Users of depth data have felt for some years now that echo-sounder surveys are inadequate both for navigation with close tolerances and for various engineering works below the waterline.

Sidescreen sonar helped ecnormously in increasing the awareness of the degree of inadequacy of line scanning echo-sounding surveys. However, sidescan has never quite become a survey equipment for a number of reasons. It is difficult to relate sidescan, in anything but the broadest sense, to a deoth survey. It is selective in what it shows, depending on the scan aspect to a feature or target. Sidescan sonar, used with caution, is a diagnostic tool, i.e. it can indicate the presence of a something which requires further examination. Therefore, in one respect, it has tended to increase the time spent on depth surveys, although as a consequence the latter were improved.

Interferometric swather sounding now adds depth values to the shadowgraph information of sidescan sonar. The Bathymetric swathe sounder, operating at 300 kHz, succeeds in producing depth data at a density which is about equal both down-track and cross-track, where as the echo-sounding down-track to cross-track ratios are almost invariably appreciably in excess of 1:100.

The swathe sounding system has proved capable of coping with gentle and steep sloped, and has already operated successfully in inclement weather conditions from small and large vessels. It does not need shadow information to delineate seabed features, and has been described as a "one-pass" system.

The Principle Of The System

The system has been developed on the basis of an acoustic interferometer which was the subject of SERC-funded research project from 1979 to 1985, at the University of Bath.

The concept was described in a paper first presented at the 1982 Oceanology Conference, and subsequently published in the Journal of the Royal Institute of Navigation (Cloet et al., 1982). The aim was to achieve depth data, laterally from the survey ship's track, of a quality equal to that which could be attained by means of an echo-sounder along the ship's track, i.e. to fill-in the space between survey sounding lines with soundings rather than sidescan sonar shadowgraphs.

The system was designed to be mounted in a towed vehicle which needed to be sufficiently small to be deployable from small vessels. Towing was adopted for the sake of transferability from ship to ship, and to enable it to be lowered below the seasonal thermocline when necessary. The ability to operate from a small vessel was to exploit to the maximum the possibility to measure depths in navigationally hazardous areas from a stand off position without endangering the vessel.

The interferometers operate at 300kHz. A beam 1°wide in azimuth and at 60° elevation is transmitted on one transducer and received, so that two interferometers are generated the transducers forming the interferometers are spaced slightly differently so that a difference phase ramp is produced which allows the normal phase ambiguity to be resolved. This ambiguity is caused by the physical dimensions of the sidescan transducer arrays.

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