深水大地电磁数据采集的若干理论要点与仪器技术

深水环境下进行大地电磁数据采集,信号幅值微弱,高频分量被严重衰减;仪器承载的环境压力巨大,且伴随底流推曳、淤泥吸附等不利因素;海上作业情况复杂多变,带给设备投放与回收诸多困难.针对这些特殊问题,从理论上分析了大地电磁场在海水及其以下层状介质中的传播规律,计算了电场与磁场随不同水深的衰变比值,研讨了与仪器上浮速度相关的物理因素;从技术上阐述海底大地电磁仪的工作原理,包括信号传感器、数据采集器、声控释放单元以及机械组装部件等等.整套仪器信号分辨率达到nV级,频带宽度为10~0.3×10^-3 Hz,最大工作水深4000 m.该仪器搭载“海洋六号”科学考察船,在我国南海中沙群岛海域进行了国内首次深水环境下的大地电磁数据采集试验.结果显示,所研发的仪器性能指标吻合先前的理论推算值,达到设计要求.我国的海底大地电磁探测技术已完全享有自主的知识产权.

Several theoretical points and instrument technology of magnetotelluric data acquisition in deep water

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 signal resolution of the instrumentation systems is up to nV level with band width 10~0.3×10^-3 Hz, and the maximum working depth is 4000 m. Not long ago, equipped with the scientific research ship "Ocean VI", the magnetotelluric data are acquired for the first time at water depth about 4000 m in 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.