Category: General Article
Sunday, 16 August 2015
Submitted by Rachel Teasdale
Summary: Learn how scientists at Axial Seamount are using a sophisticated underwater robot — officially known as an Autonomous Underwater Vehicle — to map the area of the volcano where the April eruption occurred.
This afternoon we launched the Autonomous Underwater Vehicle (AUV) Sentry to start a 24 hour survey of Axial’s north rift zone where the largest lava flows were erupted 4 months ago. Preliminary low-resolution mapping of this area was completed in July 2015 by the University of Washington, but Sentry’s work today will provide much high-resolution multibeam sonar mapping of the area.
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 197 feet (60 m) above the seafloor during its mapping survey.
During a multibeam sonar survey, sound waves are emitted from the AUV and are reflected from the ocean floor, and they return to receivers on the AUV. The time between the emitted and reflected pulses is used to determine the distance to the seafloor. Depths are represented on maps with colors indicated in the map legend. By emitting a vast number of pulses in a grid pattern, AUV Sentry can map a broad area, which in today’s dive will cover approximately 24-32 mi2 (15-20 km2). In addition to the multibeam sonar that AUV Sentry uses to map the seafloor, there is an array of sensors onboard for other measurements such as water temperature and salinity and other hydrothermal plume sensors. In this way Sentry can map the seafloor and the plumes above the seafloor at the same time.
At 2570 lb (1250 kg), the AUV Sentry is launched from the deck of the Thompson using a large onboard crane. The AUV is then lowered into the sea and released. AUV Sentry descends at approximately 132 ft/min (40 m/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 19,685 feet (6,000 m) depth, the dives at Axial Seamount will not exceed 5760 feet (1800 m).
Today’s AUV dive is planned to last approximately 24 hours and can maintain an operating speed up to 2.3 knots or 1.4 mph (1.2 m/s). 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 Autonomous Benthic Explorer (ABE) 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.
More information about the AUV Sentry is available from Woods Hole Oceanographic Institute, including videos and an animation of her making a dive.