If losing an expensive package of scientific instruments on the bottom of the ocean is a painful fact of life for the sea-going oceanographer then finding something that was once lost is a glorious moment.
The TTIDE leg 2 team was unable to bring home one of their bottom mounted ADCP’s, lost somewhere off the northeast of Tasmania in about 80 metres of water. “ADCP” is an acronym for Acoustic Doppler Current Profiler, an instrument capable of measuring the velocity of ocean currents from the shelf floor to the surface. But something unexpected and disastrous occurred and a recovery float, the ADCP’s lifeline to the surface, became separated from its companions.
So for two nights on TTIDE leg 3 the R/V Revelle patrolled the last known position of the lost gear, dragging heavy trawl wire and grapples in a crisscross pattern. Last night at 2am, after many hours of searching, suddenly the tension on the line monitor jumped to over 1000 pounds. Was this just a spike in wire tension, a temporary snag? No, the tension was steady and that meant the TTIDE team had caught something significant. “You don’t imagine that you can actually collect this thing that has a tiny footprint of about 20 metres on the bottom,” said Amy Waterhouse, lead scientist at the time of the recovery.
If snagging the ADCP was impossibly improbable getting it back to the surface and on deck safely was potentially perilous. In what condition and orientation would this mess of anchor, grapple, wire, and technology arrive aft of the R/V Revelle? Slowly, over a operation carefully executed by the skillful resident technician Josh Manger, each part of the tangle was brought aboard step by step in the dark. Finally, two hours later the ADCP was lying on the deck surrounded by a happy bunch of TTIDE crew celebrating out on the Tasman Sea at 4 in the morning.
The R/V Revelle quickly set course south for the next task, the recovery of mooring “T7”.
As dawn broke off Tasmania’s Freycinet Peninsula the TTIDE team and crew of the R/V Revelle faced an increasingly angry sea and a challenging 72 hour weather forecast. Strong westerly winds, gusting up to 54 knots, make for demanding working conditions on the back deck. After a brief meeting on the bridge a decision was made to temporarily cancel mooring recovery operations and switch efforts into “yoyo” mode.
“Yoyo” is a term that refers to mounting a large number of scientific instruments on a specialised frame and hanging that frame off a powerful and capable winch system. This package of instruments is slowly lowered to great depth and then brought back to the surface taking measurements of the ocean’s properties. The up and down motion of the instrumentation package on the wire gives “yoyo” operations their name.
With great patience each up and down dip of the frame, known as a CTD cast, adds one snapshot of information about the internal state of the sea. Ideally we will stay on station for at least 24 hours to capture an entire tidal cycle, the beating pulse of the internal waves the TTIDE team is chasing.
With our CTD casts underway we have one eye on the winch and one eye on the weather. When we get a break in the storm force winds it will be back to our line of moorings to recover “T7”.
Thomas Moore, for the TTIDE team