中心回线式直升机TEM资料的电导率-深度成像方法

以理论上的中心回线式直升机航空瞬变电磁系统为例,提出一种快速的电导率-深度成像方法,将感生电动势瞬变响应数据变换为地下介质电导率-深度断面。该方法针对成像计算中视电导率非唯一性问题和成像深度难以确定的困难,首先给出一个关于均匀半空间模型电导率?的数据函数?(?)的变换公式,由此建立数据表???来查询视电导率值,可以得到可靠的视电导率值。其次用均匀半空间中的感应电场最大幅值对应深度值来定义视深度,建立视电导率-视深度的数据表,并在2个相邻时间道期间的最大电场深度变化的基础上定义成像深度,从而导出CDI结果。该方法的特点是理论简单、物理意义明确,且利用了数据表查询技术大大提高了计算速度,使得实时CDI处理成为可能。最后用理论模型进行试算,结果表明,电导率值的查询范围宽至10-4～102S/m时,高阻薄层、低阻薄层及二维目标体模型均有较好的成像效果。     

海底油气藏地质模型的冲激响应

海洋可控源电磁法(mCSEM)的时间域冲激响应特征可以反映海底油气高阻薄层.本文计算了水平电偶极子源均匀大地半空间,海洋均匀双半空间和海洋四层模型的阶跃响应和冲激响应,提出了瞬变冲激时刻的概念.分析了水平电偶源瞬变冲激时刻与介质电导率的指示关系.对于海底油气高阻薄层宜采用多偏移距同时测量方式,由于在低电导率介质中电磁能量传播得要快,在适当的收发距瞬变冲激时刻会提前到达,提出的瞬变冲激时刻道间变化量可以明确指示高阻薄层的存在及埋深.文中还分析了海水深度对瞬变冲激时刻的影响.由于“天波”干扰,瞬变冲激响应受到一定收发距观测的限制.消除 “天波”影响是时间域和频率域mCSEM数据处理的研究热点.     

电性任意各向异性且分块连续变化CSAMT三维有限元数值模拟

考虑地球介质电导率任意各向异性且随空间位置连续变化的情况,本文实现了直接求解电磁场的可控源音频大地电磁测深（CSAMT）三维有限元数值模拟.首先给出了电导率任意各向异性介质中CSAMT二次电场满足的控制方程及其相应变分问题,然后采用任意六面体单元对研究区域进行剖分,在网格单元中对任意各向异性电导率进行线性插值,解决了实际工作中岩矿石电导率各向异性且连续变化的情况,将变分问题转化为线性代数方程组的求解.电导率各向异性且连续变化一维模型三维有限元数值模拟结果与电导率各向异性且分层均匀渐进模型解析解结果对比验证了方法的有效性;三维地电模型电导率随位置线性变化且各向同性、主轴各向异性、方位各向异性和倾斜各向异性的数值模拟结果表明,电导率各向异性且连续变化对CSAMT视电阻率和相位数据均有明显的影响.     

论固结地壳电导率的性质

用实验室测量的结果相比较,大陆同地壳深部电导率的增高通常是用电解质或石墨薄层来解释的,汇集的大量大地电磁资料得以揭示出深部电导率有下列特点：它与地质年代有关,也与地热流强度有关,还具有与弹性波波速和地震分层性的相关性。这些特点与地壳的含水模型是一致的。     

基于全区视电阻率的半航空瞬变电磁法对低阻薄层的探测能力探讨

针对使用感应电动势定义全区视电阻率会出现"二值性"的问题,本文采用与电阻率一一对应的垂直磁场分量结合二分搜索算法计算全区视电阻率,这使得全区视电阻率具有唯一性和可靠性.将全区视电阻率的概念引入到半航空瞬变电磁法中,在此基础上,建立以中间层为低阻薄层的三层地电模型,分别绘制了不同低阻薄层埋深与层厚比、围岩与低阻薄层电阻率比的全区视电阻率异常曲线图,并分析其全区视电阻率异常曲线形态及变化规律,通过计算其相对误差,分析并归纳了半航空瞬变电磁法对低阻薄层的探测能力.结果 表明全区视电阻率对地下低阻薄层分辨率较好,能较好反映地电断面信息,这对半航空瞬变电磁法有效探测低阻薄层具有实际的指导意义.     

基于神经网络的时间域直升机电磁数据电导率深度成像

采用G-S变换以及高斯数值积分法,形成了时间域直升机的航空电磁响应正演样本集,分析了飞机测量过程中吊舱高度变化对电磁响应的影响,并将吊舱高度的变化等效成电导率为零的假层厚度的变化,以去除高度计等的影响.以假层半空间模型为基础,研究了基于人工神经网络的电导率深度成像算法,通过分析两个三层模型的电导率深度成像结果得出,神经网络方法计算时间域航空电磁探测的视电导率精度较高,特别是对高阻层的视电导率计算.     

三维大地电磁激电效应特征研究

在积分方程法的大地电磁三维正演模拟中引入Cole-Cole模型研究激电效应特征,对均匀半空间中存在极化体时的三维大地电磁测深响应进行了理论计算,分析了极化参数的影响规律.结果表明:激电效应使观测视电阻率值变低;地下高阻极化体比低阻极化体的激电效应强,特别是极化率大的高阻极化体对大地电磁响应影响大,使观测结果出现假异常.     

井地电磁法勘探深度研究

结合国家深部资源勘探的迫切需求,应用电磁场理论,从麦克斯韦方程出发推导出井地电磁场公式.通过两层大地模型与均匀半空间井地电磁场响应比较,研究了井地电磁法勘探深度,并给出了不同发射电流、发射频率、线源长度及两层大地电导率差异对勘探深度的影响,为井地电磁探测的野外勘探参数的选择奠定了理论基础.计算结果表明,加大发射电流、线源长度、两层大地电导率差异和降低发射频率可以增大接收信号的强度,提高有效勘探深度,从而,有利于进行深部资源勘探.     

电导率分层连续变化的水平层的大地电磁正演

用有限单元法进行电导率分层连续变化水平层的大地电磁正演计算，与解析法的计算结果进行对比，两者非常符合．算例表明，地表浅部岩层电导率的不均匀性对大地电磁测深曲线的高频段有较大影响．     

套管参数对瞬变电磁测井响应的影响

瞬变电磁测井是一种利用套管井内的瞬变响应获取套管外地层电导率的有效方法,但是该方法受套管参数(电导率、磁导率、厚度)的影响很大.本文用频谱法分别在一维和三维三层介质套管井模型中求得了套管井内瞬变电磁响应的精确解,研究了套管对瞬变电磁响应的影响.在一维模型中,套管内表面的反射系数随套管电导率和厚度增加而增大,随磁导率的增加而减小;套管外表面的透射系数随套管电导率和厚度的增加而快速减小,然而其受磁导率的影响较小.在三维模型中,套管内表面的反射系数随套管电导率的增加而减小,随磁导率和套管厚度的增加而增大;套管外表面的透射系数则随套管电导率、磁导率和厚度的增加而减小.依据Doll电磁感应原理,用两个相邻井内深度点测量的瞬变电磁响应相减可以有效消除与地层电导率无关的无用信号.响应相减后的幅度与不同地层电导率的差在400 Hz以下的频率范围内呈线性关系.而0～100 Hz频率范围是瞬变电磁信号各频率分量的主要分布范围,因此用井内瞬变电磁方法对地层电导率进行测量具有可行性.     

Geostatistical integration of gravity and magnetotelluric data to enhance resolution of geologic structure

To enhance the spatial resolution, two types of complementary integration methods were developed using gravity and magnetotelluric (MT) data. The first method involves the enhancement processing of gravity vertical resolution using MT data. This is called a layer density correction (LDC) process which makes the density distribution more sensitive to geologic structure. The second method involves the spatial expansion processing of MT data using the enhanced gravity data. In this process, non-linear indicator transformation (NLIT) and simple kriging with varying local means (SKlm) methods were employed. The assumptions are that while the results are analyzed by different physical properties, each method senses the same underlying geologic structure and thus there is a relationship among the physical properties. The proposed geostatistical integration methods were tested using synthetic models and field data. The experiment illustrates that the integration method proposed in this study can provide improved structures. The method integrates constructively the gravity information having a wider spatial distribution and the MT information having higher vertical resolution.     

三维大地电磁测深中的薄层近似方法在地震预报研究中的应用

本文将Ｓ．Ｐａｒｋ１９８４年提出的解决三维大地电磁测深问题的薄层近似方法，用于甘肃省河西地区的大地电磁测深定点复测结果的资料解释，展示了这种方法应用于复杂介质结构下的大地电磁测深工作的良好前景。依据该方法理论而编制的程序有较高的计算精度。     

利用大地电磁资料对玄武岩成像的可行性

The magnetotelluric （MT） method has been among the favorite supporting tools for seismic imaging of sub-salt and sub-basalt targets. In this paper we present an example from Kachchh, India （where basaltic rocks overlie Mesozoic sedimentary rocks）, and discuss the feasibility of using MT method as an exploration tool in this geological setting. Our results highlight the difference in magnetotelluric response caused by the thin intrabasalt layering. The key issue addressed in this paper is what MT can and cannot provide in such geological settings. First, we compute apparent resistivity and phase response curves using representative resistivity-depth models and borehole data from the study area. Later, we compare these results to assess the plausibility of using MT to image the sub-volcanic sediments at Kachchh. Finally, we substantiate our discussion through one-dimensional inversion of the field observed MT data from this region that exhibits poor sensitivity of MT for thin basalt layers.     

基于自适应收缩可行域的遗传算法反演TEM导电薄层

将自适应收缩可行域的遗传算法与视纵向电 导微分成像法相结合,反演瞬变电磁测深中的导电 薄层,计算结果证明:该算法通过对可行域的自适 应收缩,加快了计算速度、提高了计算精度;并在 一定程度上克服了瞬变电磁测深的等值性,缩小了 等值范围。通过对导电薄层模型的模拟计算和实例 分析,并与常规反演曲线进行对比,使用本法反演 成像的结果,明显地显示出导电薄层的存在。     

Appraisal of equivalence and suppression problems in 1D EM and DC measurements using global optimization and joint inversion*

Global optimization with very fast simulated annealing (VFSA) in association with joint inversion is performed for 1D earth structures. The inherent problems of equivalence and suppression in electromagnetic (EM) and direct current (DC) resistivity methods are studied. Synthetic phase data from multifrequency sounding using a horizontal coplanar coil system and synthetic apparent resistivity data from Schlumberger DC resistivity measurements are inverted individually and jointly over different types of layered earth structures. Noisy data are also inverted. The study reveals that global optimization of individual data sets cannot solve inherent equivalence or suppression problems. Joint inversion of EM and DC measurements can overcome the problem of equivalence very well. However, a suppression problem cannot be solved even after combination of data sets. This study reveals that the K-type earth structure is easiest to resolve while the A-type is the most difficult. We also conclude that the equivalence associated with a thin resistive layer can be resolved better than that for a thin conducting layer.     

Solutions of the inherent problem of the equivalence in direct current resistivity and electromagnetic methods through global optimization and joint inversion by successive refinement of model space

The problem of equivalence in direct current (DC) resistivity and electromagnetic methods for a thin resistive and conducting layer is well‐known. Attempts have been made in the past to resolve this problem through joint inversion. However, equivalence still remains an unresolved problem. In the present study, an effort is made to reduce non‐uniqueness due to equivalence using global optimization and joint inversion by successive refinement of the model space. A number of solutions derived for DC resistivity data using very fast simulated annealing global inversion that fits the observations equally well, follow the equivalence principle and show a definite trend. For a thin conductive layer, the quotient between resistivity and thickness is constant, while for a resistive one, the product between these magnitudes is constant. Three approaches to obtain very fast simulated annealing solutions are tested. In the first one, layer resistivities and thicknesses are optimized in a linear domain. In the second, layer resistivities are optimized in the logarithmic domain and thicknesses in the linear domain. Lastly, both layer resistivities and thicknesses are optimized in the logarithmic domain. Only model data from the mean models, corresponding to very fast simulated annealing solutions obtained for approach three, always fit the observations. The mean model defined by multiple very fast simulated annealing solutions shows extremely large uncertainty (almost 100%) in the final solution after inversion of individual DC resistivity or electromagnetic (EM) data sets. Uncertainty associated with the intermediate resistive and conducting layers after global optimization and joint inversion is still large. In order to reduce the large uncertainty associated with the intermediate layer, global optimization is performed over several iterations by reducing and redefining the search limits of model parameters according to the uncertainty in the solution. The new minimum and maximum limits are obtained from the uncertainty in the previous iteration. Though the misfit error reduces in the solution after successive refinement of the model space in individual inversion, it is observed that the mean model drifts away from the actual model. However, successive refinement of the model space using global optimization and joint inversion reduces uncertainty to a very low level in 4–5 iterations. This approach works very well in resolving the problem of equivalence for resistive as well as for conducting layers. The efficacy of the approach has been demonstrated using DC resistivity and EM data, however, it can be applied to any geophysical data to solve the inherent ambiguities in the interpretations.     

海底大地电磁场二维正演TE模式空气层选取

在对比空气和海水介质中不同频率电磁波的穿透深度差别的基础上,根据大地电磁场二维正演理论及算法编写了海洋大地电磁场二维正演程序,并验证程序的正确性.对比相同地质条件下,含空气层与不含空气层模型的TE模式正演响应结果,发现薄海水层完全可以取代空气层,使TE模式下海水面上的电场和磁场分量接近于一个常数.研究同时也得到了一些其他结果.     

Horizontal magnetic dipole over an inhomogeneous earth model with exponential variation of conductivity

The paper outlines the formulation of the problem of calculating the electromagnetic field components due to a horizontal magnetic dipole placed over a multilayered earth model with one of the layers having exponential variation of conductivity with depth. Analytical solutions and numerical computations are performed for three-layered earth models possessing the described conductivity variation in the transition layer. It is assumed that the conduction currents dominate the displacement currents. Results presented here show the influence of the transition layer thickness and the conductivity contrast between the top and the bottom layers on electric and magnetic field components. The results show a characteristic dependence on the conductivity inhomogeneity.     

Layer averaged Richard's equation with lateral flow

In this paper a formulation for layer averaged sub-surface moisture transport, that models the lateral flow, is developed using the Richard's equation. This formulation is consistent with the approach currently adopted in contemporary models used in land–atmosphere interaction studies. Explicit expressions are derived for layer averaged lateral transport contribution from diffusion, gravity, dispersion, and convergence due to land–surface curvature. It is argued that lateral contribution can be a significant component of the total soil-moisture flux. Ignoring these contributions can result in significant model error leading to inaccurate prediction or unrealistic calibration of parameters that compensate for these errors.     

大地电磁资料精细处理和二维反演解释技术研究(七)——云南盈江——龙陵地震区深部电性结构及孕震环境

本文对一条布设在滇西盈江—龙陵地区的大地电磁剖面(苏典—中山剖面)数据进行了精细处理和二维反演解释,得到了测区较高置信度的二维电性结构.该电性模型纵向上表现为高阻-低阻-高阻的"三明治"式岩石圈电性结构,上地壳为平均厚度约为10km的高阻地层,在约6～16km地壳深度范围发育有电阻率为几欧姆米的显著高导层,下地壳底部和上地幔顶部表现为电性较为均匀的相对高阻层.横向上自西向东划分出以大盈江断裂带、龙陵—瑞丽断裂带为限的3个主要构造区域.壳内分布的高导层沿剖面表现出一定的横向不均匀性,其在龙陵—瑞丽断裂带下方消失,在该处形成了腾冲地块和保山地块的电性构造边界.电性结构表明,大盈江断裂附近高导层顶界面浅,两侧高阻体厚度小,因此难以形成较大规模的相互作用,致其附近浅震源、小震级的地震活跃;龙陵—瑞丽断裂两侧的高阻体较厚,易积累较大的应力,具有大震的深部孕震环境,故其附近发生过多次7级以上强震.