Today has seen the James Cook pressing ever northwards and we have been surrounded by fog for most of the day. There is now a distinct chill in the air since crossing the Charlie-Gibbs fracture zone and it will soon be time to break out the extra jumpers!
Our long CTD transect is now almost over and we have succeeded in collecting huge amounts of data that will no doubt keep many of us busy in our labs over the coming winter months.
Our first task on arriving at our northern working stations is to again make some detailed maps of the seabed terrain using the multibeam swath bathymetry system. This is a vital part of this first ECOMAR cruise as many of the areas we are studying have not been visited before. As well as providing us with a means to target suitable trawling grounds and locations for the landers and long-term moorings, these bathymetric maps will be invaluable for planning future Isis ROV dives in 2009.
As these surveys are so important to our project is probably a good idea to give you an insight into how we can visualise what lies 3miles beneath the ship.
Echo sounding is the key method scientists use to map the seafloor today. The technique, first used by German scientists in the early 20th century, uses sound waves bounced off the ocean bottom. Echo sounders aboard ships have components called transducers that both transmit and receive sound waves. Transducers send a cone of sound down to the seafloor, which reflects back to the ship. Just like a torch beam, the cone of sound will focus on a relatively small area in places where the ocean is shallow, or spread out over the size of a football pitch when water depths reach greater than 3000 metres. The returned echo is received by the transducer, amplified electronically, and recorded on graphic recorders. The time taken for the sound to travel through the ocean and back is then used to calculate water depths. The faster the sound waves return, the smaller the water depths and the higher the elevation of the seafloor. Echo sounders repeatedly “ping” the seafloor as a ship moves along the surface, producing a continuous line showing ocean depths directly beneath the ship.
This single-beam echo sounding is very useful for showing the depth of the seabed beneath the ship and it can give some limited information of the seafloor topography, but it is limited to a narrow strip and it would take many passes of the ship to build up a map of the seabed.
So how can we visualise a greater area of seabed beneath the ship? Well, the concept is simple really…..more beams = more area covered. The entire concept of multibeam bathymetry is based on the simple fact that more beams are better than one. Instead of just one transducer pointing down, “multibeam bathymetry systems” have arrays of 12 kHz transducers, sometimes up to 120 of them (we are using a Kongsberg-Simrad EM 120 on the James Cook), arranged in a precise geometric pattern on ships’ hulls. The swath of sound they send out covers a distance on either side of the ship that is equal to approximately two times the water depth. The sound bounces off the seafloor at different angles and is received by the ship at slightly different times. All the signals are then processed by computers on board the ship, converted into water depths, and automatically plotted as a bathymetric map with an accuracy of about 10 metres.
In this way, the James Cook travelling at speeds of up to 10 knots (11.5mph/18.5km hr-1) can produce a swath, rather than a line, of water-depth information. Multibeam bathymetry systems are now routinely used during research cruises to map areas of seafloor as large as thousands of square kilometres.
So tonight will be spent producing the best maps we can of our working area so hopefully by tomorrow we will have identified some good, clear, trawling grounds, a site for the long-term mooring and of course some interesting features that will warrant closer inspection with the ROV!
University of Newcastle, UK