瞬变电磁法三维正演及非线性共轭梯度反演研究

发布时间：2018-03-07 03:16

本文选题：瞬变电磁三维正反演　切入点：伪谱法　出处：《中国地质大学(北京)》2016年博士论文　论文类型：学位论文

【摘要】：20世纪80年代后,瞬变电磁法(TEM)迅速地发展,现已广泛应用于油气勘探、工程勘查、矿产勘查等众多方面。但目前瞬变电磁法二、三维正反演方法技术还处于探索研究阶段,离实用化还有很大距离。因此,本文围绕瞬变电磁法三维正反演问题开展研究。本文应用伪谱法(PSM)计算和分析全空间介质中高阻体和低阻体的响应特征,计算和分析海洋中电偶源激励的低阻体响应。结果说明伪谱法的优点是计算的效率高、精度高,可以呈现出丰富的时间域的电磁场信息,可以连续地计算电磁场的扩散过程及与低阻体响应的全过程。伪谱法潜在的应用领域是海洋瞬变电磁法和矿井瞬变电磁法。如果适当地处理地表空气边界,就能应用于计算地面瞬变电磁场。本文从麦克斯韦方程组出发,详细推导了瞬变电磁法时域交错采样有限差分法(FDTD)时间分步法迭代公式。应用交错采样有限差分法计算半空间回线源的正演响应,并与频率域的计算结果进行对比,验证算法的正确性。然后分别计算低阻体模型、高阻体模型和组合模型的响应,分析说明TEM的响应特点及时间域电磁法的优点。由于TEM方法要处理的观测数据量巨大,一般的方法进行反演的效率很低。本文借鉴地震偏移成像的思想,使用非线性共轭梯度法进行反演。利用地表差异场计算反向传播场,直接在时间域用入射电场和反传电场做卷积来计算数据拟合差的梯度,避开计算雅可比矩阵,应用非线性共轭梯度法(NLCG)来搜索目标函数的极小值,找到拟合观测数据的地电模型。最后本文展示地表差异场对于地下异常体的反映特点,用电场差异场做场源计算反传的电磁场,使用NLCG反演拟合几组合成的回线源观测数据。结果说明TEM对低阻体的识别优于高阻体,多源的数据,会增加约束,提高反演的效果,反演得到的目标体的位置更接近真实的目标体的位置。
[Abstract]:Since 1980s, the transient electromagnetic method (temm) has developed rapidly and has been widely used in many fields, such as oil and gas exploration, engineering exploration, mineral exploration and so on. There is still a long way to go from practical use. Therefore, this paper focuses on the 3-D forward and inverse modeling of transient electromagnetic method. In this paper, pseudospectral method is used to calculate and analyze the response characteristics of high and low resistive bodies in the whole space medium. The results show that the pseudospectral method has the advantages of high efficiency and high accuracy, and can present abundant electromagnetic field information in time domain. The diffusion process of electromagnetic field and the whole process of response to low resistivity body can be calculated continuously. The potential application fields of pseudospectral method are ocean transient electromagnetic method and mine transient electromagnetic method. It can be used to calculate the transient electromagnetic field on the ground. In this paper, we proceed from Maxwell's equations. The iterative formula of time step method of time staggered sampling finite difference method (FDTD) for transient electromagnetic method is derived in detail. The forward response of half-space loop source is calculated by using staggered sampling finite difference method and compared with the calculated results in frequency domain. The validity of the algorithm is verified. Then the responses of the low resistivity model, the high resistivity model and the combined model are calculated respectively. The characteristics of the response of TEM and the advantages of the time domain electromagnetic method are analyzed. Because of the large amount of observation data to be processed by the TEM method, The inversion efficiency of the general method is very low. In this paper, the nonlinear conjugate gradient method is used for inversion using the idea of seismic migration imaging. The reverse propagation field is calculated by using the surface difference field. The gradient of the data fitting error is calculated by convolution of incident electric field and inverse electric field in time domain directly. The nonlinear conjugate gradient method (NLCGG) is used to search the minimum value of the objective function, and the Jacobian matrix is avoided. A geoelectric model for fitting the observed data is found. Finally, the characteristics of the surface differential field reflecting the underground anomalous body are shown. The field source of the electric field difference field is used to calculate the reverse-propagating electromagnetic field. The NLCG inversion is used to fit the observed data of several combined loop sources. The results show that the identification of low resistivity bodies by TEM is superior to that of high resistivity bodies. The data of multiple sources will increase constraints and improve the effect of inversion. The position of the object obtained by inversion is closer to that of the real object.
【学位授予单位】：中国地质大学(北京)
【学位级别】：博士
【学位授予年份】：2016
【分类号】：P631.325

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