By Rachel Teasdale
|Bathymetric map of Axial Seamount caldera and north rift zone, with 2015 lava flows outlined with black lines.|
AUV Sentry was built by engineers at WHOI, including Dana Yoerger who is aboard the Thompson and leading the AUV operations for the 2015 Axial Seamount Expedition. AUV Sentry is pre-programmed to fly a pattern of tracklines that go back-and-forth over the seafloor in a grid pattern. Today, Sentry is “flying” approximately 60 m above the seafloor during its mapping survey.
|Diagram of multibeam sonar in which the AUV Sentry emits energy (sound waves) that reflect from the ocean floor, from Woods Hole Oceanographic Institute.|
At 1250 kg (2570 lb), the AUV Sentry is launched from the deck of the Thompson using a large onboard crane (Photos below). The AUV is then lowered into the sea and released. AUV Sentry descends at approximately 40 m/min (132 ft/min), assisted by two sets of dive weights, which assist the descent. One set of weights is released at the seafloor to keep the vehicle neutrally buoyant, and then before ascent, the last set of weights is released. While AUV Sentry is capable of diving to 6,000 m (19,685 feet) depth, the dives at Axial Seamount will not exceed 1800 m (5760 feet).
Today’s AUV dive is planned to last approximately 24 hours and can maintain an operating speed up to 1.2 m/s (2.3 knots or 1.4 mph). Once AUV Sentry is recovered tomorrow, it will take 10-16 hours to recharge the Lithium Ion batteries in preparation for the next mission.
AUV Sentry is the second of a family of AUVs, which started with ABE (Autonomous Benthic Explorer) in 1996 but was lost in March 2010 during a dive exploring the subduction zone off the coast of Chile. Sentry’s work started in 2009, utilizing improvements made from lessons learned with ABE. According to Yoerger, the hydrodynamic shape of AUV Sentry allows for faster ascent and descent, more stability and less drag than the torpedo-shaped ABE. The red foils (wings) also allow AUV Sentry to maneuver, including hovering to hold a position and changing directions. AUV Sentry also has upgraded navigation, speed, range, and maneuverability than ABE.
|Images of AUV Sentry launch on 16 August 2015.|
More information about the AUV Sentry is available from Woods Hole Oceanographic Institute (http://www.whoi.edu/main/sentry), including videos and an animation of her making a dive.