Edge of the Arctic Shelf

Edge of the Arctic Shelf
Well, last night's red sky didn't exactly bring us a "sailors' delight" in terms of weather... Ominous overcast skies and the occasional sleety rain heralded another gray day in the Beaufort Sea. The poor weather didn't stop the Healy's aviation detachment from running a number of training flights this morning, though. They practiced lowering a sling onto the deck - this is how they would hoist someone out of the ocean in a rescue situation. Crew members also got rides in the helicopter during the training flights as a morale boost.

It was another full day of preparation for the mooring team. After lunch, Ryan Schrawder tested two acoustic releases at 6,000 feet to make sure they were operating properly at depth. Even though they all worked fine during the recovery, it never hurts to double check. Meanwhile, John Kemp measured out the proper amount of line for the deepest mooring, cut it to size, and added the connectors that join it to the rest of the mooring chain.

Since the WHOI mooring array is sited on the edge of a steep submarine cliff, proper positioning of the moorings is critical. Each mooring is very precisely measured - if you put one in water that is too shallow, then the top of the mooring could hit the ice keels in winter.

Val Schmidt from the Lamont-Doherty Earth Observatory of Columbia University in New York is the seafloor mapping expert on our science team. He uses data from the ship's acoustic depth-finder to make detailed bathymetry charts (maps of the ocean bottom). I asked him to explain how the depth-finder works and how he makes use of the bathymetry data.

"Multi-beam sonar of the type installed on the Healy is used for surveying the ocean floor. It's called a "swath mapping" sonar system, as it maps out a large stripe, or swath, of the ocean floor beneath the ship as we drive along.

The sonar consists of two large transducer arrays on the bottom of the ship. The "projector" sends out pulses of sound into the ocean, while the "receiver" listens for the echo produced when the sound pulse bounces off the sea floor. We measure the time difference between each ping and echo to calculate the distance to the bottom.

It would be an easy calculation if sound traveled in a straight line in the ocean (If you drive 60 miles per hour down the highway for an hour, you've gone 60 miles.) But temperature, salinity (how much salt) and pressure (the water depth) all cause changes in the speed that sound travels in water, and these speed changes cause bends in the sound's path. So we have to know how the speed of sound changes through the water column all the way to the bottom, and we measure this periodically with data from the CTD or an expendable probe. Then with some fancy math we can calculate the correct distance to the bottom taking all this into account.

By using an array of projectors instead of just a single one, we can measure this distance in up to 120 places beneath the ship with a single ping! After driving back and forth "mowing the lawn," we can collect data from many many pings, and create a three dimensional surface of the sea floor topography.

Lately, we've been surveying the edge of the continental shelf where we will deploy 8 moorings in the coming days. The sonar allows us to locate relatively flat places on which to anchor the moorings. The drop off the continental shelf is akin to the Appalachian mountains. It is important to find a stable location to anchor the instruments."

Late tonight we suffered a minor setback - Ryan Schrawder discovered a problem in the electronics of one of the moored profilers. This means another day of testing and communication with the manufacturer. While we're not happy about waiting another day to deploy the moorings, the proper performance of the instruments is critical to the data quality.