Anna Crawford

Determining Suspended Sand Size and Concentration from Multifrequency Acoustic Backscatter

An inversion algorithm for extracting suspended sand size and concentration from simultaneous backscattered acoustic pressure amplitude at three operating frequencies is presented. The algorithm is based on the differences in signal amplitude between different frequency pairs, and is tested using laboratory measurements of multifrequency backscatter from a turbulent sediment‐carrying jet. Concentration and size profiles inverted from field and laboratory data are compared with results from a previously developed algorithm based on signal ratios. The difference inversion scheme is less sensitive to errors arising from low signal levels, allowing the size/concentration measurement range to be extended to regions of lower concentration. The concentrations from the field data agree well with independent optically determined estimates. The results demonstrate sensitivity to the backscatter form factor.

Linear transition ripple migration and wave orbital velocity skewness: Observations

Field observations were made in 3–4 m water depth of linear transition ripple geometry and migration using a high-resolution laser-video bed profiling system and acoustic scanning sensors during both the growth and decay phases of an autumn storm event. Linear transition ripples are long-crested, low-steepness bedforms of the anorbital ripple type and were observed to occur here at relatively high wave energies just below the flatbed threshold, with wavelengths of 8.5±0.5 cm and heights of 0.3±0.1 cm. The maximum observed migration rate was 0.7 cm/min. Migration was offshore during storm growth and onshore during storm decay. The observed ripple migration velocities were highly correlated (r2 > 0.7) with nearbed wave orbital velocity skewness in both cross-shore directions. During storm growth the incident wave spectrum was bimodal and the orbital velocity skewness was negative. During storm decay the wave spectrum was unimodal and the velocity skewness was positive. Bispectral analysis shows that the main contribution to negative velocity skewness during storm growth was due to a difference interaction between the two principal (sea and swell) components of the bimodal velocity spectrum. Positive skewness during storm decay was due to self-self interaction of the narrowband residual swell. The negative velocity skewness observed during storm growth is consistent with prediction by a two-frequency second-order wave theory.

A simple system for laser-illuminated video imaging of sediment suspension and bed topography

A simple underwater video system has been developed for simultaneously imaging sediment suspension and monitoring bed profiles under waves and combined wave-current flows. This consists of a diode-laser-generated light plane and a black-and-white underwater video camera. The laser light illuminates suspended material in section and also provides a bed profile at the bottom. Orthogonal laser/camera pairs are used to obtain both cross-shore and alongshore views. During deployments, the system has been augmented by acoustic backscatter devices for measurement of sediment concentration. As with all video techniques, visibility is a limiting factor, but where turbidity is low to moderate, the results are encouraging. Results on cross-shore and alongshore bed elevation variations and suspension event stales obtained with the video/laser system are presented for two experiments: one at the National Research Council Wave Research Flume in Ottawa, Canada, and the other in the field at Queensland Beach, N.S., Canada,.

Evaluation of portable high‐frequency sonars for diver identification

Obtaining a positive identification is a critical step in most tiered harbour protection strategies for countering underwater intruders. It is generally recognised that sonar is one of the best tools for underwater imaging, however operating in a harbour environment presents challenges. As part of an on‐going harbour protection research project, small easily portable high‐frequency sonar systems are being investigated as a means to equip small response craft with intruder identification capability. Several systems are being considered, with the most comprehensive testing by DRDC to date being done on small Canadian‐made sonars. Tests were conducted in local harbour waters in Halifax, Canada, and in La Spezia, Italy, through participation in the NATO Undersea Research Centre Response Against Diver Intrusions (RADI) joint trial, conducted in November 2007. A variety of small sonars and manned and unmanned response craft were used during the RADI trial. Evaluation of the performance of these devices for the ...

Observations from Demonstrations of Several Commercial Diver Detection Sonar Systems

As part of its ongoing Maritime Force Protection Technology Demonstration Project, Defence R&D Canada - Atlantic has recently concluded a series of four commercial diver detection sonar demonstrations. The one-week-long demonstrations took place off the Canadian Forces Base Shearwater jetty, near the entrance to Halifax Harbour, with the participation of divers from the Canadian Forces Fleet Diving Unit Atlantic. The sonar systems were all deployed within 100 m of the jetty in 10-15 m water depth. Experience at this and similar past demonstrations has shown the importance of fusion of the sonar data with environmental data, and therefore measurements of the local acoustic propagation environment were made to aid in interpretation of sonar performance.

Autonomous shallow water bathymetric measurements for environmental assessment and safe navigation using USVs

The application of unmanned surface vehicles for autonomous shallow water bathymetric measurements, for naval mine counter-measures and hydrographic charting, and as a navigation assist for high valued ships is discussed. Defence Research & Development Canada has developed a prototype unmanned surface vehicle based on a commerically available catamaran hull-form integrated with a hydrographic quality bathymetric sonar, side-scan sonar, and an echo sounder. The unmanned surface vehicle is also equipped with a WHOI underwater acoustic modem and a 2.4 GHz RF radio to facilitate above and below water communications. The vehicle is also integrated with a mission-planner that has an advanced autonomy framework to facilitate the development and implementation of more complex robotic behaviors towards capabilities for the above-mentioned applications. This autonomous system has undergone validation and testing in the Canadian Arctic and numerous local trials in Halifax Canada. The efficacy of, and lessons learned from, using unmanned surface vehicles for these applications are discussed.

Automatic detection of sand ripple features in sidescan sonar imagery

A novel image processing technique, based on fingerprint analysis, is applied to sidescan sonar data in order to automatically extract seabed ripple orientation, wavelength, and defect density parameters. The technique is applied to seabed imagery collected on repeat passes over the same ripple field at headings distributed over 360° in order to evaluate ripple parameter extraction across a range of relative angles between the sonar and dominant ripple orientation. The presence of ripples increases the difficulty of identifying objects of interest in sidescan sonar seabed imagery. The potential for utilizing the density of ripple defects as an objective parameter for the quantification of this difficulty in the context of object detection operations is discussed. Results suggest a number of sidescan sonar data analysis applications that are highly compatible with unsupervised detectors and autonomous mission planning processes.

Field observations of bedforms in high sediment transport conditions

In the first reported use of MHz‐frequency acoustics to study small‐scale bedforms in the near shore zone, Dingler and Inman were able to observe the evolution of transition ripples under energetic groupy waves near the flat bed threshold. They found that these low‐relief, long‐crested bedforms appeared to be obliterated under the largest waves in a group, and subsequently reformed in one or two wave periods [J. Dingler and D. Inman, Proc. 15th Coastal Conf., Vol. II, 2109–2126 (1976)]. More recently, measurements of linear transition ripples were made during an autumn storm event using scanning wide beam and fixed narrow beam sonars, as well as a high resolution laser‐illuminated video system. These observations also indicate the occasional apparent disappearance of these ripples, and also their reformation at the same locations. The question arises as to whether the disappearance is apparent (the result of increased near bed sediment concentration and the definition of bed elevation based on reflection ...