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导电海水的衰减作用以及天然电磁场功率谱中大约1Hz处的能量最弱,导致了周期小于1000s的海底电磁场非常微弱。正是由于这一原因,传统的海洋大地电磁方法只能用周期大于10~3s的低频信号探测深部地壳结构。要使大地电磁法能够用于海洋石油勘探,大地电磁测深技术必须探测到1～1000s周期的信号。这一目的可以使用一电场放大器和感应线圈来实现,因为海底电场平稳,所以在记录之前可以极大地放大;对于磁场,在深水中(1km),洋流噪声对磁场测量影响很小;在浅水区洋流噪声严重影响磁场测量质量,但可以用陆上记录来代替,因而可获得有效的海洋大地电磁响应。Scripps海洋学院研制了自动洋底数据记录仪,这种仪器在水中1km深的野外试验中得到了3～1000s的大地电磁响应,而且质量很好;浅水区也能够得到最小几赫兹的响应。
The decay of conductive seawater and the weakest energy at about 1 Hz in the natural electromagnetic field power spectrum result in very weak seafloor electromagnetic fields of less than 1000 s. It is for this reason that the conventional marine MT method can only detect deep crustal structures with low-frequency signals with periods longer than 10 ~ 3s. For the MT method to be used for offshore oil exploration, MTF must detect a signal of 1 ~ 1000s cycle. This purpose can be achieved by using an electric field amplifier and induction coil. Because the seafloor electric field is stable, it can be greatly amplified before recording. For the magnetic field, the ocean current noise has little effect on the magnetic field measurement in deep water (1km) Ocean current noise seriously affects the quality of the magnetic field measurements, but can be replaced by land-based recordings, resulting in an effective electromagnetic response to the marine earth. Scripps Marine Institute has developed an automatic ocean floor data logger that produces a 3 to 1000 second geomagnetic response in a field depth of 1km in water and is of good quality; shallow water can also respond to a minimum of several Hertz.