> The locations of pretty much every cable laid in the last half century is very precisely tracked and one of the first things that has to happen when preparing a new cable route is to undertake a high resolution side-scan sonar survey of all or part of the planned route.

When the cables aren't well buried, they can migrate. No link, but earlier today I saw a description of a repair that was significantly delayed because the cable ends were 15 km away from where they were expected to be.

That said, I think there's a lot of area on the sea, and not a lot of cables, so chances of overlapping are low; especially if a survey is done of the new route immediately before. Although if they're buried, maybe you can't see them so well. And a little overlap here and there probably isn't a big deal, because most of the time cables are brought up, it's because they were severed, so likely it doesn't disturb the other cable too much on its way up.