Dives of AUV "r2D4" to Rift Valley of Central Indian Mid-Ocean Ridge System

the Autonomous Underwater Vehicle "r2D4", constructed in 2003, is a cruising type AUV weighing 1,600 kgf with a length of 4.4 m. It is fitted with a side scanning sonar, an interferometry sonar, a CTDO, and an in-site chemical analyzer "GAMOS" for measurement of manganese concentration. The AUV "r2D4" has been deployed off Sado-ga-shima Island along a fault line in the Sea of Japan (2003), over Kuroshima Knoll (2003), North West Rota 1 Underwater Volcano (2004) where it took video images of plumes, and Myojin-sho Underwater Crater (2005), and demonstrated its high performance in autonomy and reliability in hostile environments. During NH06-4 Leg 3 cruise in December 2006, we deployed "r2D4" in the rift valley of Segments 15 and 16 of the Central Indian Mid-Ocean Ridge System. The first area "r2D4" surveyed is "Roger Plateau (tentative name)" at 19_34S, 65_51E. The AUV "r2D4" performed three separate dives and found some anomalies in the concentration of manganese, but we could not identify the location of the origin of these anomalies. The second area is a narrow gorge in the middle of segment 16. The AUV "r2D4" descended to the east peak of the rift valley, with a depth of 2,400 m, near the center of the gorge. After calibration of its magnetometer, the AUV jumped off the cliff to the bottom of 2,700 m deep gorge. The AUV "r2D4" cruised over the bottom of the gorge for about 6 hours at 3.