FREQUENCY-DEPENDENT COMPLEX RESISTIVITY OF ROCK-SALT SAMPLES AND RELATED PETROPHYSICAL PARAMETERS1

The presence of water is one of the main concerns of nuclear waste disposal in rock-salt. It can be investigated using electrical properties of the rock. Laboratory measurements of frequency-dependent resistivity and other petrophysical parameters, such as porosity, water content, and specific internal surface area, have been carried out on rock-salt from the Asse mine in Germany, in order to obtain characteristic resistivity responses for the evaluation of geoelectric field methods and to develop new methods for the estimation of the water content and saturation. The laboratory method, on a.c. half-bridge for very high impedances, allows measurements of the resistivity spectrum of rock-salt in the frequency range from 15 Hz to 10 kHz. The saturation of the samples was varied artificially and was approximately 5%, 10%, 20% and 100%. The porosity varies between 0.1% and 0.5%, the water content is approximately 0.05% or less, and the initial saturation is less than 50%. The resistivity ranges from 10 MΩm at the initial saturation down to 1 kΩm for fully saturated samples. In the low-frequency range up to 100 Hz, an Archie-type relationship may be used to estimate the water content of the rock-salt from resistivity measurements. The Archie exponent m is found to be approximately 2. The resistivity is observed to be strongly dependent on frequency. The resistivity decreases with increasing frequency, with a greater decrease for small saturations and vanishing frequency dependence at complete saturation. The relative dielectric constant was found to be 6 ± 1. Saturation dependence was not observed within this error range. The measurements imply that, by measuring resistivity in rock-salt, estimations of water content and saturation, and thus the porosity, can be made in situ. This is particularly important for the safety of nuclear waste disposal in rock-salt.     

Measurement of electrical properties of contaminated soil1

We present the measured dielectric constant and conductivity of soil samples contaminated by diesel oil. Measurements of the electrical properties of contaminated soil were carried out using a guarded-electrode sample holder and a parallel-plate sample holder in the frequency range 2–250 MHz. Two different soil samples were measured. Both the dielectric constant and the conductivity of the contaminated soils and uncontaminated soils are compared. The measurement results show that the change in the dielectric constant of soils before and after diesel oil contamination is small but significant. These results provide a basis for using ground-penetrating radar or other high-frequency electromagnetic sensors in the detection of soil contamination.     

Time domain reflectometry measurement principles and applications

Time domain reflectometry (TDR) is a highly accurate and automatable method for determination of porous media water content and electrical conductivity. Water content is inferred from the dielectric permittivity of the medium, whereas electrical conductivity is inferred from TDR signal attenuation. Empirical and dielectric mixing models are used to relate water content to measured dielectric permittivity. Clay and organic matter bind substantial amounts of water, such that measured bulk dielectric constant is reduced and the relationship with total water content requires individual calibration. A variety of TDR probe configurations provide users with site‐ and media‐specific options. Advances in TDR technology and in other dielectric methods offer the promise not only for less expensive and more accurate tools for electrical determination of water and solute contents, but also a host of other properties such as specific surface area, and retention properties of porous media. Copyright © 2002 John Wiley & Sons, Ltd.     

Electrical properties and geochemistry of carbonate rocks from the Qasr El‐Sagha Formation,El‐Faiyum,Egypt

Understanding petrographical, geochemical and electrical properties of rocks is essential for investigating minerals. This paper presents a study of the petrographical, geochemical and A.C. electrical properties of carbonate rock samples. The samples collected show six lithostratigraphic rock units. Electrical properties were measured using a non‐polarizing electrode at room temperature (～20°C) and a relative atmospheric humidity of ～50% by weight in the frequency range from 42 Hz to 5 MHz. The difference in electrical properties between the samples was attributed to the change in composition and texture between the samples. Electrical properties generally change with many factors (grain size, chemical composition, grain shape and facies). The dielectric constant decreases with frequency and increases with conductor composition. The conductivity increases with the increase of conductor paths between electrodes. Many parameters can contribute to the same result of the electrical properties. The main objective of the present study is to shed more light on the relation between the texture and geochemical composition of measured samples (carbonates that contain clays and quartz grains) through electrical laboratory measurements (conductivity and dielectric constant as a function of frequency).     

Low frequency complex conductivity measurements of microcrack properties

The frequency dependence of complex electrical conductivity in the IP frequency range (10^–3 to 10³ Hertz) has been investigated for a variety of microcracked rocks from the German continental deep drilling project (KTB), Northern Bavaria. The laboratory measurements were made with a computer controlled four-electrode system on plugs saturated with brine of different salinity. It has been found that the complex nature of the conductivity is caused solely by the capacitive behaviour of the interlayer region between the solid matrix and the electrolytic pore solution. The resulting main feature of the conductivity spectra is a constant phase angle over the investigated frequency range combined with a nearly identical power law frequency dependence of the real as well as the imaginary parts. The low-frequency exponent is in the order of about 0 to 0.05. It is related to common IP-parameters. The relationships between the frequency exponent and microcrack properties are of special interest. The results of the study show that the frequency exponent is (1) proportional to the surface area to porosity ratio, (2) inversely proportional to water salinity, and (3) dependent on water composition. Complex conductivity measurements allow an uncomplicated separation of electrical volume and interface effects. Moreover, the results suggest that determination of specific surface area of microcracked rocks directly from complex electrical measurements can be made.     

Evaluation of transport and storage properties in the soil and groundwater zone from induced polarization measurements1

Spectral induced polarization as well as complex electrical measurements are used to estimate, on a non-invasive basis, hydraulic permeability in aquifers. Basic laboratory measurements on a variety of shaly sands, silts and clays showed that the main feature of their conductivity spectra in the frequency range from 10^-3 to 10³ Hertz is a nearly constant phase angle. Thus, a constant-phase-angle model of electrical conductivity is applied to interpret quantitatively surface and borehole spectral induced polarization measurements. The model allows for the calculation of two independent electrical parameters from only one frequency scan and a simple separation of electrical volume and interface effects. The proposed interpretation algorithm yields the true formation factor, the cation exchange capacity and the surface-area-to-porosity ratio, which corresponds to the inverse hydraulic radius. Using a Kozeny–Carman-like equation, the estimation of hydraulic permeability is possible.     

A contribution to the measurement of frequency dependence of electrical conductivity of rocks

Summary Some errors of method occurring in A. C. measurements of the electrical conductivity of rocks are discussed. It is demonstrated that the difference between A.C. and D.C. conductivities, at given frequency, depends mostly on the magnitude of the D.C. conductivity and magnitude of the dielectric constant.     

含流体砂岩地震波频散实验研究

为了研究孔隙流体对不同渗透率岩石地震波速度的影响,在实验室利用跨频带岩石弹性参数测试系统得到了应变幅值10-6的2~2000Hz频段下的地震波速度和1 MHz频率下的超声波速度,利用差分共振声谱法得到了频率600Hz岩石干燥和完全饱水情况下岩石声学参数.实验表明,在低饱和度下,致密砂岩在地震和超声频段下没有明显的频散;在高饱和度下纵波速度的频散变得明显.从干燥到完全水饱和条件,不同频率测量的致密砂岩的体积模量随岩石孔隙度增高而降低,且体积模量的变化量受岩石微观孔隙结构的影响较大.高孔、高渗砂岩无论在低含水度下还是在高含水饱和度下频散微弱,并且在地震频段下围压对于岩石纵横波速度的影响要大于频率的影响.高孔、高渗砂岩和致密砂岩不同含水饱和度下的频散差异可应用于储层预测,油气检测等方面,同时该研究可以更好地帮助理解岩石的黏弹性行为,促进岩石物理频散理论的发展,提高地震解释的精度.     

Thermal effect of magma intrusion on the electrical properties of magnetic rocks from Hammamat sediments, Cairo, Egypt

The thermal effects of magmatic intrusion on the conductivity and dielectric constant of magnetic rocks from Hammamat sediments, NE desert, Cairo, Egypt (latitude ∼27° and longitude ∼33°) were investigated experimentally in the laboratory using nonpolarizing electrodes. Granitic magma was intruded into the Hammamat sediments, which are a mixture of mainly magnetite with sandstone and due to the thermal effect the area around was extensively heated and altered to different degrees. Due to this magma intrusion, magnetite was transformed (by heating) to hematite to different degrees according to its location from the intrusion. Complex impedance measurements were performed in the frequency range of 10 Hz to 100 KHz at normal temperature (∼20°C) and at a relative humidity of ∼50% RH. Samples were collected at different locations perpendicular to the core of the magma intrusion. Experimental data indicate that the electrical properties vary strongly as we move away (with distance) from the magma intrusion. The conductivity of hematite is ∼10⁻² S/m and that of magnetite is ∼10⁴ S/m. As we move from magnetite to hematite (to the core of the magma intrusion) it is supposed that the conductivity will decrease but it was found that the conductivity increases (which is supposed to be abnormal). The conductivity increases with increasing frequency from ∼10⁻⁸ S/m to ∼10⁻⁵ S/m with almost power‐law dependence on frequency. The conductivity increases in the order of one decade due to the variation from magnetite to hematite. The increase of conductivity, as we move from magnetite to hematite, was argued to be due to the heating that partially or completely melts the samples, thus the porosity of the samples was decreased and accordingly the conductivity and dielectric constant increased. It was also supposed that the grains of the conductor in the samples are coated or isolated with insulator material. A percolation behaviour for the conductivity and dielectric constant, characteristic of random conductor‐insulator mixtures, was found with distance, where continuous paths of the conductive material occur accompanied by peaking of the dielectric constant. Complex impedance plots show that as we move in the direction of altered samples (towards hematite) the relation between real and imaginary impedance changes from a linear form to an arc of a depressed semicircle and increases in depression as we move in the direction of the altered samples, which is consistent with the above interpretation.     

Fluid injection monitoring using electrical resistivity tomography — five years of injection at Ketzin,Germany

Between the years 2008 and 2013, approximately 67 kilotons of CO₂ have been injected at the Ketzin site, Germany. As part of the geophysical monitoring programme, time‐lapse electrical resistivity tomography has been applied using crosshole and surface‐downhole measurements of electrical resistivity tomography. The data collection of electrical resistivity tomography is partly based on electrodes that are permanently installed in three wells at the site (one injection well and two observation wells). Both types of ERT measurements consistently show the build‐up of a CO₂‐related resistivity signature near the injection point. Based on the imaged resistivity changes and a petrophysical model, CO₂ saturation levels are estimated. These CO₂ saturations are interpreted in conjunction with CO₂ saturations inferred from neutron‐gamma loggings. Apart from the CO₂–brine substitution response in the observed resistivity changes, significant imprints from the dynamic behaviour of the CO₂ in the reservoir are observed.     

混合物电导率和介电常数的研究

应用电场理论研究混合物的电性，得出了混合物的电导率公式和介电常数公式．这一结果表明：混合物的电导率决定于混合物各成分电导率的均值和方差；混合物的介电常数决定于混合物各成分介电常数的均值和方差．作为所得结果的应用，给出了岩石的电导率公式和介电常数公式．     

Acoustic and electromagnetic properties of soils saturated with salt water and NAPL

Electrical, seismic, and electromagnetic methods can be used for noninvasive determination of subsurface physical and chemical properties. In particular, we consider the evaluation of water salinity and the detection of surface contaminants. Most of the relevant properties are represented by electric conductivity, P-wave velocity, and dielectric permittivity. Hence, it is important to obtain relationships between these measurable physical quantities and soil composition, saturation, and frequency. Conductivity in the geoelectric frequency range is obtained with Pride's model for a porous rock. (The model considers salinity and permeability.) White's model of patchy saturation is used to calculate the P-wave velocity and attenuation. Four cases are considered: light nonaqueous phase liquid (LNAPL) pockets in water, dense nonaqueous phase liquid (DNAPL) pockets in water, LNAPL pockets in air, and DNAPL pockets in air. The size of the pockets (or pools), with respect to the signal wavelength, is modeled by the theory. The electromagnetic properties in the GPR frequency range are obtained by using the Hanai–Bruggeman equation for two solids (sand and clay grains) and two fluids (LNAPL or DNAPL in water or air). The Hanai–Bruggeman exponent (1/3 for spherical particles) is used as a fitting parameter and evaluated for a sand/clay mixture saturated with water.Pride's model predicts increasing conductivity for increasing salinity and decreasing permeability. The best-fit exponent of the Hanai–Bruggeman equation for a sand/clay mixture saturated with water is 0.61, indicating that the shape of the grains has a significant influence on the electromagnetic properties. At radar frequencies, it is possible to distinguish between a water-saturated medium and a NAPL-saturated medium, but LNAPL- and DNAPL-saturated media have very similar electromagnetic properties. The type of contaminant can be better distinguished from the acoustic properties. P-wave velocity increases with frequency, and has dissimilar behaviour for wet and dry soils.     

Relating electrical conduction of alluvial sediments to textural properties and pore‐fluid conductivity

Electrical conductivity of alluvial sediments depends on litho‐textural properties, fluid saturation and porewater conductivity. Therefore, for hydrostratigraphic applications of direct current resistivity methods in porous sedimentary aquifers, it can be useful to characterize the prevailing mechanisms of electrical conduction (electrolytic or shale conduction) according to the litho‐textural properties and to the porewater characteristics. An experimental device and a measurement protocol were developed and applied to collect data on eight samples of alluvial sediments from the Po plain (Northern Italy), characterized by different grain‐size distribution, and fully saturated with porewater of variable conductivity. The bulk electrical conductivities obtained with the laboratory tests were interpreted with a classical two‐component model, which requires the identification of the intrinsic conductivity of clay particles and the effective porosity for each sample, and with a three‐component model. The latter is based on the two endmember mechanisms, surface and electrolytic conduction, but takes into account also the interaction between dissolved ions in the pores and the fluid‐grain interface. The experimental data and their interpretation with the phenomenological models show that the volumetric ratio between coarse and fine grains is a simple but effective parameter to determine the electrical behaviour of clastic hydrofacies at the scale of the representative elementary volume.     

An experimental investigation of nitrogen gas produced during denitrification

In situ denitrification relies on indigenous microorganisms to reduce nitrate to N(2) gas. However, when initial nitrate concentrations are large, produced gas volumes also can be very large, potentially resulting in reduced water saturation and hydraulic conductivity in the treatment zone. In this study, we investigated the fate of N(2) and other gases produced during denitrification in a laboratory flow cell containing packed sediment. Denitrifying activity was stimulated by additions of nitrate and ethanol. Microbial activity was monitored by measuring nitrate, nitrite, and ethanol concentrations; gas saturations were measured during the experiment using a gamma imaging system. Biomass was measured using phospholipid fatty acid analysis of sediment samples. Bioenergetic calculations calibrated to measured nitrate consumed and biomass produced predicted that 1.2 L N(2) gas/L water should have been produced following the addition of 100 mM nitrate. However, the maximum measured gas saturation was only 23%, indicating substantial gas loss from the sediment pack. Temporal gamma images and visual observations confirm that small gas bubbles formed in the sediment pack coalesced into larger bubbles and migrated upward through gas-filled channels to the sediment pack surface. Although gas saturations increased, there was no significant change in sediment pack hydraulic conductivity. These results suggest that in permeable reactive barriers used for in situ denitrification, gas production will not necessarily lead to unlimited gas accumulation in the pore space and that the effects of gas production on water saturation and hydraulic conductivity may be relatively minor.     

地震深部电性变化前兆特征及其机理的初步探讨

本文着重讨论了地壳内埋深约10—25km的高阻层原地条件下的导电机理,指出该层岩体导电性主要取决于其裂隙及孔隙情况和孔隙水在其中的分布。根据对部分熔体导电性的研究,对实际岩体电导率进行数字模拟发现,连接权因子增加可导致该层原地体积电阻率极大的减少。本文还分析了孔隙水参与下震源及其外围区的应力应变特征,阐明了可能出现的深部电性变化前兆的分区性及其区域的体积电阻率、电性各向异性变化特征,初步探讨了这些变化的物理机理,粗略地估计了可能的变化量值。     

Multifluid flow in bedded porous media: laboratory experiments and numerical simulations

Understanding light nonaqueous-phase liquid (LNAPL) movement in heterogeneous vadose environments is important for effective remediation design. We investigated LNAPL movement near a sloping fine- over coarse-grained textural interface, forming a capillary barrier. LNAPL flow experiments were performed in a glass chamber (50 cm×60 cm×1.0 cm) using two silica sands (12/20 and 30/40 sieve sizes). Variable water saturations near the textural interface were generated by applying water uniformly to the sand surface at various flow rates. A model LNAPL (Soltrol® 220) was subsequently released at two locations at the sand surface. Visible light transmission was used to quantitatively determine water saturations prior to LNAPL release and to observe LNAPL flow paths. Numerical simulations were performed using the Subsurface Transport Over Multiple Phases (STOMP) simulator, employing two nonhysteretic relative permeability–saturation–pressure (k–S–P) models. LNAPL movement strongly depended on the water saturation in the fine-grained sand layer above the textural interface. In general, reasonable agreement was found between observed and predicted water saturations near the textural interface and LNAPL flow paths. Discrepancies between predictions based on the van Genuchten/Mualem (VGM) and Brooks–Corey/Burdine (BCB) k–S–P models existed in the migration speed of the simulated LNAPL plume and the LNAPL flow patterns at high water saturation above the textural interface. In both instances, predictions based on the BCB model agreed better with experimental observations than predictions based on the VGM model. The results confirm the critical role water saturation plays in determining LNAPL movement in heterogeneous vadose zone environments and that accurate prediction of LNAPL flow paths depends on the careful selection of an appropriate k–S–P model.     

结晶岩的水化膨胀研究

室温下的应变实验表明 ,低孔隙度的结晶岩如花岗岩、片麻岩、闪长岩和变基性岩 ,水饱和时膨胀 ,干燥时收缩 ,这种效应是可重复的 .用水的吸附和解吸 （在相对湿度下 ） ,也可以观察到膨胀和收缩 .膨胀效应是由于表面力 （VanderWaals吸引、双电层排斥及溶剂化排斥 ）引起的 .水饱和体应变约为其孔隙度的 1 /1 0 .用庚烷饱和的体应变比用水饱和的小 .用CaCl2 溶液饱和的应变随着盐克分子浓度的增加而减小 .杨氏模量E、切变模量G、品质因数Q的准静态测量表明 ,随着相对湿度的增加 ,E、G、Q减少 ,吸附水的增加改变了孔隙内表面分子的相互作用力 .大约在岩石孔隙表面吸附达到 3个水分子层时 ,可观察到最强烈的膨胀效应 .     

Influence of Degree of Saturation in the Electric Resistivity–Hydraulic Conductivity Relationship

The relationship between aquifer hydraulic conductivity and aquifer resistivity, either measured on the ground surface by vertical electrical sounding (VES) or from resistivity logs, or measured in core samples have been published for different types of aquifers in different locations. Generally, these relationships are empirical and semi-empirical, and confined in few locations. This relation has a positive correlation in some studies and negative in others. So far, there is no potentially physical law controlling this relation, which is not completely understood. Electric current follows the path of least resistance, as does water. Within and around pores, the model of conduction of electricity is ionic and thus the resistivity of the medium is controlled more by porosity and water conductivity than by the resistivity of the rock matrix. Thus, at the pore level, the electrical path is similar to the hydraulic path and the resistivity should reflect hydraulic conductivity. We tried in this paper to study the effect of degree of groundwater saturation in the relation between hydraulic conductivity and bulk resistivity via a simple numerical analysis of Archie’s second law and a simplified Kozeny-Carmen equation. The study reached three characteristic non-linear relations between hydraulic conductivity and resistivity depending on the degree of saturation. These relations are: (1) An inverse power relation in fully saturated aquifers and when porosity equals water saturation, (2) An inverse polynomial relation in unsaturated aquifers, when water saturation is higher than 50%, higher than porosity, and (3) A direct polynomial relation in poorly saturated aquifers, when water saturation is lower than 50%, lower than porosity. These results are supported by some field scale relationships.     

Stream bottom resistivity tomography to map ground water discharge

This study investigates the effectiveness of direct current electrical resistivity as a tool for assessing ground water/surface water interactions within streams. This research has shown that patterns of ground water discharge can be mapped at the meter scale, which is important for understanding stream water quality and ecosystem function. Underwater electrical resistivity surveys along a 107-m stream section within the Burd Run Watershed in South Central Pennsylvania identified three resistivity layers: a resistive (100 to 400 Ωm) surface layer corresponding to the streambed sediments, a conductive (20 to 100 Ωm) middle layer corresponding to residual clay sediments, and a resistive (100 to 450 Ωm) bottom layer corresponding to the carbonate bedrock. Tile probing to determine the depth to the bedrock and resistivity test box analysis of augered sediment samples confirmed these interpretations of the resistivity data. Ground water seeps occurred where the resistivity data showed that the residual clays were thinnest and bedrock was closest to the streambed. Plotting the difference in resistivity between two surveys, one conducted during low-stage and the other during high-stage stream conditions, showed changes in the conductivity of the pore fluids saturating the sediments. Under high-stream stage conditions, the top layer showed increased resistivity values for sections with surface water infiltration but showed nearly constant resistivity in sections with ground water seeps. This was expressed as difference values less than 50 Ωm in the area of the seeps and greater than 50 Ωm change for the streambed sediments saturated by surface water. Thus, electrical resistivity aided in characterizing ground water discharge zones by detecting variations in subsurface resistivity under high- and low-stream stage conditions as well as mapping subsurface heterogeneities that promote these exchanges.     

高温高压下角闪片麻岩的电导率研究

在1.0 GPa压力、343～962 K温度和0.1～106Hz频率的条件下,使用Solartron 1260阻抗-增益/相位分析仪对含角闪石的片麻岩从平行和垂直面理两个不同方向分别进行了电阻抗的测定,并且进一步分析了片麻岩的微观导电机制.高温高压实验结果表明:片麻岩的复阻抗对温度、频率表现出明显的依赖性.片麻岩的电导...