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ABSTRACT: In order to use micro-seismic data for understanding flow-pathway structure in subsurface, we considered in detail the process of pressure propagation to cause micro-seismicity in hydraulic stimulation. Then we found that pore pressure distribution along flow-pathway and its variation with time could be estimated by analyzing the data of micro-seismicity. Furthermore, the estimated pore pressure allows us to estimate the location of flow-pathways and the distribution of hydraulic conductivity along them. To do this, we assume an appropriate model of flow-pathway structure and adjust it as the pore pressure distribution computed by the model agrees well with that estimated from micro-seismicity. Finally, by a numerical experiment for one dimensional case, we demonstrated how we can optimize the model of flow-pathway according to the input data of pore pressure. 1. INTRODUCTION For oil production, a network of natural fractures plays a role of flow-pathways to connect oil reservoirs with production wells. The fracture network is used also as heat exchanger in the advanced geothermal heat extraction system, i.e. the Hot Dry Rock system. Thus for designing and managing the oil-flow system and the heatexchanging system, it is important to know the flow-pathway structure [1-3]. We will present here a new concept to detect both of the location of flowpathways and the distribution of hydraulic conductivity along them by analyzing the data of micro-seismicity accompanying hydraulic stimulation. 2. PRESSURE PROPAGATION CAUSING MICRO-SEISMICITY IN HYDRAULIC STIMULATION Hydraulic stimulation is carried out to improve the fracture connectivity and also to connect the fracture network with production / injection wells by creating new fractures and re-activating old fractures. In this operation, massive fluid is injected into subsurface rock through drilled wells. Then a number of micro-seismicity is commonly observed. It is believed that the stimulation raises pore pressure in pre-existing fractures, the elevated pressure reduces friction between the fracture planes, and finally shear sliding occurs for optimally-oriented fractures to cause microseismicity. Based on such consideration, the detected source location of micro-seismicity has been used to estimate the location of flow-pathway. However, the source locations are usually distributed as a "cloud". The cloud-like distribution tells us just rough alignment of flow-pathways basically, and it does not allow us quantitative estimation of hydraulic conductivity along them while such kind of approach has been advancing year after year. In order to detect more precise flow-pathway structure from micro-seismic data, let us consider in more detail the process of pressure propagation to cause micro-seismicity in hydraulic stimulation. We assume a model case that micro-seismicity occurs around the injection well as schematically shown in Fig. 1(a). As described above, there should exist one fracture at each location of microseismicity, and its sliding to cause micro-seismicity occurs due to the increase in pore pressure inside of the fracture. The additional pressure should be transferred to the fracture from the injection well through some flow-pathways. (available in full paper) There may be treelike flow-pathways as shown in Fig. 1(b). Thus high pressure at the injection well propagates through major pathways, and it finally reaches fractures by way of relatively short branches connecting the fractures and the major pathways.
- North America > United States > Texas (0.28)
- Asia > Japan (0.28)
- Energy > Oil & Gas > Upstream (1.00)
- Energy > Renewable > Geothermal > Geothermal Resource > Hot Dry Rock (0.34)
Estimation Of Intergranular Bond Strengths By Core Scratching: A Comparison Between A Laboratory Experiment And A Numerical Discrete Particle Simulation
Larsen, I. (SINTEF Petroleum Research) | Li, L. (SINTEF Petroleum Research) | Holt, R.M. (NTNU Petroleum Technology & Applied Geophysics, and SINTEF Petroleum Research)
ABSTRACT: The potential for using discrete particle modeling as a tool to compute rock mechanical parameters based on microscopic information is being explored. In order to achieve this, the numerical model needs to be calibrated vs. simple and controlled model experiments. The most essential input parameters for the numerical model are interparticle bond strengths and stiffnesses. We present here a series of rock mechanical (UCS) and core scratch tests on artificial rock-like samples of glass beads cemented with epoxy, and discrete particle simulations of the same tests. We conclude that both numerical and experimental data show the same features, such as correlation between UCS and peak force in the force distribution obtained from the scratch tests. The force required to break one intergranular bond is the same size of order (within a factor . 4) when evaluated from physical and numerical simulations. 1. INTRODUCTION Due to enhanced computer speed and power, it is becoming more and more attractive to model rock mechanical behavior with discrete particle models. Cores from oil field are scarce, and only a limited number of mechanical tests can be performed. Cores may also be altered during the coring process, so that the data obtained needs to be corrected for core damage. Furthermore, certain experiments such as creep tests may be very time-consuming, and not feasible within practical time limits. It is therefore important to have numerical models which can be used to simulate the mechanical behavior of the cored material. In addition, the use of numerical models may in itself provide insight into the physical processes of rock deformation and failure. In order to use a discrete particle model as a "numerical laboratory" as outlined above, one needs to obtain the input parameters necessary to describe classes of rock materials based on petrographical information (such as microscope images) plus simple experiments that preferably can be done with small amounts of recovered core material. In order to establish the numerical laboratory, a calibration of the numerical model is required. The work presented here is part of such a calibration. We use the Particle Flow Code (PFC, [1]) (provided by HCItasca) as the sample generator and as the test machine of our numerical laboratory. A rock like material is generated numerically as an assembly of particles. The mechanical behaviour is controlled largely by the intergranular bond strength and stiffness. In this study we aim at providing guidance for choosing these numerical parameters by performing experiments on a controlled granular material. 2. EXPERIMENTAL SETUP The numerical model is based on spherical particles which can be cemented with a bond, defined by shear and tensile strength, plus shear and normal stiffness. In its basic version, the discrete particle model (PFC) uses spherical particles as its building blocks. In order to calibrate the numerical bond parameters through controlled experiments, we therefore chose to use an artificial material of spherical glass beads, with epoxy as cement between the particles. 2.1. Sample preparation The samples were made of glass beads with diameters in the range 1000-1180 microns with physical properties listed in Table 1.
- Research Report > Experimental Study (1.00)
- Research Report > Strength High (0.94)
ABSTRACT : This paper describes the successful measurement of stress changes induced in a crystalline rock mass adjacent to a full size hydraulic fracture. A hydraulic fracture was initially formed using water and subsequently reopened using cross linked gel. The stress changes measured during these treatments are compared with the results of 2-D and axisymmetric models. The full three dimensional stress changes were measured using four ANZI stresscells installed and tested in situ prior to the start of hydraulic fracturing. The instruments were logged at 15 second intervals throughout the hydraulic fracture treatments to provide a time history of the complete three dimensional stress changes that occurred as each hydraulic fracture grew toward and then passed close to the instruments. Analysis of the stress change data provides information about the fracture rate and mode of growth, orientation, and about the excess pressure acting inside the fracture to open it. The stress change data confirms that the hydraulic fractures grew as openingmode rather than shear mode fractures. Stress change calculated using a radial fracture geometry model with the fracture opened by a uniform pressure fit the measured stress change quite well. The fit obtained helps establish the orientation and growth rate for the fractures generated at the site. 1. INTRODUCTION Hydraulic fracturing has long been used to induce fractures in rock masses for reservoir stimulation and other purposes. However, it is rare to have the opportunity to measure the actual stress changes that are caused by a full scale hydraulic fracture as it grows. Recently, a study aimed at assessing the potential for using multiple hydraulic fractures to pre-condition a rock mass to increase caveability and reduce fragment size provided just such an opportunity. The details of the site and the overall study results remain confidential as a condition of using the data, but the results of the stress change monitoring are of interest from a scientific perspective unrelated to the outcomes of the caveability study. This paper describes the results of three dimensional stress change monitoring about two full size hydraulic fractures induced in a crystalline rock mass. An initial hydraulic fracture was made using water as the injection fluid. This fracture was later reopened using cross-linked gel. 2. STRESS CHANGE MONITORING The stress change in the rock near the hydraulic fractures was measured using ANZI cells. These instruments are described in detail in Mills [1]. The ANZI cell is a soft, inflatable stress measurement instrument that measures the strain changes on the wall of a borehole induced by stress changes in the rock mass. In each instrument, eighteen electrical resistance strain gauges of various orientations are pressure bonded directly onto the borehole wall. A multiple linear regression analysis of the strain changes measured by the instrument allows the complete three dimensional stress changes to be calculated. A high level of redundancy (only six of the eighteen strain readings are required to determine the full stress change tensor) provides a strong measure of the confidence that can be placed in the integrity of the result.
- Oceania > Australia (0.28)
- North America > United States > Texas (0.28)
- Well Completion > Hydraulic Fracturing (1.00)
- Reservoir Description and Dynamics > Reservoir Characterization > Reservoir geomechanics (0.71)
- Reservoir Description and Dynamics > Reservoir Characterization > Faults and fracture characterization (0.68)
- Reservoir Description and Dynamics > Unconventional and Complex Reservoirs > Naturally-fractured reservoirs (0.49)
ABSTRACT: In this paper, we describe a method to estimate, in real-time, the volume of a hydraulic fracture from tiltmeter measurements recorded during the treatment. The resolution of fracture dimensions and orientation is discussed at first. A limit, expressed in terms of the distance between the tiltmeters and the fracture, above which only the volume and orientation of the fracture have an effect on the tilt field is established. The proposed method of analysis takes advantage of this, often neglected, fundamental properties of the elastic kernels (St. Venant principle). We recognize that, in most cases without additional information, only the fracture volume and orientation can be accurately estimated from tiltmeter measurements. The knowledge of the fracture volume estimated from tilt data together with the injected volume at time t furnish the efficiency of the treatment at this time. Furthermore, because the method employed to analyse the tilt data is computationally efficient, the volume and efficiency can be obtained in real time (i.e. as the data arrive) throughout a hydraulic fracturing treatment. 1. INTRODUCTION Hydraulic fracturing is a technique widely used for the stimulation of oil and gas reservoir (Economides & Nolte, 2000). Fracture orientation must be known to determine well layout and spacing, especially in reservoirs with permeability anisotropy. In some environments, there is considerable uncertainty about whether fractures are growing in horizontal or vertical planes, which can significantly effect the overall size of the fracture formed. Fractures treatment schedule designs which assume a vertical orientation will often fail in execution if the actual fracture is horizontal. Ultimately, the success of a hydraulic fracture treatment depends on placing the correct amount of proppant in the fracture to produce a conductive channel with a size and shape that provides an overall optimal stimulation effect. Fracture olume and efficiency are important in design because these parameters help determine timing and volumes of for pumping clean and slurry fluid stages. In order to "map" hydraulic fractures, several type of indirect measurements are typically used such as micro-seismic acoustic monitoring, tiltmeter mapping, and treatment pressure analysis. In this communication, our interest lies in the analysis of tiltmeter measurements performed during hydraulic fracturing jobs. Despite the now common use of tiltmeters to map hydraulic fractures in the petroleum industry (Wright et al., 1999), it is not precisely clear what information on the fracture can and cannot be obtained from such measurements. Based on practical experience, (Cipolla & Wright, 2000) lists some of the fracture quantities better resolved by surface or borehole tiltmeters. In this paper, we present formal results we have recently obtained regarding the issue of determining the geometry of hydraulic fractures from tiltmeter measurements. We then discuss practical implications of these results. Specifically, we show that due to the elastostatic nature of the problem, the effect of the dimensions of the fracture fades extremely fast spatially such that, in most case, tiltmeters only sample the far-field effect of the hydraulic fracture. This far-field deformation pattern only depends on the orientation of the fracture (anisotropic displacement field) and the volume of the fracture (intensity of the recorded tilts), the two being coupled.
ABSTRACT: The point estimation method can be applied to the safety factor (SF) equation for any specified rock slope failure mode (such as plane shear, step path, or wedge) to obtain reliable estimates of the mean and standard deviation of the SF probability distribution. A gamma probability density function is recommended for modeling this probability distribution, because it allows only for positive values and is flexible enough to provide symmetrical shapes and right-skewed, exponential-type shapes for the SF distribution. The mean and standard deviation define this distribution, which then can be integrated numerically from 0 to 1 to obtain the probability of sliding, PS (portion of the SF distribution where SF<1.0). The overall probability of failure, PF, for the potential slope failure mass is the joint probability that the rock discontinuities are long enough to allow kinematic failure (PL) and that sliding occurs along the rock discontinuities (PS); that is, PF = PSPL. This method for estimating the probability of sliding is extremely efficient computationally, and thus, expedites slope stability simulation routines used by NIOSH software to stochastically describe rock slope behavior and assist the engineer in designing catch benches for large rock slopes. Enhanced bench design translates into increased operational efficiency and safer working conditions in open pit mines and quarries. 1. INTRODUCTION Stochastic simulations of fractured rock masses can provide valuable information for the engineering design of rock slopes, particularly when the natural geologic discontinuities may form potential slope failure modes. An essential component of such engineering simulations is being able to compute the probability of sliding for a given potential failure mass once the geometry of that failure mass has been identified through spatial rock-fracture simulations. In cases where several thousand (or ore) simulations of possible failure geometries are needed to provide a realistic representation of the rock slope, computational efficiency is essential for the repetitious protocol used to calculate the probability of sliding. An example of applying this type of geotechnical approach to rock slope design was presented by Miller and others [1] in a paper focused on the design of catch benches for open pit mines and quarries. This issue is important to NIOSH (National Institute for Occupational Safety and Health) as part of its research mission to improve safety and health in the mining industry. Between 1995 and 2003 there were 42 reported fatalities dueto slope failure accidents at surface mines in the United States, at least one and as many as eleven each year. Additional accident statistics collected by the Mine Safety and Health Administration (MSHA) have shown that loose material from slope and bench failures can pose significant safety hazards to miners. To address these concerns the NIOSH Spokane Research Laboratory has been developing and testing rock slope stability software ver the past few years to provide advanced technical tools for analyzing bench stability. A key element of this software is a module used to compute the probability of sliding for a given viable failure geometry that has been simulated for the bench under study.
- Materials > Metals & Mining (1.00)
- Government > Regional Government > North America Government > United States Government (1.00)
Rock Mass Characterization For Slope/Catch Bench Design Using 3D Laser And Digital Imaging
Nasrallah, Joshua G. (Department of Mining and Geological Engineering, University of Arizona) | Monte, Jamie (Department of Mining and Geological Engineering, University of Arizona) | Kemeny, John (Department of Mining and Geological Engineering, University of Arizona)
Abstract: Slope stability designs are largely dependent on rock mass and more specifically discontinuity characterization. Traditional discontinuity characterization methods (scanline and cell mapping) have many problems associated with time, safety, accuracy, and human bias. This study shows how 3D imaging using ground based LiDAR scans and digital photography can be used to collect discontinuity information for slope stability analysis. 3D imaging allows for potentially larger data sets to be collected rapidly while eliminating or highly limiting the problems with traditional methods. Rock mass information for a benched road cut was gathered using traditional and 3D imaging methods. A set of NIOSH slope stability programs were then compared to two other commercial programs using the two different data sets gathered. 3D imaging was used to create an "as built" of a benched road cut. This image was used to test the accuracy of the three slope programs. The comparison between programs resulted in similar conclusions despite which data set was used. All slope modeling programs results contained slight inaccuracy when compared to the actual slope. 1. INTRODUCTION Predicting the stability of a rock slope is vital to the construction and mining industries. Slope instability is a leading cause of fatalities in surface mining [1]. Correctly designing stable slopes increase safety and production as well as reducing construction time and cost. Slope stability designs are largely dependent on the characteristics of the rock mass and more specifically discontinuity characterization. Discontinuities in this study refer to: joints, fractures, bedding planes, or any other visible planes of weakness within the rock mass. Traditional methods for characterizing discontinuities include scanline surveys and cell mapping [2,3]. Scanline (detailed line) surveys are conducted by laying a tape measure along a rock face and discontinuity characteristics are measured for every discontinuity crossed. Cell (area) mapping is conducted by breaking the rock mass into multiple areas of equal size. Discontinuity characteristics are measured for each set within these different areas (cells). Although these methods have proven to be very effective there are many drawbacks associated with them, including time, accessibility, and human bias [4]. Recent advances in imaging technologies show the potential to eliminate or highly reduce these problems. These advantages will be discussed later in this paper. 1.1. Imaging Technologies The two predominate imaging technologies, 3D LiDAR scanning and digital photography, show the most potential for rock mass characterization purposes. Below is a brief discussion of how each technology works. For more detailed analysis on how the different technologies work or other possible imaging technologies please see Baltsavias [5,6,7]. LiDAR (Light Detection And Ranging or Light RADar) uses a time of flight light pulses to generate a 3D image of a surface. A light pulse is emitted from the source, reflects off the surface of an object and returns to the source. A high precision counter measures the travel time and intensity of the returned pulse. Thus the distance from the source to a point on some surface as well as the reflectivity of that surface can be measured with great accuracy.
- Media > Photography (1.00)
- Health & Medicine (0.95)
- Materials > Metals & Mining (0.86)
- (2 more...)
Using Borehole Data To Estimate Size And Aspect Ratio Of Subsurface Fractures
Wang, Xiaohai (Department of Civil and Environmental Engineering) | Mauldon, Matthew (Department of Civil and Environmental Engineering) | Dunn, William (Department of Geological Sciences, University of Tennessee) | Heiny, Chris (Department of Geological Sciences, University of Tennessee)
ABSTRACT: This study focuses on estimating fracture size and aspect ratio of subsurface fractures in sedimentary rocks. Fractures in sedimentary rock are typically elongated in one direction and their shapes can be considered rectangles. The study shows how information about sizes and aspect ratios of rectangular fractures can be discerned from study of borehole-fracture (or core-fracture) intersections. Based on the possible geometric relations between a fracture and a sampling cylinder, six types of intersection: complete, long-edge, short-edge, corner, end, and pierced, are defined. The probabilities of occurrence of these different intersection types are related to the sizes and aspect ratios of fractures. The sizes and aspect ratios of fractures are then estimated directly from the observed counts of different types of intersection in a borehole or rock core.
1. INTRODUCTION
Fracture information is essential in rock engineering and engineering geology as well as in hydrogeology. Tunnels and boreholes often provide the main sources of data from which fractures are characterized. However, those data are still underutilized [1, 2, 3, 4, 5, 6, 7, 8, and 9]. Recently researchers and engineers have been working on developing new techniques to obtain more information about fractures through borehole and tunnel data [3, 4, 6, 7, and 10]. In this paper we give a method to estimate the size and shape of fractures in sedimentary rocks using borehole data.
Since fractures in sedimentary rock are commonly elongated in one direction, rectangles are considered to be good assumptions for their shape. For this study we assume a single set of parallel rectangular fractures with constant width W and length L = W, and with long axes aligned in the same direction. This fracture set is intersected by a borehole of diameter D, oriented normal to the fractures (Fig. 1).
(available in full paper)
2. BOREHOLE-FRACTURE INTERSECTIONS
Based on geometric relations (Fig. 2) between a rectangular fracture and a sampling cylinder, six types of intersection are defined. They are:
Type A Complete intersection
Type B1 Long-edge intersection
Type B2 Short-edge intersection
Type B3 Corner intersection
Type B4 End intersection
Type C Pierced.
(available in full paper)
Because fracture locations at depth are unknown, the boring locations can be assumed independent of specific fracture locations [3, 4]. Independence holds regardless of any grid or other pattern used to choose locations for borings. Probabilities of occurrence of the different intersection types, assuming independence between the borehole and the fracture population and assuming, in this case, boreholes perpendicular to fractures, depend only on the sizes and aspect ratios of fractures and the size of the borehole.
We consider two cases, W>D and W
- North America > United States > Colorado (0.30)
- North America > United States > Tennessee (0.28)
- Geology > Geological Subdiscipline > Geomechanics (0.98)
- Geology > Rock Type > Sedimentary Rock (0.95)
ABSTRACT: Methods of automatically extracting rock discontinuity orientations from digital images and 3D models generated using current laser scanning technology are under development. Using a series of image processing algorithms, fracture traces can be delineated from digital images of exposed rock surfaces and in turn, used to determine discontinuity data. Field studies suggest that the discontinuity data collected from digital images compares favorably to data collected using more traditional manual methods. Algorithms for the processing of raw point clouds, created by laser scanning exposed rock surfaces, have been developed. A novel method of triangular mesh generation rapidly creates a 3D model of the scanned surface. The triangular elements of the mesh are grouped together using their normals as a similarity measure, resulting in the identification of larger fracture patches that represent discontinuity surfaces. Refinement and validation of this process has been initiated through a series of field studies. Additional algorithms that expand the application of automated rock mass characterization using digital images and 3D laser scans are under development, with the ultimate goal of creating a software package integrating both technologies and adaptable to any field requiring rock discontinuity analysis. 1. INTRODUCTION Rock, by its nature, is a complex geologic material and the design of structures in and on rock can be a multifarious problem. Rock engineering is further complicated by the typically discontinuous state of rock masses, where individual elements of rock material are separated by structural discontinuities that include bedding planes, faults, joints, and other types of fractures. For most rock engineering applications, the material strength of the intact rock between discontinuities is high relative to the expected stresses. In these cases the deformation of the rock mass is generally controlled by the discontinuities, and the behavior of the rock mass, under varying conditions of stress and strain, is dependent upon the nature, distribution, and properties of the discontinuities [1]. These properties include orientation, surface roughness, length, persistence, aperture, spacing, filling, and termination [2]. The most crucial property is orientation (i.e., the direction that a discontinuity is dipping and the angle of its dip), which influences the potential for a rock mass to move, the direction of movement, and the volume of material in motion [3]. There are various engineering situations where knowledge of discontinuity properties is important, and a variety of approaches can be taken in order to analyze the stability and behavior of a rock mass given those characteristics. In mining, discontinuities are central to blast design, blasted fragment size and shape, downstream mineral processing operations (i.e., crushing, grinding, and leaching), open pit slope stability, and the design and stability of underground workings. In civil/geotechnical engineering, collection and analysis of discontinuity data is critical to the design and support of foundations/abutments, dams, tunnels, and road cuts. Likewise, the effect of discontinuities on the stability of underground storage areas and the amount of fluid flow around and through those openings, or in fractures themselves, is of utmost importance in the fields of environmental and petroleum engineering.
- North America > United States > Arizona (0.29)
- North America > United States > Texas (0.28)
- North America > United States > Massachusetts (0.28)
- Media > Photography (1.00)
- Energy > Oil & Gas > Upstream (1.00)
ABSTRACT: A new semi-empirical model that predicts fracture deformation under normal compressive loading is presented. The development of a simple exponential model is given first after which a modified and more general exponential model, with an additional degree of freedom in the model parameters, is presented. The simple and the modified exponential models are then compared to available fracture closure models, namely the empirical Barton-Bandis hyperbolic model, and a power-law model based on Hertzian contact theory, to determine how good they fit the results of fracture closure experiments conducted under monotonically increasing normal compressive loading. A new parameter called the half-closure stress, s½, is introduced and is used, in addition to the maximum fracture closure, ¿¿m, in the model fitting procedures for the Barton-Bandis and the simple and generalized exponential models. The half-closure stress is shown to be related to the initial normal stiffness, Kni, used in the original Barton-Bandis model. An additional parameter, n, is used in fitting the modified exponential model to the experimental data. Of the models presented herein, the modified exponential model was found to provide the best fit to the experimental data, for the same values of s½ and ¿¿m, over the entire range of compressive stresses. The power-law model based on Hertzian contact theory was found to be unsuitable for accurate prediction of fracture normal deformation behavior. 1. INTRODUCTION Fracture deformability resulting from normal compressive stress is of fundamental importance to the study of the hydraulic and mechanical behavior of rock discontinuities. Fracture deformation directly affects the factors that govern the hydraulic conductivity of single fractures such as aperture distribution, contact area distribution, and spatial connectivity of the apertures [1,2]. Since fracture geometry networks and fluid flow through single fractures govern the hydraulic behavior of fractured rock masses, it follows directly that deformability of single fractures due to the action of compressive stress would affect the hydraulic properties of a rock mass. It is also generally understood that the mechanical behavior of rock masses is controlled significantly by the deformation of discontinuities [3]. The most fundamental properties of the bounding surfaces of a fracture affecting fracture deformation include the rock type, weathered state and matedness of the surfaces, and the spatial and size distributions of asperities on the surfaces. The roughness of each bounding fracture surface is directly related to the size and spatial distributions of the surface asperities, whereas the aperture size and spatial distributions, and the contact area distribution are functions of the cross-correlation of the surface asperity spatial and size distributions. The mechanical strength and deformability of the asperities are functions of the rock type and weathered state of the bounding surfaces. It is also a well known experimental observation that the shear displacement of the bounding surfaces of a fracture, that governs fracture mating, drastically affects the aperture and contact area distributions, even for zero to moderate normal loads, for which the asperity spatial and size distributions of the individual surfaces are practically unchanged [4,5].
- North America > United States > Arizona (0.29)
- North America > United States > Texas (0.28)
ABSTRACT: The mechanical properties of an earth material are related to it's dielectric permittivity, or the per-unit dimensional extent of which an electric charge distribution in a material can be polarized or distorted by the application of an electric field. The activation of an electric charge distribution within the material is dependent on the composition of the material, and the electrical resistances of the material's components. Material composition is related to the material's mechanical properties. Dielectric permittivity can be measured through the earth material's absorption of wave energy. The degree of absorption is applied to calculating the material's physical properties such as density, porosity, and also the material's shear strength values at varying depths in terms of cohesion and internal friction angle. Dielectric permittivity is also used to calculate the moisture content and porewater pressures. The level of the phreatic surface is inferred through the permittivity in relation to the degree of absorption of wave energy, the porosity, and the degree of saturation 1. INTRODUCTION Geotechnical data was collected at a copper sulphide tailings basin, primarily through cone penetrometer testing and logging. Field data was gathered in September and October of 1997. Included was data pertaining to tailings density (¿), cohesion (c), angle of internal friction (F), and the depth to the phreatic surface (l). Multispectral imagery of the site was acquired in April of 1999. This imagery had a ground resolution of one meter, and was in 8-bit format. For each test site, pixels were analyzed on the imagery and geotechnical parameters were calculated from the pixel values, which represent the degree of wave signal remission. Results were then compared with the field data. 2. DETERMINATION OF DIELECTRIC PERMITTIVITY The relative dielectric permittivity er of the material is determined through signal reflectance at varying frequencies. By observing that the earth material to be studied is a dielectric material, and assuming negligible refraction losses of waves hitting earth material, one can ideally assume that reflectance is complementary to absorbance, or [1]: A(f) + R = 1 (1) where A(f) = frequency-dependant attenuation or signal decay (absorption), and R is the signal reflectance (or radiance), and is comprised of wave components: R = .5(Rs + Rp) (2) Rs and Rp are the reflectance/remittance of s- and p-waves of a transmitted wave signal. The value of R can be found, in terms of percent reflectance, by examining the digital number of a single picture element (pixel) of an image, which merely represents the strength of the remitted signal. Most imaging systems use 8-bit binary data formats, which means that pixel values range from 0 (no signal remission) to 2, or 256. The pixel's brightness value, or digital number (DN) will fall within this range for 8-bit data, 256 will be the brightest possible value of any pixel in the image. The dielectric permittivity of the earth material can be found through the amplitudes of the propagation vectors of the material's electric fields, if the reflectances of the s- and p-wave components of the transmitted signal Rs and Rp can be determined.
- North America > United States > Texas (0.28)
- Africa > Cameroon > Gulf of Guinea (0.24)
- Geology > Mineral (1.00)
- Geology > Geological Subdiscipline (0.70)
- Geophysics > Seismic Surveying (0.68)
- Geophysics > Borehole Geophysics (0.66)
- Energy > Oil & Gas > Upstream (1.00)
- Government > Regional Government > North America Government > United States Government (0.46)