Research on clock synchronization method of marine controlled source electromagnetic transmitter base on coaxial cable

Abstract. Marine controlled source electromagnetic (MCSEM) method is widely used to reveal the electrical structure of shallow media below the seafloor. It is an indispensable geophysical means in the exploration of marine oil and gas exploration, natural gas hydrates and seafloor geological structures. The transmitter and receiver in electromagnetic detection equipment need to maintain a high temporal consistency, usually using high-stability pulse-per-second (PPS) generated by GPS or BeiDou navigation modules as a synchronization signal. Coaxial cable is a widely used tow cable, so it is necessary to design a clock synchronization method of marine controlled source electromagnetic transmitter using coaxial cable. This paper proposes a method for synchronizing the internal clocks of the transmitter with PPS using ship-borne power supply when coaxial cable is used as tow cable. In this method, the ship-borne high-power supply outputs a high-voltage AC signal that is synchronized with the 400 Hz signal output from GPS; the coaxial cable transmits AC high power electrical energy and control commands; the AC signal transmitted via the coaxial cable is converted into a stable and continuous 1 Hz signal by step-down, waveform shaping and frequency division for synchronizing the internal time pulses of the transmitter. The test result shows that the 1 Hz signal obtained by this method has a deviation of about 504 ns relative to the PPS. This deviation meets the need of MCSEM transmitter for clock synchronization.