Date:17. June, 2004
Author: Cairistiona Anderson (University of St Andrews, UK) and Stein Kaartvedt (University of Oslo, Norway)
During a research cruise like this one there is a basic pattern of sampling that we complete at each station. This means that normally we have a routine to follow that we repeat over and over again in different places. Today we did something totally different and deployed an “acoustic” lander. This piece of equipment is designed to float above the sea bottom tethered to some large weights (old train wheels), and to look towards the surface with a SIMRAD echosounder similar to that which looks down from the ship’s hull. The plan is to leave the lander recording data until the second leg of the cruise, when it will be recovered and we will be able to see the data it has collected.
Bottom topography at the lander site
However, deploying a lander is not a simple process. First you have to find a large enough flat piece of seabed at the right depth. The equipment on the lander will be damaged if it goes too deep, and the securing mechanisms may be damaged if it is put down among sharp rocks. To make it more complicated, the lander is not carefully placed on the seabed, but is let go from the ship at the surface and then falls for around 1000 m through the water until it reaches the bottom. Away from the Mid-Atlantic Ridge finding a good site would be quite a simple task, as most of the ocean floor is nearly flat and very muddy, so that you can find a soft landing site quite easily. This is not true here in the Charlie Gibbs Fracture Zone where geologic forces over millions of years have moved and broken up the underwater mountain range of the Mid Atlantic ridge. The topography (seabed geography) is very complicated with deep canyons and quite small shallow peaks, some of which are the right depth for the lander but which may be very rocky on top.
Preparing the ROV Aglantha on deck (Photo by Uwe Piatkowski)
This morning 7 1/2 hours was spent carefully mapping the selected area to find exactly the right place before the lander was deployed. Once it was in the water, there were a few anxious moments as we watched on the ship’s echosounder to see what happened. Soon, we picked up the acoustic signal the lander was transmitting and we could see the acoustic reflections from the weights and the lander itself, so we knew that it was turned on and at the right depth. Usually, that would be all you could find out until the lander was retrieved, but we are lucky enough to have the ROV (Remotely-Operated Vehicle) Aglantha from Argus Remote Systems on board. This is a small submarine that is attached to the ship by an umbilical cable. The cable allows the operator (and the scientists) on the ship to see the pictures the ROV is sending and to send commands back to the ROV to control it.
An echogram from the ship’s echosounder, produced using SonarData’s Echoview, showing fish in the main acoustic layer moving away from the path of the ascending ROV Aglantha (red line).
The ROV was deployed to inspect the lander (and to see what else it could see in the water column on it’s way up and down). Fortunately, the lander was working exactly as it was supposed to, and as a bonus we got some very clear pictures of a variety of animals including squid, jellyfish and comb jellies (ctenophores) (see video from the ROV at http://www.mar-eco.no/Shiptoshore/g._o._sars/video). It is always fascinating to see these animals alive in their natural habitat rather than as specimens in the laboratory. Unfortunately, we failed to film any fish. The acoustic specialists had hoped that actual observations of fish would add to the information obtained from the trawl catches about the composition of the acoustic layers seen from the ship’s echosounder. However, in this case the echosounders aboard G.O. Sars documented how the fish avoided the ascending ROV, with the passing vehicle leaving a “hole” in the acoustic layer.