Operating synthetic aperture sonar

Paul Boughton
Synthetic aperture sonar (SAS) improves on the range, resolution and positioning of conventional SSS by advanced digital signal processing. iXSurvey has purchased a SHADOWS SAS (synthetic aperture sonar) system manufactured byIXSEA SAS.

Resolution approaches 500kHz quality but at up to 300m range with sub-metre positional accuracy. A real time georeferenced 'mosaic' of the seabed is produced. A forward looking 'Gap Filler' sonar is used to image directly beneath the fish, negating the need for 200 per cent coverage and halving survey time.

The ability of a of a conventional SSS to resolve a discrete seabed target is controlled by beam width and pulse length; add to this uncertainty in the position, heading, roll and pitch of the towfish and errors increase still further.

To illustrate, a conventional sidescan with an array length of 25 centimetres at 100 KHz will have a beam width of about one degree.

At 300m, 1degree of arc covers about 5m along track - the smallest resolvable object. Narrowing this beam width requires a physical 100kHz transducer many metres long.

In SHADOWS, this requirement is replaced by a virtual transducer integrating multiple pings with towfish position and attitude. This comes from two sources, 1) the onboard Inertial Navigation System (INS) and 2) the position correlation between consecutive pings. This assumes that each ping has all the information needed to determine the change in fish position and attitude by identifying the same target on two consecutive pings and resolving the movements needed to place the target at the same point on the seabed. At 300m range and 5knots a target will be insonified by over 35 separate pings.

iXSurvey undertook extensive testing, before commercial operations began, to determine optimum settings and scenarios. Systematic variation of power output, pulse length, range and fish height were made whilst surveying point targets and seabed features, eg sands, gravels and muds. Image quality and absolute positioning accuracy were then compared.

The system was found to excel at broad, regional seabed mapping where it gives high quality data in very efficient timescales, but is also able to acquire stunning quality images over specific targets at short range.

SHADOWS represents a huge advance in technology, and for economic use it requires a similarly large change in mindset for survey planning. For example, the traditional line plan for a 1000m pipe or cable route survey may be reduced by two thirds by replacing 15 lines of SSS and sub-bottom profiler with only 2 lines of SHADOWS to cover the entire area. The real time mosaic would then be used to determine the optimum route, by passing hazards, and this would then be surveyed with three lines at 25m spacing. This reduces the survey time and focuses data in the most important zones.

iXSurvey recently used SHADOWS on a seagrass mapping project in Cavailaire, South of France.

Enter 12 or at √ www.engineerlive.com/ihss

Dominic Koe is with iXSurvey, Edinburgh, Scotland. www.ixsurvey.com

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