Duncan E. McGehee

Three-dimensional modeling of acoustic backscattering from fluid-like zooplankton

Scattering models that correctly incorporate organism size and shape are a critical component for the remote detection and classification of many marine organisms. In this work, an acoustic scattering model has been developed for fluid-like zooplankton that is based on the distorted wave Born approximation (DWBA) and that makes use of high-resolution three-dimensional measurements of the animals outer boundary shape. High-resolution computerized tomography (CT) was used to determine the three-dimensional digitizations of animal shape. This study focuses on developing the methodology for incorporating high-resolution CT scans into a scattering model that is generally valid for any body with fluid-like material properties. The model predictions are compared to controlled laboratory measurements of the acoustic backscattering from live individual decapod shrimp. The frequency range used was 50 kHz to 1 MHz and the angular characteristics of the backscattering were investigated with up to a 1 degree angular resolution. The practical conditions under which it is necessary to make use of high-resolution digitizations of shape are assessed.

Effect of orientation on broadband acoustic scattering of Antarctic krill Euphausia superba: Implications for inverting zooplankton spectral acoustic signatures for angle of orientation

Acoustic scattering experiments involving simultaneous acquisition of broadband echoes and video footage from several Antarctic krill were carried out to determine the effect of animal orientation on echo spectral structure. A novel video analysis technique, applied to extract krill angle of orientation corresponding to each insonification, revealed that echo spectra from krill near broadside incidence relative to the incident acoustic wave exhibited widely spaced, deep nulls, whereas off-broadside echo spectra had a more erratic structure, with several closely spaced nulls of variable depth. The pattern of changes in echo spectra with orientation for the experimentally measured acoustic returns was very similar to theoretically predicted patterns based on a distorted wave Born approximation (DWBA) model. Information contained in the broadband echo spectra of the krill was exploited to invert the acoustic returns for angle of orientation by applying a newly developed Covariance Mean Variance Classification (CMVC) approach, using generic and animal-specific theoretical and empirical model spaces. The animal-specific empirical model space was best able to invert for angle of orientation. The CMVC inversion technique can be implemented using a generic empirical model space to determine angle of orientation based on broadband echoes from individual zooplankton in the field.

FTV: a sonar for tracking macrozooplankton in three dimensions

Abstract A three-dimensional sonar imaging system, Fish TV (FTV), was developed to observe the three-dimensional trajectories of macrozooplankton in situ . Consisting of two arrays of eight transducers, the sonar uses one of the arrays as a projector and the other as a receiver. In this way, a combination of 8 × 8, or 64 beams, is constructed. Spatial resolution is approximately 2° by 2° and the range resolution is 3 cm. The sonar is capable of recording up to five images every second. The data is stored digitally in a three-dimensional matrix which can be postprocessed. Tests performed in the lab with both test targets and freshwater shrimp indicate that the system has the sensitivity and accuracy to track euphausiids in the ocean. The system was deployed at sea on an ROV (Remote Operational Vehicle) and the tracks of euphausiid sized objects were recorded.

Effect of animal orientation on acoustic estimates of zooplankton properties

It is well known that the behavior of zooplankton and, in particular, their orientation distribution dramatically affects the level of backscattered acoustic energy. As a result, interpretation of acoustic survey data in the ocean is subject to error. In order to quantify these effects, laboratory data from two important classes of animals were collected. The data involved broad-band (350-650 kHz) acoustic signals insonifying individual animals whose orientation was varied over the range 0/spl deg/-360/spl deg/ in 1/spl deg/ increments. The animals were from two major anatomical groups: fluid-like (decapod shrimp; Palaemonetes vulgaris) and elastic-shelled (periwinkles; Littorina littorea). The data were analyzed both in the time domain (with pulse compression processing) and the frequency domain. Averages of the laboratory data over different orientation distributions illustrate the variability in average target strength that can be expected in the ocean environment. The average target strength of the shrimp varied by 3 dB when averaged over orientation distributions centered around broadside and end-on incidence. In addition, size estimates from pulse compression processing of the broad-band echoes were made for various orientation distributions for both the shrimp and periwinkles. These results show the necessity of animal orientation information for the proper interpretation of acoustic backscatter data.

High-frequency scattering from saturated sand sediments

Measurements of bottom scattering strengths were made with a multi-frequency echo sounder mounted on a tower on a sandy bottom off West Destin, FL. Data were measured at five frequencies in the range 265–1850 kHz, at subcritical grazing angles ranging from 9° to 20° and at azimuths up to ±90° around the tower. Scattering strength increased at about f1.4 up to >400 kHz, peaked between 700 and 1100 kHz, and decreased at higher frequencies.

Multifrequency acoustical volume backscattering patterns in the Arabian Sea—265 kHz to 3 MHz

High-frequency acoustical volume backscattering was examined during three periods of the annual cycle of winds in the Arabian Sea-the Winter Northeast (NE) Monsoon, the Summer Southwest (SW) Monsoon, and the Fall Intermonsoon-using a multifrequency acoustic profiling system deployed on an undulating towed body. This system measured acoustical volume backscattering strengths at six frequencies ranging from 265 kHz to 3.0 MHz. Measurements were made from near the surface to 250 m depth along a ship track that paralleled the coast of Oman, then proceeded away from the coast towards the middle of the Arabian Sea. At all frequencies, volume backscattering strength was highest near the surface, decreasing with increasing depth. Contrary to expectation, backscattering was generally lower during the Summer SW Monsoon than during the Winter NE Monsoon. Also contrary to expectation, backscattering was not appreciably higher near the coast than it was offshore during the Summer SW Monsoon, although it was higher near the coast during the Winter NE Monsoon. Generally speaking, regional and seasonal differences were smaller than expected, and much smaller than the fine-scale spatial and temporal variability, particularly at frequencies below 1 MHz. There was a daily pattern of increased nighttime backscattering in the upper 100 m at 265 and 420 kHz. This pattern was less evident at 1.1 MHz and above.

Beamforming with dense random arrays: The development of a spatially shaded polyvinylidene fluoride acoustic transducer

This article describes an approach to array design that results in the construction of transducers producing precisely controlled radiation patterns. The method of dense random arrays employs an array of equally weighted elements, randomly distributed with a density distribution that matches a desired shading. As the spatial density of elements increases, the theoretical radiation pattern approaches that of the ideal shading. If the ideal shading is real (e.g., for a symmetric beam), all elements in the random array will be either in phase or in phase opposition, and the array will require a single amplifier coupled with a phase inverter. If the ideal shading is both real and positive (e.g., Gaussian or Blackman–Harris shading), all elements will be in phase and will require only one amplifier. This approach was used to design a linear array to generate a 2° wide Gaussian beam. The elements had a minimum spacing of one‐quarter of a wavelength and were placed using a Monte Carlo approach. A theoretical sidelobe level of −21.8 dB was achieved, with good agreement in the central beam (2.4° beamwidth predicted). A 500‐kHz acoustic transducer with this element distribution was built using 110‐μm‐thick polyvinylidene fluoride (PVDF). Experiments reveal a radiation pattern very close to the predicted pattern, with −20.8‐dB sidelobes.

Calibration and use of a broadband ADCP to measure zooplankton volume scattering strengths

A 1200 kHz broadband ADCP from RD Instruments was calibrated and used to collect quantitative volume backscattering data from zooplankton in the Gulf of Mexico. Voltages on the four receivers, reported in terms of ‘‘counts,’’ were linear with respect to received sound pressure level in dB re 1 μPa, with sensitivities of 2.376 to 2.434 counts/dB, depending on the receiver. Source levels ranged from 212.9 to 214.6 dB re 1 μPa at 1 m. The ADCP was mounted on the C/V Admiral Semmes and used to survey volume backscattering around an oil platform in the Gulf of Mexico. Volume backscattering strengths measured between −40 and −90 dB, which were generally consistent with independent observations of the numbers and sizes of scatterers present in the water column. These results suggest that, with careful calibration, ADCPs may be used for quantitative zooplankton research.

Killer whale caller localization using a hydrophone array in an oceanarium pool

A system to localize calling killer whales was designed around a ten‐hydrophone array in a pool at SeaWorld San Diego. The array consisted of nine ITC 8212 and one ITC 6050H hydrophones mounted in recessed 30×30 cm2 niches. Eight of the hydrophones were connected to a Compaq Armada E500 laptop computer through a National Instruments DAQ 6024E PCMCIA A/D data acquisition card and a BNC‐2120 signal conditioner. The system was calibrated with a 139‐dB, 4.5‐kHz pinger. Acoustic data were collected during four 48–72 h recording sessions, simultaneously with video recorded from a four‐camera array. Calling whales were localized by one of two methods, (1) at the hydrophone reporting the highest sound exposure level and (2) using custom‐designed 3‐D localization software based on time‐of‐arrival (ORCA). Complex reverberations in the niches and pool made locations based on time of arrival difficult to collect. Based on preliminary analysis of data from four sessions (400+ calls/session), the hydrophone reporting t...

Acoustical study of the prey field and supporting food web for whales in the Ligurian Sea north of Corsica

The Sound, Oceanography, and Living Marine Resources (SOLMAR) program is a study coordinated by the NATO SACLANT Undersea Research Centre to develop ways of mitigating risks to whales from anthropogenic sound sources. One objective of the SOLMAR program is to examine the correlation between the distribution of whales in parts of the Mediterranean Sea and that of their prey, lower trophic levels, and basic oceanographic parameters. A multiship field study was conducted in the Ligurian Sea north of Corsica in August 1999, as will be a follow‐up study in August 2000. The primary role of the R/V Ammiraglio Magnaghi is to map basic physical oceanographic parameters with a CTD; phytoplankton with a fluorometer; small zoo‐plankton (copepods and smaller) with a six‐frequency TAPS (Tracor Acoustic Profiling System) and small plankton net; and large zooplankton (euphausiids and larger) with a 120‐kHz split‐beam echosounder. Measurements in 1999 were consistent with a counterclockwise circulation in the basin causin...