A system for mapping and characterizing a hydrocarbon plume in seawater by measuring seawater capacitance. Multiple streamer cables are towed in the sea behind a ship, each at a different depth, simultaneously. Each streamer cable includes transmitters and receivers at the free end thereof. The free ends of the streamer cables pass through the plume and the transmitters transmit an electrical current into the plume. The receivers detect any secondary signals produced by capacitive effects of the hydrocarbon or hydrocarbon and dispersant surrounded by conductive seawater in response to the inducing electrical current. Pre-amplifiers connected to the receivers and a two-step calibration procedure and various grounding and shielding steps provide noise rejection. An electronics system on board the ship processes the secondary signals to provide immediate development of detailed maps of plume location, and to provide tracking and characterization of how the plume changes shape and character over time.