Indicator loops .
Anti-submarine ‘indicator loops’ are long lengths of cable laid on the seabed in shipping channels of strategically important ports. They rely on the production of an induced current in a stationery loop of wire when a magnet (in this case, a submarine) moves overhead.
If TDW could make a mine with asdic then this would serve the purpose i would think . This hopefully would trigger nearby ships to respond .
(Harbor defense asdic is what i wanted in sh5 by using a mine that has asdic)
http://indicatorloops.com/loops.htm