Originally Posted by JD-EE
[ROVs] are lowered in a cage and turned loose with a tether back to their transit cage. The tether's length is limited. So their ability to move along in a search is limited. They'd have to search an area and come back to their cage to be moved to the next block lest cords get tangled.
Agreed that the cage or depressor weight (2-body launch and recovery) is the usual configuration for a deep ROV. However, it is not so that the ROV has to return to the cage to move the ship. When the ship moves, additional scope must be payed out from the ship to keep the depressor at the right altitude, and the ROV can tow its neutral tether as the depressor moves. When the ship stops, the additional scope is retrieved as the depressor settles under the ship. With a rough bottom, it is necessary to ensure that the neutral tether is not dragging on the bottom, lest it fetch up on something. I don't know if this is attempted with more than one ROV/ship; that would surely be more tricky. Agreed though, for reasons already discussed, that ROVs are not the right tool for a wide area survey.