Time synchronization and data transfer method for towed electromagnetic receiver.

Abstract

Deep marine controlled-source electromagnetic (CSEM) prospecting has attracted extensive interest because it enables high efficiency and high horizontal-resolution prospecting of gas hydrate and oil. However, the elimination of time errors between the transmitter and the receiver and realization of long-distance high-speed real-time data transmission (submarine towed body status information and raw electromagnetic field data stream) are worthwhile challenges that require continuous effort. We developed a novel towed CSEM system using double-vessels that have high time synchronization accuracy and real-time data transmission. The near-seafloor-towed CSEM receiver contains a deck user terminal, master node, slave nodes, tail buoy, and neutrally buoyant towed cable. The deck user terminal generated and transmitted a pulse per second to the master node through a fiber converter and optical fiber. The RS-485 transceiver then turned the pulse signal into a differential signal and transmitted it to each slave node for error-free synchronization. The time information was also transmitted from the deck user terminal to various nodes through ethernet switches, optical fibers, and serial to ethernet converters. The deck user terminal can conveniently communicate with each node cascaded by the ethernet switch through ethernet and fiber optic communication technology. During an offshore experiment involving oil and gas exploration in the South China Sea, the towed CSEM receiver continuously acquired all electromagnetic components and status information, which achieved a preliminary prospecting result. The maximum transfer rate of real-time data can reach 10 Mbps with 300 m distance between each slave node, and the time synchronization error between transmitter and receiver is less than ±3 µs.