Where No Man Can Go: Robot Surveys the Deep

The first scientific mission using Sentry, a robot capable of diving 5,000 m, or 3.1 miles, into the ocean, was successfully completed by scientists and engineers from the Woods Hole Oceanographic Institution (WHOI) and the University of Washington (UW). The vehicle surveyed and helped locate several proposed deep-water sites for seafloor instruments that will be used in the National Science Foundation’s (NSF)’s planned Ocean Observatories Initiative (OOI).

Once launched, Sentry operates without being attached to a ship and is pre-programmed for the areas it is to map. Credit: University of Washington

Sentry is a state-of-the-art, free swimming underwater robot that can operate independently without lines or other connections to a research ship. The autonomous underwater vehicle (AUV) is pre-programmed with guidance for deep water surveying. However, it can make its own decisions about navigation on the terrain of the sea floor. Sentry will soon conduct high-resolution oceanographic surveys that, until now, have been impossible.

Recently, working in tandem with sonar instruments a UW-operated research vessel and with photo-mapping by WHOI’s TowCam seafloor imaging system, Sentry gathered the most precise maps to date of the seafloor features known as Hydrate Ridge and Axial Volcano off the coast of Oregon and Washington. Sentry made six dives during the expedition and surveyed 212 linear kilometers of seafloor, or about 53 sq km, as it traced parallel lines like a lawn mower making a pattern.

Sentry, a one of a kind undersea vehicle, was used for the first time during an oceanographic cruise off the US West Coast from July 22-August 5, 2008. Photo credit: University of Washington

The AUV can collect the data needed to make seafloor maps at a resolution of less than one meter. On its first cruise, Sentry collected as many as 60 million individual soundings of seafloor depth in a single dive.

Powered by more than 1,000 lithium-ion batteries, Sentry dove for as long as 18 hours and 58 km. The AUV is designed to swim like a fish or fly like a helicopter through the water. Its hydrodynamic design allows it to descend quickly from the sea surface to the depths (approximately 3,500 meters/hr). The novel shape also gives the vehicle stability and balance while cruising through bottom currents. It has thrusters built into its foils which allow the AUV to gain lift or drag or directional momentum as needed.

When necessary, Sentry can hover over the bottom for close-up inspections, navigational decision-making, and rising up and down over rugged terrain. The design allows the vehicle to start, stop, and change directions. It steers itself with a magnetic compass; long-baseline (LBL) navigation triangulated from underwater beacons; a sophisticated inertial guidance system (INS); and, when within 200 meters of the bottom, an acoustic sensor that can track the vehicle’s direction and speed with precision.

This diagram shows the formation of an asphalt volcano and associated release of methane and oil. Photo credit: Jack Cook, WHOI

Recently, Sentry was used to investigate asphalt volcanoes at the bottom of the Santa Barbara Channel. The largest of the volcanoes lies at a depth of 700 ft, too deep for scuba diving. Some of the volcanoes are said to be longer than a football field and up to six stories high – all made completely out of asphalt. Sentry was used to conduct a detailed survey of the area. In addition, researchers used a mass spectrometer, carbon dating, microscopic fossils, and comprehensive, two-dimensional gas chromatography to determine that the volcanoes are made of asphalt and were probably formed when petroleum flowed from the seafloor about 30,000-40,000 years ago.