带地形频率域可控源电磁法三维反演研究

单一的地震勘探不能满足所有地质地形条件下的油气勘探开发需求，试验证明与非地震勘探方法尤其是电磁法的联合应用可以有效提高钻井成功率，而对带地形三维可控源电磁法的正反演研究可以有效克服地形对勘探结果的影响，进一步提高勘探精度。采用基于二次电场的有限差分方法进行三维正演，与前人有限元大地电磁计算的经典地质模型在远区卡尼吉亚视电阻率正演方法相比，结果基本一致，证明该正演算法的可靠性和准确性；应用非线性共轭梯度（NLCG）方法进行三维反演，提出并使用初值优化方法，能减少每次反演迭代所需时间，从而提高反演效率。带地形的地质模型反演结果表明：反演的地下电阻率模型能够较好地还原异常体的形态和位置；初值优化方法能够明显缩短反演时间，为三维反演的实用化带来可能。

3D controlled source electromagnetic inversion with topography in the frequency domain

Seismic exploration is sometimes unsuccessful in some areas with difficult geological and topographical conditions such as complex mountainous area. Experiments show that the joint application with seismic and non-seismic methods especially electromagnetic (EM) is an effective way to ensure high quality prospecting. This paper discusses the theory and algorithm of three dimensional forward modeling and inversion of controlled-source electromagnetic method (CSEM) with topography, which is tested to be accurate in theoretical data inversion. The paper consists of two parts:3D forward modeling and non-linear conjugate gradient (NLCG) inversion. For 3D forward modeling, the finite difference based on scattered fields is implemented, and the obtained apparent resistivity in far field is similar with that of magnetolluric (MT) method on a typical model. Therefore the forward modeling algorithm is suitable and accurate. On the other hand, NLCG is used in the 3D inversion and initial solution optimization is adopted, which can efficiently reduce computation time consumption. Numerical experiments illustrate that the distribution of anomalous bodies can be well recovered by the proposed inversion and the initial solution optimization can dramatically reduce computation time consumption.