" ..........................the use of a sonobuoy pattern in the ocean to detect and localise a target is one of the first and basic detection methods used in Anti Submarine Warfare for over 50 years."
As an Ex-Navy pilot, I'm wondering why some agency, aircraft company, or even the CVR/FDR makers, didn't previously contract for a thousand or two sonobuoy's especially configured to detect these signals, perhaps with a cable depth option of greater than 1000 feet. There should have been a stack of these devices sitting in some warehouse just waiting for the next airliner to go down at sea, especially where the water is deep or the exact crash location isn't known.
These special sonobuoys could then be flown anywhere in the world and scattered, if necessary, over a wide area without interfering with any other concurrent search effort. Plenty of aircraft are capable of deploying these devices.
I don't understand why they have to modify an anti-submarine sonobuoy a month after MH370 went down, especially given the Air France search of several years ago.