Mark V. Trevorrow

Directionality and maneuvering effects on a surface ship underwater acoustic signature

This work examines underwater source spectra of a small (560 tons, 40 m length), single-screw oceanographic vessel, focusing on directionality and effects of maneuvers. The measurements utilized a set of four, self-contained buoys with GPS positioning, each recording two calibrated hydrophones with effective acoustic bandwidth from 150 Hz to 5 kHz. In straight, constant-speed runs at speeds up to 6.2 m s(-1), the ship source spectra showed spectral levels in reasonable agreement with reference spectra. The broadband source level was observed to increase as approximately speed to the fourth power over the range of 2.6-6.1 m s(-1), partially biased at low speeds by nonpropulsion machinery signals. Source directionality patterns were extracted from variations in source spectra while the ship transited past the buoy field. The observed spectral source levels exhibited a broadside maximum, with bow and stern aspect reduced by approximately 12-9 dB, respectively, independent of frequency. An empirical model is proposed assuming that spectral source levels exhibit simultaneous variations in aspect angle, speed, and turn rate. After correction for source directionality and speed during turning maneuvers, an excess of up to 18 dB in one-third octave source levels was observed.

Comparison of multifrequency acoustic and in situ measurements of zooplankton abundances in Knight Inlet, British Columbia

An investigation of midwater zooplankton aggregations in a coastal fjord was conducted in November 2002. This study focused on quantitative comparisons between a calibrated, three-frequency (38, 120, and 200 kHz) vessel-based echo-sounder, a multinet towed zooplankton sampler (BIONESS), and a high-resolution underwater camera (ZOOVIS). Daytime layers of euphausiids and amphipods near 70-90-m depth were observed in lower parts of the inlet, especially concentrated by tidal flows around a sill. Quantitative backscatter measurements of euphausiids and amphipods, combined with in situ size and abundance estimates, and using an assumed tilt-angle distribution, were in agreement with averaged fluid-cylinder scattering models produced by Stanton and Chu [ICES J. Mar. Sci. 57, 793-807, (2000)]. Acoustic measurements of physonect siphonophores in the upper inlet were found to have a strong 38-kHz scattering strength, in agreement with a damped bubble scattering model using a diameter of 0.4 mm. In relatively dense euphausiid layers, ZOOVIS abundance estimates were found to be a factor of 2 to 4 higher than the acoustic estimates, potentially due to deviations from assumed euphausiid orientation. Nocturnal near-surface euphausiid scattering exhibited a strong (15 dB) and rapid (seconds) sensitivity to vessel lights, interpreted as due to changing animal orientation.

Response of Euphausia pacifica to small-scale shear in turbulent flow over a sill in a fjord

Zooplankton in the ocean respond to visual and hydro-mechanical cues such as small-scale shear in turbulent flow. In addition, they form strong aggregations where currents intersect sloping bottoms. Strong and predictable tidal currents over a sill in Knight Inlet, Canada, make it an ideal location to investigate biological behaviour in turbulent cross-isobath flow. We examine acoustic data (38, 120 and 200 kHz) collected there during the daylight hours, when the dominant zooplankters, Euphausia pacifica have descended into low light levels at ∼90 m. As expected, these data reveal strong aggregations at the sill. However, they occur consistently 10–20 m below the preferred light depth of the animals. We have constructed a simple model of the flow to investigate this phenomenon. Tracks of individual animals are traced in the flow and a variety of zooplankton behaviours tested. Our results indicate that the euphausiids must actively swim downward when they encounter the bottom boundary layer (bbl) to reproduce the observed downward shift in aggregation patterns. We suggest that this behaviour is cued by the small-scale shear in the bbl. Furthermore, this behaviour is likely to enhance aggregations found in strong flows at sills and on continental shelves.

Susy, Seascan, and other acoustical contraptions

In the early 1990s, the acoustical oceanography research group, led by Dr. David Farmer, developed several high-frequency sonar platforms for near-surface oceanography. These platforms, suspended approximately 25 m below the surface, typically supported six frequencies of upward-looking echo-sounders and up to four steerable 100 kHz sidescan sonars. Additionally, the SUSY platform had four extensible arms, 4.5 m in length, each supporting a wideband hydrophone. The platforms had sufficient batteries and data recording for up to 40 hours of autonomous operation, which could be extended through scheduled on/off periods. The intent of these platforms was to investigate properties of near-surface bubble due to breaking waves, with additional Doppler velocity measurements of surface waves and convective circulations. These platforms were also used for other studies, such as tidal convergence zones and ship wakes. The SEACAN platform was also used to assess zooplankton and fish populations in lake in Japan. Thi...

Accuracy and resolution of bearing measurements for crossed-dipole receiver

A well-known system for estimating the bearing to an underwater acoustic source consists of two crossed dipole sensors and an omni-directional sensor. At DRDC Atlantic, several bearing estimation methods for this three-channel system in the presence of ocean noise have been examined with the goal of minimizing the bearing error. Example algorithms are arctangent and maximum likelihood beamformer. These algorithms have trade-offs between estimation accuracy and computational efficiency, because any practical bearing-estimation algorithm must remain efficient enough for implementation in a real-time sonar processor. The accuracy of these algorithms can be demonstrated through comparison with the Cramer-Rao lower bound. Previously the calculation of bearing accuracy was based on the assumption of isotropic noise and a single signal source. However, the case of anisotropic noise and multiple-sources is more realistic, especially for bistatic or multistatic active sonar. Adaptive methods such as the Minimum Variance Distortionless Response (MVDR) beamformer will be explored to address this problem. The paper will discuss the potential improvement of bearing estimate accuracy and bearing resolution through numerical models.

The use of moored inverted echosounders for monitoring near‐surface processes

The near‐surface ocean is a difficult place to conduct measurements of physical and biological processes. High‐frequency inverted echo‐sounders (IES), moored a short distance below the surface, provide a simple means for sustained, high‐resolution monitoring. In particular, long duration deployments are useful in capturing transient events, such as storms or solitary wave trains, and placing their occurrence within the context of longer term trends. Lessons learned from use of these moored IES systems can be applied to modern ocean observatories. A number of long duration deployments of a self‐contained 200kHz IES were made in the late 1990s. Three specific examples will be discussed. Firstly, an investigation of calibrated volume scattering from near‐surface bubbles in the NE Pacific Ocean showed the frequent occurrence of vertical, plume‐like structures drawn downwards up to 25 m. Within the plumes, the backscatter cross‐section exhibited an exponential decay with depth, with e‐folding scale in the ran...

Mapping fish schools with a high‐resolution multibeam sonar

In late 2002 an evaluation of a high‐resolution multibeam sonar (RESON SeaBat 8125) for mapping mid‐water fish schools was performed in Knight Inlet, BC. The ship‐mounted multibeam sonar operated at 455 kHz and provided 240 one‐half‐degree beams in the across‐track plane with one‐degree along‐track beam width. Simultaneous measurements were made with a conventional 200‐kHz echo‐sounder. Acoustic calibrations of this multibeam sonar will be discussed, showing some range‐ and angle‐dependent variations in sensitivity. The multibeam sonar was able to provide well‐resolved mid‐water images of fish schools (likely juvenile herring), sampling many fish schools missed by the echo‐sounder and revealing cross‐track fish school shape. Comparison of near‐vertical volumetric scattering strengths between the multi‐beam sonar and echo‐sounder show reasonable agreement. Discussions on the combined use of conventional and multibeam sonars for fish surveys will be given.

Numerical and Experimental Investigations of Transformations of Near-field to Far-field Scattering Measurements

Due to the size of a target and measurement constraints, the measurements of monostatic acoustic scattering are often made in the near field of the target. For many applications, it is an estimate of the far‐field target scattering strength, which is desired. In this presentation some model‐based inversion techniques, which can be used to predict far‐field monostatic target strengths as a function of aspect angle, are derived. In particular, a set of near‐field measurements is used in an inversion scheme to compute coefficients for either a linear array of point scatterers or for a generalized Kirchhoff model of the target. The resulting model is then used to predict the far field scattering amplitudes. Simulated and experimental data are used to illustrate the accuracy of the proposed methods.

In situ and experimental observations of the relationships between euphausiid orientation, vessel lights, and acoustical scattering

During a collaborative investigation of zooplankton aggregations near a coastal fjord (Knight Inlet, British Columbia), surveys were conducted using down‐looking echosounders and a digital imaging system (ZOOVIS). Two surveys conducted at night revealed that the presence or absence of external illumination on the vessel had a pronounced influence on measured acoustical scattering. The extremely short latency between shifts in illumination and changes in acoustical scattering suggested that differences in the orientations of scatterers were responsible for this phenomenon. High‐resolution, in situ images of euphausiids (Euphausia pacifica) from ZOOVIS indicated that these organisms were present in a range of orientations ranging from horizontal to vertical, relative to the incident acoustical beam. Theoretical scattering models based on digitizations of in situ ZOOVIS images suggest that the magnitude of the observed changes in scattering, in response to altered illumination, may be accounted for by change...

Multifrequency acoustic observations of zooplankton in Knight Inlet, B.C

A collaborative investigation of midwater zooplankton aggregations in a coastal fjord was conducted in November 2002. Midwater aggregations of zooplankton in a coastal fjord were sampled and mapped using a calibrated, three‐frequency (38, 120, and 200 kHz) vessel‐based echo‐sounder system, a multinet towed zooplankton net (BIONESS), and a high‐resolution in situ camera system (ZOOVIS). Dense daytime layers of euphausiids and amphipods near 70‐ to 90‐m depth were found in the lower reaches of the inlet, especially concentrated by tidal flows around a sill which rises above the layer. Quantitative euphausiid and amphipod backscattering measurements, combined with in situ species, size, and abundance estimates, were found to agree closely with recent size‐ and orientation‐averaged fluid–cylinder scattering models produced by Stanton et al. Also, in situ scattering measurements of physonect siphonophores were found to have a much stronger low‐frequency (38 kHz) scattering strength, in agreement with a simple bubble scattering model. [Work supported by Dr. J. Eckman, ONR code 322BC.]