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MYRTLE test release.

MYRTLE - Multi Year Return Tidal Equipment
At one second after midnight, January 10th 2001, deep down on the ocean floor, where the pressure is greater than 5000 pounds per square inch, in the darkness there was a momentary flash from an explosive bolt. A 17-inch diameter glass capsule began its long, graceful ascent after being held captive for over a year. On reaching the rolling surface it began to send back piece-by-piece its stored data to an orbiting satellite and then on to a bunch of relieved and much elated huddle of scientists and engineers!

The project is called MYRTLE and its mission is to send back sea level and temperature information from the deep ocean over a period of five years. Each year, for four years, a Benthos glass sphere is automatically released from a frame on the seabed, a central data logger pumps the data simultaneously into all the spheres. In the fifth year, the frame, data logger and sensors can be released from the seabed, recovering the equipment and the whole five-year data set.

The MYRTLE instrument is a is a circular, tubular frame, with four 17-inch spheres to provide buoyancy and two 17-inch acoustic releases to fire the explosive bolts to release the data capsules. The four Releasable Data Capsules (RDCs) are 17-inch spheres containing the data loggers and an Argos multi-channel satellite transmitter. The RDCs are fitted with Benthos radio beacons for surface tracking when manual release is required. Finally, the main frame is fitted with a flashing light and radio beacon.


MYRTLE2 on deck of RRS James Clark Ross.

A central data logger, in an aluminium pressure housing, records sea pressure and temperature from two highly accurate sensors and stores the data in solid-state memory. Simultaneously the data is sent to the four glass data capsules by infrared data telemetry directly through the glass. This is done to avoid having cable connections to the spheres, which would somehow have to be cut to release the capsules.

The data loggers inside the RDCs are first 'woken-up' by an infrared signal and then record the infrared data stream and return to their 'sleep' state. This low current state enables the data loggers to continue operating for periods of over five years. A countdown timer ticks away and releases the capsules at pre-determined, yearly intervals. Alternatively if a ship is in the area, the Benthos releases can be used to retrieve capsules manually. The British Antarctic Survey operates vessels nearby to service their Antarctic Research Stations. They closely collaborate with POL to recover and redeploy our sea level recorders in the deep ocean and at remote island sites.



Capsule at surface.

The first prototype MYRTLE was deployed from the RRS Bransfield in November 1992 at the Antarctic end of the Drake Passage in the Southern Ocean and was finally recovered in November 1996 by the RRS James Clark Ross, after successfully releasing its capsules and providing the world's longest deep ocean sea level data set. After extensive modifications to the data loggers and satellite telemetry systems, MYRTLE2 was deployed again, west of the South Orkney Islands in the Scotia Sea. As stated in the opening paragraph the first capsule was released in January 2001 during the austral summer and has now sucessfully relayed its data back to POL via satellite.


MYRTLE2 position (red circle).

You may say "Why go to all this trouble when I can see the tide going in an out down at the beach"? We measure sea level in the deep ocean because that is where tides are generated and what we see at the coastline is just an indication of what is going on, a bit like looking through a telescope the wrong way round you can't quite see what's going on. To understand tides it is necessary to measure them where they are created. Leaving the pressure sensors on the seabed and just recovering the data by capsules preserves the datum of the sea level measurements. Excursions to these remote areas are thereby kept to a minimum and the quality of data from the sensors greatly improved.

There have been "spin-offs" from MYRTLE technology and one has been to use the capsule idea to make a cost-effective sea level recorder that is small enough to be easily deployed from almost any ship. These consist of just one Benthos sphere that is released from a ballast frame, and can be recovered from the surface or made disposable by using a satellite transmitter to get the data back. Recently, four single-capsule instruments were deployed in the Antarctic area adjacent to the Weddell Sea. During one recovery, the conditions were marginal with patches of ice on the surface. It was mostly open water and because chances to get to that region are few and far between, the decision to "pop-up" the gauge was taken. Unfortunately the capsule popped-up under some thin surface ice but the Benthos radio beacon came on and the ship was able to track the position of the capsule through the ice and eventually the yellow hardhat could be seen through the ice. It was possible to release it by some judicious "ice-breaking"!

Proudman Oceanographic Laboratory has been taking sea level measurements in the Antarctic and Southern Ocean for over fifteen years. The study was part of Britain's contribution to the World Ocean Circulation Experiment (WOCE) and interacted with international work from the United States, South Africa, Australia, France and Germany. POL's contribution is now focused with the replacement for WOCE, the Climate Variability and Predictability Programme (CLIVAR).

The principal objective is to study variations in the flow of the Antarctic Circumpolar Current (ACC) on large time and space scales. The ACC influences global climate by acting as a conduit for interchange of heat between the major ocean basins.

The original MYRTLE project team members were Ian Vassie, Bob Spencer and Peter Foden. Bob has recently taken early retirement, while Geoff Hargreaves and Steve Mack have joined the team.

We would like to thank the Captains, Officers, Crew and Staff of the British Antarctic Survey, without their support, little of this research could have been done. In addition we would like to specially thank Robert Catalano of Benthos for his personal support of our projects over many years.

Article by Pete Foden


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