Acoustic Backscattering Strength Research Articles

A salinity front and current rip near Cape Hatteras, North Carolina

Shipboard hydrographic and acoustic Doppler current profiler data collected in September 1991 show a shallow front separating brackish coastal water from denser Gulf Stream water advancing onto the continental shelf near Cape Hatteras. The surface expression of the front has the hallmarks of a current rip: a corrugated line of flotsam (indicating surface convergence) and an adjoining band of short, steep waves (indicating wave‐current interaction). Sinking flow of 15 cm/s and a 10‐dB change in acoustic backscatter strength occur over a cross‐front distance of 10–20 m. While northward surface currents in the Gulf Stream water move at about 60 cm/s, the front advects northward at only 35 cm/s; this difference implies frontal propagation to the south relative to the deeper fluid at a speed consistent with theoretical expectations. The surface convergence across the front is estimated to be about 50 cm/s, and the horizontal surface strain rate is estimated to be 0.025–0.05 s−1. This supports the large strain rate needed by Jansen et al. (1993) to simulate the O(10 dB) radar modulations measured at the rip by Askari (1992). Similar current rips may account for curvilinear high‐backscatter features appearing in historical synthetic aperture radar images of the shelf area north of Cape Hatteras.

Estimation of seafloor microtopographic roughness through modeling of acoustic backscatter data recorded by multibeam sonar systems

A method is described for estimating parameters related to the power-law frequency spectrum of seafloor interface roughness from acoustic backscatter strength measured from hull-mounted multibeam sonar arrays. Scattering parameters for a sediment-free seafloor are inversely derived from backscatter data using a Kirchhoff-based interface scattering model developed by Jackson et al. [J. Acoust. Soc. Am. 79, 1410–1422 (1986)]. A variety of data reduction routines are employed, including slope correction, beam-pattern and pulse-width corrections [de Moustier et al., J. Acoust. Soc. Am. 90, 522–531 (1991)], ping cross correlation, and an indirect method to account for the lack of system calibration. The method is applied to well-studied areas of the Juan de Fuca Ridge and the results compared to geological ground truth. Preliminary results indicate that the method may be valuable as a survey tool for routine mapping of seafloor acoustic properties.

Seafloor acoustic remote sensing with multibeam echo-sounders and bathymetric sidescan sonar systems

This paper examines the potential for remote classification of seafloor terrains using a combination of quantitative acoustic backscatter measurements and high resolution bathymetry derived from two classes of sonar systems currently used by the marine research community: multibeam echo-sounders and bathymetric sidescans sonar systems. The high-resolution bathymetry is important, not only to determine the topography of the area surveyed, but to provide accurate bottom slope corrections needed to convert the arrival angles of the seafloor echoes received by the sonars into true angles of incidence. An angular dependence of seafloor acoustic backscatter can then be derived for each region surveyed, making it possible to construct maps of acoustic backscattering strength in geographic coordinates over the areas of interest. Such maps, when combined with the high-resolution bathymetric maps normally compiled from the data output by the above sonar systems, could be very effective tools to quantify bottom types on a regional basis, and to develop automatic seafloor classification routines.

Interpretation of acoustic data at two frequencies to discriminate between Antarctic krill (Euphausia superba Dana) and other scatterers

A visual classification was carried out of echo trace characteristics derived from 113 integration intervals of 1 n mi (nautical mile) each taken from the Southern Ocean in the area of South Georgia. This separated the echo traces into five categories: three biotic and two abiotic in origin. The acoustic data were subsequently analysed in detail by comparing the acoustic parameter Mean Volume Backscattering Strength (MVBS) at 120 kHz with that at 38 kHz. The three biotic categories separated visually had significantly different mean AMVBS values (δMVBS = MVBS 120 kHz - MVBS 38 kHz). To investigate if the biotic categories could be identified solely on the basis of δMVBS values, maximum-likelihood techniques were used to analyse the multimodal distribution of a further 1286 δMVBS records. These analyses allowed ranges to be defined for AMVBS which separate three biotic categories. On the basis of the target strength properties of the most likely scatterers in the region, the three categories were identified as krill, organisms larger than krill (fish or squid) and organisms smaller than krill (small zooplankton). The acoustic techniques described in this paper indicate a useful methodology in the study of Southern Ocean pelagic organisms.

Differences in backscattering strength determined at 120 and 38 kHz for three species of Antarctic macroplankton

The ability to acoustically separate zooplankton species is an important requirement for ecological studies and to improve biomass estimates. In order to distinguish between Euphausia superba and other swarm-forming macroplankters we used a dual frequency echo-sounder (120 and 38 kHz) and echo-integrator during a series of Longhurst Hardy Plankton Recorder (LHPR) hauls near South Georgia. We compared the acoustic parameter Mean Volume Backscattering Strength (MVBS) according to the equation: DELTAMVBS (dB) = MVBS 120 kHz - MVBS 38 kHz. Mean values of DELTAMVBS for E. superba, Themisto gaudichaudii and E. frigida were 4.6, 9.7 and 15.6 dB, respectively, and were significantly different, allowing the 3 species to be separated acoustically.

Tomographic measurements of sediment inhomogeneities and determination of statistical parameters critical to sediment volume backscattering

A tomographic method was used for measuring sound-speed fluctuations in the top 5 m of sediments in Biscayne Bay, Florida. Sub-bottom porosity and density fluctuations were estimated from measured sound-speed fluctuations by using Gassmann’s [Geophysics 16, 673–685 (1951)] equations. Statistical parameters for volume backscattering calculations such as vertical correlation length, horizontal correlation length, and variance of acoustic impedance were obtained from tomographic measurements. Volume backscattering strengths for the experiment site were calculated using the theoretical results given by Crowther [AcousticsandtheSeabed, edited by N. G. Pace (Bath University, Bath, 1983), pp. 147–155]. Measurements of acoustic backscattering strengths were also conducted at the same site. Comparisons between measured backscattering strengths and calculated volume backscattering strengths indicate that, for this particular location, scattering is mostly due to bottom roughness. [Work supported by ONR and UM-Research Program.]

Gas bubbles in ocean sediments and high-frequency acoustic backscattering strength

The influence of trapped gas bubbles on the high-frequency acoustic bottom backscattering strength of ocean sediments will be examined. Statistical trends in the extant backscatter database at shallow grazing angles suggest that gas bubbles may be a dominant factor. The effect may be modeled in terms of an increase in the volume scattering strength of the sediment. Results from model and laboratory studies will be presented. [Work supported by ONR under NOARL management.]

On ambient noise generation by “soliton” surface waves

Wave‐wave interaction is one of the more promising mechanisms for wind‐generated ambient noise. However, levels calculated with the linear gravity/capillary dispersion model for surface waves are found to be much too low above about 10 Hz. This discrepancy may rest in the wave model rather than the mechanism itself. Surface‐wave measurements in flumes show no evidence of dispersion at higher wavenumbers. To account for dispersionless propagation, a nonlinear model in which the surface fine structure is treated as a random ensemble of solitonlike hydraulic bumps rather than dispersive waves has been proposed. The “soliton” model appears to account for measured acoustic backscattering strengths and associated Doppler spectra at moderate sea states. Effects of the model on ambient noise prediction are examined.

Minimising errors in estimating fish population and biomass densities using the ‘acoustic volume backscattering strength’ method

Abstract The basic acoustical methods for estimating the volume backscattering strength and the absolute biomass density are commonly known. However, the detailed techniques for minimising errors in fitting the calibrated parameters of an echo sounding system in these computations are still not adequately examined and discussed. This paper describes and discusses the following computing techniques involved in the absolute biomass estimation: (i) the fitting of the calibrated time varied gain (TVG) curve, the selection of its operating interval, and the correction of its response; (ii) the compensation for the limitation of the signal bandwidth; (iii) the computation of the equivalent beam width of a transducer using its calibrated beam patterns; and (iv) the accuracy in the conversion from the volume backscattering to the absolute fish density using individual fish target strength measurements.

Intercalibration of Two Echo Integration Systems based on Acoustic Backscattering Measurements

A joint Canada/U.S. survey for Pacific hake (Merluccius productus) was performed in 1983. As part of this survey two vessels made 350 pairs of acoustic backscattering strength measurements while the vessels steamed along side-by-side. A stochastic model was developed to analyse these data and to test for any departure from the null hypothesis of equal measured backscattering strengths. The null hypothesis was rejected. Several possible explanations for the observed differences are examined.

Acoustic scattering from the seafloor

Acoustic bottom backscattering strength measurement results from an experiment conducted in the shallow waters of the North Sea are presented. The data were obtained using a bottom-mounted high resolution (narrow beamwidth) pulsed parametric sonar transmitter and conventional receiver. Scattering strength values were obtained as a function of frequency (3–20 kHz) and azimuth (100° sector) at a grazing angle of 9°. The average scattering strength values are dependent on frequency and exhibit little azimuthal dependence. Instantaneous scattering strength values obtained from point scatters within the insonified area of interest exhibit a high degree of frequency and azimuthal dependence. The data are found to be reasonably consistent with the experimental data of Urick, and Bunchuk and Zhitkovskii [Sov. Phys. Acoust. 26(5)(Sept.-Oct. 1980).] [Work supported by NAVSEA.]

Biological sound scattering in an ocean eddy

Acoustic volume-backscattering measurements and midwater net-trawls were conducted in the vicinity of an eddy in the Tasman Sea in 1979 and 1980. At acoustic frequencies greater than about 8 kHz, there is generally good agreement between the measured acoustic backscattering strengths and the predictions based on the results of the trawls. The main difference is that, although the trawls did not catch any fish heavier than 3 g, the acoustic results indicated that many heavier fish were present within the trawling regions. The acoustic scattering inside the eddy was several decibels higher than the scattering in the frontal region or outside the eddy during two of the three cruises that were conducted. For the remaining cruise, there was no significant difference.

Bottom backscattering in the Java Trench area and the Tasman Sea

Acoustic backscattering strengths of the sea floors in the Java Trench, and the Tasman Sea near Sydney, have been measured for the first time. The results obtained for the Java Trench area are, on average, several decibels lower than the Tasman Sea results, which are similar to results that have been published for the bottom of the Atlantic Ocean. In each area, the standard deviations of the station-to-station variability are generally around 5 dB. All of the backscattering strengths increase with frequency; the rate of increase is greater at the lower grazing angles. The backscattering strengths also increase with grazing angle, and the rate of increase is greater at the higher grazing angles. Sea floors at depths from 4 to 7 km have similar backscattering strengths to each other, whereas sea floors at 1 to 2 km depth tend to have lower backscattering strengths.

Backscattering of Underwater Sound in Saanich Inlet, British Columbia, Including Observations of Scattering from a Fish School

The frequency dependence of acoustic volume backscattering strength has been measured at depths of 100, 200, 300, and 400 ft in Saanich Inlet, British Columbia, Canada, over a 24-h period. The measurements were made for comparison with open-ocean (Pacific) results, since the biology of the inlet has been extensively studied, and it serves to some extent as a reference water volume. In the course of carrying out the above measurements, the backscattering spectral characteristics and strength of a school of young hake, of apparent mean size of 11 in., was obtained. The time series measurements revealed unchanging scattering characteristics at the 300- and 400-ft depths, with broad peaks near 1.2 and 4.8 kHz superimposed on otherwise flat spectra. Greater variability was observed at the 200-ft depth, while the greatest variability was observed near the surface, i.e., at the 100-ft observation depth. The fish school, which was observed to be closely packed (less than a fish length spacing near the surface) and randomly oriented, was shown to have a flat backscattering spectrum between 1 and 9 kHz and an estimated backscattering strength of −47±3 dB.

Statistical Studies of Back-Scattered Sound near 28 kHz

A relatively simple but sufficiently general expression for the autocorrelation function of the complex modulation envelope of back-scattered underwater sound has been derived under certain geometrical and environmental limitations from the model for acoustic reverberation proposed by Middleton. [“Statistical Theory of Reverberation and Similar First-Order Scattered Fields,” IEEE Trans. Information Theory, IT-13 3 (July 1967)]. The parameters of that expression are estimated for some conditions from the first- and second-order statistics of narrow-band reverberation data collected at 28.2 kHz in the Strait of Juan de Fuca. In particular, the sample means, variances, and normalized auto-covariances have been computed as a function of range from the transceiver. From the sample variances, average acoustic back-scattering strengths have been obtained. Comparative examples are given for short-, long-, and burst-pulse transmissions. The estimation problems caused by inhomogeneities in the scattering medium are clearly indicated. The necessity for environmental stationarity and the resulting statistical homogeneity is discussed.

An ultrasonic thickness measuring instrument using transistor logic techniques: Elliot, B.L. Electrical Engineering, 35, No. 429, p. 738 (1963)

Acoustic backscatter from multi-beam echo-sounder (MBES) and dual-frequency single-beam echo-sounding systems (SBES) operable at 95 kHz and 33/210 kHz, respectively, were used to study the distribution of sediment texture and benthic macro-fauna along the central part of the western continental shelf of India (off Goa). To characterize the continental shelf seafloor, single-beam and multi-beam backscatter signals were acquired along with grab sediment samples. The relationships between processed acoustic backscatter strength, grain size, and benthic macro-fauna abundance were demonstrated employing clustering technique (PCA) and Geographic Information System (GIS) based mapping. The clustering analysis delineated that the backscatter values at three frequencies are strongly correlated with both substrate type and faunal functional groups. The preferences of deposit feeders (soft body benthic macro-fauna) for the fine-sediment regions and filter feeders (hard body benthic macro-fauna) for coarse sediment regions were linked to the variations in sediment granulometry as well as backscatter strengths in the study area. This study further demonstrates the utility of high frequency backscatter data employing echo-sounding systems towards the interpretation of seafloor sediments and benthic habitat characteristics across large areas of seafloor.