Acoustic calibration

I always enjoy calibrating the acoustic instruments on our research vessels around South Georgia. It usually means taking the ship into a sheltered harbour called Stromness Bay. As you enter the bay you enjoy stunning views of three now uninhabited whaling stations (Leith, Stromness and Husvik), fur seals and penguins huffing and puffing on the beaches, and entertain some small satisfaction of looking at the salvation of Shackleton’s epic rescue of his crew of the Endurance, stranded 800 miles away on Elephant Island. 

As most of the scientific personnel on-board the ship enjoy a well-deserved break from intensive sampling (and become excited sight-seers), I get to work. My scientific interests revolve around observing animals (zooplankton and micronekton – e.g. krill and fish) in the water and how they interact with their environment and their predators. I use an echosounder fitted to the ship’s hull, a device that transmits sound and records the quantity of the echo returned from targets in the ocean. It enables me to observe krill swarms and fish schools within the water, and to estimate their biomass. In order to get the biomass I have to calibrate the echosounder with a known reference target (to create an echo)– in this case a small metal (copper or tungsten carbide) ball, the size of a large marble.

With the help of the ship’s crew and some agreeable colleagues we place three winches (essentially electric fishing rods) with fishing line strategically around the ship, two on one side and one on the other. Passing one line under the ship (an interesting challenge in itself with a ship that is ~100m long and 18m wide, with a variety of thrusters, anodes and sea chests sticking out on the hull), the three are joined together, a metal ball attached and then lowered beneath the ship and located on the echosounder. We can then use the winches to move the metal ball around under the ship and in front of the echosounders. It sounds challenging to locate a small marble under such a large ship, but it is a large target on the echosounders and as long as you have done your maths right (ships size, location of sphere and a little Pythagoras!) it shows up easily. Then the katabatic winds kick in, the ship moves and the whole process takes many more hours than you thought it would.

Having finished the calibration, we pack up the equipment in darkness. The ship is quite silent, the sight-seers long gone to bed, and the loudest sounds are the wind with a subtle huffing of fur seals in the background (and a slightly pervading smell of ammonia emanating from them!). Satisfied that what I now see on the echosounder can be quantified, I say good bye to Stromness for another year.

Sophie Fielding

British Antarctic Survey

28 November 2017