This site might interest you. Look at the electro-magnetic detector sled, they are lowering overboard. That thing is 13 feet long by 13 feet wide by 2 feet high.
As an electronics buff that has built several Pulse inductor metal detectors from scratch,(one with a 4 foot search coil, that I wound myself,} I would love to see the schematics of that thing. It has one large transmit coil and three individual receive coils.