Research on the clock synchronization method of the marine controlled-source electromagnetic transmitter based on coaxial cable

Abstract

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