20/07/01 - Day 19
Sonar finds "highly promising target"
Rob White reports
Conducting a sonar search for a never-before found wreck is a long and painstaking
business. It's not quite as bad as looking for the proverbial needle in a haystack -
more like trying to find it with a powerful magnet in your hand.
But you've still got to get your magnet in the right position over the haystack to have a chance of finding it!
The sonar goes down
The sonar sits 350 metres above the sea bottom on its first pass, sending out two
"swathes" or circular sweeps, one on either side of it. This covers an area 2,200
metres out to left and right on the first "low definition" pass.
This signal is then electronically treated and coloured to give the seabed image - like a contoured map - that you can see here. But the sonar can't look directly beneath itself - that's the big dark band in the middle. David Mearns located 'Bismarck' right on the edge of that band - a close-run thing.
And this, magnified to make it clearer, is the sort of thing you have to look out
for - a "hard target". Straight lines and right angles usually mean a man-made object,
and the darker the image the harder the target.
Go down deeper and you get a more "high-resolution" pass picking out more detail, which can give you an image almost as good as a photograph. But knowing which target to go for is the trick - that only comes with experience.
A "promising target" on the monitor screen
And for the rest of the team? We can only "rest on our arms reversed" as the military drill
says. Coiled springs the lot of of us.
STOP PRESS: David Mearns has identified a "highy promising" target. We're taking the
ROV down to look at it. There's a chance we might have the first pictures of the Hood tomorrow!