双相各向异性介质中弹性波传播有限元方程及数值模拟

基于Biot双相各向异性介质理论和动态问题的哈密顿原理，推导出任意双相各向异性介质中弹性波传播的有限元方程，并给出双相各向异性介质中弹性波有限元方程的数值解法．最后进行有限元法的数值模拟，对双相各向异性介质中弹性波传播特征进行了模拟与分析．       

EDA各向异性层状介质MT正演模拟

推导了ＥＤＡ型层状各向异性介质大地电磁测深正演计算公式,对多种模型从不同的频率,测量方向计算了地表视电阻率曲线。结果表明,一般的各向异性介质的地表视电阻率值不仅与测量频率有关,而且与测量方向相对电性主轴的交角有关。     

各向异性层状介质中视电阻率与磁场响应研究

针对任意各向异性地层,利用极向型和环向型标量位函数,导出相应的直流电视电阻率和磁电阻率的磁场响应关系.计算了各向异性地层的直流电视电阻率和磁电阻率响应特征,重点分析了电阻率测深方法与磁电阻率探测方法对地下各向异性介质的探测能力.文中采用状态矩阵的分析方法,首先采用极向型和环向型标量位构造了各向异性层状介质电场与磁场的通解,利用各层界面电场、磁场的连续性及地面激励源的耦合条件,推导了不同层之间电磁场的状态矩阵,建立了空间电场与磁场的递归计算关系.其次,针对递归计算中指数项数值计算的不稳定性,借用状态矩阵的性质,导出了将不稳定指数计算项转化为稳定的指数项的转换关系.针对横向各向同性(TI)介质中极向位与环向位解耦的特点,导出了电磁场的直接积分解.最后,采用解析解验证了算法的正确性,给出了多层各向异性地层模型的视电阻率和磁场响应曲线,分析了直流电法探测裂缝性地层、估计裂缝分布性状的可能性.     

大地电磁全张量响应的一维各向异性反演

目前大地电磁(MT)测深资料反演主要基于各向同性介质,但随着MT实际应用的需要,各向异性研究已逐渐引起关注.我们采用广泛应用的广义逆法对一维MT水平层状各向异性介质模型反演进行了探索性研究,并实现了MT全张量响应(即所有的阻抗张量的视电阻率和相位)的一维各向异性反演.理论模型试验表明,无论理论观测值中是否含有噪声,这种方法都能够较好地恢复真实模型,验证了其正确性和有效性.将此方法用于MT实测资料时,能够同时拟合4对视电阻率和阻抗相位曲线,说明本方法可以用于实测资料的处理解释,具有一定的实用价值.     

S—s方式瞬变电磁测深资料处理方法研究

根据半无限均匀介质空间中电磁场的表达式，本文分析了S-s方式瞬变电磁测深法的全区视电阻率定义的的基本性质并讨论了求解方法。在此基础上，引入了基于感应电动势的“虚拟全区视电阻率”概念，从而解决了全区视电阻率的解的唯一性和存在性等问题。理论算例表明，“虚拟全区视电阻率”能较好的反映地下一维电性层电性垂向变化特征，故对瞬变电磁测深资料的定性解释有一定的实际应用价值，同时对一维反演（构制初始模型和确立目标函数）也有重要意义。     

新沂地震台地电阻率反向年变分析

结合新沂地震台电测深曲线、 测区地质剖面资料, 建立了三维非均匀层状介质有限元模型. 以第一层介质电阻率变化模拟表层介质电阻率随季节性降雨的变化, 在模型中计算了3个测道的地电阻率年变化形态. 计算结果表明, EW和N45°E测道地电阻率随表层介质电阻率的增减同向增减, 而NS测道地电阻率则与表层介质电阻率变化相反. 该结果符合新沂台3个测道实际观测的年变形态．      

阳原地震台井下地电阻率观测系统分析

通过对层状介质影响系数的计算,得出阳原台井下地电阻率观测系统记录数据主要体现该装置所在介质层的电阻率变化,反映了深层介质电阻率的变化情况;结合井下地电阻率观测曲线与降雨、气温等的对比分析可以看出,井下地电阻率抑制地表干扰能力较强,变化形态稳定且年变幅度较小,更能凸显应力作用下各向异性变化情况,有利于地震异常的分析和判别。     

煤矿巷道直流三极法超前探测的可行性

全空间巷道超前探测由于受到掌子面前方及上下左右岩土介质的影响异常要复杂许多,应用直流电阻率法进行超前探测也会遇到同样的问题.本文从理论计算和三维数值模拟两个方面分析了直流电阻率三极测深法超前探测的能力,结果表明:在理想情况下,当掌子面前方存在一定规模的低阻体时只能引起巷道底板上三极测深视电阻率曲线单调下降,且影响很小,...     

S-s方式瞬变电磁测深资料处理方法研究

根据半无限均匀介质空间中电磁场的表达式,本文分析了S-s方式瞬变电磁测深法的全区视电阻率定义的基本性质并讨论了求解方法.在此基础上,引入了基于感应电动势的"虚拟全区视电阻率"概念,从而解决了全区视电阻率的解的唯一性和存在性等问题.理论算例表明,"虚拟全区视电阻率"能较好的反映地下一维电性层电性垂向变化特征,故对瞬变电磁测深资料的定性解释有一定的实际应用价值,同时对一维反演(构制初始模型和确立目标函数)也有重要意义.     

基于VTI各向异性介质的频率域海洋可控源电磁三维约束反演

近年来,海洋可控源电磁法(MCSEM)被引入油气勘探领域以降低勘探风险.在海洋环境中,受沉积因素所造成的电阻率各向异性的影响,地电模型往往会非常复杂.为更好地反映地下电性结构,本文实现了基于VTI各向异性介质的频率域海洋可控源电磁三维反演.其中,正演采用基于Yee氏交错网格的三维有限差分算法,所形成的离散线性系统通过大规模并行矩阵直接求解器(MUMPS)进行求解.反演采用基于不等式约束的有限内存BFGS(L-BFGS)算法.最后,利用VTI各向异性介质合成数据,分别进行了电阻率各向异性覆盖层和电阻率各向异性高阻层的三维反演,结果表明:(1)基于并行直接法的MCSEM非常适用于海洋电磁所特有的多场源问题;(2)针对各向异性覆盖层模型进行三维各向异性约束反演,提高了解的可靠性;(3)针对电阻率各向异性高阻层,Inline和broadside数据覆盖的反演结果对异常体位置有很好的反映.     

Apparent resistivity of azimuthal anisotropy layered media

Introduction The method of apparent resistivity with direct current (DC) measurement, adopted from geophysical exploration, is one of the most important precursory approaches in earthquake prediction, in which the geo-electric constitutive model is illustrated by the scalar theory Archies law and correspondingly a set of calculation methods for apparent resistivity of layered media has been established (QIAN, et al, 1985; YAO, 1989). But this is not consistent with the fact that in earthqu…     

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.     

STUDYING THE INTERCONNECTION OF GROUND AND SURFACE WATERS IN REPRESENTATIVE BASINS UNDER THE PROGRAMME OF THE INTERNATIONAL HYDROLOGICAL DECADE

Abstract

Four geoelectrical soundings were measured with a combination of Schlumberger and azimuthal or equatorial dipole electrode arrays on a Carboniferous limestone basin of the Condroz area, Belgium. The measuring technique is briefly outlined as well as the interpretation procedure, which follows a closed-loop scheme with control of calculated model curves. Some general problems of interpretation of geoelectrical sounding curves are tackled, as far as they have a practical bearing on the treatment of Condroz soundings.

The problem of determining the very high resistivity of limestone is approached through ARCHIE's formula, an empirical relation between the bulk rock resistivity, the porosity and the electrolyte resistivity. An evaluation of the latter two parameters, combined with electrical horizontal conductance measurements directly made on resistivity sounding curves, offers a possibility for fast determination of the total water storage in a limestone aquifer. Such storage determinations could be applied whenever an aquifer shows up as a conductive layer interbedded between two highly resistant layers (e.g. nonsaturated limestone and compact, non-fractured limestone).     

Difficulties in determining electrical anisotropy in subsurface investigations

Surface electrical and electromagnetic methods have a limited resolution capability for determining the conductivity structure of the earth. In one-dimensional modelling a collection of many thin layers is frequently considered as one composite layer, which is then macro-anisotropic. Neither galvanic methods nor inductive methods alone can resolve the anisotropy of the ground, but a joint inversion of galvanic and inductive data may do so. The necessity of including the coefficient of anisotropy in the joint inversion of galvanic and inductive sounding data is demonstrated. An analysis is made of the combined use of geoelectrical and transient soundings to resolve the coefficient of anisotropy of a subsurface layer for varying thickness, resistivity and coefficient of anisotropy. It is found that the coefficient of anisotropy is well resolved only for layers that are many times thicker than the overburden and for coefficients of anisotropy that are not too small. The ability of the joint inversion of geoelectrical and transient sounding data to resolve macro-anisotropic layers is tested using realistic earth models determined from electrical logs.     

GEOELECTRICAL MODELS INVOLVING LAYERS WITH A LINEAR CHANGE IN RESISTIVITY AND THEIR USE IN THE INVESTIGATION OF CLAY DEPOSITS*

A model has been set up for the interpretation of geoelectrical sounding data for certain kinds of clay deposits containing gradually varying amounts of sand. The model assumes that in the so-called gradient layer, resistivity varies linearly with depth. Model calculations show how such a layer can be replaced by two homogeneous layers. An inversion procedure using the Marquardt algorithm has been developed for the interpretation of sounding data obtained with the Schlumberger array; it assumes a gradient layer beneath several covering layers. The procedure is demonstrated on two clay deposits. A comparison is made between the newly developed interpretation, the traditional approach using model curves, and computerized inversion for a homogeneous layer model.     

A qualitative approach to electrical resistivity interpretation

Summary The formula for apparent resistivity can be interpreted in terms of two quantities more easily identified with the physical process involved: the distribution of current flow in the earth, as it affects the current density along a line between the potential electrodes, and the true resistivity along this line. The analysis permits a qualitative prediction of resistivity sounding and profiling data over a variety of structures. Examples are given for faults, layers, and a dipping dike.     

AN INTERPRETATION SYSTEM FOR GEO-ELECTRICAL SOUNDING GRAPHS *

For the two and three layer cases geo-electrical sounding graphs can be rapidly and accurately evaluated by comparing them with an adequate set of standard model graphs. The variety of model graphs required is reasonably limited and the use of a computer is unnecessary for this type of interpretation. For more than three layers a compilation of model graphs is not possible, because the variety of curves required in practice increases immensely. To evaluate a measured graph under these conditions, a model graph is calculated by computer for an approximately calculated resistivity profile which is determined, for example, by means of the auxiliary point methods. This model graph is then compared with the measured curve, and from the deviation between the curves a new resistivity profile is derived, the model graph of which is calculated for another comparison procedure, etc. This type of interpretation, although exact, is very inconvenient and time-consuming, because there is no simple method by which an improved resistivity profile can be derived from the deviations between a model graph and a measured graph. The aim of this paper is, on the one hand, to give a simple interpretation method, suitable for use during field work, for multi-layer geo-electrical sounding graphs, and, on the other hand, to indicate an automatic evaluation procedure based on these principles, suitable for use by digital computer. This interpretation system is based on the resolution of the kernel function of Stefanescu's integral into partial fractions. The system consists of a calculation method for an arbitrary multi-layer case and a highly accurate approximation method for determining those partial fractions which are important for interpretation. The partial fractions are found by fitting three-layer graphs to a measured curve. Using the roots and coefficients of these partial fractions and simple equations derived from the kernel function of Stefanescu's integral, the thicknesses and resistivities of layers may be directly calculated for successively increasing depths. The system also provides a simple method for the approximative construction of model graphs.     

ELECTRICAL SOUNDING ON THE WATER SURFACE AT KHOR KUNDI EL-BAHARI IN EGYPT*

Electrical resistivity sounding was attempted with success in 1960 on the surface of the Nile water in Khor Kundi El-Bahari, the starting place for the construction of the offset channel of the High Dam. The method was applied to determine the depth to the upper surface of the granite below the alluvium and Nile water. The success of the method conducted at two stations in the Khor is attributed to the favourable geological conditions of the section which consists of nearly homogeneous layers besides the good earthing conditions of electrodes dipped in water. The high resistivity contrast between water and granite facilitates interpretation of sounding curves. Factors interfering with the results of interpretation are mentioned.     

A deep geoelectric model of the Bol’she-Bannyi hydrothermal system,Kamchatka

This paper reports the results of magnetotelluric sounding (MTS) in the area where the Bol’she-Bannyi hydrothermal springs are discharged. The MTS curves were inverted on the assumption of a twodimensional inhomogeneous model using longitudinal and transverse curves of apparent resistivity. It was found that the geoelectric section contains a nearly vertical anomaly of high electrical conductivity at depths of 5.5–8 km, which is the signature of a deep-seated fault. The resulting geoelectric section for the upper crust and data from regional magnetotelluric soundings were used as a basis for developing a conceptual deep model of the Bol’she-Bannyi hydrothermal system. We quote an approximate estimate of rock porosity. According to the model, deep fluids come from a crustal layer into the subvertical deep-seated fault then penetrate via fissures into the sedimentary–volcanogenic cover, and finally arrive at the ground surface in zones of high rock permeability. We provide a recommendation for drilling a deep well in order to determine the potential of the Bol’she-Bannyi hydrothermal field.