二维海底地层可控源海洋电磁响应的数值模拟

Numerical simulation of marine controlled-source electromagnetic (MCSEM) response to 2-D seabed formation

（1．中国石油大学（北京）资源与信息学院地球物理系，北京102249；2.中国石油集团公司物探重点实验室，北京102249;3.中国石油大学（北京）油气资源与探测国家重点实验室，北京102249）

Geophysical Department, Faculty of Resources and Informatics, China Petroleum University, Beijing 102249，China

利用等参有限元方法对不同类型的偶极子源在二维海底地层上方产生的电磁场响应进行了数值模拟。通过对构造走向进行Fourier变换，将全三维电磁问题转化为一系列二维电磁问题，并在波数域求解，极大地减小了计算量；导出了适用于二维海底地层中任意方向的电偶极子或磁偶极子响应的波数域电磁场方程，并在x-z平面内采用等参有限元方法求解，利用Fourier逆变换得到空间域海洋电磁响应；采用具有一定面积的伪δ函数表达源电流分布，提高了数值解的精度；利用层状含高阻油气储层和低阻水层模型的数值模拟结果，分析了不同方向的电偶极子源和磁偶极子源的敏感性。模拟结果表明，水平电偶极子激发测得的两个电场分量和一个水平磁场分量，均对海底高阻层响应灵敏，适合于油气储层直接探测；水平磁偶极子激发的两个磁场分量和一个水平电场分量，尽管响应幅度较低，但在高阻储层能产生一定的异常，可以为水平电偶极子激发油气探测提供补充信息。

Iso-parametric finite element method is applied to simulate the marine controlled-source electromagnetic (MCSEM) responses generated by different dipole source on 2-D seabed formation. Entire 3-D electromagnetic problem is transformed into a series of two-dimensional problems through Fourier transformation on structural trend, and is calculated in wave-number domain, which minimizes computation amount. The electromagnetic equation derived in wave-number domain can be adapted to electric (or magnetic) dipole source in any directions and it can be solved in x-z plane by using iso-parametric finite element method. The marine electromagnetic response in spatial domain has been obtained through the inverse Fourier transformation. The accuracy of numeric solution is enhanced by using a pseudo-delta function that distributes a dipole source current in a specific area. The sensitivity of electric and magnetic dipoles in different directions is analyzed by the numerical simulation results of layered model with high-resistivity hydrocarbon reservoir and low-resistivity water layer. The simulation results show that the two electric components and one horizontal magnetic component stimulated by a horizontal electric dipole are sensitive to high-resistivity seabed formation response and suitable for direct hydrocarbon detection. Whereas, the two magnetic components and one horizontal electric component stimulated by the horizontal magnetic dipole has lower response, but high-resistivity reservoir can generate anomaly to some degree, which can provide supplementary information for hydrocarbon detection through horizontal electric dipole stimulation.