Multibeam Swath System
Hull-mounted on the RV Tangaroa, the EM302 multibeam echosounder maps the seafloor using a fan of 288 acoustic beams, producing up to 864 soundings per ping in dual swath mode, providing 100% coverage of the seabed. The resulting surveys show far greater detail than the earlier method of multiple lines using a single-beam sounder, as well as greatly reduced ship survey time and cost.
A Kongsberg EM-300 multibeam echosounder, originally installed in 2000, was upgraded to an EM302 on the Tangaroa during 2010. The EM302 is a high resolution 30-kHz multi-beam system, which produces excellent bathymetry and imagery in depths up to approximately 8000 meters. While complying with the performance standards defined in the International Hydrographic Organization’s performance standards, S-44 edition 4, it also meets Land Information New Zealand’s (LINZ) Hydrographic standards. Its beam width measures 1-degree along track by 2-degree across track and incorporates the facility to fully steer the beams to compensate for vessel movement using roll, pitch and yaw stabilisation. Overall swath coverage is up to 5.5 times water depth, increasing with depth to a maximum width of 7000 meters at an approximate 2500 meters, where it remains relatively constant until 5000 m, then slowly decreasing until extinction.
While bathymetry data reveals the shape and depth of the ocean floor, imagery data can also be produced using the strength of the return signal (backscatter). This data can indicate variations in bottom type and therefore what habitats may occur. In addition to bathymetry and backscatter, the system records water column information which can be used to identify seeps and plumes, fish schools and other features not normally imaged in the bathymetry data. The system is also mammal friendly, having the ability to reduce the system power if required.
The EM302 installation includes a POS/MV 320, which integrates positioning and motion compensation. The POS provides precise GPS navigation solutions, in addition to heave, pitch, roll, and yaw information for EM302. The precision of the POS/MV motion sensor allows for high calibration accuracy of the multibeam system and provides a freedom from acceleration errors induced when cornering.
Data are recorded using Kongsberg's SIS software, and processed on board using CARIS HIPS & SIPS and QPS Fledermaus along with ArcInfo, the end product being bathymetry data, geotiffs and backscatter imagery for use in both Hydrographic surveys and research programs.
The EM302 and its predecessor (EM300) have now been used for mapping the New Zealand realm, from the Equator to the Antarcti,c and in the 12 years since first installation in excess of 500,000 km2 of the seafloor has been mapped.
Note that no part of these figures are to be reproduced or interpreted without prior, written permission of NIWA.
A video explaining how the multibeam system on the R.V. Tangaroa works and is used can be watched above, or in our video gallery.
John Mitchell [ email@example.com ]