Sunday, 19 February 2012

Back to work

We've returned to East of Celtic Deep rMCZ to complete the survey work started during part A of this survey.  Initially we finished off the  multibeam echo-sounder work (see multibeaming East of Celtic Deep ) after a CTD calibration.  The CTD is a small cylindrical probe housed in a robust stainless steel cage that is deployed over the side on a wire and allowed for drop rapidly to close to the seabed.  This takes some judgement as you don't want to let the CTD hit the bottom (hence the steel cage just in case) but you want to take measurements through as much of the water column as possible.  Although you know the depth of water under the boat when you release the CTD you need to compensate for the fact that wind and tide can effect the ship and the CTD differently so the wire is seldom vertical.

CTD in its protective cage. c.50cm long

Why do we need it?  The CTD measures Conductivity, Temperature and Depth every 0.5s as it descends (the downcast), parameters need to to calculate the speed of sound through the water.  This sound profile information is used to calibrate the multibeam echo sounders so that data taken over a period of time can be integrated into a single 'picture' of the seabed.  Analysis of the sound backscatter can provide a broad scale analysis of the seabed type, which is then ground truthed with grab and camera samples.

Talking of which we're at our next station so I need to get back on duty!