MT方法中Basokur视电阻率定义的研究

电法勘探中不同的视电阻率定义可以带来不同的反演效果,好的视电阻率定义可以提高反演分辨率。传统的Cagniar视电阻率(1953)只是利用了波阻抗的模值,而Basokur 1994年提出的视电阻率利用波阻抗的实部和虚部。本文做了4个不同模型的Basokur视电阻率响应及反演,结果显示新的视电阻率对薄层有很好的反应,对常规模型也有不错的表现。     

A field test of imaging properties of rotational invariants of the magnetotelluric impedance tensor

A part of the Békés Basin (an extensional sub‐basin of the Pannonian Basin, where the basement under thick Pannonian sediments is well known from deep boreholes and from seismic measurements, and where many magnetotelluric (MT) soundings have been carried out for frequencies ranging from 1 to 10^?3 Hz) was selected as a test area to assess the imaging performances of various apparent‐resistivity definitions computed with rotational invariants of either the real part of the complex impedance tensor, or its imaginary part, or both. A comparison (based on earlier 3D numerical studies) has been made between the magnetotelluric images obtained in this way and the depths to the high‐resistivity basement, as known from boreholes and seismic investigations. The correlation coefficient between the series of basement depth values at 39 MT sites and the apparent‐resistivity values was found to be stronger and high correlation appeared at a shorter period when it was computed with apparent resistivities based on the real tensor rather than with apparent resistivities based on the imaginary tensor. In the light of our studies, ρ_{Re Z} and the impedance phase seem to be more informative than any other combination of magnetotelluric interpretation parameters.     

大地电磁测深资料的联合反演研究

本文从大地电磁扬的色散关系出发,阐述了进行大地电磁资料一维联合反演的物理意义。论证了对于以色散关系相联系的一对大地电磁响应函数,特别是阻抗实部和虚部视电阻率进行联合反演的必要性和可能的应用前景。对比研究了大地电磁阻抗各单参量及其联合反演的效果。结果表明,在加快反演迭代速度,限定反演解的非唯一性、保证反演迭代的稳定进行等方面,联合反演比单参量反演具有明显的优越性;在大多数情况下,阻抗实部视电阻率和阻抗虚部视电阻率联合反演的效果要比通常所采用的常规视电阻率和阻抗相位的联合反演效果好。实际大地电磁资料的试验表明,各种不同平均阻抗的利用和综合反演解释将有助于获得更为可靠的结果。     

层状地基端承桩扭转动力阻抗研究

本文采用Novak薄层法推导粘弹性地基的扭转动力阻抗,并将其用于考虑桩土相互作用的单桩扭转动力阻抗,又通过传递矩阵法将此公式推广到求取层状地基单桩扭转动力阻抗。而且本文以工程中常用的端承桩为例,推导了层状地基中端承桩扭转动力阻抗的简化计算公式。根据此公式,分析了频率、上覆软土层厚度、上覆软土层刚度等因素对单桩扭转动力阻抗的影响,分析结果表明,随着振动频率提高,层状地基中端承单桩的扭转动力阻抗的实部有缓慢下降的趋势,而虚部则增大;随着上覆土层厚度的增加,层状地基中端承单桩的扭转动力阻抗的实部和虚部均减小;随着上覆土层刚度的减小,层状地基中端承单桩的扭转动力阻抗的实部减小,虚部在低频段减小,而在较高频率段则增大。     

THE DISPERSION RELATION, OF EARTH''S ELECTROMAGNETIC RESPONSE FUNCTIONS AND THE JOINT INVERSION OF DATA

On the basis of the dispersion relations of MT field, the necessity and applied prospects of the joint inversions using a pair of MT response functions which are correlative with the dispersion relations, are infered. A filter coefficient algorithm is made, with which the corresponding impedance phase data can be estimated using a set of apparent resistivities. The tests for the observed MT data show that when comparing the impedance phase estimated using the dispersion relation with the ob served phase, it can be checked whether the dispersion relation between observed apparent resistivity and phase data is satisfied or not, and that the use of the phase data corrected using the dispersion relation in the joint inversion is advantageous to obtain more confident results. It is shown that joint inversions are more advantageous than single parameter inversions, and that in the most case the joint inversion using the apparent resistivities of impedance real and imaginary parts is more advantageous than the jointinversion using the normal apparent resistivity and impedance phase. The existence of the dipersion relations between the ratio apparent resistivity and corresponding impedance phase of the orthogonal electric and magnetic field horizontal Components in the frequency EM sounding with horizontal electric dipole(FEMS) are discussed, the better effect of the joint inversion using the pair of EM response functions is obtained. The problems on the one-dimensional joint inversion for the MT and FEMS apparent resistivities, for which the observed frequency bands partly overlape each other, are studied. It is shown that this joint inversion is applicable and effective:the joint inversions of the practical data for two kinds of EM methods at two sites give the results well corresponding to the drilling data. The simulated MT inversions for the data of two kinds of EM methods are made, and more confident results also are obtained.     

Electric potential and apparent resistivity in rocks containing non-uniformly distributed cracks

IntroductionIn multidisciplinary research of earthquake predication, the geoelectric method is one of the important branches. It takes the electric phenomenon or the electric property changing of the earth medium in seismogenic process as the studying objects, and it is the important composing portion of the earthquake-development comprehensive research. The electric parameters of the earth medium, mostly the resistivity, are the main body in geoelectric research.Experiments indicate (LU, et …     

电偶源频率电磁测深二维阻抗视电阻率计算的源效应校正法

研讨了频率域电磁法中不同源装置的大地电磁测深、线源频率电磁测深和偶极源频率电磁测深阻抗视电阻率的源效应影响特征。在唯象分析的基础上，提出了几种电磁测深法阻抗视电阻率的相互换算法──源效应校正法（大地电磁测深二维ＴＥ极化视电阻率和其它两种电磁法的赤道装置二维阻抗视电阻车）。模型试验表明，利用这一源效应校正法可以由大地电磁二维视电阻率近似地计算出线源频率电磁二维阻抗视电阻率。这一方法被尝试应用于由线源频率电磁二维阻抗视电阻率估算偶极源频率电磁二维阻抗视电阻率。     

电导率随深度分层线性变化的大地电磁响应

本文在电导率随深度分层线性变化的地球模型上,研究了大地电磁的正演问题.由Airy微分方程导出了电场和磁场水平分量的解、地面阻抗的递推表达式和视电阻率的计算公式;并证明了分层线性模型与均匀层状模型一样,也存在高阻薄层的H等值性和低阻薄层的S等值性;总结了电导率随深度的变化率α,对大地电磁测深曲线的影响规律.     

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.     

大地电磁资料的灵敏度研究

本文研究了大地电磁资料对地球电性结构模型参数的灵敏度,它在反演解释中具有重要意义.通过研究均匀半空间中视电导率资料的灵敏度函数与计算分析实际观测资料的Fréchet导数和斜率表明,视电导率资料的灵敏度在地表最大,从地表向下呈指数衰减,频率愈高,电导率愈大,衰减也愈快.视电阻率资料和阻抗相位资料对高频、浅层和低阻层具有更高的灵敏度,几何参数的平均灵敏度高于电性参数的平均灵敏度,通常阻抗相位资料比视电阻率资料更灵敏.同时,根据阻抗实部定义的视电阻率与Kunetz定义的时间域大地电磁响应,反映地层电性差异的灵敏度最高. 文章还指出,在实际的反演解释中,如何正确有效地应用阻抗相位资料,提高分辨能力,仍是急待解决的问题.     

Airborne VLF measurements and mapping of ground conductivity in Sweden

Airborne VLF data are routinely collected by The Geological Survey of Sweden (SGU) as part of its bedrock mapping programme. In this paper we demonstrate that the novel Tensor VLF technique developed at Uppsala University and SGU can provide useful qualitative and quantitative information about the electrical conductivity distribution in the upper few hundred meters. Single transmitter scalar VLF maps emphasize those conductive structures that have dominant strikes in the direction of the transmitter. The tensor tipper (essentially the vertical magnetic field from currents along the strike direction) calculated from multiple transmitters is dependent only upon the underlying conductivity structure. Transformation of the tipper into the peaker (the horizontal divergence) has proven to enhance the lateral resolution while the transformation to the apparent resistivity can be used to discriminate different rock types. Two case histories from the application of VLF data are presented in this study. Two dimensional structures can be quantitatively modelled by modern inversion methods developed originally for deep electromagnetic MT soundings. Direct inversion of the real and imaginary parts of the tipper provides more quantitative information about the subsurface resistivity distribution.     

一种新型随钻电阻率测井仪器的响应和刻度

对一种新型随钻电阻率测井仪器在均质地层中的电磁场分布和接收线圈的响应特性进行分析,并对仪器在真空中的刻度方法进行研究.推导出了圆柱坐标系下径向成层介质中磁流源并矢Green函数的矢量本征函数展开式,从而得到研究该仪器电磁场分布、接收线圈响应、刻度方法所必需的解析计算式.计算发现,钻铤的存在大大提高了仪器的定向探测性能.计算还发现,不同的钻铤尺寸、线圈系到钻铤不同的距离对线圈系在真空中的响应值具有较大影响.钻铤在地层中产生的实部信号可近似认为恒定,而当地层电导率大于001 S/m时,钻铤在地层中产生的虚部信号亦可近似认为恒定.数值计算表明,仪器偏心程度越大,钻铤半径越大,仪器中刻度环电阻所对应的视电导率值越小.     

水平多层大地上垂直磁偶极频率测深的全波视电阻率

本文根据视电阻率定义的原则,以及用不同的场量定义的视电阻率效果不同这一事实,提出一种新的全波视电阻率定义.在全区同时用均匀大地上电磁场的三个分量来分区定义祝电阻率.在远区视电阻率由磁场的水平分量求出,在近区由磁场的垂直分量或其实分量定义,而在过渡区则由电场的水平分量确定.用这种方法定义的视电阻率为电磁响应的单值函数,它随频率变化的曲线显著改善,能直观地反映地层电阻率随深度的变化,数值比较接近地层的真电阻率值,假极值效应明显压低.在计算中用切比雪夫多项式分段拟合均匀大地电磁响应的反函数,并给出一套系数,由此算出的视电阻率误差小于1％.     

Definitions of apparent resistivity for the presentation of magnetotelluric sounding data1

A new definition of apparent resistivity for the presentation of magnetotelluric sounding data is proposed. The new definition is based on the frequency-normalized impedance function. Both the existing and proposed definitions of apparent resistivity are analysed theoretically and are compared using model curves computed for a 1D earth model. Apparent resistivity curves computed using the proposed definition are a better approximation to the true resistivity values of the subsurface layers. In addition, the layers are more noticeable on the apparent resistivity curves, which is an advantage, especially for the ascending and descending type of apparent resistivity curve.     

地电阻率的数值模拟和多极距观测系统

本文利用高精度的多层水平层状介质视电阻率计算公式,模拟了不同结构剖面、不同装置极距、不同规律的真电阻率变化,以及这种变化在介质中所处不同部位条件下地表视电阻率变化的计算结果;讨论了发生于介质中的真电阻率随时间变化在上述各种不同条件下,与在地表观测到的地电阻率(即视电阻率)变化两者之间的关系;指出为了解次异常变化与干扰变化的识别问题,应当将现行单一极距观测改造为多极距观测,并研究和发展相应的地电阻率随时间变化的反演技术和方法,以促进地电阻率法预报地震从看图识字的被动状态向物理预报方向转化。      

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%.     

ON SOME RESISTIVITY LOG INTERPRETATION CHARTS*

The paper relates primarily to the borehole and the side bed correction charts for Latero-logs 3 and 7. Versions of some of these charts published by different companies—or even by the same company in different years—exhibit significant variations. Usually, such publications do not contain adequate information on how the charts were constructed and do not explain why discrepancies occur. Because of these reasons, an attempt has been made in this paper to reconstruct the borehole correction chart for Laterolog 7 and the shoulder bed correction charts for Latero-logs 3 and 7. For the latter two, the results found differ substantially from those published earlier. The paper demonstrates how departure (response) curves and correction charts for the lateral and the Laterolog 7 sondes can be computed from those for the normal sonde. An apparent resistivity formula is suggested for Laterolog 7 in which all currents that exist in the ground at the time of measurement and that produce the signal are monitored and used. Response curves and correction charts for Laterolog 7, based on such a formula, are presented as illustrations.     

地电阻率法中勘探深度和探测范围的理论讨论和计算

本文对地电阻率法中两个常用的基本概念——勘探深度和探测范围进行了理论探讨和计算,分析了电法勘探中关于这两个概念的若干问题和不足。为了适应地电阻率法比较台址条件和研究其探测能力的需要,在分析这些问题的基础上,运用层状介质视电阻率的理论公式提出了勘探深度的新定义。用统一的标准给出了若干不同电性结构、不同装置下的勘探深度的定量结果,并具体讨论了探测范围上、下界面理论位置与介质表层及深部电性变化量大小、装置极距、仪器精度等参数之间的关系。指出在均匀介质、四极对称装置下,利用精度为1级的仪器探测下界面的最大深度可达极距AB的1.5倍左右的新结论,以及给出了加大探测范围的理论途径。最后本文还讨论了两个概念的差异和联系以及在一般非均匀介质条件下求得勘探深度的原则方法。     

二维频率测深边界单元法正演计算

在二维层状介质中含有横向非均匀体的地电断面情况下，对电偶源的偶极长度作了延长，并选取了适当的基本解及无穷远边界条件，因而将相应的含源谐变电磁场的边值问题转化为较简单的边界积分方程．用边界单元法求出了相应的电磁场分量，进而计算阻抗视电阻率，并对这一算法作了相应的检验．     

华北地区上地幔及过渡带电性结构研究

采用远参考道和Robust技术,处理了华北地区14个地磁台站资料,得到了相干度超过0.8的地磁测深响应函数.并将其转换为大地电磁测深的响应函数,获取了105~107 s周期范围内的视电阻率和相位.应用ρ+理论对数据进行了一致性检验和反演,结果表明417km,850km深度附近可能存在电性间断面.同时采用基于一维最光滑模型的Occam反演方法得到了300~1000km范围的地幔电性结构,并与前人在其他地区的研究结果进行了对比.发现华北地区地幔过渡带的电导率在大兴安岭—太行山重力梯度带东西两侧表现不同,重力梯度带附近及西侧台站下方过渡带深度的电导率和北美的Tucson地区相当,而华北地区东部的电导率在地幔过渡带范围高出西侧约2~5倍,这很可能和太平洋板块的俯冲有关.