Wolf,
Good question.
What you do in soft sediment is deploy the beacon atop a long rope which is is attached to a bloody great big clumpweight and has a floatation collar on itself.
This keeps the beacon above the seabed, hopefully at a similar height above seabed level to that of its brethren in the array so that its depth does not vary much with the others in the array if the seabed current changes direction as a consequence of tidal or other current movements. If the current changes direction, the entire array sways and the resultant fix just goes with the flow, so to speak. They are generally five or so metres above the local seabed, but that depends on the planning of the array vis-a-vis intervisibility acoustically for the calibration of the array. They have to be able to 'talk' to eachother acoustically for them to to be able to self-calibrate trilaterally so that all of the baselines among the array are measurable.
The flot collar is there anyway to recover the beacon at the end of the job. There is an acoustically addressable release device at the base of the beacon. When you want to send the beacon up to the surface you send the command acoustically and the acoustic release releases the physical connection to the clump weight and the buoyancy of the transponder's flot collar sends it upwards. The flot collar is chamfered at its upper end so the the transponder floats nose down in the water at the surface and enables it to give ranging information to the recovering vessel. They cost several tens of thousands of Dollars/Pounds/Euros apiece and are not regarded as being expendable like the sonobouys that the military use with great abandon.
I've had a few rug-munching encounters with bean-counting bosses, without tea and biccies, when having to explain my failures to recover such beacons.
In the North Sea, they usually end up on the beaches of the Fresian islands. Rich rewards to be had by the cattlemen there!
Last edited by Cazalet33; 26th May 2016 at 23:54.