DICT participates in recent Bathymetric survey

July 12, 2013 by dyertrust

DICT participates in recent Bathymetric survey aroundthe Dyer Island system and inshore Reefs.

The Dyer Island Conservation Trust has recently collaborated with Underwater Surveys (PTY) Ltd, a private marine services company based in Cape Town. Mark Prowse and Hydrographic Surveyor Andrew Matthew towed the company survey boat from Cape Town and spent two full days at sea in the Gansbaai area. The 6.7m custom-finished vessel Blue Dolphin is a computer lab on water and is fully kitted out with a high-resolution multibeam echosounder which is pole-mounted underwater over the stern and complemented by high-accuracy Real-time Kinematic (RTK) GPS positioning and vessel attitude sensing equipment. Huge amounts of data are collected and logged onto a high-powered PC running some really advanced survey software which displays bathymetric data in real-time for the operators. The aim of this visit was to conduct a detailed bathymetric survey and produce high resolution three dimensional charts of some interesting areas around the Dyer Island system and the inshore regions seafloor and topography.

The DICTs resident Marine Biologist Alison Towner was invited along to observe Mark and Andrew demonstrate the system for a while on their second day of survey. She explained how she enjoyed the experience of joining the surveyors immensely, she was even fortunate enough to assist as they surveyed over a few favourite reefs to capture data and produce real-time digital terrain models.

For best results this type of work ideally requires perfectly flat sea conditions and a lot of skill on the surveyors and boat skippers part Alison said. Dyer Island, being densely encapsulated by Kelp forests and reefs is a challenging environment to conduct bathymetric work. In fact, the only data available for the region prior to this survey are standard navigation charts based on old naval data. I couldnt believe how quickly the data is acquired and displayed on the screen - it literally happens in real time. The guys worked very fast and efficiently, with meticulous attention to detail says Alison.

This equipment is used for bathymetric and engineering surveys for civil engineering, oil and gas, dredging projects and general hydrographic mapping for navigation purposes. It is deployed in harbours, inshore areas, offshore and even dams and rivers. Far more sophisticated than traditional singlebeam echosounders, multibeam echosounders generate a swath (narrow fan perpendicular to vessel track) of return echoes from transmitted pings which are integrated with GPS position and corrected for vessel motion and attitude by navigation software which depicts and logs the data as accurately geo-referenced points on the Earths surface. As the vessel surveys along a track, the individual multibeam data swaths accumulate and form a 3D Digital Terrain Model (DTM) which shows the seabed in great detail. Typically, swath widths 4 times the depth of water can be seen below the vessel, meaning that while surveying in 10m of water, 40m of seabed swath is visible. By mowing the lawn and surveying many lines across an area, a detailed map is built up. After some data validation is done back in the office, cleaned and accurate point data can be exported from the DTM for use in engineering or mapping applications. Some of the more interesting uses are searching for underwater objects, such as wrecks or lost aircraft and by the navy to search for underwater threats.

Mark Prowse oceanographer from underwater surveys commented: Mapping high-density shark areas around our coastline should contribute to a better understanding of their habitat, and some of the reasons that sharks choose to spend more time in certain areas the benthic characteristics of such areas are usually not well documented and this demonstration over well-known areas around Dyer Island has revealed a wealth of detail never seen before and surprisingly similar to the rocky York Shoal in False Bay which is another shark hot-spot. Mark also mentioned another well-known site theyve surveyed for the oil and gas industry. We hope to collaborate in this area again knowing that this information will go towards enriching our knowledge in terms of the shark tagging research for DICT.

The reefs around the island and inshore are incredible and completely blew my mind in terms of their size and complexity commented Alison. The features we mapped range from deep channels and troughs particularly South of Geyser Rock, to vast expansions of rocky reef sloping off inshore from the Island. The details of these features are simply outstanding and very interesting. I am now beginning to understand further why our acoustically tagged White sharks patrol these sections of Dyer Island so extensively!