Several theoretical points and instrument technology of magnetotelluric data acquisition in deep water
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Magnetotelluric data acquisition in deep water faces some difficulties,such as weak signal,serious attenuation of high-frequency components,great ambient pressure on instruments,disadvantages of the bottom currents push drag and adsorption,and highly variable sea conditions which make deployment and retrieval of equipment difficult.Concerning these special issues,this work analyzes theoretically earth electromagnetic field propagation in sea and below layered media,calculates the decay ratios of the electric field to the magnetic field with different water depths,and discusses the physical factors relevant to the upward-floating speed of instruments.The working principles of the seafloor electromagnetic instrument are presented,including the signal sensor,data acquisition,voice release unit and mechanical installation parts.The signalresolution of the instrumentation systems is up to nV level with band width 10~0.3×10-3 Hz,and the maximum working depth is 4000m.Not long ago,equipped with the scientific research ship"Ocean VI",the magnetotelluric data are acquired for the first time at water depth about4000min the Zhongsha Islands of the South China Sea.The results show that the measured electromagnetic field is consistent with theoretical calculations and the instrument has achieved the designed specifications.It demonstrates that the marine magnetotelluric technique of our country has been of completely independent intellectual property rights.