摘要
瞬变电磁法(Transient electromagnetic methods)是时间域电磁法之一(Time domain electromagnetic methods)简称TEM，属于地球物理电法勘探，是一种以电磁感应原理为基础的时间域人工场源的电磁勘探方法。利用线圈或接地电极观测二次场，通过对所观测的数据进行分析、提取和处理，从而到达解释地下矿体及相关物理参数的目的。大定源回线是瞬变电磁法的常用工作装置之一，其采用人工源方法，具有随机干扰小、发射磁矩大、分辨率高、勘探的深度大等优点。现在广泛应用于能源、矿产、水文、工程、环境等领域。
本文首先回顾了瞬变电磁法的发展历程，简要的介绍了瞬变电磁法的工作原理、特点、装置类型。从数据处理和解释方法、仪器等方面介绍了国内外发展现状及发展趋势。
其次在瞬变电磁法的基本理论知识基础上分别详细推导了在均匀半空间和各项同性水平层状介质的谐变电磁场、瞬变电磁场。通过汉克尔变换和Gaver‐Stehfest逆拉普拉斯变换将大定源回线频率域电磁场表达式转换成时间域电磁场表达式。最后根据瞬变电磁场求解出早期和晚期视电阻率解析表达式。因在实际工作中瞬变电磁法采集到的早期数据受关断效应的影响而表现出非常复杂的性质，用于解释会产生错误的结果，一般舍弃不用而都采用晚期视电阻率公式。故本文也采用晚期视电阻率方案。
再次利用大定源回线正演的数值算法，运用Matlab语言编写一维正演程序，实现正演模拟过程。分析了在二、三、四层地电模型下进行对比试验而得到的瞬变电磁响应和视电阻率的曲线特征，对瞬变电磁的资料处理解释具有指导意义。
最后在野外数据处理方面，针对大定源回线野外实际观测数据采用C++语言在Microsoft Visual2008平台上设计开发了一个数据资料处理软件，完成了实测数据的转换，实现单通道感应电动势、视电阻率衰减曲线图和多通道感应电动势、视电阻率剖图。这样有利于推断出地下地质体的感应电动势和视电阻率的衰减方向，反映出地质体的异常位置和形态，有利于推测其地质体的走向。对大定源回线资料定性解释有一定辅助作用。
The transient electromagnetic method is one of the Time domain electromagneticmethod, referred to the TEM, belonging to geophysical electrical prospecting.It is atime domain Artificial source electromagnetic exploration method based on theprinciple of electromagnetic induction. Useing the unground loop or groundingelectrode to observe the secondary field. Through analyzing,extracting and processingof the observed data, reach to the purpose of explaining the relevant physicalparameters of the underground minerals. Because the artificial source methods has theadvantaves of smaller random disturbances and larger launching magnetic moment andhigh resolution and depth of exploration, Now is widely used in energy,mineralresources,hydrology,engineering,environmental and so on.
Firstly,It is reviewed the development of the transient electromagnetic method,andBriefly introducted the working principles, characteristics and the device type of thetransient electromagnetic method.And also introducted current development and thefuture trend at the processing and interpretation methods of the data in domesticand international.
Secondly, the derivation process of forward formula and forward modeling of alarge-fixed loop TEM method was introduced in detail. Starting from the basictheoretical knowledge of the transient electromagnetic method, respectively, Theelectromagnetic field in frequency-domain and time-domain are derived in detail in thehomogeneous half space and the level of a large-fixed loop isotropic layered media.The electromagnetic field in frequency domain of was converted into time-domainelectromagnetic field expression by Hankel transform and the Gaver-Stehfest inverseLaplace transform. Finally, according to the transient electromagnetic field intime-domain solve the analytical expression of the early and late stages of the apparent resistivity.Because the early data of transient electromagnetic method which wascollected in the actual work are largely affected by turn-off effect, the late apparentresistivity is adopted in this paper. according to the forward numerical algorithm of thelarge-fixed loop, the one-dimensional forward was programmed by Matlab language toachieve the process of forward modeling.the characteristics of curve of the response oftransient electromagnetic and the apparent resistivity are analyzed andcompared,which are significance to guide the data processing and interpretation oftransient electromagnetic.
Finally, on the aspect of data processing, on account of observation data of a large‐fixed loop a data processing software was designed and developed by C++language in the Microsoft Visual2008platform.the conversion of measured data was Completed to achieve attenuation curves of the single‐channel induced electromotive force and apparent resistivity and sectional drawing of the multi‐channel induced electromotive force and the apparent resistivity. That is beneficial to deduce the direction of attenuation of the induced electromotive force and the apparent resistivity of the subsurface geology and to Reflect the abnormal position and shape and trend of the geological body. To some extent complement the Qualitative interpretation of date of the large‐fixed loop.
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