Sunday, 26 February 2012

Little did they know...

The Nightshift (midnight to noon) went to bed thinking that at the start of their shift we would be on passage east towards an additional site south of the Isle of Wight (so called 'Wight-Barfleur Extension, of which more in a future post).
Filling in the gap - the white triangle

Since then our plans have changed and we are now gathering multibeam echosounder data for an area of the Western Channel rMCZ for which we don't have detailed data.  The multibeam data provides an acurate picture of the shape of the seabed, but the 'backscatter' of the echoes can be further analysed to give an indication of what the seabed type is.  This can then the confirmed ("ground-truthed") by the camera and grab samples that we've already taken.

Indicative survey lines - there are really 30+ lines
The process of collecting this full-coverage multibeam data involved steaming along a series of parallel lines 300m apart.  The map on the right gives an  idea of the lines we are following - but in reality there will be over 30 lines.  It will take approximately 20 hours to complete this piece of work.  It keeps the helmsman busy as he steers a 3000 tonne, 78m ship within metres of a pre-determined line, but for the science staff its not the most exciting aspect of cruise...

Data acquisition
Two echo-sounder systems working at different frequencies are being run at the same time.  Between them they generate about 400mb of data per hour, which means there is a lot of processing work to be done to produce the final images.  To get accurate results a number of additional factors have to be compensated for: sound velocity through the water, ships heading, tidal height, and ships motion.  

Ultimately, the processed data will yield a detailed seabed image.  The example below shows an area north of Anglesey.
Drumlins, north of Anglesey