Inductively coupled underwater modem

An inductive modem telemetry system for use in collecting data in seawater, the system comprising:

• an electrically insulated wire rope immersed in seawater;
• a plurality of subsurface instruments removably attached to the wire rope at preselected locations;
• master modem means for inductively coupling a plurality of subsurface instrument specific wake-up command signals to each subsurface instrument through the wire rope with a return path through the seawater;
• a transducer in each subsurface instrument to generate a data signal related to the data to be collected;
• a slave modem in each subsurface instrument interconnected with the transducer to generate modem encoded signals related to the data signal, and receive modem encoded signals from the master modem;
• an inductive coupler in each subsurface instrument enveloping the wire rope and interconnected with the slave modem, the inductive coupler constructed to induce and receive electrical signals in accordance with the modem encoded signals in a signal path to the master modem including substantially equal length path legs in the wire rope and in the seawater;
• switch means in each subsurface unit operable in a sleep mode for reducing power consumption of the subsurface instrument by removing power from the transducer and slave modem;
• and
• a continuously powered wake-up detector in each subsurface instrument for operating the switch means to apply power to the transducer and the slave modem in response to the wake-up signal specific to that subsurface instrument.