Polar Discovery :: Expedition 2: Polar Mail, July 9

Dear Lauren,
Oceanographers seem to like to create acronyms for the things they talk about. For example, they refer to “mid-ocean ridges, as “MORs.” Specific mid-ocean ridges, such as the Mid-Atlantic Ridge or the East Pacific Rise became the “MAR” and the “EPR.” Our device that measures “conductivity, temperature, and depths” in the ocean is referred to as a “CTD.”

It’s often the same with vehicles. One of WHOI’s underwater robot vehicles is called “ABE,” which stands for “Autonomous (which means it is swims on its own, untethered by a cable to the ship) Benthic (which means “at the bottom of the ocean”) Explorer.

Puma roughly stands for “Plume Mapper,” because that is its main mission: to swim in the deep and search for and map hydrothermal vent plumes. Our second robot has more or less the same design as Puma, and the two are meant to work together, so it seemed natural to have that robot be named after a big cat—Jaguar. In this case, the name came first, scientists then thought about what it might stand for. In a whimsical mood, they came up with “Just Another (“Grand,” or “Gosh-darned”—pick one!) Underwater Autonomous Robot.”

Our third vehicle, Camper, was designed to investigate hydrothermal vent sites and the communities of animals around them. It has a camera to take images and two types of hydraulic systems to take samples of rocks and organisms. So Camper is a combination of “camera” and “sampler.”

Sincerely,
Lonny Lippsett

Question:

Can you explain a bit more why seismometers on top of ice floes give reasonable data?

Also - any consideration for having your robots drop off a seismic package or two on the ocean floor?

Dave Huntsman
Cleveland, Ohio USA

Answer:

Dear Dave,

Earthquake waves are transmitted from the seafloor to the ice floe and are recorded there among lots of noise from the ice, mainly icequakes. The ice floes we use as recording platforms are about two kilometers in diameter. We deployed four three-component seismometers on each ice floe.

Icequakes travel horizontally in the ice floe and will arrive at one seismometer after the other and will produce strong signals on the horizontal component of the seismometers.

Earthquakes arrive from steeply below more or less simultaneously on all four seismometers and their signal is stronger on the vertical component than on the horizontal component. In that way, we can distinguish between earthquakes and icequakes.

The Arctic Ocean is very quiet. There is hardly any noise from waves and the ice floes are very big and all move in unison, so the conditions are not too bad. Of course, they are not ideal survey conditions, but in the Arctic one has to take what one can get.

Concerning your second question, standard instrumentation would be ocean bottom seismometers, but recovery in 95 percent ice-covered ocean is difficult. OBSs are too heavy to be carried by the robots. A special instrument would have to be designed, but there are no other applications for that than studying ice covered oceans. So using land seismometers is the cheapest way to get data.

Vera Schlindwein

Answer No. 2:

Dear Dave,

Thanks for your insightful question. Seismologists would love to get seismic data from stations on the Arctic seafloor, but this is a difficult technical problem.

First of all, you have to have an instrument that works on the deep seafloor. Those do in fact exist, and we call them ocean bottom seismometers. In the open ocean, we deploy them simply by dropping them off the side of the ship and letting them sink to the bottom. They are typically equipped with anchors that can be released with acoustic signals, at which point they float back to the surface for recovery.

That scenario breaks down in the Arctic because the instruments would more likely than not come up under ice, which would make it quite difficult to find and recover them—not impossible, but prohibitively difficult from an operations standpoint. The problem is therefore recovery, not deployment. We don't really need robots to drop off the seismometers; we could actually do that perfectly well from the icebreaker. But then
how do we get them back?

The cool thing about your question is that it hearkens back to the challenge that motivated me to develop robots to work under the ice in the first place. I have done a lot of seismological research, and the first robots I designed actually carried seismic packages inside the vehicle. They were designed to sit stationary on the seafloor under the ice for long periods of time and then swim back to an icebreaker for recovery.

The problem with this idea is that it takes a highly valuable, mobile, robotic asset and parks it on the seafloor for a long time. From a mission standpoint, this is an inefficient use of the asset. Moreover, most seismic studies require at least four stations to allow for accurate localization of the earthquake source, or hypocenter. That means we would need four robots to do the most rudimentary seismic study—again, very inefficient. For these reasons we modified the design and replaced the seismometer with sensors that take advantage of the robot’s mobility.

Rob Reves-Sohn