by Neill MacKay
It’s been a busy few days for this blogger as 4 out of 5 of the NOC moorings have been recovered and redeployed, and I’ve also had the chance to see what’s involved for the first time. Compared with the NIOZ (the Royal Netherlands Institute for Sea Research) moorings also being serviced on this cruise, the NOC moorings present some additional challenges when it comes to recovery. Rather than just the big buoyant smartie on the top, the NOC moorings are designed with smaller packages of buoyancy – glass spheres surrounded by orange plastic – at various points along their length. The additional buoyancy means that if the top part of the mooring breaks away due to some violent weather or some other event during the year, the rest of the instruments below remain supported and so will continue gathering data until the mooring is recovered. The downside of this design becomes apparent when we come to recover the moorings, as they have a tendency to become tangled! So rather than coming on board in an orderly fashion, the instruments arrive in a number of tangled spaghetti-like bundles, and not necessarily in the order you expected! In the worst cases the cable has to be cut to sort out the mess, and great care is taken not to drop any loose end, as this would mean any instruments attached and still in the water would be lost forever!
Once we have recovered all the instruments, we download the data onto a computer and do some initial processing to make sure everything has worked OK. In one or two cases we may not have a full year’s worth of data from an instrument – this can happen for example because of a faulty battery, but from almost all the instruments recovered from the first 4 moorings there are no obvious deficiencies which is good news! The next step is to carry out a ‘cal dip’ – attaching mooring instruments to the CTD and ‘dipping’ them into the water (actually sending them to the bottom!) to check how well they are working. In the case of the microcats, we compare their measurements of temperature, salinity and pressure with those on the CTD and with each other for consistency. The information from the cal dips is used when deciding which instruments to deploy on the subsequent moorings, and which ones to swap out.
The final preparations are then made for redeployment of the mooring, including putting shiny new chains on the buoyancy packages to hold them together in groups of between 2 and 7 spheres. Then it’s time to put it all back in the water! This job is rather easier than the recovery because we can stream the wire onto which all the mooring instruments will be attached out behind the ship, maintaining a steady 1-2 knots to make sure that things do not get tangled. The instruments are then attached to the wire one by one and checked off as they go; these moorings contain microcats and current meters like the NIOZ moorings, but also have an Acoustic Doppler Current Profiler, or ADCP, near the bottom. The ADCP gives a vertical profile of the current speed and direction over a range of depths, obtaining its measurement by firing sound waves into the water and detecting a change in frequency – or ‘doppler shift’ of the waves as they return to the sensor having been reflected off of particles moving with the water. Finally the anchor is attached – and rather than the solid blocks used on the NIOZ moorings, these have giant chains weighing 900kg which are hoisted onto the deck with the crane and then lowered over the water before being dropped in.
The anchor having been dropped, we then take the acoustic transducer which we use to communicate with the mooring acoustic release mechanism and monitor its progress as it sinks to the bottom at a rate of around 100m per minute. When it reaches the bottom, the ship then moves about a mile away from the anchor drop location to begin triangulating the mooring’s final position. At 3 points surrounding the drop location we range to the acoustic release, and use the 3 ranges to do the triangulation. So we know where to look when we come back to retrieve the data next year!