Publication Abstract

Adaptive survey strategies for acoustic surveys, with guidance on their application for surveying hard substrate reef habitats

Adaptive survey strategies for acoustic surveys, with guidance on their application for surveying hard substrate reef habitats

R. Coggan*, K. Vanstaen*. Report for Defra Project ME1102 30/03/2009.

This project was involved with studies of the seabed in the central English Channel and addressed our need for improved evidence about the distribution and nature of seabed habitats so that we can better manage the area for conservation and sustainable use of resources. The project involved ship based surveys of broadscale areas in the central Channel, searching for rocky reef habitats, investigating the faunal communities associated with gravel substrates, and developing new approaches to broadscale surveys targeting hard substrates (stony and rocky reefs). The survey work used acoustic remote sensing techniques (sidescan- and multibeam- sonar) and directed ground-truth sampling (underwater video and Hamon grabs). The three distinct parts of the project are referred to as the Reef study, the Aggregates study and the Survey Strategies study. The latter of these is reported here.

The aim of this study was to develop a survey strategy that would optimise the efficiency of future exploratory acoustic (sonar) surveys searching for hard-substrate habitats (reefs). The traditional approach to acoustic mapping surveys is to scan the seabed in a pre-determined pattern of closely spaced parallel lines to build up a sonar mosaic (image) of the whole area (like a lawn mower systematically cutting an area of grass). With this method, many hours of survey time could be spent covering expansive areas of soft substrate that are clearly unlikely to contain the target reef features. An alternative approach is an Adaptive Survey Strategy, where the position of the survey lines is not pre-planned, but developed in real-time according to what is ‘seen’ by the acoustic instruments as the survey progresses. In this way, the survey can be more effectively target the features of interest.

We developed two rule-based designs under the Adaptive Survey Strategy, one for targeting a feature of interest, such as a reef, and the other for following a border; such that frequently exist between two seabed types (e.g. rock and sand). The first ‘Systematic’ design starts by running a few widely spaced parallel lines (e.g. three lines each 100 km long, spaced 4 km apart) and then applies a simple rule that whenever two adjacent lines both show a feature of interest (rock reef) at a similar distance along the line, a shorter ‘in-fill’ line is run mid-way between them. This rule is applied again and again until a full-coverage mosaic is made of the feature. The design can be applied to any feature of interest and minimises time spent on non-target features.

The second ‘Adaptive’ design uses the same pilot lines as the first, but where a border between two contrasting seabed types is found, a very short (2 km) in-fill line was run immediately adjacent to the pilot line to pick up the feature. The rule is then applied that as soon as the border has again been crossed by the acoustic sensors, the ship makes an about-turn and runs another short line, parallel and next to the previous one.

In practice, our reef survey unexpectedly revealed a vast expanse of outcropping rock along the initial ‘pilot’ lines and there was insufficient survey time to cover it all. Consequently, we were obliged to revert to a nested survey design, using in fill lines across most of the survey area and targeting a small sector (32 sq km) for full coverage with high definition multibeam sonar to help characterise the reefs. The Adaptive survey design was tested successfully to follow two linear features within the reef system.

These new survey designs utilise the survey vessel in an unfamiliar manner, requiring more rapid turns than is usual practice. The report discusses certain operational constraints and precautions that are necessary when employing the new designs, and compares them to the designs of a number of recent habitat mapping surveys carried out by Cefas for the Nature Conservation Agencies. Experience from these surveys is used to provide guidance on best practice.

The report concludes with five recommendations

• A seabed terrain model is developed by SeaZone Ltd based on Digital Survey Bathymetry.
• Where no DSB is available, an Adaptive Survey Strategy should be adopted, beginning with a Pilot survey to reconnoitre the area.
• If extensive outcrops are encountered, a nested survey design should be employed, using planned in-fill lines and obtaining 100% multibeam cover over representative sample areas.
• Where patchy reef is encountered, the survey should proceed using the Systematic and Adaptive survey designs developed by this project.
• Prior to any survey, opportunity should be taken to consult other stakeholders to identify and deal with any potential conflicts that may arise during survey operations.

R. Coggan*, K. Vanstaen*

Project report to defra: ME1102 Survey Design Report 300309_full.doc