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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"Conducting high-resolution geophysical surveys by AUV would enable ROVs to be deployed for tasks to which they are better suited." The GEOMAR Abyss AUV, a Hydroid Inc. (Pocasset, Massachusetts) REMUS 6000 depth-rated to 6,000 meters, in- cluded a dedicated launch and recovery system and was contained in two stan- dard 20-foot containers. The primary mapping payload was either a RESON A/S (Slangerup, Denmark) SeaBat 7125 400-kilohertz MBES system or EdgeTech (West Wareham, Massachusetts) 2200 410-kilohertz side scan sonar system. In addition, this marked the maiden cruise for a three-axis flux-gate magnetometer AUV payload built by GEOMAR. Mag- netometer data was acquired concur- rently with MBES or side scan sonar primary payloads, along with CTD and turbidity data. The AUV was deployed in fully au- tonomous mode by laying a two- transponder long baseline array over each dive site. Having additional batter- ies and long baseline transponders avail- able on board during the cruise allowed dive sites to be prepared ahead of the AUV, and the AUV to be operated in a continuous cycle, enabling a fast turn- around time between dives. The support vessel was able to engage in other activ- ities while the AUV was surveying, in- cluding vessel-based MBES and magnetometer survey operations, and camera tow and seafloor sampling activ- ities. Results The AUV mapped a total area of 114.71 square kilometers during the cruise, achieving 1,199 line kilometers of survey. Of those 1,199 line kilometers, 1,007 line kilometers of actual data were recovered, due to some data loss en- countered on two dives. The AUV un- dertook surveys 46 percent of the 240.58 hours total dive time during the cruise, with only 5 percent time utilization at- tributed to equipment failure that caused launch delays. Eighteen percent of oper- ations time was spent on vessel transits between dive sites; 27 percent on AUV dive preparation, launch and recovery, including long baseline acoustic transponder deployment and recovery; and 4 percent on dealing with inclement weather. The AUV completed surveys of 10 out of a planned 14 target areas, constituting 16 dives in total. From the data acquired, it was possible to map SMS chimney field targets using MBES and side scan sonar. The magnetometer data also pro- vided useful information related to struc- tural trends and anomalies key to understanding SMS mineralization for- mation processes. CTD and turbidity data provided useful detailed-scale infor- mation pertaining to the location and distribution of plumes from active hy- drothermal sources. AUV Versus ROV Survey Efficiency Comparing AUV and ROV survey ef- ficiencies using historical data from Nau- tilus Mineral exploration programs over an equivalent cruise duration shows that AUVs are at least 5 to 6 times more effi- cient; a total of 212.54 line kilometers for ROVs compared to 1,199 line kilo- meters for AUVs, or 1,007 line kilome- ters if considering data loss experienced during the Kilo Moana cruise. A comparison of area coverage statis- tics shows nearly double square kilome- ters of coverage using AUV technology; an average of 9.4 square kilometers per AUV dive compared to an average of 4.8 square kilometers covered per ROV dive. However, it must be noted that this is a comparison of an AUV traverse line spacing of 120 meters versus an ROV traverse line spacing of 200 to 300 me- ters. Future survey efficiency improvements could realistically enable AUV traverse line spacing to be opened up to 200 to 300 meters to gain greater area coverage efficiencies over ROV survey as geolog- ical targeting confidence increases with ongoing data set familiarity from map- ping sensors onboard the AUV. The platform stability and payload flexibility afforded by AUVs enables 100 percent seafloor coverage with high-res- olution MBES or side scan sonar to a res- olution of 50 or 5 centimeters, res- pectively, which enables much higher 12 st / AUGUST 2012 www.sea-technology.com

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