CSRMT正交水平发射源电磁场分布规律研究

    Distribution patterns of the electromagnetic fields of orthogonal horizontal magnetic dipoles as sources in CSRMT

    • 摘要: 可控源射频大地电磁法(controlled source radio-magnetotellurics, CSRMT)测量通常使用发送频率1~1 000 kHz的人工场源, 正交水平电偶源和正交水平磁偶源是人工源电磁法众多发射源中实现张量电阻率测量的优质场源。为此, 本文基于水平电偶极子源和水平磁偶极子源电磁场解析公式, 计算了均匀半空间模型正交水平电偶极子源和正交水平磁偶极子源的电磁场。结果表明, 发射频率大于100 kHz时, 应考虑位移电流; 而张量视电阻率和阻抗相位在远区可以忽略位移电流的影响; 固定模型电阻率、改变收发距时, 模型计算表明高频电磁场远区测量范围更大; 固定收发距、改变模型电阻率时, 模型计算表明电磁场远区范围受电阻率影响较大, 高阻模型需要更高的频率才会出现远区观测条件。磁偶源相较于电偶源, 在张量视电阻率和相位上与实际值的偏差更小, 更适于地质分析。

       

      Abstract: Controlled source radio-magnetotellurics (CSRMT) measurements typically use artificial field sources transmitting at frequencies ranging from 1 to 1 000 kHz. Among the many transmitting sources of the artificial source electromagnetic method, the orthogonal horizontal electric dipole source and the orthogonal horizontal magnetic dipole source are preferred field sources for tensor resistivity measurements. Hence, using the analytical formulas for electromagnetic fields based on the horizontal electric dipole source and the horizontal magnetic dipole source, this study calculated the electromagnetic fields based on the orthogonal horizontal electric dipole source and the orthogonal horizontal magnetic dipole source in the homogeneous half-space model. The results show that: (1) The displacement current needs to be considered at transmitting frequencies above 100 kHz; (2) The effects of displacement current on the tensor apparent resistivity and the impedance phase can be ignored in the far zone; (3) With a constant model resistivity and varying distances between transmitter and receiver, model calculations indicate a larger measurement range in the far zone of the high-frequency electromagnetic field; (4) With a constant distance between transmitter and receiver and varying model resistivities, model calculations suggest that the far-zone range of the electromagnetic field is significantly influenced by resistivity, and that the high-resistivity model requires higher frequencies for achieving far-zone observation conditions.Compared with the electric dipole source, the magnetic dipole source exhibits smaller deviations on the tensor apparent resistivity and impedance phase with the actual value, which is more suitable for geological analysis.

       

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