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ABSTRACT: A limestone for which mechanical behavior is well determined is used as a basis for a study of the effects of mechanical property variations on apparent seismic yield for contained nuclear explosions. The resulting variations in apparent seismic yield, up to 25% for this study, form an irreducible lower limit on the accuracy of seismic yield determination if further event-specific information on the mechanical properties is unavailable. 1 INTRODUCTION The need for understanding the methodologies used for verifying nuclear testing treaties has been increasing. The two predominant methods for estimating yield from foreign underground tests both rely on measurements of the stress waves created by an explosion and transmitted through rock or soil. The hydrodynamic method measures shock radius versus time near the explosion in the region where peak stresses are greater than a few GPa (King, et. al 1989). In this regime, the shock radius versus time curve can be scaled to a curve for known yield giving the unknown yield from the scaling factor. In this regime, where strength effects are negligible, only the pressure-volume response of the rock at high pressures influences the results. In contrast, the seismic methods measure the elastic wave at large ranges. Not only does the response of the rock in the high stress regime affect the outgoing wave, but the final signal is influenced by the rock response in all regimes down to the low stress levels where linear elasticity applies. In this paper we explore the sensitivity of the seismic signal to various aspects of rocks mechanical behavior. 2 REDUCED DISPLACEMENT POTENTIAL The fundamental measure of yield for seismic yield estimation is the late time value of the reduced displacement potential (RDP) (Denny and Good- man 1990, Patterson 1966). For a spherically symmetric wave in a linear elastic medium, the solution can be described by the RDP defined by: where u is the displacement, ô is the radius, c is the longitudinal sound speed, and ô = i — r/c is the retarded time (Timoshenko and Goodier 1970). The RDP has had the geometric spreading removed by the 1/r term in equation 1 and so, in the absence of attenuation, it is independent of location at distances great enough that only linear elastic response is exhibited. Typical RDP's (see Fig. 2) may be crudely approximated as a step function with some oscillations superimposed at early retarded time. Because of preferential attenuation of the higher frequencies the amplitude of a seismic signal in the far field is proportional to the value at large retarded times of the RDP calculated in the linear elastic regime near the explosion (Latter, Martinelli, and Teller 1959; Haskell 1961, Patterson 1966). Although the higher frequency components of the seismic signal give important information about the size of the non-linear region around an explosion, the apparent yield is proportional to the low frequency amplitudes which are in turn pro- portional to the asymptotic value of the RDP at late retarded time (Denny and Goodman 1990).
- Reservoir Description and Dynamics > Reservoir Characterization > Seismic processing and interpretation (1.00)
- Well Drilling > Wellbore Design > Rock properties (0.84)
- Reservoir Description and Dynamics > Reservoir Characterization > Reservoir geomechanics (0.84)
ABSTRACT: The U. S. Bureau of Mines is using several new promising techniques to study the spatial and temporal patterns of microseismicity associated with active stope mining before and after rock bursts. To illustrate these techniques, we use data from two stopes that have suffered rock bursts in a silver-copper mine in northern Idaho. One technique involves analyzing the degree of clustering of the event location pattern. The degree of spatial clustering, measured by the fractal dimension, changes before and after a rock burst for the stopes studied. The same technique can be applied to the post-burst time intervals between all event pairs, and also shows the fractal nature of the seismicity. Another technique displays the time and space context of individual events by grouping events according to the patterns of triggered stations. Finally, another technique searches for planarity in the microseismic event locations. 1 INTRODUCTION The Bureau of Mines maintains an extensive microseismic network in an underground silver-copper mine in northern Idaho for the purpose of attempting to understand and predict rock bursts. Installed as part of a cooperative agreement between the Bureau of Mines and ASARCO Incorporated, the operator of the mine, the network monitoring system is designed to continuously track and display microseismic activity in a number of areas of the mine. The network consists of 8 local nets of 16 geophones each, deployed near active stopes in the mine. Rock bursts occur in conjunction with blasting (Riefenberg, 1991) and other mining activities. These methods display and analyze patterns that develop in time and space as microseismic activity progresses. Examples of preliminary results are presented, along with descriptions of the methods. A subsequent publication will present more detailed results and conclusions. 2 THE DATA The data presented here come from two stopes (49-189 and 49-307) in ASARCO's Galena mine. Both stopes progress from the 4900 to 4600 level and have been actively mined for the last 3 years. The data are for several days' time encompassing damaging rock bursts in July 1989 and January 1990 (stope 49-307), and August 1989 (stope 49-189). Several hundred to several thousand events for each time period are included. Raw data files contain information about the time when each geophone in the local network exceeded a minimum threshold for any particular event. Additional data are derived from the raw data, such as event origin time and location in mine coordinates, time and distance to next or nearest event, total number of geophones over threshold per event, event rate, etc. These derived data, as well as the original raw data, form the basis of the information extraction techniques discussed below. 3 SPATIAL AND TEMPORAL FRACTAL PATTERNS Recently, a number of studies have shown that earthquake spatial and temporal patterns are fractal. That is, if the earthquake location or occurrence time is considered a point in space or time, there is self-similarity of the point pattern over a range of spatial or temporal scales.
- Materials > Metals & Mining (1.00)
- Energy > Oil & Gas > Upstream (1.00)
- Government > Regional Government > North America Government > United States Government (0.34)
- Well Completion > Hydraulic Fracturing (1.00)
- Reservoir Description and Dynamics > Reservoir Characterization > Seismic processing and interpretation (1.00)
- Information Technology > Data Science (0.49)
- Information Technology > Artificial Intelligence (0.49)
ABSTRACT: The delimitation of zones of potential earthquakes and the evaluation of maximum possible intensity is based on the analysis of complex tectonical and geophysical data. Statistical methods of prediction were used in the case of rockbursts. For multichannel prediction were used wiener extrapolation, adaptive filtration and autoregresive method (Bayessan solution). The efficiency of statistical prediction was discussed for various lengths of prediction operators and various members of auxiliary data channels (convergency, acoustic emision) and for both predicted series - daily number of rockbursts and daily released seismic energy. 1 INTRODUCTION According to macroseismical evidence of historical earthquakes in the Bohemian Massif, intensity I did not exceed the value of 8 MSK-64 with a focal depth up to 30 km. For number N of these earthquakes the empirical relation of Krn, k V., Schenk V., Schenkov Z, 1981. With respect to this moderate seismicity prediction are important only in localities of structures with a high degree of seismic vulnerability, e.g. nuclear power plants. The assessment of seismic hazard is based on the estimate of maximum possible magnitudes in seismoactive seismotectonic zones and lineaments. For the small number of historical earthquakes the seismotectonic methods of prognosis are much more important than seismostatistical methods. 2 THEORETICAL BACKGROUND The rock massif in the vicinity of workings is strained by both primary and secondary states of stress induced by mining activities. As a rule, workings are situated in tectonically disturbed regions, i.e. in the regions of increased rock massif heterogeneity; the concentrations of the state of stress are locally increased here, particularly in the surroundings of tectonic disturbances and structural heterogeneities. As a result of the progress of mining activities, not only is the stress state distribution in the rock massif variable, but also the orientation and magnitude of the principal stress tensor components. In a massif thus loaded there occur creep deformations as well as mechanical instabilities, i.e. brittle fracturing of various dimensions, beginning from microfracturing to major ruptures - rockbursts. Rockbursts are thus the manifestation of a gradual compensation of localconcentrations of the state of stress. With respect to the distribution of the tectonic state of stress and the degree of the massif disturbance, rockburst foci need not be necessarily confined to the immediate surroundings of workings /Pribyl A., Rudajev V., 1969/. A rockburst can be physically described as a brittle failure of rock massif, under which the accumulated deformation energy is released. This is first of all transformed into the work of disturbing the massif continuity, then into the kinetic energy of irreversible displacements in the focus and finally, into the energy of seismic waves (less than 1 % of the total deformation energy). A prominent deviator stress in the proximity of the limit state is also manifest by changes of different physical characteristics of the massif. These are mainly- changes in effective elastic moduli, changes in mechanical anisotropy, changes in the density of magnetic susceptibility, electric conductivity and coefficients of inner thermal conductivity.
- Materials > Metals & Mining (1.00)
- Energy > Oil & Gas > Upstream (1.00)
- Energy > Power Industry > Utilities > Nuclear (0.54)
ABSTRACT: A hydraulic-fracture operation was undertaken in the Inglenook oil field near Kindersley, Saskatchewan. The associated microseismic activity was recorded on three-component geophones and hydrophones installed in the treatment well and in adjacent observation wells. Calibration shots were fired in five boreholes to estimate the P- and S-wave velocities in the rock mass, orient the three-component sensors, and estimate the accuracy of two independent source-location techniques. The microseismic signals recorded after the fracturing operation were source located and used to estimate the orientation and geometry of the fracture. Preliminary results yielded a definite fracture orientation and a lower bound for fracture length. 1 INTRODUCTION The injection of fluids under high pressure into a rock mass has several scientific and industrial applications such as development of deep exchangers in hot dry rock geothermal projects, disposal of liquid wastes, and measurement of regional stress fields. In the oil and gas industry, hydraulic fracturing is carried out to increase the productivity of oil and gas wells. Better predictions of reservoir deliverability and optimization of hydrocarbon recovery could result from a reliable estimation of the geometry of hydraulic fractures. Among the different techniques which can achieve this objective, monitoring and processing of the microseismic activity associated with fluid percolation through the rock mass is considered to be the most reliable, especially for injections at great depths (Albright and Pearson 1980, Pearson 1981, Bachelor et al. 1983, Talebi and Cornet 1987). This paper summarizes the results of a hydraulic-fracture monitoring experiment which took place in June 1990 in North Canadian Oils' Inglenook field in south- western Saskatchewan. The main objective of the project was to obtain an accurate estimate of fracture orientation (azimuth) and geometry (height and length) using the microseismics technology. Two distinct methods were used for source locating the microseismic events: P- and S-wave first-arrival times on several sensors, and polarization analysis of P waves on three-component geophones. The results of these two methods are presented separately in Sections 4 and 5 of this paper. 2 SITE DESCRIPTION The Inglenook oil field is located south of the town of Kindersley in southwestern Saskatchewan. The oil-bearing Viking sandstone is 5 m to 8 m thick, is surrounded by massive shaly horizons, and has an average depth of about 700 m at the test site. The reservoir porosity ranges between 20% and 22%. The orientation of the principal components of the in-situ stress field in the western Canadian basin has been investigated by several authors (Bell and Gough 1979, Gough and Bell 1981). Stress trajectories inferred from breakout orientations in the area (Bell and Babcock 1986) indicate that the maximum horizontal stress component points in the N40°E direction. A micro-fracture done prior to the experiment showed that the minimum in-situ stress component is of the order of 15 MPa in the upper and lower shales and 9.5 MPa in the pay zone. This difference in stress magnitude will tend to contain the hydraulic fracture within the pay zone.
- Geology > Geological Subdiscipline > Geomechanics (1.00)
- Geology > Rock Type > Sedimentary Rock > Clastic Rock > Sandstone (0.34)
- North America > United States > Alaska > Arctic Ocean > Arctic Basin > Amerasia Basin > Canadian Basin (0.99)
- North America > United States > Texas > Fort Worth Basin > Bell Field (0.93)
- Well Completion > Hydraulic Fracturing (1.00)
- Reservoir Description and Dynamics > Reservoir Characterization > Seismic processing and interpretation (1.00)
Modeling Elastic Waves In Fractured Rock With the Kirchhoff Method
Nihei, K.T. (Department of Materials Science and Mineral Engineering, University of California) | Cook, N.G.W. (Department of Materials Science and Mineral Engineering, University of California) | Johnson, L.R. (Department of Geology and Geophysics, University of California)
1 INTRODUCTION Interconnected fractures can serve as major conduits for fluid flow and can significantly alter the mechanical properties of rock. Information about the location, orientation, and mechanical and hydrologic properties of fractures are therefore of great importance in many problems encountered in the earth sciences. To obtain this information, crosshole seismic imaging methods such as ray tomography (Majer, 1990) and diffraction tomography (Tura, 1990) are currently being developed. The success of seismic imaging methods for locating and characterizing fractures depends on our understanding of the interaction of an elastic wave with a fracture. In this study, the effects of spatial variations in the mechanical properties of fractures are investigated. A numerical approach based on the elastic Kirchhoff method is presented for modeling elastic wave transmission and reflection from fractures with arbitrary shapes and stiffness distributions. 2 THEORY For wave propagation problems involving reflection and transmission of waves from surfaces, it is often convenient to work with an integral form of the elastodynamic equation. The advantage of using the integral representation for seismic waves is that it does not require absorbing boundary conditions and extensive gridding of the entire medium that are necessary in finite difference and finite element methods. A potential drawback of the method is that a Green's function is needed to propagate waves between surfaces. However, if the medium is homogeneous and isotropic, the simple whole-space Green's function can be used. Numerical evaluation of equation (1) for the displacement at a point in the medium requires a knowledge of the terms appearing in the integral. When the medium is homogeneous and isotropic, the whole-space expressions for G and Ó can be used. Once the values of stress and displacement are known along S for a particular source location, the displacement can be evaluated for a receiver located anywhere in the medium. 2.2 Kirchhoff Approximation A problem with the BIEM approach is that for high frequency, three-dimensional, elastic problems typically encountered in fracture studies it requires large matrix inversions that are computationally intensive. A more feasible approach is to approximate the surface displacements and tractions using ray theory and plane wave reflection and transmission coefficients. This approximation, which is called the Kirchhoff or tangent plane approximation, is valid when the incident wave is of sufficiently high frequency (i.e., the wavelength is much smaller than the correlation distance of any variation in material properties) that locally its amplitude decay is described by geometric ray theory and plane-wave reflection and transmission coefficients (Scott, 1985). The Kirchhoff approximation has the following implications: (1) that every point on the surface of material discontinuity reflects the incident wave as though there were an infinite plane tangent to the surface at that point, and (2) that the values of displacement and traction at a point are independent of the boundary values at other points. This independence of displacement and traction between neighboring elements suggests that interface waves and multiply scattered waves will not be included in Kirchhoff method solution.
- Geophysics > Seismic Surveying > Seismic Processing (0.70)
- Geophysics > Seismic Surveying > Seismic Modeling (0.62)
Prediction of Rock Hardness And Drillability Using Acoustic Emission
Jung, S.J. (Department of Met. & Mining Engineering, University of Idaho) | Prisbrey, Keith (Department of Met. & Mining Engineering, University of Idaho) | Wu, Guanglin (Exploration Engineering Department, Chengdu College of Geology)
ABSTRACT: The performance and efficiency of drilling depends to a large extent on the rock properties and geological characteristics. To predict drillability Acoustic Emission(hereafter AE) techniques were adopted for this study. Experimentally, flat bottom punches were forced into confined rock specimens while stress-displacement curves and AE data were recorded simultaneously. Nine different types of rocks were used for this experiment to determine the relationship between indentation hardness and AE parameters such as accumulated number of events, total energy, peak mean amplitude and integrated mean amplitude. The AE signature can represent valuable information on material behavior using different parameters for assessment of mechanical properties such as strength and hardness of rock. INTRODUCTION Today's mining and tunneling machines, including drills, mainly employ mechanical methods to fracture the rock. All mechanical methods of breaking rock depend on the application of high contact stresses to the rock surface. So indentation hardness or breaking strength is also a fundamental property of rock in the field of comminution. The AE rate and the total number of events are the usual measurement parameters for determining the intensity of AE. Many investigators have used them to study failure mechanisms and mechanical properties of geological materials and to predict the stability of geological structures. Nakamura (1977) pointed out that counting the number of emission events required a more sophisticated processing of emission signals, yet information on signal strength was lost in counting. Brown (1966) conducted an investigation of microseismic activity in rock under tension using AE energy rate. It was noted that there was a direct correlation between energy emitted and strength for rocks which had grain sizes of the same order of magnitude. He considered that the energy rate was probably a more precise measure of creep deformation than AE rate. But Hardy (1972) noted that the energy data quoted by Brown and Singh should be utilized with caution since energy determinations are highly sensitive to the characteristics of the monitoring facilities. Pollok (1977) predicted that AE energy should be used as the principal parameter for quantifying emission data. In general the accumulated microseismic activity can be correlated to strain-time behavior during rock cyclic loading (Haimson and Kim, 1977). Eisenblatter (1980) noted that registration of the mean amplitude (RMS) using a high frequency volt meter is simpler and physically of more use with continuous acoustic emission. Trombik and Zuberek (1977) reported that the amplitude distribution could be considered as a particular case of energy distribution of microseismic activity. Also Schmitt-Thomas (1980) stated that amplitude analysis of AE signals offered the possibility of obtaining information of the energy of events. During indentation it was discovered that both the signature of AE rate and AE mean amplitude of rock had a regular change and might be divided into five regions corresponding to micropore closure, linear elastic deformation, micro-crack growth, fracture propagation and chipping, and post failure relaxation. The AE rate and AE mean amplitude rose rapidly and a strong burst signal was generated as the indentation stress was reached.
ABSTRACT: The viscoelastic closure of the underground cavities and the dissolution of the mine structures through fresh water flooding caused subsidence above the deep kainite San Cataldo mine, Sicily (Italy), opened in the early 60s. The present paper describes the instruments installed and measurements taken to observe the phenomena. The systems used were: automatic deep wire extensometers in the rock mass above the mine; measurements by surveying methods on the area above the cavities and in the neighbourhood; inclinometers in the slope above the mine; piezometer sensors in the mine; microseismic monitoring; remote sensing. A simple model has been taken to interpret all the various data collected, and to compute the time for the fresh water to fill all the underground cavities. 1 INTRODUCTION The San Cataldo mine is in central Sicily, near Cal anissetta. The mine was opened in 1962, and production stopped in 1979. The original owner of the mine, Montecatini, built the industrial treatment plant on a gentle slope 150 m above the salt deposit. Kainite from neighbouring mines was also treated at this plant, and so the plant went on working until 1986, when it had to be closed and fenced for safety reasons. In the late 60s a significant amount of water had started leaking into the mine from the rock mass above the salt, and the flooding had to be contained by continuous pumping. At the same time the tanks of the floating plant on the surface began tilting towards the foot of the slope. In 1983 a geotechnical, hydrogeological and rock-mechanics related study was begun in order to assess the origin of the tilting, quantify the subsidence in the area above the mine, and propose possible remedial measures. The study revealed that the phenomena were related to the behaviour of the underground cavities. Initially the elasto - plastic behaviour of the rock mass (in particular: a) the weakening process due to salt dissolution; b) the approach of failure conditions of the rock mass above the cavities) was considered to be the cause. However, after several years of observation and monitoring, it became apparent that the viscoelastic behaviour of the orebody was the main factor. The present paper describes the investigations carried out so far, the monitoring methods used and the most important results obtained. 2. BACKGROUND The surface layout of the San Cataldo mine is shown on Figure 1. The slope where the plant and the tanks are located is at 320 * 350 m above sea level. Mining was carried out on 5 main levels, from +185 to -142 m asl (Figure 2). Both the room and pillar method and the sub-level sloping method were used to mine 3 kainite layers. Mining started at the top and worked downwards with occasional filling. Between +185 and 0 m asl the 3 layers are approximately vertical. Below this level, the layers dip about 45°. The total volume of cavities created is about 2.5 Mm³, due to the high extraction ratio.
- Geology > Mineral > Halide > Halite (1.00)
- Geology > Geological Subdiscipline > Geomechanics (1.00)
- Reservoir Description and Dynamics > Reservoir Characterization > Seismic processing and interpretation (1.00)
- Reservoir Description and Dynamics > Reservoir Characterization > Reservoir geomechanics (1.00)
- Reservoir Description and Dynamics > Reservoir Fluid Dynamics > Integration of geomechanics in models (0.92)
ABSTRACT: Applied rock engineering logically forms the technological base for the dimensioning and sequencing of underground mine openings. Key factors influencing this technology have included the development of rock mass classification systems, numerical modelling and empirical design methodologies. In addition, mining induced microseismicity provides information about the reaction of fractured rock masses to mining activity on a mine wide scale and in real time. This paper discusses the general philosophy of utilizing microseismic data for calibration of numerical models in both tactical and strategic mine design. 1 INTRODUCTION The geology of a deposit and its structural characteristics present constraints which, combined with available mining equipment capabilities, determine how a deposit should be mined. The most important geomechanics contributions to the mine design process are made in the dimensioning of stopes and pillars and in the control of energy release related to the sequencing of excavations. Until very recently, the dimensioning and sequencing of openings was based almost strictly on experience. Over the last decade, however, the science of rock mechanics has evolved to the point where it provides a sound framework for underground design. Considerable uncertainty remains in the predictions from rock mechanics analyses since the mechanical behaviour of large fractured rock masses are not well understood. The constitutive parameters used to characterize the rock mass response are specific to a particular mine and therefore field monitoring is essential for verification purposes. During the past 15 years applied rock engineering has matured into an operationally useful tool for the mining industry. The function of rock engineering personnel is to prevent unexpected failures in order to improve safety and efficiency at operations and to optimize resource recovery. Rockbursts represent probably the most serious mode of unexpected rock mass failure and are of particular concern in deep hardrock mining. Research and development into the rockburst phenomenon has been ongoing for over 40 years. The development of microseismic monitoring systems has led to a better understanding of this phenomenon. Considerable research has been directed toward using such systems to predict rockburst occurrences in real time. The results of rockburst prediction research have often proven to be less than satisfactory. Over the same period numerical stress analysis tools have also evolved rapidly. Initial hardware and software limitations have largely been overcome and today these techniques offer powerful tools for the analysis of mine induced stress redistribution and attendant stability problems. It is generally accepted that major mine induced rock engineering problems can only be realistically addressed through the design process. Many investigators now believe that this holds true for both standard rock engineering problems and for rockburst occurrences. To achieve this will require real time monitoring of large volumes of ground and subsequent interpretation of this data in terms of mining stress redistribution and the influence of key geological structure. Microseismic monitoring technology represents one of the few rock mass instrumentation methodologies capable of sampling large rock volumes in near real time. The largest microseismic systems can monitor an entire underground operation.
- Europe (0.93)
- North America > Canada > Ontario (0.68)
- Materials > Metals & Mining (1.00)
- Energy > Oil & Gas > Upstream (1.00)
- Well Completion > Hydraulic Fracturing (1.00)
- Reservoir Description and Dynamics > Reservoir Characterization > Seismic processing and interpretation (1.00)
- Reservoir Description and Dynamics > Reservoir Characterization > Reservoir geomechanics (1.00)