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1.
G. FISCHER 《Geophysical Prospecting》1985,33(5):716-722
The magnetotelluric phase in the medium of incidence over a good conductor varies from its value at the conductor surface, usually close to 45°, to the characteristic standing-wave phaseshift of 90° over a distance of a few conductor skin-depths, much shorter than the vacuum wavelength at the frequencies considered. Over the top of a graben or at the surface of a two-layer structure the basement can be looked upon as the matrix. If the graben or the overburden is a better conductor it will carry an extra current density in phase with the electric field, thus giving rise to an additional magnetic field also in phase with the electric field. This drives the argument of the surface impedance toward smaller values. For a more resistive graben or overburden the current is depressed, corresponding to a reduction of the surface magnetic field by an amount in phase with the electric field. This effect drives the phase of the impedance to values above the characteristic 45° of a uniform conductor. 相似文献
2.
Two case histories are presented in which near-surface water-bearing formations and their overburden are examined. This survey utilized a combination of traditional geoelectric methods (direct current resistivity and time domain induced polarization) and a special shallow-depth engineering geophysical sounding method. This latter method was developed in Hungary to investigate near-surface unconsolidated formations such as clay, silt, sand, gravel and other similarly “penetratable” formations.For a gravel terrace, in addition to the usual parameter maps (resistivity, thickness, polarizability), combined multiparametric characterization maps have been plotted. These maps illustrate the hydrogeologic value of the water-bearing formations and the protecting capacity of the overburden better than the conventional parameter maps.Surface measurement results together with filtration coefficient values show how an inexpensive geophysical survey can successfully be applied in estimating the dangerous or protected situation of near-surface reservoirs and how such a survey can be used for siting reservoirs. This work enabled the near-surface geological structure to be determined, hereby providing orientation for more expensive, highly detailed surveys. 相似文献
3.
Large sand intrusions often exhibit conical morphologies analogous to magmatic intrusions such as saucer-shaped or cup-shaped sills. Whereas some physical processes may be similar, we show with scaled experiments that the formation of conical sand intrusions may be favoured by the pore-pressure gradients prevailing in the host rock before sand injection. Our experiments involve injecting air into a permeable and cohesive analogue material to produce hydraulic fractures while controlling the pore pressure field. We control the state of overpressure in the overburden by applying homogeneous basal pore pressure, and then adding a second local pore pressure field by injecting air via a central injector to initiate hydraulic fractures near the injection point. In experiments involving small vertical effective stresses (small overburden, or high pore fluid overpressure), the fracturing pressure (λfract) is supralithostatic and two dipping fractures are initiated at the injection point forming a conical structure. From theoretical considerations, we predict that high values of λfract are due to strong cohesion or high pore fluid overpressure distributed in the overburden. Such conditions are favoured by the pore pressure/stress coupling induced by both pore pressure fields. The dips of cones can be accounted for elastic-stress rotation occurring around the source. Contrary to magmatic chamber models, the aqueous fluid overpressure developed in a parent sandbody (and prevailing before the formation of injectites) may diffuse into the surrounding overburden, thus favouring stress rotation and the formation of inclined sheets far from the parent source. For experiments involving higher vertical effective stresses (thick overburden or low pore fluid overpressure), the fracturing pressure is lower than the lithostatic stress, and a single fracture is opened in mode I which then grows vertically. At a critical depth, the fracture separates into two dilatant branches forming a flat cone. We make use of a P.I.V. (Particle Imaging Velocimetry) technique to analyse plastic deformation, showing that these inclined fractures are opened in mixed modes. Close to the surface, they change into steep shear bands where fluids can infiltrate. The final morphology of the fracture network is very similar to the common tripartite architecture of various injection complexes, indicating that different mechanisms may be involved in the formation of dykes. Feeder dykes under the sill zones may open as tensile fractures, while overlying dykes may be guided by the deformation induced by the growth of sills. These deformation conditions may also favour the formation of fluid escape structures and pockmarks. 相似文献
4.
瞬变电磁场的直接时域数值分析 总被引：44，自引：9，他引：35
为了深入了解瞬变电磁场的勘探原理,直接在时间域对负阶跃脉冲激发的二维瞬态场进行了数值分析.采用的方法是从反映电磁场基本规律的麦克斯韦方程组出发,导出时域电场的齐次扩散方程,对所研究的空间区域作差分离散,源作为初始条件加入,利用准静态近似处理空中边界,然后进行时间的逐步递推,由此展现瞬变电磁场在地下扩散随时间发展的全过程.通过模拟计算不同时刻瞬态电场在地下的分布形态及地面上感生电动势相应的变化,揭示了低阻异常体对感应涡流的聚集作用,低阻覆盖层对瞬变场扩散的减速作用,及瞬变场的延时效应.因此,瞬变电磁法对低阻体是敏感的,有上覆低阻层时探测同样的深度需要较长的时间,而延时效应瞬变场的晚期时段可反映埋藏较浅的异常体. 相似文献
5.
B. S. RATHOR 《Geophysical Prospecting》1978,26(3):550-561
Expressions for the I. P. activity coefficient for an inhomogeneous sphere with the conductivity increasing/decreasing from the core towards the periphery according to a power law have been derived. In certain geometrical situations of the electrode configurations a negative I. P. signal is obtained. The negative I. P. may be understood as due to modifications in the discharge current from the polarized medium during the discharge process. The results of the investigation may be useful for more accurate interpretation of I. P. anomalies due to isometric inhomogeneous bodies. 相似文献
6.
"嫦娥"一号、二号绕月飞行经历地球磁尾边界层区域时,分别在2007年11月26日—2008年2月5日和2010年10月3日—2011年2月28日,发现了15次月球轨道0.1~2 MeV电子急剧增加(Bursts of 0.1~2 MeV Energetic Electrons,BEE),卫星周围等离子体离子加速的现象.统计研究表明,这类现象发生在稳定太阳风和弱行星际磁场条件下,且无显著空间环境扰动事件发生时,离子的加速滞后于高能电子爆发,离子能量的变化与高能电子通量的时间演化正相关,地球磁鞘内侧或边界层过渡区域是该类现象的高发区,离子能量增加时卫星表面电位大幅下降可达负几千伏.为了研究高能电子爆发与绕月卫星表面电位变化的关系及其对月球表面电位的影响,本文用电流平衡法建立绕月卫星和月球表面充电模型,并假设能量电子(2eV~2 MeV)满足幂律谱的分布,模拟急剧增加的能量电子对卫星和月球表面电位的影响.模拟结果表明,能量电子急剧增加使得绕月卫星和月球表面电位大幅下降;能量电子总流量1011 cm-2时,绕月卫星和月球表面充电电位可达负上千伏;月球充电到大的负电位的时间仅为卫星充电时间的1/10.鉴于高能电子急剧增加事件的高发生率(~125次/年),能量电子急剧增加使得绕月卫星表面电位大幅下降的发生率应大于实测等离子体离子加速现象的发生率(~25次/年). 相似文献
7.
JACQUES BERTIN 《Geophysical Prospecting》1968,16(4):401-426
In the first part, the author, recalling the principle of d/c current measurements, shows by means of some examples, different experimental results relative to the induced polarization phenomenon. He presents the case of the negative induced polarization which can be explained by geometric effects. He gives some examples of saturation curves and discusses the problem of the linearity of the I.P. phenomenon. Then he shows some aspects which demonstrate that I.P. decay curves do not always conform to the general law. He concludes by showing the influence of the current electrodes, the position of which, relative to the polarizable bodies, may alter the shape of the I.P. anomalies. In the second part, the author presents different case histories of I.P. mineral surveys where I.P. is compared to other geophysical methods and drilling results. 相似文献
8.
本文根据ＬＥＯ中等离子体温度低，密度高，德拜长度小的特点，采用薄鞘层近似，计算了离子收集电流；根据电流平衡方程，考虑表面二次电子发射，计算了大型航天器表面在沉降电子流辐照下的充电特性。 相似文献
9.
R. SCHM
LLER 《Geophysical Prospecting》1982,30(6):735-751
There are two types of masked layers in seismic refraction work: the velocity reversal (low-velocity layer) and the hidden layer (insufficient velocity contrast or layer thickness). On the basis of an analytical formulation of the general case of a masked layer under an overburden of plane and parallel multiple refractors the two limiting cases are discussed: the solution resulting from an uncritical interpretation of the measured time-distance curve and the blind zone solution. Between these two limiting cases there is a variety of possible masked layer solutions. These no-blind zone solutions—as well as the blind zone solution itself—are formulated separately for the velocity inversion and the hidden layer case. For the evaluation of some no-blind zone solution a diagram is presented which can be used for any case of multiple refractors in the overburden of the masked layer. However, it is only for the three- and the four-layer case that a blind zone interpretation by use of diagrams is advisable. Such diagrams are presented together with the basic sets of formulae which contain as parameters only ratios of velocities and layer thicknesses. As the velocity of the masked layer is usually unknown the diagrams are principally constructed to show the dependence on the masked layer velocity. This is useful for estimation of the largest possible error. 相似文献
10.
Influence of subsurface injection on time‐lapse seismic: laboratory studies at seismic and ultrasonic frequencies 
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下载免费PDF全文 Seismic monitoring of reservoir and overburden performance during subsurface CO2 storage plays a key role in ensuring efficiency and safety. Proper interpretation of monitoring data requires knowledge about the rock physical phenomena occurring in the subsurface formations. This work focuses on rock stiffness and elastic velocity changes of a shale overburden formation caused by both reservoir inflation induced stress changes and leakage of CO2 into the overburden. In laboratory experiments, Pierre shale I core plugs were loaded along the stress path representative for the in situ stress changes experienced by caprock during reservoir inflation. Tests were carried out in a triaxial compaction cell combining three measurement techniques and permitting for determination of (i) ultrasonic velocities, (ii) quasistatic rock deformations, and (iii) dynamic elastic stiffness at seismic frequencies within a single test, which allowed to quantify effects of seismic dispersion. In addition, fluid substitution effects connected with possible CO2 leakage into the caprock formation were modelled by the modified anisotropic Gassmann model. Results of this work indicate that (i) stress sensitivity of Pierre shale I is frequency dependent; (ii) reservoir inflation leads to the increase of the overburden Young's modulus and Poisson's ratio; (iii) in situ stress changes mostly affect the P‐wave velocities; (iv) small leakage of the CO2 into the overburden may lead to the velocity changes, which are comparable with one associated with geomechanical influence; (v) non‐elastic effects increase stress sensitivity of an acoustic waves; (iv) and both geomechanical and fluid substitution effects would create significant time shifts, which should be detectable by time‐lapse seismic. 相似文献
11.
12.
本文给出一种用积分方程法配合有限差分法计算位于导电介质中三维导体的电磁响应的新途径,在二层大地条件下给出了具体计算方法和结果。详细讨论了围岩介质的导电性对导体异常的影响。这对于开展低阻覆盖层地区的电磁法工作具有一定的指导意义。 相似文献
13.
The influence of a partially conducting overburden/host rock on the electromagnetic response of a horizontal, tabular conducting ore body, investigated with the aid of quantitative scale model experiments, was discussed in an earlier paper (Gaur, Verma and Gupta 1971), which will be referred to as I. This paper presents the results of more comprehensive experiments subsequently carried out to study the combined effect of various geological parameters, namely: the dip and depth of burial of the ore body, its electrical conductivity and that of the zone surrounding it. These results obtained for four different transmitter-receiver configurations confirm the general enhancement of response in varying degrees, brought about by a conducting overburden. However, the transformation of the shapes of the anomaly profiles with increasing overburden conductivity is observed to depend on the dip of the ore body, being more drastic for gently dipping ones. Variations in the inphase and quadrature components as well as in the phase of the anomaly have been studied for varying depths of burial of the ore body and for a number of values of the solution conductivity. Anomaly index diagrams have been constructed with a view to predicting possible values of the geological parameters from a knowledge of the anomaly components. It is felt that the notable overburden effects are caused by a drastic redistribution and concentration of currents, mostly in a narrow loop at the top of the ore body, brought about by its galvanic contact with an extended medium of relatively poorer conductivity. 相似文献
14.
P.D. JACKSON 《Geophysical Prospecting》1981,29(4):601-626
The “Laterolog 7′’and the “Microlaterolog’ have been studied theoretically for the case of a conducting halfspace containing a single overburden, using a technique based on the method of images. The results have shown these focussed arrays to be more sensitive to the lower medium (i.e., having greater depth of investigation) than unfocussed ones of the same dimensions, when the “correct’ geometric factor is used. The geometric factor of a focussed electrode array is somewhat involved, and is explained with reference to an ideal focussed array; such an array would pass a measuring current of constant intensity into a fixed geometrical shape of conducting material, irrespective of any layering or any other heterogenities that may be present, using auxiliary current sources of variable magnitude. This concept of a constant amount of current flowing in a beam of fixed geometry and current density, is the basis of focussed arrays, and it is shown that the geometric factor, used to calculate the apparent resistivity, is the one derived when considering the homogeneous case. The value of the constant measuring current, alone, is used to convert the measured potential difference into a resistance, the ratio between this value and the combined intensities of the auxiliary current sources, for the homogeneous case, being incorporated into the geometric factor. Surprisingly good agreement was found between the theoretical models and practical experiments using a focussing seabed resistivity probe which is a substantial modification of the “Microlaterolog”. Both show similar deviations from the ideal case which are explained in terms of refraction at the overburden interface. All experiments indicated that focussed arrays have a greater depth of investigation than similar unfocussed ones for a single overburden, whether it be resistive or conductive. 相似文献
15.
由于激发极化效应的影响,时间域航空电磁晚期道信号经常会出现变号现象.基于电阻率的传统反演方法无法对变号数据进行正确反演,因此通常在数据处理中予以剔除.为深入了解极化介质的电磁扩散特征,认识航空瞬变电磁负响应的产生机理,本文研究时间域航空电磁系统的电磁扩散特征.我们以均匀极化、非极化半空间及层状介质模型为例,通过直接积分的方法求解频率域电场响应,并由欧姆定律得到电流响应,再经过汉克尔变换得到时间域电流响应.通过研究电流随时间在地下极化介质中的传播特征研究电磁扩散过程;通过对比不同激电参数对电磁扩散的影响,研究极化介质中感应电流与极化电流的扩散规律,从而合理地解释极化介质中负响应的产生机理.基于本文研究和分析结果,可加深对时间域航空电磁法中激电效应的认识. 相似文献
16.
A seismic source consisting of a 700 kg weight that could be dropped vertically or projected down a ramp inclined at 45° to the vertical was tested as a source of P, SV and SH waves within a crystalline rock body at Chalk River, Ontario. The seismic energy was recorded by arrays of both horizontal and vertical-component geophones at distances between 30 and 600 m from the source, which was operated over glacial overburden varying in thickness from less than a meter to a few tens of meters. Seismic energy was more efficiently generated when the overburden thickness was at least several meters. The signals identified visually as S are generally true S, though some may be the converted wave PS. The SV amplitudes are generally larger than those of P, regardless of the type of shot, while the signal frequencies are roughly 60 Hz and 90 Hz, respectively. The horizontal-component seismograms for the inclined shots showed no evidence of SH polarization, and the SH amplitudes were only rarely enhanced relative to P and SV amplitudes on changing from vertical to inclined shots. These unexpected results are attributed to the combined effect of the high velocity and density contrasts and the irregularity of the boundary between the glacial overburden and the rock body. 相似文献
17.
Responses of a multifrequency, multicoil airborne electromagnetic (AEM) system were modelled numerically for 3D electrical conductors embedded in a resistive bedrock and overlain by an overburden of low to moderate conductivity. The results cover a horizontal coplanar coil configuration and two frequencies, 7837 Hz and 51 250 Hz. The models studied are single or multiple, poor conductors (conductance lower than 0.1 S) embedded in a host rock of high but finite resistivity (5000 Ωm) and overlain by a layer of overburden with finite thickness and low to moderate conductivity (conductance up to 2 S). On the basis of the modelling results, limits of detectability for poor conductors have been studied for the various model structures. The results indicate that the anomaly from a steeply dipping, plate-like conductor will decrease significantly when the conductor is embedded in a weakly conductive host rock and is overlain by a conductive overburden. However, an anomaly is obtained, and its magnitude can even increase with increasing overburden conductivity or frequency. The plate anomaly remains practically constant when only the overburden thickness is varied. Changes in overburden conductivity will cause the plate-anomaly values to change markedly. If the plate conductance is less than that of the overburden, a local anomaly opposite in sign to the normal type of anomaly will be recorded. Another major consequence is that conductors interpreted with free-space models will be heavily overestimated in depth or underestimated in conductance, if in reality induction and current channelling in the host rock and overburden make even a slight contribution to the anomalous EM field. The lateral resolution for the horizontal coplanar coil system was found to be about 1.7 times the sensor altitude. Similarly, the lateral extension of a horizontal conductive ribbon, required to reach the semi-infinite (half-space) behaviour, is more than three times the sensor altitude. Finally, screening of a steeply dipping plate, caused by a small, conductive horizontal ribbon, is much more severe than screening of the same plate by an extensive horizontal layer. 相似文献
18.
Xavier Emery 《Stochastic Environmental Research and Risk Assessment (SERRA)》2005,19(5):348-360
This work deals with the geostatistical simulation of a family of stationary random field models with bivariate isofactorial
distributions. Such models are defined as the sum of independent random fields with mosaic-type bivariate distributions and
infinitely divisible univariate distributions. For practical applications, dead leaf tessellations are used since they provide
a wide range of models and allow conditioning the realizations to a set of data via an iterative procedure (simulated annealing).
The model parameters can be determined by comparing the data variogram and madogram, and enable to control the spatial connectivity
of the extreme values in the realizations. An illustration to a forest dataset is presented, for which a negative binomial
model is used to characterize the distribution of coniferous trees over a wooded area. 相似文献
19.
全球大气电模式是指大气和地表构成的闭合回路,其中考虑雷暴为主要充电电源,并将雷暴作为一个垂直电偶极子来考虑.本文从电流连续方程出发,采用球谐函数展开法,边界条件采用下边界为地表大气电势,上边界为电离层底部电势,非齐次电流源函数项采用垂直偶极子假设.通过所做的非线性函数变换,得出了考虑雷暴活动的全球电模式的近地稳态解析解,并证明其能展开为有限多项式级数之和.所得结论与过去的理论分析结果和实验观测是一致的.该文的工作是对以前相关工作仅能得到近似解的一种改进,同时本文所得公式可用于今后对大气电参量进行相关的数值计算及模拟之中. 相似文献
20.
R. SCHULZ 《Geophysical Prospecting》1985,33(8):1240-1253
Gradient measurements in a homogeneous electrical primary field can easily be interpreted for simple models. The simplified solution (conducting or resistant body in a homogeneous space in a homogeneous electrical field) is often sufficiently accurate, as comparisons with the exact solution (body of finite resistivity in a homogeneous half-space in a quasihomogeneous electrical field) show. The exact geometry of the body cannot be determined by gradient measurements; the same anomaly of apparent resistivity can be caused by different bodies. In particular, the similarity between a sphere and a cube of the same volume is very high. There is a distinct influence of the resistivity of the overburden: the higher this resistivity is, the stronger is the effect caused by a buried body. If a deviation of 10% of the apparent resistivity is assumed as the lower boundary at which a buried body can be detected by gradient measurements, the depth of investigation for a three-dimensional body is approximately equal to its width; in the two-dimensional case the thickness of the overburden can be twice the width. If the overburden has a resistivity which is higher than the resistivity of the substratum, these depths are greater. The greatest possible depth is approximately three times the width of the body. 相似文献