The Sikuliaq has several powerful instruments and sensors on board, and we brought along a few of our own in addition to those, but the up-close and personal magnetic data from the oldest Jurassic crust in the Pacific — our main goal — just can’t be gathered from the surface. For that, we need the help of the Autonomous Underwater Vehicle Sentry and the team of engineers from Woods Hole Oceanographic Institution who run it.
Sentry is an AUV — an autonomous underwater vehicle. This puts it in contrast with many other oceanographic research vehicles which are often manned submersibles or ROV’s — remote-operated vehicles. Autonomy provides a variety of benefits: there are no passengers to take up weight and volume that could be used for batteries or instruments, and the lack of an operator controlling each move means there is no latency between new data and navigation decisions.
Even among AUV’s, Sentry’s design gives it several advantages as a platform for deep sea science. For one thing, the vehicle’s shape and vertical orientation make for easier and more flexible navigation. Sentry has about a forty-five degree field of view and is capable of avoiding obstacles if it sees them. Similar to the way a helicopter exceeds the maneuverability of a plane, Sentry’s four props on movable fins give it much finer control over its motion than typical torpedo-shaped vehicles whose propulsion is much more directional.
Each of these propellers is made of carbon fiber, and they actually come from a specialized Czech model airplane vendor. In combination, they can drive the vehicle at speeds of up to about 2 nautical miles per hour. In practice, stability of Sentry’s travel decreases as speed increases, and the limiting factor for all considerations is the finite resource on board: battery life. It takes more power to maintain a stable trajectory at a higher speed. The tradeoff is that much of what Sentry does — including collecting data with a magnetometer for us — involves traveling over large spans of the ocean floor, so speed and distance count for a lot. As the vehicle continues to accumulate data from each dive, the Sentry engineers are always analyzing the speed/power tradeoff, including factors like water temperature and currents, to coax even better performance from the vehicle. So far on this cruise, we’ve seen Sentry break its all time distance record and resurface with battery power to spare on a number of occasions, indicating that the most impressive dives are still ahead.
Sentry is constructed of syntactic foam with a ceramic housing. It is reconfigurable depending on the needs of each individual science assignment; different instruments can be replaced or added to suit the particular type of data being collected. Fully outfitted with the magnetometer and other instruments for our cruise, Sentry weighs about 3000 pounds. After dropping its expendable steel ballast (not recovered, but environmentally harmless), it is neutrally buoyant and can use its unique propulsion configuration to navigate, ascend, and descend with ease.
On our cruise, Sentry has been operating, roughly, on a 48 hour cycle with about 30 hours of dive time bookended by ascent, recovery, recharging, launch, and descent. While on deck here in tropical latitudes, Sentry’s interior computers and instruments are protected from the heat by a water cooling system which pumps 48 degree F water through the AUV to remove waste heat. When submerged, the ocean itself keeps the components cool.
Sentry’s capabilities are crucial to our mission: without it, we would be unable to get a magnetometer close enough to the seafloor to get the high-resolution readings of the magnetic record in the ocean crust that we need to complete the survey of the history of field reversals during the Jurassic. As the vehicle continues to exceed expectations, we’re looking forward to returning with even more of this record. After this cruise, as we return to the lab to process and interpret these data, Sentry will continue its own mission of enabling unprecedented oceanographic science.
You can read a lot more about Sentry at its own page on Woods Hole’s website