Acoustic manoeuvres in the dark

Late 2004 will see the implementation in the Mediterranean Sea of 12 detection lines carrying more than 1000 photomultipliers at depths from 2300 to 1950 m. These underwater eyes will form the ANTARES telescope, which will observe the Cerenkov emissions of up-going muons from the conversion of high-energy neutrinos in the deep water and seabed. Such neutrinos arrive undeviated from a variety of sources of astrophysical interest, and may also be produced in the annihilation of dark-matter neutralinos. In the closing weeks of 2002, the ANTARES collaboration passed two important milestones in the preparation for the final array.

At 12.10 a.m. on 9 December 2002, the underwater electro-optical junction box – the nerve centre of the future array – touched down on the seabed south of l’Ile de Porquerolles near Toulon, France. Twelve days later, a 60 m detection line carrying photomultipliers and prototype readout electronics was successfully anchored nearby. The deployment of the junction box, which involved a 36 h boat mission, was a trickier operation, requiring the dredging and lifting of 2.5 km of the 40 km undersea electro-optical cable that had been laid to the ANTARES shore base more than a year previously.

The operations had begun in calm conditions at 8.30 p.m. on 7 December, when the GPS dynamical positioning ship Castor 02 set out from the Foselev Marine quay at La Seyne for the 3 h trip to the ANTARES site. Aboard were a CNRS film crew, two divers and eight ANTARES personnel with their test and underwater acoustic navigation equipment.

In the early hours of the following morning, an acoustic transponder deployed over Castor’s side received a response from acoustic beacons attached to the undersea cable. Taking position fixes from a net of acoustic transponders on the seabed, Castor manoeuvred a metre at a time on a pre-planned course perpendicular to the lay of the cable, dragging a chain grapple across the seabed nearly 2.5 km below. At about 9.00 a.m., the movement of a beacon showed that the 400 m “dredging tail” extension to the cable had been successfully snagged on the first pass, and cable lifting could begin. Steadily winching at 30 m per minute, Castor slowly reversed 3 km along the cable lay. A little before lunchtime, the end of the cable was landed on Castor’s deck.

Throughout the cable lift, the ANTARES shore station team regularly checked the attenuation in each of its 48 optical fibres. With the cable on Castor’s deck, the optical parameters were still acceptable, so the boat team set to work connecting the cable to the junction box’s titanium pressure sphere. Some 3 h later, the optical attenuations through the 16 junction box outputs had been checked and signed off. The insulation resistance of each of the 16 galvanically-isolated secondaries of the internal transformer had passed the acceptance criteria, and telemetry data were arriving back at the shore station through the undersea cable for the first time.

Weathering the storm

However, sea conditions had worsened, with torrential rain and waves lashing Castor’s deck. The test gear was sheeted down. Rain clouds cut short the daylight, so the remaining operations would be carried out under Castor’s powerful arc lamps. By 6.00 p.m., the swell exceeded 2 m. It was too dangerous to power the underwater cable to its full operating voltage with the junction box on deck, but Ohm’s law and a little resourcefulness with a car battery verified the continuity and resistance of the 40 km cable.

An hour later, the junction box was hanging far out over the fantail on Castor’s 25 tonne crane. On shore, the telemetry from the junction box inclinometers attested to the pounding, but with 2.5 km of cable now hanging from the junction box, the deployment had to continue. The divers donned lifelines. Some 20 m below the surface chop they would detach the kevlar slings from the crane, leaving the junction box suspended from a transfer cable spooled on Castor’s deep-sea winch. The crane block was paid out, and the junction box was lowered into the swell, breaking the surface twice before disappearing into the calmer water below trough level. So far so good; telemetry data was still coming in.

With the junction box 400 m below the surface, the descent was paused. An acoustic beacon and anchor weight were fitted to the transfer cable, which would become the dredging tail should the junction box need to be raised in the future. A duplex acoustic release was fitted to the winch cable. From here on up, all the cable would be respooled onto Castor’s winch once a coded acoustic signal had triggered the release shackles. Descent continued steadily until 11.30 p.m. By then, communications with the seabed transponder net were difficult due to the excessive noise from the sea swell and the efforts of Castor’s positioning motors, working flat out to keep her on the precalculated cable re-laying track. Suddenly, with the junction box around 400 m above the seabed, the indicated cable tension dropped by 2 tonnes – the weight of the junction box, its dredging tail and anchor. Had the cable parted, precipitating the junction box to the seabed?

Satellite phone calls to the shore station revealed that telemetry was still arriving. Pitch and roll were nominal. The swell decreased a little and the junction box acoustic beacon revealed that it was close to its correct descent position above the seabed; the fault was in the tensiometer and not the rigging. The last 400 m were carefully paid out, and the junction box was placed on the seabed within a few metres of the planned position. Not bad for acoustic manoeuvres in the dark! The dredging tail was aligned. The undersea cable was energized to 3700 V, and the correct current measured through the junction box transformer. The acoustic release was triggered and the deep-sea cable winched back to the surface, leaving the junction box in communication with the shore.

A rare window of good weather allowed the subsequent deployment of the 60 m test line to proceed smoothly. Following procedures developed during a rehearsal using “triplets” of empty pressure spheres, the test line deployment concluded at around 3.00 p.m. on 21 December, with the line anchor acoustic beacon revealing it at the correct touch-down location on the seabed, about 200 m from the junction box.

No further deployments were possible in the last few days of 2002. An instrumentation line carrying monitors for underwater environmental parameters including water transparency, undersea current profile and seabed seismic activity is awaiting good weather for deployment. The hook-up operations to the junction box plug board, using cables with underwater-mateable electro-optical connectors, must await the return of the Nautile submersible of France’s IFREMER oceanographic research agency from its capping operation on the wreck of the oil tanker Prestige off the Spanish coast. The collaboration hopes to make these hook-ups some time in March, allowing the instrumentation to be fully tested under real conditions.