Hi @Salome, welcome to the forum! 
Sounds like an interesting (and hopefully fun) project!
I’d say that depends on how accurate the initial position record is, and the visibility and current strength at the time of recovery combined with the skilfulness of the operator(s).
It’s not something I’ve personally done at those depths.
This retrieval example from @btrue may be worth a look, although your use-case is quite a bit deeper so will need lighting from the ROV to see what you’re looking for.
That may be possible, although depending on the constraints of the use-case it could be quite challenging to achieve. Hardware wise it would require at minimum a transmitter at the bottom (with the loggers) and a receiver on the ROV or the boat, but the most efficient and practical retrieval approaches would likely make use of both transmitting and receiving on each end.
Behaviour wise the main approaches I can think of are:
- bottom transmitter transmits at regular intervals, and is searched for and detected by the moving receiver
- too frequently could be wasteful of power, and may cause confusion for other vessels that come by if they have a sonar sensor with a similar frequency, and could potentially be disruptive to nearby marine life
- too infrequently could make searching and finding slow and challenging
- bottom receiver waits for a transmission, and when it detects one it transmits back so it can be detected (resulting in faster ping-ponging as the sensors get closer)
Beyond that, if you know you’ll only be searching after a set amount of time, or within a particular time period during each day, then that could be used to help save power by turning off the bottom sonar while it’s not needed (albeit potentially with some extra risk, because if your control computer somehow loses track of time it may change to a range that’s misaligned with the search time).
Technologically speaking that kind of use-case could be achieved with a couple of Ping Sonars being controlled by some Arduinos or Raspberry Pi boards or similar, although it would require custom firmware on the sonars to support the non-standard usage. At the moment we don’t provide firmware source code (i.e. that would allow adding custom messages and/or logic), so unless that changes custom firmware would need to come from us.
The ~30 degree beamwidth on the Ping Sonar means detection would be possible within a radius of about 10m for an ROV 35-40m above the bottom transmitter