河北兴济台地电阻率年变特征分析

基于电测深曲线反演结果,利用地电阻率转换函数的递推公式定量模拟分析了地下水位对兴济台地电阻率的影响。结果表明,地下水位下降会造成地电阻率上升,这与兴济台N30°E向的地电阻率观测结果相符;N60°W向的地电阻率的趋势变化与N30°E向的相反,且年变幅较大,这是由于测区供电极和测量极之间存在因取土而积水成坑这个局部异常体引起的。     

三维电阻率测深数据Zohdy近似反演方法

Zohdy方法近似反演三维电阻率测深数据。正演计算采用有限单元法。反演初始模型由测量视电阻率数据给出。通过比较实测视电阻率值和预测模型计算的视电阻率值对数差来修改模型网格电阻率．为了解决任意电极距测深数据的反演,采用大、小双网格剖分。大网格反映地下电性分布情况。小网格用于实际有限元正演计算．在电阻率调整公式中加入一个迭代系数,能够加快收敛速度．并对加5％随机噪声的模型理论视电阻率测深断面数据进行反演,得到的电阻率分布与模型电阻率基本一致．     

层状方位各向异性介质的视电阻率计算

从电性各向异性的欧姆定律出发，推导了直流电法层状方位各向异性介质中的电位分布、边界条件及视电阻率计算公式．以四极对称装置系统为例，对具有相同各向异性系数的4层模型采用核函数递推法作了理论数值模拟，得到了不同方向的电阻率测深曲线及其等值线形态．结果表明理论公式是正确的，测深曲线既反映了分层介质的电阻率差异，又反映了各层中电阻率的各向异性特征．      

尖锐边界反演在临江市六道沟可控源音频大地电磁测深数据反演中的应用

在实际大地电磁测深数据反演过程中,为了得到更加清晰的电性单元分界面,传统的Occam平滑反演方法遇到了挑战,本文讨论了sharp boundary inversion(SBI)反演的理论及其应用.通过模型试算,对比Occam和SBI的反演效果,证明了SBI反演划分层状介质的可靠性.本文对临江市六道沟地区实测可控源音频大地电磁测深(CSAMT)数据进行了全区视电阻率转换,然后对全区视电阻率进行Occam反演和SBI反演.最后对比已有地质资料,表明SBI反演能够用于CSAMT数据的处理中.     

瞬变电磁测深早期数据的修正

通过时间-频率转换关系,TEM数据可以转换成平面波场测深数据,从而可以对TEM资料进行拟平面波场处理解释.在对瞬变电磁视电阻率数据向平面波场测深视电阻率数据转换时,发现由于瞬变电磁使用晚期计算公式及装置问题,使测深曲线早期数据发生畸变.文中建立了视电阻率曲线进入晚期所满足的关系式,从理论上给出不同情况下瞬变电磁测深视电阻率曲线进入晚期的临界点.以瞬变电磁、大地电磁、CSAMT为例,对大量的模型进行正演计算,对计算结果进行对比分析,建立不同地表电性结构、不同时间延迟情况下,瞬变电磁早期数据误差的校正量板.     

电测深曲线的遗传算法反演

电测深曲线作为地下介质电阻率和深度的非线性函数，其解具有高度的非唯一性．常规的基于局部线性化的最优化反演方法易使解估计陷入局部极大值中，而且严重地依赖初始模型的选择．遗传算法作为一种全局最优化方法，对初始模型的依赖性大为减弱，且不易陷人局部极大值之中，从而能有效地解决这类非线性最优化问题．本文首次将遗传算法用于电测深解释并对实测曲线进行反演，效果很好，显示了遗传算法独特的优越性．     

电测深曲线的遗传算法反演

电测深曲线作为地下介质电阻率和深度的非线性函数，其解具有高度的非唯一性．常规的基于局部线性化的最优化反演方法易使解估计陷入局部极大值中，而且严重地依赖初始模型的选择．遗传算法作为一种全局最优化方法，对初始模型的依赖性大为减弱，且不易陷人局部极大值之中，从而能有效地解决这类非线性最优化问题．本文首次将遗传算法用于电测深解释并对实测曲线进行反演，效果很好，显示了遗传算法独特的优越性．     

MT激电效应的模拟研究及在油气检测中的应用

本文引入Cole-Cole模型来模拟大地的激发极化效应,对三层水平地层且中间层为极化层的大地电磁测深的视电阻率进行了理论计算,分析了极化参数对视电阻率曲线的影响规律.采用广义逆方法对三层水平地层且中间层为极化层的模型进行了反演研究,结果表明该反演方法能够较好地确定地层电阻率的同时获得地层激电参数,应用于实际资料的反演时,反演结果与已知含油气地层的实际参数十分吻合.     

大地电磁测深资料的二次函数逼近非线性反演

将二次函数逼近非线性优化首次应用于大地电磁测深反演问题,该反演方法利用二次函数有唯一最小值的特点进行逼近大地电磁反演模型,从而避免了常规的迭代反演过程中陷入局部极小问题,实现了对目标函数求全局极小,较好地解决了非唯一性问题;同时该方法不用求灵敏度矩阵,且对初始模型无任何要求。通过理论模型检验、井旁MT点反演结果与测井曲线的对比及MT测线的反演电阻率深度剖面与地震测线的时间剖面对比均表明,本文方法取得较好的应用效果。     

海岸效应对近海地区大地电磁测深数据畸变作用研究

在近海地区采集的大地电磁测深数据通常受到海岸效应的影响,使得大地电磁测深数据发生畸变,因而很难利用大地电磁测深资料较为可靠地获得地下深部的电性结构.本文通过正演模拟方法,分析和总结海水深度变化和海底地形变化对近海地区大地电磁测深数据的畸变影响.当测区与海岸线的距离小于目标频率的大地电磁场趋肤深度时,高导海洋的存在会严重影响测区内电磁场的分布.由于海岸效应的影响,大地电磁测深视电阻率曲线和相位曲线均会发生不同程度的畸变,在低频部分,这种畸变作用尤为明显.大地电磁测深一维Occam反演方法和二维非线性共轭梯度反演方法,对近海地区浅部地层具有较好的反演效果.随着海水深度的增加和海底地形的复杂变化,两种反演方法均会出现不同程度的假异常,为地质解释工作造成了影响.近渤海地区的实测大地电磁测深数据在低频部分可能受到海岸效应的影响而导致视电阻率曲线的严重畸变.     

三层倾斜电性层模型的电测深曲线反演

本文根据对三层倾斜电性层模型的电测深曲线反演结果,解释了地电台址的电测深曲线的复杂性,对如何较精确地确定各电性层的埋藏深度作了探讨。     

DEFINITION AND APPLICATION OF THE FREQUENCY -EFFECT TRANSFORM FUNCTION TO THE INTERPRETATION OF IP SOUNDINGS*

This paper deals with a new method of quantitative interpretation of induced polarization soundings in the frequency-domain. From the general expression of the apparent frequency-effect for soundings carried out on a multi-layered earth the application of Hankel's inversion theorem allows to introduce a new function, called here the “frequency-effect transform”. The new interpretation method consists of two steps: 1) the inversion of field data to obtain the frequency-effect transform graph and 2) the analysis of this graph to derive the layering parameters. The first step is performed by means of a slightly revised version of a simple numerical procedure, previously suggested by the author for the inversion of d.c. resistivity sounding data. The second step is carried out by a complete curve-matching procedure, applied directly on the transform graph. This implies suitable master curves, whose preparation doesn't meet all the mathematical difficulties which are present when preparing master curves of the apparent frequency-effect function.     

THE QUANTITATIVE INTERPRETATION OF DIPOLE SOUNDINGS BY MEANS OF THE RESISTIVITY TRANSFORM FUNCTION*

In this paper a theorem is demonstrated which allows—after the introduction of a suitable dipole kernel function or dipole resistivity transform function—to write the apparent resistivity function as an Hankel transformable integral expression. As a practical application of the theorem a procedure of quantitative interpretation of dipole soundings is suggested in which the dipole resistivity transform function obtained after inversion of the original dipole apparent resistivity data is used to control the goodness of the set of layering parameters which have been derived with our previous method of transformation of dipole sounding curves into equivalent Schlumberger diagrams.     

Joint inversion of Rayleigh‐wave dispersion data and vertical electric sounding data: synthetic tests on characteristic sub‐surface models

In the traditional inversion of the Rayleigh dispersion curve, layer thickness, which is the second most sensitive parameter of modelling the Rayleigh dispersion curve, is usually assumed as correct and is used as fixed a priori information. Because the knowledge of the layer thickness is typically not precise, the use of such a priori information may result in the traditional Rayleigh dispersion curve inversions getting trapped in some local minima and may show results that are far from the real solution. In this study, we try to avoid this issue by using a joint inversion of the Rayleigh dispersion curve data with vertical electric sounding data, where we use the common‐layer thickness to couple the two methods. The key idea of the proposed joint inversion scheme is to combine methods in one joint Jacobian matrix and to invert for layer S‐wave velocity, resistivity, and layer thickness as an additional parameter, in contrast with a traditional Rayleigh dispersion curve inversion. The proposed joint inversion approach is tested with noise‐free and Gaussian noise data on six characteristic, synthetic sub‐surface models: a model with a typical dispersion; a low‐velocity, half‐space model; a model with particularly stiff and soft layers, respectively; and a model reproduced from the stiff and soft layers for different layer‐resistivity propagation. In the joint inversion process, the non‐linear damped least squares method is used together with the singular value decomposition approach to find a proper damping value for each iteration. The proposed joint inversion scheme tests many damping values, and it chooses the one that best approximates the observed data in the current iteration. The quality of the joint inversion is checked with the relative distance measure. In addition, a sensitivity analysis is performed for the typical dispersive sub‐surface model to illustrate the benefits of the proposed joint scheme. The results of synthetic models revealed that the combination of the Rayleigh dispersion curve and vertical electric sounding methods in a joint scheme allows to provide reliable sub‐surface models even in complex and challenging situations and without using any a priori information.     

THE APPLICATION OF LINEAR FILTER THEORY TO THE DIRECT INTERPRETATION OF GEOELECTRICAL RESISTIVITY SOUNDING MEASUREMENTS*

Koefoed has given practical procedures of obtaining the layer parameters directly from the apparent resistivity sounding measurements by using the raised kernel function H(λ) as the intermediate step. However, it is felt that the first step of his method—namely the derivation of the H curve from the apparent resistivity curve—is relatively lengthy. In this paper a method is proposed of determining the resistivity transform T(λ), a function directly related to H(λ), from the resistivity field curve. It is shown that the apparent resistivity and the resistivity transform functions are linearily related to each other such that the principle of linear electric filter theory could be applied to obtain the latter from the former. Separate sets of filter coefficients have been worked out for the Schlumberger and the Wenner form of field procedures. The practical process of deriving the T curve simply amounts to running a weighted average of the sampled apparent resistivity field data with the pre-determined coefficients. The whole process could be graphically performed within an quarter of an hour with an accuracy of about 2%.     

A NUMERICAL DIRECT INTERPRETATION METHOD OF RESISTIVITY SOUNDINGS USING THE PEKERIS MODEL*

A numerical method is presented for direct interpretation of resistivity sounding measurements. The early part of the resistivity transform curve derived from field observations by standard methods is approximated by a two-layer curve. The resistivity of the first layer is determined from the arithmetic mean of the successive computations which are carried on each of three successive discrete values of the resistivity transform curve. Using this mean value of the resistivity, the thickness of the first layer is computed from the sample values in pairs of the resistivity transform curve. After these determinations, the top layer is removed by Pekeris's reduction equation. The parameters of the second layer are obtained from the discrete values of the reduced transform curve (which corresponds to the second part of the resistivity transform curve) by the same procedure as described for the first layer. The same computational scheme is repeated until the parameters of all intermediate layers are obtained. The resistivity of the substratum is determined from the reduction equation.     

二维地电模型多参数反演方法及其在MT测深中的应用

二维地电模型电（电磁）测深法参数化反演,宜采用多参数反演方案.文中导出了多参数反演的基本数学模型,并进行了分解处理,使之变成与剖面上反演点数相等的多个相互独立的小数学模型的组合,用广义逆法逐个求解,即可得一次迭代中整个二维模型参数的修改量.大地电磁测深理论模型和实测数据反演结果表明所提出的二维多参数反演方法可行.它具有稳定的收敛性、较快的收敛速度、能适应较复杂构造的反演和计算量小等特点.     

Direct interpretation of magnetotelluric sounding data based on the frequency-normalized impedance function

An important aspect of any non-linear inversion method is the generation of a suitable or good initial model as this controls the rate of convergence and accuracy of the result. To overcome the problem, a numerical method is presented for direct interpretation of magnetotelluric sounding data based on the frequency-normalized impedance (FNI) function. The expressions used to calculate the parameters are developed, first for a two-layer case under the assumption that deeper layers do not contribute to the early part of the FNI curve, and they are then generalized for an n -layer situation. The parameters of the first layer are computed by using successive sample values and the final estimate is obtained from the arithmetic mean of selected values by excluding unacceptable results in the logarithmic space. The top layer is then removed using a reduction equation. The repetition of the procedure on successive branches of the FNI function gives successive layer parameters, the resistivity of the substratum being obtained at the final step, when the reduction equation becomes equal to the square root of that resistivity.   The proposed method can be used as a complementary method for iterative inversion as it creates an initial guess which is close to the optimal solution. The solution produced by the direct interpretation may also be modified by the interpreter to incorporate prior geological information before being input to iterative interpretation schemes.