The study on synchronization technology for marine controlled source electromagnetic survey

As a potential strategic resource in the future, natural gas hydrates has been highly appreciated by governments and scientists all over the world. The Marine Controlled Source Electromagnetic (CSEM) method has become one of the high techniques to explore this resource in seas. This paper presents the developed equipment for marine CSEM, including the towing controlled source electromagnetic emitter and receiver, as a part of the National 863 Program Marine CSEM survey technology for natural gas hydrates (2006AA09A201). Based on the EM theory, the measured field sources in a large range are correlated at the same time. To measure the regional field source signals accurately, it is required that the working clocks between the emitter and receiver as well as between different receivers must be consistent, so that the subsequent data processing and interpretation will have unified time coordinates. Much different from surveying on land, the instruments on seafloor for marine CSEM survey cannot receive GPS signals from satellites. Thus we need to study the synchronization technology for the instruments on seafloor. It contains two items. One is to perform time lock to the emitter and receiver consistent with the GPS time and put the GPS time into RTC chip before the instrument is put into the sea. The other is to keep the inner clocks of all instruments on seafloor absolutely consistent with errors less than 10^-8s/s during the whole survey period (about half a month), so that the emitter sends out controlled source signals and the receiver gets the signal both at planned times accurately. The development and realization of the technology mentioned above ensure the temporal consistency of the marine survey network, providing a unified time base for subsequent data inversion and interpretation.