Figure 1 shows the present state of surveys over the UK continental shelf. Out of an area of some 173 000 square miles contained within the British Fishery limits, 36 per cent has been surveyed by echo sounder, but not necessarily to a scale suitable for modern standards, and only surveys to full modern standards are sufficient to ensure safe navigation within r m of the seabed; a modern standard survey must be carried out using a modern echo-sounder and include a thorough sonar sweep. All hips of the Survey Flotilla are now fitted with a Kelvin Hughes MS48 echo sounder and an OAL side-scan sonar.

New technology, such as the towed side-scan system, has enabled the surveyor to examine the whole of the sea bed between a regular pattern of sounding lines, spaced to ensure that the sonar beam from one line searches at least 50 m beyond the adjacent lines either side. This ensures that all wrecks, rocks and other obstructions on the sea bed which could be a danger to navigation are located, and subsequently closely examined for the least depth over them. Side-scan sonar is best operated at a speed of 5 or 6 knots, which means that the time taken to carry out a survey to full modern standards is far longer than twenty years ago, but during the last five years almost 8 per cent of the UK continental shelf has been fully surveyed to modern standards.

Our Survey Flotilla comprises eleven vessels, four Ocean Survey Ships – Heda, Hecate, Herald and Hydra; four Coastal Survey Vessels – Bulldog, Beagle, Fox and Fawn; and three Inshore Survey Craft – Egeria, Enterprise and Echo.