Some of you are probably wondering why ships in the south Indian Ocean still need to run grids when they already heard pings from those black boxes. Why not just draw a straight line from the ship to the box?
People who read The Hunt For Red October know the answer already, but as a former oceanographer I’ll summarize. In most places the ocean has a sharp boundary between warm-ish surface water and the cold deeper zone where you find giant squids, godzilla and cthulu. We call that sudden temperature change the thermocline. Sound moves faster through warm water, so if it hits the thermocline at an angle it can refract in a weird direction or even bounce off and disappear back into the deep sea. Submarines use this all the time – if you have ears above the thermocline, even when you can hear a sub running underneath it the sound will come and go like light rippling off disturbed water. As a result you have a hell of a time pinpointing where it is. Some whales like to find places where a note can hit the thermocline at just the right angle and reach friends half the world away by bouncing back and forth forever between the warm(ish) and cold zones.
A couple of ships have picked up on again / off again signals that almost certainly come from the flight’s black boxes. Not much else makes a similar ‘ping’ at exact one second intervals. That narrowed the search area way down, but you still cannot pinpoint the box for the same reason that you cannot just shoot at a submarine on the wrong side of the thermocline. They really need some towed arrays or sub-hunting autonomous vehicles in deeper water more or less today, because in another day or two the warrantied thirty day life of those batteries will go kaput.
From reading a little more I see that the boat did use a towed array below the thermocline, but the sea floor is still much, much, much farther down than that. So the thermocline is not specifically responsible here, although the two vertical miles of water the boxes sit in still have plenty of layers with different temperatures and salinities that will make sound go screwy. If you want to get a sense of just how far down the boxes are, check out this helpful graphic. Then pour a drink.