And they are no countermeasures that can fool them. You don't have to worry about decoys (as long as the torp has found the wake you wanted.)
On a side note: I've done some digging into Kapitan's comments and he's actually right on. Check out these space views of ships wakes. From space some of these things are seen to be a 200km long.
http://disc.gsfc.nasa.gov/oceancolor...b/oss_94.shtml
So it seems like the effects of the turbulence say around for a while. The obvious followup questoin would be: But how long is the turbulence significant enough to be detectable by a sonar sensor? The only people that probably know are the engineers that build the torps in the few place... and the navy brainiacs that are working to counter them.