【作者】 张自力；

【导师】 魏文博；

【作者基本信息】 中国地质大学（北京） ， 地球探测与信息技术， 2009， 博士

【摘要】 随着世界各国开发海洋资源步伐的加快,海底大地电磁法(MT)得到越来越广泛的应用。在海洋环境条件下,海底大地电磁探测受人类活动产生的电磁噪声干扰小,但是受海洋环境的电磁噪声影响比较大。受高导电海水层的影响,入射到海底的大地电磁场不同于陆上的情况,海水相当于低通滤波器,对高频电磁波有较强的吸收作用。海洋中的潮波、海流、波浪、涌浪等多种海水运动形式都能因切割地磁场而产生电磁场,该电磁场是海底大地电磁探测数据中的主要噪声,影响了探测结果数据处理与解释的精度。为了提高探测结果的精度,本文从理论上建立了海水运动产生电磁场的数学物理模型,该理论从麦克斯韦的电磁理论出发,完成了海浪、海流、内波三种主要的海水运动形式在地磁场中感应产生电磁场的理论研究工作,根据理论计算初步认识了运动海水产生电磁场的特点及分布规律,并结合实测数据证明了该理论的可行性。本文是以中国地质大学(北京)负责的国家海洋863课题《海底大地电磁探测与电磁成像技术》(2002AA615020)项目为依托完成的。前期工作主要集中在理论模型的建立和理论结果的分析;后期工作主要是结合项目组在我国南海、黄海等海域进行的海底大地电磁探测试验所获得实测数据,以及试验海域海水运动和地磁场的实际情况,运用海水运动产生电磁场的理论分析海底大地电磁探测数据中的电磁噪声特点。在南黄海试验点的海底大地电磁探测数据处理与解释中,探讨了该理论的应用以及运动海水产生电磁噪声的处理方法,经过去噪处理后的数据所得到的海底大地电磁测深曲线和一维反演结果,真实地反映了海底地下介质层的电性。本文取得了以下研究成果:(1)总结了海域固体地球的电导率,推演了海底阻抗、电磁衰减比率的计算方法。(2)研究了海底大地电磁场的特征,通过理论模拟计算和对实测海底大地电磁数据的分析,表明高导海水及海水运动对海底大地电磁场有重要的影响。(3)描述了海浪、海流和内波的运动规律及特征,给出了它们速度场的简化数学表达结果。(4)结合实测资料,总结出我国近海及临近海域海浪和海流的特征;参考国际地磁参考场( IGRF ),描绘出我国海域正常地磁场、地磁倾角与偏角的分布情况。(5)在麦克斯韦电磁理论的基础上,建立了海浪在地磁场中感应电磁场的理论模型。得到海洋的表面波浪、涌浪在地磁场中感应产生的电场、磁场与它们的周期、振幅密切相关。对于海洋中常见的表面波,其振幅一般在几十厘米到几米之间、周期在几十秒范围内,它们产生的电场、磁场在海水较大的深度范围内都具有可观测性,海面处磁场一般具有零点几到几个nT的强度值、电场一般具有几个μV /m的强度值。(6)理论上计算了海浪产生电磁场的频谱特性,实际海浪产生的磁场频谱具有单峰性,频带宽度较窄,峰值频率和频带宽度与海洋表面的风速有关。(7)在麦克斯韦电磁理论的基础上,建立了海流在地磁场中感应电磁场的理论模型。得到海流在地磁场中感应产生的电场、磁场主要取决于海流的规模,主要参数为海流的速度及海流厚度。感应电场的垂直分量与海水流速和地磁场的水平分量成正比;感应电场的水平分量与海水流速和地磁场的垂直分量成正比,与海水流向垂直。当海水流动速度为1.5m/s时,感应电场的量值约为几十μV/ m。感应磁场在海底与海面上的量值约为十几个nT ,方向与水流方向平行。(8)在麦克斯韦电磁理论的基础上,建立了海洋内波在地磁场中感应电磁场的理论模型。得到海洋内波在地磁场中感应产生的电场、磁场类似于海浪,也主要取决于它们的周期(或波长)、振幅。感应产生的磁场水平分量和垂直分量在内波界面处都存在极值点,磁场的水平分量为极大值,磁场的垂直分量为极小值,且磁场的水平分量大于垂直分量。对于一般规模的内波,它能在一定的海水深度内产生几个nT的感应磁场。(9)分析、总结了海底大地电磁探测数据中电磁噪声的特征;结合南海的实测数据,探讨了海水运动产生的电磁噪声对海底大地电磁探测工作的影响,验证了海水运动产生电磁场理论的可行性。(10)在南黄海海底大地电磁探测数据处理与解释中,探讨了海水运动产生电磁噪声的消除方法,对3个试验点的海底观测数据经过去噪处理,提高了信噪比,所得海底大地电磁测深曲线以及一维反演拟合曲线较真实地反映了海底地下地层的电性。更多还原

【Abstract】 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.更多还原