Sea Technology

AUG 2012

The industry's recognized authority for design, engineering and application of equipment and services in the global ocean community

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(Left) A RESON SeaBat 7125 MBES data mosaic (50-centimeter resolution) showing SMS chim- neys over the Fonualei Spreading Center off- shore Tonga. (Bottom) A 3D drape of AUV magnetometer data showing magnetic anomalies associated with regional structure and possible hydrother- mal alteration within a caldera. conducted closer to the seafloor. Historically, this detailed mapping has included use of deep-tow side scan sonar tech- nology, which is limited by reduced survey speed (maximum 2 knots), poor positional control of the tow-body due to the long length of tow cable behind the survey vessel and limita- tions of tow altitude (100 to 300 meters above the seafloor) re- quired to avoid collision in predominately rugged volcanic terrain. The result is limited resolution of typically 1-to-2-meter cell size. Another method for detailed seafloor mapping uses ROV technology. Although ROVs can host numerous geophysical mapping sensors and are well-suited to ground intervention and opportunistic sample recovery (geological rock specimens or hydrothermal chimney fragments), they are an inefficient means of mapping the seafloor and limited to maximum survey speeds of 0.5 to 0.7 knots. In addition, ROVs are not well- suited for continually traversing rugged volcanic terrain, and it is difficult to sustain long periods of productivity and minimize equipment breakdown unless two ROVs are utilized. Despite their limitations, a combination of these techniques has yielded a high success rate in the discovery of SMS prospects for Nautilus Minerals on its exploration tenements. AUV Technology AUVs could bridge the gap between target generation from vessel-based MBES acquisition and detailed, prospect-scale ROV intervention for SMS exploration. AUVs provide the op- erational flexibility to survey with a variety of sensor payload configurations at different scales of resolution, provide higher- resolution mapping than is possible with conventional deep- tow technology and offer significant operational productivity gains over ROVs. Conducting high-resolution geophysical surveys by AUV would enable ROVs to be deployed more effectively for other tasks to which they are better suited, such as selective follow- up ground truthing, sampling and electromagnetic surveys. AUV Proof-of-Concept Survey In November 2011, Nautilus Minerals sponsored a collab- orative marine science research cruise initiative with the Uni- versity of Hawaii and the Leibniz Institute of Marine Sciences at the Christian-Albrechts Universität zu Kiel (IFM-GEOMAR) over Nautilus Minerals' tenements in the northeast Lau Basin, Tonga. With the RV Kilo Moana from the University of Hawaii and Office of Naval Research, and GEOMAR's Abyss AUV, the 21-day cruise provided an opportunity to undertake a proof- of-concept evaluation of AUV technology for SMS mapping applications. The AUV was used for detailed follow-up map- ping of hydrothermal plume anomalies discovered during an- other collaborative cruise initiative with the Commonwealth Scientific and Industrial Research Organisation (CSIRO), Aus- tralia, using its research vessel MV Southern Surveyor in 2009. www.sea-technology.com AUGUST 2012 / st 11

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