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Putting PIES into the ocean

In 2006 nine so-called “PIES” – Pressure sensor equipped Inverted Echo Sounders - were deployed to measure changes in surface level and transport of water mass of the Antarctic Circumpolar Current: Yesterday, we arrived at the first of three positions where a new PIES with fresh batteries was deployed. This PIES will measure water pressure at the ocean floor during the next 2 to 4 years.

Picture above:
Andreas Macrander takes a few final photographs of the first PIES he deploys.

Andreas Macrander from Alfred Wegener Institute for Polar and Marine Research in Bremerhaven, Germany was here in 2006 to deploy the PIES. He has now returned to service three of the PIES. Andreas has made this contribution to the cruise journal:

Down at 4300 m depth, the water pressure is 430 kilogram per square centimeter – thus the glass sphere that contains the electronics and batteries of the PIES has to withstand a force of 2500 tons. A sensor measures this pressure with such a resolution that changes in the sea level of just a few millimeters are detected. And these small changes are what I am looking for: They correspond to changes of the overlying mass of water, and serve as a reference for gravity field measurements of the GRACE satellite mission. Since 2002, the GRACE satellites have observed the Earth’s gravity field and have already proven the loss of mass on Greenland due to ice melt as well as seasonal changes in the tropical monsoon regions. The aim of the German government-funded project that I am working for is to employ GRACE measurements also for world-wide observation of mass changes in the ocean, e.g. due to variability in density structure and ocean currents, since the in-situ observation network is rather coarse. Detection of oceanic variability in tropical oceans is still a challenge for GRACE. In high latitudes like the Southern Ocean, however, the performance is already astonishingly accurate. The figure shows a typical example of PIES data and monthly GRACE gravity fields obtained during an earlier deployment in the Antarctic Circumpolar Current.

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Ocean Bottom Pressure (OBP) in the Southern Ocean at 50°S 1°E from 2002 to 2005. The solid black line shows the pressure anomalies observed by the PIES, the vertical scale corresponds to monthly sealevel variations of up to 0.06 m. The dashed grey line shows the corresponding estimates based on GRACE gravity fields. The overall agreement is quite good given the fact that the PIES measures OBP at one spot, while GRACE obtains averages over a region several hundreds of kilometers in diameter. Figure: Andreas Macrander.

Additionally, PIES measure the travel time of an acoustic signal from bottom to surface and back – essentially like a ship’s echo sounder. The combination of pressure and travel time series makes it possible to determine the mean temperature of the water column. Furthermore, the combination of several PIES allows an integral measurement of the total volume transport between the instruments. Thus, the entire Antarctic Circumpolar Current is monitored by an array of only 9 PIES, three of which are now served by G.O.Sars. The remaining six are exchanged by the German icebreaker Polarstern, which is supposed to leave Cape Town today.


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The PIES is deployed.

Yesterday, the plan was to recover the first PIES and then deploy another PIES. When G.O.Sars arrived at the site around 20:00 UTC, a hydrophone was lowered into the water, and we tried to establish communication with the PIES by means of a coded acoustic signal. Since sound has a wide range under water, it is normally not a problem to receive a clear reply from the PIES that is 4300 m below the ship’s keel.
When the PIES receives the “Release” command, it should start burning a wire that holds it to its anchor weight, and about 2 hours later the instrument should reach the surface where it can be detected by a VHF radio signal and a flashlight.

However, yesterday we had bad luck: There were only very weak signals indicating that the PIES was still there. During the first 30 minutes there was also some weak indication that the instrument started to burn the release wire – but later we could not detect anything. Repeating the release command many times didn’t help either – silence under water. Perhaps the batteries were already too low, perhaps something was blocked mechanically, perhaps it came to the surface unnoticed, and in the darkness we could not locate it without VHF radio and flashlight. We don’t know it. Four hours later, we gave up. 


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Just before the PIES is released.

Finally, we deployed a new PIES equipped with fresh batteries to continue the observations for the next 2 – 4 years. The instrument was lowered to the side of G.O.Sars and the slip-hook released – and then the mooring fell down 4300 meters to the sea floor.

This evening we will reach the second station where one PIES will be recovered an another deployed. I hope that we will have luck this time. The recovered PIES will be refurbished with new batteries and deployed on the third (and last) PIES station that we will reach in another two days.

NOTE (Feb. 10): Last night at the second station Andreas made contact with the PIES several thousand meters below the vessel. The PIES was sucsessfully released and retrieved when it reached the surface. Also the deployment went without a hitch.


 

Written by: 
Andreas Macrander
Alfred Wegener Institute for Polar and Marine Research (Bremerhaven, Germany)