井下地电阻率观测的探测深度初步研究

对井下地电阻率观测的探测深度进行了研究,计算了均匀半空间和给定结构参数的水平层状介质模型在不同装置电极埋深下的探测深度,分析了探测深度与装置电极埋深和介质电阻率结构之间的关系,得到如下结果:①与地表观测相比,在供电极距为1 km左右时,探测深度随装置电极埋深的增大而增大,且增大的速度与装置电极埋深密切相关;当装置电极埋深h < 100 m时,探测深度的增大速度远小于装置电极埋深h≥100 m时. ②当装置电极埋深h < 50 m时,与地表观测相比探测深度增加很小,不超过10 m;当装置电极埋深相同时,供电极距越大,与地表观测相比探测深度增加得越小. ③对于水平层状电阻率均匀分层结构,在装置电极埋深相同的情况下,下伏低阻结构的探测深度显著大于下伏高阻结构.本文的研究结果表明,为了观测到深部电阻率的变化情况,首先需要查明测区电性结构,再进行综合分析,以确定井下地电阻率观测的装置电极埋深,其结果为深部电阻率变化研究提供了理论基础.      

COMPUTER CALCULATION OF RESISTIVITY PSEUDOSECTIONS OF A BURIED SPHERICAL CONDUCTOR BODY*

The use of the electrical image method in calculating the potential of a source in the presence of a buried conductive sphere proves to be difficult because of the great number of terms making up the potential formula. The disadvantages contained by this method are avoided here by resorting to the theory of graphs. In view of this aim the “propagation” of the electrical images due to the sphere has been represented by an infinite graph to which an infinite matrix has been attached. Taking this as a basis several FORTRAN IV programs for the computation of resistivity pseudo-sections for various electrode configurations have been coded, and a great deal of such pseudo-sections have already been calculated with an IBM-360 computer. Using these results, recommendations regarding both the methodology of the geoelectrical prospecting of spherical shaped conductors and the most suitable way of building resistivity pseudo-sections are developed.     

地震过程中的大地电场变化特征

根据国产ZD9A型大地电场仪在宝低地电台的记录结果,研究了1998年1月10日河北省张北地区6．2级地震过程中的大地电场变化特征．主要表现为：1．震前36天至发震当日出现大地电场日变异常簇;2．地震前后大地电场E的方位发生转动;3、大地电场E具有日变倍9（天）周期性和振幅｜E｜变化特征;4大地电场EW分量的极性变化．根据对这些特征的分析结果表明,它们有可能做为孕震进入短临阶段的前兆信息及识别标志．     

THE INFLUENCE OF INDUSTRIAL STRAY CURRENTS ON THE MEASUREMENT OF EARTH POTENTIALS AND THEIR ELIMINATION*

Simultaneous observations of electrical earth-potentials along parallel measuring lines offer a simple but useful method to study telluric and stray-current potentials. Different correlation-factors between the records of both observation points can help to separate telluric from industrial stray-current potentials. A method is suggested to eliminate disturbing impulses of stray currents from continuous telluric current recordings as those made at observatories. The influence of stray currents can also be eliminated from potential readings of an artificial current as applied in geoelectric measurements with the four-point-method. The elimination is possible by a simultaneous observation of the stray-current potentials on an equipotential line of the artificially generated field. The recording examples show an appreciable improvement of the signal-noise ratio without increase in the current. The method is useful for geoelectrical prospecting in areas which are contaminated by industrial noise.     

Electromagnetic measurements in the vicinity of the KTB drill site. Part II. Magnetotelluric results

Summary In addition to the magnetovariational measurements across an array in Western Bohemia, close to the KTB ultradeep borehole (Germany), discussed in part I of this paper [1], magnetotelluric results from pivot sounding point Ostrvek within the array are presented here. Good quality of long-period magnetotelluric data (period range from 30 s to about 1 hour) allowed structural dimensionality of the medium to be analysed in detail. The geoelectrical structure was identified as a slightly distorted two-dimensional regional substratum, with dominating E - W strike, overlaid by a heterogeneous subsurface layer with extremely strong and anisotropic galvanic distortion effect on the magnetotelluric data. Estimating the total static shift distortion tensor by fitting the local magnetotelluric curve to the curve of the global magnetovariational soundings (for the European continent), the static distortions were identified as of generally multidirectional origin. The resulting telluric ellipse is, however, strongly anisotropic, indicating an approximately SW - NE apparent local strike, which is in the approximate agreement with remote reference magnetovariational results. Finally, the magnetotelluric results from the station Ostrvek are compared with long-period data from the immediate neighbourhood of the KTB borehole on the German territory.     

基于遗传算法的大地电磁阻抗张量分解方法研究

本文针对Groom-Bailey分解法在消除地表电性不均匀体对MT数据的畸变效应时存在的问题引入遗传算法,对基于传统线性最优化方法的GB分解算法进行改进,提出基于遗传算法的大地电磁阻抗张量分解方法.通过对理论合成数据以及三维/二维模型正演数据的分解试验,并且对青藏高原北缘阿尔金断裂地区实际MT数据进行分解处理,证明了基于遗传算法的GB分解能够更加有效地校正三维/二维情况下近地表三维电性不均匀体所造成的畸变影响.最后,在已有算法基础上,研究了基于遗传算法的多频率GB分解算法和MT数据静校正算法,并通过实际MT数据的处理证明了这些算法的有效性.     

Modelling and straight‐ray tomographic imaging studies of cross‐hole radio‐frequency electromagnetic data for mineral exploration

Radio‐frequency electromagnetic tomography (or radio imaging method) employs radio‐frequency (typically 0.1–10 MHz) electromagnetic wave propagation to delineate the distribution of electric properties between two boreholes. Currently, the straight‐ray imaging method is the primary imaging method for the radio imaging method data acquired for mineral exploration. We carried out synthetic studies using three‐dimensional finite‐element modelling implemented in COMSOL Multiphysics to study the electromagnetic field characteristics and to assess the capability of the straight‐ray imaging method using amplitude and phase data separately. We studied four sets of experiments with models of interest in the mining setting. In the first two experiments, we studied models with perfect conductors in homogeneous backgrounds, which show that the characteristics of the electromagnetic fields depend mainly on the wavelength. When the borehole separations are less than one wavelength, induction effects occur; conductors with simple geometries can be recovered acceptably with amplitude data but are incorrectly imaged on the phase tomogram. When the borehole separations are longer than two wavelengths, radiation effects play a major role. In this case, phase tomography provides images with acceptable quality, whereas amplitude tomography does not provide satisfactory results. The third experiment shows that imaging with both original and reciprocal datasets is somewhat helpful in improving the imaging quality by reducing the impact of noise. In the last experiment, we studied models with conductive zones extended into the borehole plane with different lengths, which were not accurately recovered with amplitude tomography. The experiment implies that it is difficult to determine the extent of a mineralised zone that has been intersected by one of the boreholes. Due to the large variation of the wavelength in the radio‐frequency range, we suggest investigating the local electric properties to select an operating frequency prior to a survey. We conclude that straight‐ray tomography with either amplitude or phase data cannot provide high‐quality imaging results. We suggest using more general methods based on full electromagnetic modelling to interpret the data. In circumstances when computational time is critical, we suggest saving time by using either induction methods for borehole separations less than one wavelength or wave‐based methods (only radiation fields are considered) for borehole separation larger than two wavelengths.     

井下地电阻率观测中地表电流干扰影响计算

本文把地电阻率台站地下介质简化为水平三层均匀介质模型,以点电流模拟地表干扰电流,针对对称四极观测装置,计算了在不同电性结构中的不同深度观测时,地表电流干扰源对对称四极装置地电阻率观测的影响,计算得到:地表干扰源对电阻率观测的影响取决于电性结构的类型和层参数、供电电极和测量电极的埋深以及避开干扰源的距离.本文研究结果对实施井下电阻率观测中台址电性结构的选择、电极埋深、干扰源避让距离等有参考意义.     

DIFFERENTIATION DIRECTION OF TELLURIC CURRENTS IN THE SOUTHERN SECTION OF THE TANLU FAULT ZONE

The Tancheng-Lujiang Fault is an important tectonic boundary in eastern China. The southern part of the Tancheng-Lujiang Fault is located south of Baohai Bay, which is an area with a dense population and frequent economic activities. It is worth conducting an in-depth study on the southern section of the fault, especially in the aspect of geophysical exploration and seismicity analysis. Electrical structure detection is an important way to interpret the structural activity of the fault. It can also analyze and explore the influence of the fault on the physical properties of both sides of the fault based on the geoelectrical observation data. In the study area, there are densely distributed stations of geoelectrical observation, including 27 fixed stations distributed along the fault zone from the southern Baohai Bay to Nanjing, Jinagsu Province. The continuous observations and recording of these stations provide a favorable condition for studying the tectonic activity of Tancheng-Lujiang Fault. In the long-term observation of geoelectric observation network, the geoelectric field measurements of long- and short-spacing measuring tracks in the same direction at the same station vary significantly because of the effect of long-term stability of the observation system and the environment near the electrodes. Also, the data curve changes complicatedly and seems to be in a mess. However, there are three basic facts of observation existing in the geoelectric field change: 1)The variation amplitude of the geoelectric field changes observed on the long- and short-spacing measuring tracks in the same direction at the same station(including tidal response changes and the rapid change events such as short periods or pulses)is the same or very close; 2)The E_x and E_y components at the same station always show the same variation in the same time period, or the opposite, which is related to the anisotropy of the medium under the station; 3)The rapid changes of the minute values of the geoelectric field observed at different stations are synchronous in a wide spatial area. In this study, in order to take full advantage of these basic facts, we only use the amplitude variation of geoelectric field with time. Based on the data of 27 geoelectric field observation stations in the study area, we used the current density vector and streamline to characterize telluric current with its divergence and vorticity calculated in the southern Tancheng-Lujiang Fault in this paper. The results show that: 1)the telluric current shows the phenomenon of opposite directional differentiation in the southern part of the fault zone, the direction of the current vector is NE on the east side, while the direction is NW to SW on the west side; (2)The divergence and vorticity of telluric current also show the differentiation phenomenon along the fault, the positive/negative maximum of vorticity and divergence mainly occurs near the fault zone and the direction of alternating positive and negative gradient(or negative gradient)of vorticity or divergence is consistent with the strike of the fault zone. By analyzing the current superposition simulation results and comparing them with previous studies, an interpretation model of the above phenomenon is established in this paper. The results agree with previous studies on the electrical structure of this region. Besides, the results that telluric current differentiates along the fault zone may improve our understanding of the process of deep electrical and material migration.     

青藏高原中强地震前的地电场变异及构成解析

青藏高原及邻区的地电场常态波形存在场地的选择性现象,场地的岩石结构、裂隙及裂隙水、构造活动等因素影响地电场的常态波形,较大湖泊有助于附近出现TGF-A波形,在第四纪沉积层较厚、岩石含水度高和透水性强的地区多出现TGF-B波形,而构造活动剧烈的基岩山区易出现无日变形态的地电场.该地区中强地震前,地电场的短临异常存在时间上或同步或有十余天差的丛集现象,而在空间上可分布于该区多个断层附近,表现出离散现象,这种时间上丛集、空间上离散的现象是该区域地电场短临前兆特征.引入Σ-Δ求和方法从地电场观测数据中解析出自然电场通常的稳定性,说明了中强地震之前部分地电场的背景值跃变是一种自然电场变化现象;建立潮汐谐波振幅比T_A值的概念及计算方法,从定量角度说明了青藏高原地区TGF-A、TGF-B和无日变波形的潮汐影响在逐次降低;使用一阶差分ΔE方法,解析出地电场分钟数量级的高频电磁成分,通常这种高频突跳表现出一定程度的随机性、有限性.三种不同的分析方法,从地电场的构成中解析出自然电场、大地电场和高频电磁成分的特征,为地电场的物理解析提供了理论和方法上的基础.     

Impact of Groundwater Flow and Energy Load on Multiple Borehole Heat Exchangers

The effect of array configuration, that is, number, layout, and spacing, on the performance of multiple borehole heat exchangers (BHEs) is generally known under the assumption of fully conductive transport. The effect of groundwater flow on BHE performance is also well established, but most commonly for single BHEs. In multiple‐BHE systems the effect of groundwater advection can be more complicated due to the induced thermal interference between the boreholes. To ascertain the influence of groundwater flow and borehole arrangement, this study investigates single‐ and multi‐BHE systems of various configurations. Moreover, the influence of energy load balance is also examined. The results from corresponding cases with and without groundwater flow as well as balanced and unbalanced energy loads are cross‐compared. The groundwater flux value, 10^?7 m/s, is chosen based on the findings of previous studies on groundwater flow interaction with BHEs and thermal response tests. It is observed that multi‐BHE systems with balanced loads are less sensitive to array configuration attributes and groundwater flow, in the long‐term. Conversely, multi‐BHE systems with unbalanced loads are influenced by borehole array configuration as well as groundwater flow; these effects become more pronounced with time, unlike when the load is balanced. Groundwater flow has more influence on stabilizing loop temperatures, compared to array characteristics. Although borehole thermal energy storage (BTES) systems have a balanced energy load function, preliminary investigation on their efficiency shows a negative impact by groundwater which is due to their dependency on high temperature gradients between the boreholes and surroundings.     

地电井下观测装置技术指标的测试与认定讨论——井下观测设施构建与测试技术

观测技术系统的设施构建技术指标认定是观测技术的核心所在,本文结合地电井下观测设施构建工程^[1],着重讨论针对观测装置测试的需求、技术指标测试的方法以及认定的过程。文中以具体工程参数为例,给出4项测试结果,讨论了认定依据和认定过程,希望本讨论能对地电井下观测设施建设、指标的认定起到参考作用。     

Far field elastodynamic Born scattering revisited

This paper summarizes part of an ongoing feasibility study that investigates the possible use of the full elastic Born approximation in multipole borehole acoustics. As a first step we exclude the fluid-filled borehole with the motivation that one or two wavelengths away from the fluid-filled borehole, radiating borehole mode amplitudes (e.g., Stoneley wave, formation dipole wave, etc.) are small compared to body wave amplitudes (P-, SV- and SH-waves). Consequently, for scatterers one or two wavelengths away from the fluid-filled borehole, it suffices to only consider their interaction with body waves.In this paper we apply the contrast-source stress-velocity forward scattering (integral equation) formulation for solid configurations in its first order (Born-) approximation (De Hoop, 1995) assuming a multipole force source excitation in a zero-offset configuration. To scrutinize the validity of the Born approximation, we consider the simplest type of scatterer, i.e., one characterized by a (Heaviside) step function change in one or more of the contrast (perturbation) parameters and we derive analytic zero-offset formulas for the scattered wave particle velocity and displacement in both the space–frequency and space–time domain, respectively. We assume the scatterer to be located in the far-field. More complicated layered configurations can easily be derived by superposition of the given solution types. Explicit results are given for the dipole and quadrupole excitation, where the former is allowed to have an arbitrary orientation relative to the scatterer and where the latter one is located in a plane perpendicular to the scatterer. In the time domain it is shown, how the scattered wave field decomposes in a specular and diffuse wave field (two terms borrowed from ‘Optics’), where the former contribution vanishes in the absence of an imaging condition and where the latter is always present. For the dipole case, we subject our results to a sensitivity analysis with respect to the three independent perturbation parameters (i.e., density and two compliance parameters) and we compare these results to a full waveform benchmark code that has implemented the reflectivity method (Kennett, 1983), for a ‘horizontally’ stratified elastic medium. This allowed us to pinpoint the root cause of the observed (small) differences. As it turns out these deviations could be traced back to the inaccurateness of the first order Born scattering coefficients. An additional confirmation of this fact is provided through a comparison between the zero-offset scattering coefficients and the corresponding Zoeppritz reflection coefficients. Most notably, it was found that the PP first order scattering coefficient needs a higher than quadratic correction in two of the three independent perturbation parameters, i.e., the two compliance parameters, δΛ and δM. With respect to the density perturbation parameter (δρ) the PP scattering coefficient correction is quadratic with respect to this perturbation parameter, as is to be expected for a first order approximation. Moreover, also the SS first order scattering coefficient only needs a quadratic correction with respect to its associated perturbation parameters, i.e., δρ and δM.Finally, we give a brief outline on how to numerically implement the Born approximation (employing arbitrary offsets) in a configuration where a source–receiver pair is moving continuously relative to the ‘contrast’ (Geology), as is the case in borehole acoustic applications.     

Understanding of hydraulic properties from configurations of stochastically distributed fracture networks

A systematic investigation of the effect of configurations of stochastically distributed fracture networks on hydraulic behaviour for fractured rock masses could provide either quantitative or qualitative correlation between the structural configuration of the fracture network and its corresponding hydraulic behaviour, and enhance our understanding of appropriate application of groundwater flow and contaminant transport modelling in fractured rock masses. In this study, the effect of block sizes, intersection angles of fracture sets, standard deviations of fracture orientation, and fracture densities on directional block hydraulic conductivity and representative elementary volume is systematically investigated in two dimensions by implementing a numerical discrete fracture fluid flow model and incorporating stochastically distributed fracture configurations. It is shown from this investigation that the configuration of a stochastically distributed fracture network has a significant quantitative or qualitative effect on the hydraulic behaviour of fractured rock masses. Compared with the deterministic fracture configurations that have been extensively dealt with in a previous study, this investigation is expected to be more practical and adequate, since fracture geometry parameters are inherently stochastically distributed in the field. Moreover, the methodology and approach presented in this study may be generally applied to any fracture system in investigating the hydraulic behaviours from configurations of the fracture system while establishing a ‘bridge’ from the discrete fracture network flow modelling to equivalent continuum modelling in fractured rock masses. Copyright © 2007 John Wiley & Sons, Ltd.     

Geophysical Assessment of Groundwater Potential: A Case Study from Mian Channu Area,Pakistan

An integrated study using geophysical method in combination with pumping tests and geochemical method was carried out to delineate groundwater potential zones in Mian Channu area of Pakistan. Vertical electrical soundings (VES) using Schlumberger configuration with maximum current electrode spacing (AB/2 = 200 m) were conducted at 50 stations and 10 pumping tests at borehole sites were performed in close proximity to 10 of the VES stations. The aim of this study is to establish a correlation between the hydraulic parameters obtained from geophysical method and pumping tests so that the aquifer potential can be estimated from the geoelectrical surface measurements where no pumping tests exist. The aquifer parameters, namely, transmissivity and hydraulic conductivity were estimated from Dar Zarrouyk parameters by interpreting the layer parameters such as true resistivities and thicknesses. Geoelectrical succession of five‐layer strata (i.e., topsoil, clay, clay sand, sand, and sand gravel) with sand as a dominant lithology was found in the study area. Physicochemical parameters interpreted by World Health Organization and Food and Agriculture Organization were well correlated with the aquifer parameters obtained by geoelectrical method and pumping tests. The aquifer potential zones identified by modeled resistivity, Dar Zarrouk parameters, pumped aquifer parameters, and physicochemical parameters reveal that sand and gravel sand with high values of transmissivity and hydraulic conductivity are highly promising water bearing layers in northwest of the study area. Strong correlation between estimated and pumped aquifer parameters suggest that, in case of sparse well data, geophysical technique is useful to estimate the hydraulic potential of the aquifer with varying lithology.     

Theoretical analysis of telluric anomaly of a basin structure

The solution of a boundary value problem modelling a two-dimensional basin structure has been obtained by using the Schwartz-Christoffel conformal transformation technique and numerical methods of solving non-linear differential equations. Utilizing this solution, the telluric field and its first horizontal derivative have been theoretically computed for field directions perpendicular to the strike of the structure. On the basis of systematic analysis of a large number of such anomaly curves, two nomograms have been prepared to be used in the quantitative interpretation of telluric data. An interpretation procedure to evaluate the geometric parameters of the basin from the observed telluric data is outlined. This procedure is demonstrated on an actual field example.     

基于BS-MUSIC的定向钻孔雷达三维成像算法

定向钻孔雷达由位于井中的一个发射天线和分布于圆环上的呈等角度分布的四个接收天线组成,能确定井旁地质目标的深度和方位.然而,目标方位确定和三维成像的算法问题一直是该技术面临的挑战.本文将空间谱估计中的BS-MUSIC算法应用于定向钻孔雷达目标方位识别算法中,在此基础上提出并实现了利用四个接收天线的信号对井周地质体进行三维空间成像的算法.使用合成数据对以上算法的效果进行验证,并和通常的MUSIC算法进行比较,结果表明,MUSIC算法和基于波束空间的MUSIC算法均可以在方位确定和三维成像方面取得非常好的效果,但计算速度方面,BS-MUSIC算法相比于MUSIC算法有了非常明显的提升.     

Geoelectrical Classification of Gypsum Rocks

Gypsum rocks are widely exploited in the world as industrial minerals. The purity of the gypsum rocks (percentage in gypsum mineral in the whole rock) is a critical factor to evaluate the potential exploitability of a gypsum deposit. It is considered than purities higher than 80% in gypsum are required to be economically profitable. Gypsum deposits have been studied with geoelectrical methods; a direct relationship between the electrical resistivity values of the gypsum rocks and its lithological composition has been established, with the presence of lutites being the main controlling factor in the geoelectrical response of the deposit. This phenomenon has been quantified in the present study, by means of a combination of theoretical calculations, laboratory measurements and field data acquisition. Direct modelling has been performed; the data have been inverted to obtain the mean electrical resistivity of the models. The laboratory measurements have been obtained from artificial gypsum-clay mixture pills, and the electrical resistivity has been measured using a simple electrical circuit with direct current power supply. Finally, electrical resistivity tomography data have been acquired in different evaporite Tertiary basins located in North East Spain; the selected gypsum deposits have different gypsum compositions. The geoelectrical response of gypsum rocks has been determined by comparing the resistivity values obtained from theoretical models, laboratory tests and field examples. A geoelectrical classification of gypsum rocks defining three types of gypsum rocks has been elaborated: (a) Pure Gypsum Rocks (>75% of gypsum content), (b) Transitional Gypsum Rocks (75–55%), and (c) Lutites and Gypsum-rich Lutites (<55%). From the economic point of view, the Pure Gypsum Rocks, displaying a resistivity value of >800 ohm.m, can be exploited as industrial rocks. The methodology used could be applied in other geoelectrical rock studies, given that this relationship between the resistive particles embedded within a conductive matrix depends on the connectivity of the matrix particles.     

A new telluric KCl probe using Filloux's AgAgCl electrode

The measurement of the telluric field's long-time variations requires stable instruments in the period range above 1 day. Obviously, most problems arise from drifting voltages between the telluric probes. Good results have been achieved using a three chamber Hempfling KCl probe together with Filloux's AgAgCl electrode. However, a one chamber probe of 0.6 m length and 0.06 m diameter filled with saturated KCl solution may be sufficient for some applications and additionally allows permanent control of the electrolyte's salt concentration. In a field test the telluric field at a single site was simultaneously observed using one and three chamber probes separated by 25 and 55 m resp. For periods shorter than 1 hour the noise level of the electrodes was found to be less than 1 (mV)²/Hz whereas in the period range between 1 hour and 1 day it increases to 100 (mV)²/Hz. Consequently, surveys investigating long periods of the telluric field can be carried out using small electrode separations of a few tens of meters. Furthermore, the stability of this probe negates the need to interrupt the time series for servicing of the probe. However, the one chamber probe is somewhat inferior to the three chamber probe with regard to temperature dependence at long periods.     

Recent developments in the direct-current geoelectrical imaging method

There have been major improvements in instrumentation, field survey design and data inversion techniques for the geoelectrical method over the past 25 years. Multi-electrode and multi-channel systems have made it possible to conduct large 2-D, 3-D and even 4-D surveys efficiently to resolve complex geological structures that were not possible with traditional 1-D surveys. Continued developments in computer technology, as well as fast data inversion techniques and software, have made it possible to carry out the interpretation on commonly available microcomputers. Multi-dimensional geoelectrical surveys are now widely used in environmental, engineering, hydrological and mining applications. 3-D surveys play an increasingly important role in very complex areas where 2-D models suffer from artifacts due to off-line structures. Large areas on land and water can be surveyed rapidly with computerized dynamic towed resistivity acquisition systems. The use of existing metallic wells as long electrodes has improved the detection of targets in areas where they are masked by subsurface infrastructure. A number of PC controlled monitoring systems are also available to measure and detect temporal changes in the subsurface. There have been significant advancements in techniques to automatically generate optimized electrodes array configurations that have better resolution and depth of investigation than traditional arrays. Other areas of active development include the translation of electrical values into geological parameters such as clay and moisture content, new types of sensors, estimation of fluid or ground movement from time-lapse images and joint inversion techniques. In this paper, we investigate the recent developments in geoelectrical imaging and provide a brief look into the future of where the science may be heading.