echoes averaged over many pings. There is reasonable qualitative agreement between the predictions and single ping data, and reasonable quantitative agreement between the predictions and variability and averages of echo data.

Acoustic scattering characteristics of several zooplankton groups

The acoustic echo levels from zooplankton are strongly dependent upon the acoustic frequency and size, shape, orientation, and material properties of the animals. Because of the great number of species of zooplankton, it is practical to study the acoustic properties of species grouped by their gross anatomical similarity. Zooplankton from several major groups are discussed: fluid-like (decapod shrimp, euphausiid, salp), hard elastic shelled (gastropod), and gas-bearing (siphonophore). The results from laboratory tests show that the plots of (single ping) target strength versus acoustic frequency have a distinct pattern for each animal type. For example, the plot for euphausiids when ensonified at broadside incidence contained a series of broadly spaced deep nulls; the plot for gastropods either had more tightly spaced nulls or aflat spectrum; the plot for siphonophores either had a less consistent pattern of nulls or a flat spectrum. The nulls from the euphausiid data were sometimes as deep as 30 dB below surrounding levels. The patterns are linked to the physics of the scattering process and modeled mathematically. In addition, key results on these animals from Stanton et al. (1994a, ICES J. Mar. Sci., 51: 505‐512) are summarized to further illustrate the variability in scattering characteristics of the animal groups (for example, data from 2-mm-long gastropods show that they produce a level of echo energy per unit biomass approximately 19 000 (i.e., 43 dB) times greater than that of 30-mm-long salps). The impact of these observations on design and interpretation of acoustic surveys is discussed. Very importantly, drawing a simple relationship between echo energy and biomass for regions containing a complex assemblage of zooplankton would be greatly flawed. ? 1996 International Council for the Exploration of the Sea

Signal processing strategies for a bathymetric sidescan sonar

A dominant source of errors in swath bathymetry is acoustic interference. In 1989 the author published an analysis of these errors and predicted depth accuracies for a system which reduced their effect by averaging. This present paper shows how a considerable improvement in performance may be obtained by a variety of signal processing strategies that include the use of several widely spaced receivers and the elimination of the most unsatisfactory measurements before averaging. Simulations show how impressive sea bed profiles can be produced with a single ping, even at low signal-to-interference ratios. >

Determination of the Form Function for Rigid and Elastic Spheres from a Single Ping

A short acoustic pulse is used to determine significant portions of the form function vs ka curves for rigid and elastic spheres, from a single reflection. Excellent agreement between exact theory and experiment is obtained over limited ranges of ka for aluminum and tungsten carbide spheres measured in water and for metal spheres measured in air.