Sea Technology

NOV 2013

The industry's recognized authority for design, engineering and application of equipment and services in the global ocean community

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(Photo Credit: Masanobu Shibuya) VideoRay Sonar CoPilot can be deployed and operated by a single operator. (Bottom) MBIS used to track an oil rig to aid ROV control. for acoustic returns or echoes. Through a method known as beam forming, the transducer array is divided into a series of pseudobeams which are used to measure the range and intensity of the returned sound signal by breaking it into a fxed number of equiangular channels. The recorded signal from each channel is passed through an analog-to-digital converter and then through a signal processor before fnally being plotted to create a digital 2D image that can be visually interpreted. As with all acoustics, the usable range depends on the frequency employed (a low frequency propagates further than a high frequency) and the strength of the return from the target; itself a function of the material from which the sound is being refected. Soft sediment and sand, for example, are poor acoustic refectors making them diffcult to detect against ambient noise, whereas metal and concrete tend to give off a much stronger "ring" and can therefore be detected at greater distance. Typical MBIS units in the market range from about 200 kilohertz (which will allow stronger targets to be detected at several hundred meters) to 3 megahertz (which will detect only the brightest targets beyond about 5 meters). To date, the most common MBIS in use operates at a center frequency of 900 kilohertz. 40 st / November 2013 Importantly, unlike their predecessor, the singlebeam scanning sonar that creates a single 2D image by sweeping a pencil beam across a given area of interest, an MBIS collects data from many hundreds of beams simultaneously, multiple times per second, creating a series of images every second rather than a single image every four or fve seconds. This has the advantage of providing the user with a streaming video-like image rather than a series of captured stills. (For this reason, ROV pilots sometimes refer to MBIS as acoustic cameras). Furthermore, due to the fast update rate (modern imaging sonar can image at more than 20 hertz), multibeam imaging sonar are motion immune, i.e., they do not smear or blur when operated from a moving platform. There is no longer any need to stop and scan when navigating an ROV by sonar. This makes an ROV a more effcient tool for aiding piloting operations when visibility is poor, e.g., in areas of high turbidity or when operating in the dark such as deep water or night-time operations (during night-time operations, ROV lighting typically limits visibility to just 10 meters in clear water conditions). Imaging sonar therefore complement optical camerabased navigation by allowing their user to see through suspended sediment and far beyond the limitations of either natural or artifcial light, and are rapidly replacing scanning sonar as standard tooling across the ROV industry globally. Sonar images also provide range information where optical imaging systems cannot. While the MBIS is used today as a standard visual aid for the manual piloting of ROVs, of perhaps greater worth is its pioneered use as a sensor input for the dynamic positioning and automated control of ROVs. Using automated targetrecognition routines, range and relative bearing of selected

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