有限元法2.5维CSAMT数值模拟

2.5 D CSAMT modeling with finite element method

目前国内大多数CSAMT资料处理都用一维反演方法,即假定地球介质是水平层状地电结构,这对于横向变化较大的介质结构的反演则是不真实的.为了合理的模拟可控信号源的三维特征应该使用三维有限元方法.受计算机内存和计算速度的限制,三维方法的实用化受到约束.在许多情况下,地电结构沿走向变化很小,只沿倾向发生变化.这种地电结构是二维的,而人工源是三维的,因此CSAMT资料的观测可用2.5维有限元方法进行数值模拟.本文从麦克斯韦方程组出发,建立了2.5维有限元CSAMT数值模拟方法,其核心是把地电参数变化小的走向方向转化成波数域,用一系列波数模拟三维源的特征.用一个横向均匀的三层地电结构模型展示了2.5维数值模拟的特征,并和一维模拟结果进行了比较,证实了有限元方法2.5维CSAMT数值模拟的可靠性.在此基础上对一个地电结构已知的实际模型进行了2.5维数值模拟并和该剖面的野外实测剖面数据进行了比较,进一步有效地说明了本文介绍的2.5维CSAMT数值模拟方法是仿真和可靠的,为在此基础上的2.5维反演打下了良好的基础.

Now most of CSAMT data processing methods are developed in one dimension, that is that supposing the earth media is horizontal layers, this will cause some wrong results for an earth whose conductivity varies in two or three dimensions. For some case's, the survey lines are parallel to the geologic structure's dip direction. In such situation, to consider three dimensions source and two dimensions earth is enough. In order to reasonably Simulate the three dimension source and two dimensions earth, this paper introduces a 2.5D finite element forward model method for CSAMT data, which bases on the Maxwell's equation. The main principle of this forward modeling method is to synthesize data for three dimensional signal source with a series of different wave numbers in geologic structure's strike direction (direction x in Cartesion coordinate system). One typical three layers model is used for 2.5D forward modeling, Because for such typical model the EM caused with a 3D source has an analytical integral solution, through numerical method, The EM field values is easy to determined. Though comparison with these two methods, the reliability of 2.5D modeling method and software is verified. Then we introduce another model outlined from known geological structure in a coal deposit and compared the results modeled with 2.5D modeling method with the observed real CSAMT data in the field. The comparison shows they consist very well. The comparison results show that the 2.5D forward modeling method and correspond software in this paper provides a good tool for CSAMT data modeling because the finite element method can be used for the models with lateral variation of earth electric properties, it will be useful to model 3D source and 2D earth media.