| With rapid exploitation of ocean resources, Marine Magnetotellurics (MT) method has been applied in many fields. Under ocean conditions, application of MT is less disturbed by electromagnetic noise generated by human activities, but more affected by that of the ocean conditions. For high electric conductivity, the Magnetotellurics in ocean is different from that in land. Furthermore, similar to a low band filter, the ocean water has a high absorption effect on high frequency electromagnetic wave.Electromagnetic field in ocean is generated in many kinds due to geomagnetic field cut by tidal wave, ocean current, ocean wave, surge et al. The electromagnetic noise in marine magnetotelluric detection is mainly generated by such electromagnetic field. In order to enhance the detection sensitivity, a theory model of mathematic physics is established. Base on Maxwell theory, induced electromagnetic field generated by ocean wave, ocean current and internal wave in geomagnetic field is finished in this paper. According to the simple calculation, characteristics and distribution tendency of electromagnetic field generation by movement of ocean water is obtained. It also confirms that the theory model is feasible.This paper is supported by national ocean 863 project"telluric electromagnetic detection and electromagnetic image technology in ocean floor"charged by China University of Geosciences (Beijing)(2002AA615020). On the earlier stage, it mainly concentrates on the model establishment and theory results analysis. On the final stage, combining the experimental data collected in South China Sea and Yellow Sea, as well as the ocean movement and practical geomagnetic field, the characteristics of electromagnetic noise is analyzed. In the processing and explanation for South China Sea data, application of this theory model and treatment of the electromagnetic noise are discussed. The MT curve after noise treatment and one-dimension inversion results illuminate the practical electric property of dielectric layer in ocean floor. Main results are obtained in this paper as follow,(1) The electric conductivity in sea area solid earth is summarized, and the marebase resistance, electromagnetic attenuation ratio are induced. (2) Characteristics of the marebase electromagnetic field is investigated. Through simulation and experimental measurement, it is shown that the marebase electromagnetic field is mainly determined by the seawater for its high conductivity and movement.(3) The movement tendency and characteristics of ocean wave, current and surge are represented. And their velocity fields are given.(4) Combining the experimental measurement, the characteristics of ocean current and ocean wave in paralic and approach sea are described. Reference the IGRF, the distribution of the regular geomagnetic field, the tilt and declination angle in sea area are given.(5) Base on the Maxwell theory, a model of induction electromagnetic field is established for ocean wave in geomagnetic field. The electric field, magnetic field and their period, amplitude of the induction electromagnetic field are obtained. For a surface wave, the amplitude is in a range for a few centimeters to meters, and the period is about tens of seconds. Electric field and magnetic field can be observed in a large distance in ocean. On the ocean surface, the intensity of magnetic field is no more than a few nT, and the intensity of the electric field is a fewμV /m.(6) Spectrum characteristics of electromagnetic field generation by ocean wave are calculated in theory. However, in fact, the spectrum has single peak and narrow spectrum band. The peak frequency and duration are relative to the wind velocity on the sea surface.(7) A model of induction electromagnetic field is established for ocean current in geomagnetic field base on the Maxwell theory. It is found that the electric field and magnetic field are determined by the scale of the ocean current, the velocity and the thickness. Perpendicular component of the induction electric field is proportional to the seawater velocity and horizontal component of geomagnetic field. The horizontal component of the induction electric field is also proportional to the seawater velocity as well as the perpendicular component of geomagnetic field. And its direction is normal to the direction of seawater current. For seawater velocity of 1.5m/s, intensity of the induction electric field is tens ofμV /m. Intensity of induction magnetic field on sea floor and surface is more than ten nT, and the direction is parallel to water current.(8) A model of induction electromagnetic field is also established for ocean surge in geomagnetic field base on the Maxwell theory. Similar to the ocean wave, an electric field induced by ocean surge in geomagnetic field is determined by their period, amplitude. Maximum values are observed at surge interface for horizontal component and perpendicular component of induction magnetic field. With a maximum horizontal component, the perpendicular component is minimum and it is higher than that of horizontal component. A induction magnetic field with a few nT is generated in a certain thickness for some scale surge.(9) The electromagnetic noise in MT is analyzed and summarized. Combining the experimental data collected in South China Sea, effect of the electromagnetic noise generation by movement of ocean water on the MT is discussed. It confirms that the theory of electromagnetic field generation by movement of ocean water is feasible.(10) In the processing and explanation for South China Sea data, a method of how to remove the electromagnetic noise is discussed. After erasing the electromagnetic noise for data obtained in there observation points, the MT curve after noise treatment and one-dimension inversion results illuminate the practical electric properties of dielectric layer in ocean floor.
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