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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.


The sonar goes down

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 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.


A "promising target" on the monitor screen

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.

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!





Full Listing
26/07/01 -
Finally - a service of remembrance


25/07/01
The Hood Photo Appeal

25/07/01 -
More big discoveries


23/07/01 -
She's found!


22/07/01 -
Keeping in position


21/07/01 -
Tracking the ocean floor


20/07/01 -
Sonar finds "promising target"


19/07/01 -
Combing the search box


18/07/01 -
Transfers and goodbyes


17/07/01 -
Meet the search team


16/07/01 -
Meet the survey team


15/07/01 -
Meet the experts


14/07/01 -
Meet the film crew


13/07/01 -
Farewell to the Bismarck


12/07/01 -
A service of remembrance


11/07/01 -
More filming from the deep


10/07/01 -
The Bismarck plaque is laid


09/07/01 -
The Bismarck is found!


07/07/01 -
Staying on track


06/07/01 -
The sonar is finally launched


05/07/01 -
More frustration for the team


04/07/01 -
Rough weather halts progress


03/07/01 -
Expedition to film the Bismarck


03/07/01 -
Holding a steady course


02/07/01 -
Northern Horizon leaves Cork


28/06/01 -
David Mearns in Webchat


27/06/01 -
ITN's Rob White reports from the ship