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针对地面场源在地层介质中所产生瞬变电磁场的数值计算方法和响应分析问题,本文展开研究.解法方面,传统离散镜像法多采用复数运算、数字滤波等方法计算量大,针对这些问题,提出一种改进的离散镜像方法:基于Gaver-Stehfest概率变换算法将电磁场解式实数化,选用Prony方法对目标核函数进行指数级数逼近,根据离散镜像原理和近似系数闭合式求解瞬变电磁场.通过试算均质模型中瞬变电磁场并对比汉克尔变换的数字滤波法所得结果,证明该方法有效且具有较好的精度和适用性.继而基于该方法计算地表磁偶源在典型地电模型中产生的瞬变电磁场,对“地面激发-地层中测量”方式所得感应磁场水平分量响应进行分析并得出结论瞬变场水平分量响应与地电结构、观测时间、空间位置等因素有关,感应磁场水平分量响应反映出涡流场分布及其垂向梯度变化情况,在探测异常体的工作中应尽量选取零偏移距、较大偏移距位置钻孔或用较大观测延时以减小背景场对勘测结果的影响。文中所用离散镜像方法与正演计算结论可为相关研究工作提供参考依据。
Aiming at the problem of numerical calculation and response analysis of transient electromagnetic fields generated by ground source in formation media, this dissertation starts with the research.During the solution method, the traditional discrete image method mostly uses complex computation and digital filtering to calculate the large amount of electromagnetic fields, An improved discrete image method is proposed. Based on the Gaver-Stehfest probability transform algorithm, the electromagnetic field is real-valued. The Prony method is adopted to approximate the exponential series of the target kernel function. The transient electromagnetic field is solved according to the discrete mirror principle and the closed-form approximation coefficient. By calculating the transient electromagnetic field in the homogeneous model and comparing the results obtained by the Hankel transform digital filtering method, the method is proved to be effective and has good accuracy and applicability.According to the method, The transient electromagnetic field generated in the model is used to analyze the horizontal component response of the induced magnetic field obtained from the “ground-excited-measured” method and the conclusion is that the horizontal component response of the transient field is related to the geoelectric structure, observation time, spatial location and other factors , The response of horizontal component of induced magnetic field reflects the distribution of eddy current field and the change of vertical gradient, Work anomalies should try to select a zero-offset, offset position of the drilling or larger with a larger delay to reduce the effects observed on the background field survey results. Discrete image method used in this paper and forward calculation of conclusions can provide reference for the relevant research work.