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Introducing Sentry, Our Autonomous Secret Weapon on the Seafloor

The Autonomous Underwater Vehicle Sentry on the deck of the Atlantis. (Photo by Bridgit Boulahanis)
The Autonomous Underwater Vehicle Sentry on the deck of the Atlantis. (Photo by Bridgit Boulahanis)

Going to sea with two assets of the National Deep Submergence Facility (NDSF) is kind of like getting a driving lesson from Jeff Gordon while Dale Earnhardt Jr. hangs out in the back seat. It’s an embarrassment of riches, and taking full advantage of the collective capabilities is a daunting responsibility. In our case, the autonomous underwater vehicle (AUV) Sentry—a powerful mapping and imaging platform dressed in sleek yellow fiberglass—shares deck space on the Atlantis with Alvin, the newly refurbished human-occupied sub with a long history of transformative discovery.

To maximize our efficiency, the plan goes something like this: use Sentry at night to map areas of potential interest for future Alvin dives, taking advantage of the AUV’s larger survey footprint and unique sonar-based ability to “see” beneath the seafloor. “For example,” explains geologist and AT-36 co-chief scientist Adam Skarke, “bathymetry and water column mapping data collected by Sentry could be used to pinpoint the coordinates of a specific active gas plume that could then be targeted during a later Alvin dive.” Sentry thereby serves as a high-tech advance team, minimizing the time Alvin must spend perusing the seafloor for compelling targets while producing reams of valuable data in its own right.

The Deep Submergence Vehicle Alvin prepares for launch. (Photo by Anne Dekas)
The Deep Submergence Vehicle Alvin prepares for launch. (Photo by Anne Dekas)

“At its best, working with Sentry during a dive is a very relaxing experience,” says Bridgit Boulahanis, a PhD student in marine geophysics at Columbia University who was prominently involved in AUV operations during the expedition. Before the dive begins, the mission is planned out and uploaded onto the vehicle; the Sentry engineers go through an 11-page checklist before lowering the bright yellow instrument into the water. After launch, one team member monitors the dive, watching a shipboard monitor that relays the vehicle’s progress and reports any major malfunctions. “Everyone else goes to bed until it is time for the recovery,” says Boulahanis. “Of course, if something breaks then the entire team has to wake up and figure out how to fix it.”

That’s precisely what happened a few days into the cruise: during the previous night’s dive, Sentry hit bottom and got stuck, beset by 32 pounds (14.5 kg) of unaccounted-for weight. Only after some nimble maneuvering and full-powered upward thrusting did the vehicle wriggle free of the sediment and find its way to the surface, muddy and beleaguered.

A view from Alvin: mussels and clams coated with white microbial mat cover the seafloor as the robotic arm samples a carbonate rock. (Photo by Anne Dekas)
A view from Alvin: mussels and clams coated with white microbial mat cover the seafloor as the robotic arm samples a carbonate rock. (Photo by Anne Dekas)

But perhaps equally as alarming was the fact that the cause of the problem couldn’t be diagnosed. “We checked for any problem we could, and ruled them out one by one,” explains Carl Kaiser, the AUV Operations Manager at the NDSF. “Then we dove again to see if it got worse or not. Since Alvin was along and could provide an easy rescue, it was low risk.” Remarkably, subsequent dives went smoothly. “We still cannot identify the problem,” says Kaiser, weeks later.

Despite the technical challenges, Sentry obtained a great deal of sonar and high-res imagery data, the analysis of which is just getting underway. (The ambitious plan of using telepresence to enable the shore-based team to analyze Sentry data and thereby inform subsequent Alvin dives didn’t come to pass due to uplink data limitations.) Nonetheless, the AUV’s utility is unquestioned. “Having a vehicle that can map the seafloor at several orders of magnitude higher resolution than our global map is incredibly exciting,” says Boulahanis.

Global ocean maps max out at a spatial resolution of about a kilometer (0.6 miles), meaning that smaller objects typically go unnoticed. While roughly five percent of the seafloor is mapped to higher resolution, the limitations of the global resolution mean that even substantial objects can go unseen.

Case in point: the largest shield volcano on the planet (Tamu Massif in the northwest Pacific) was only found a few years ago.

“There are still enormous features waiting to be discovered,” explains Boulahanis, “and important details to see on smaller features. Tools like Sentry will inevitably lead to a richer world of deep ocean discovery.”