Investigation Of The Frequency Spectra Of Microseismic Activity In Rock Under Tension

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... I n most studies, however, amp tude and energy data are presented in arbitrary units (where it assumed that k = 1 and k,= 1). Assuming that n microseismims occurrl during a specific time interval (At), then the apparent accumulat~ energy is given by : and I t should be noted that accumulated activity (N) and noise rate (N1 are dependent on the sensitivity of the monitoring system and its sign; 4 to-noise ratio. The parameters, amplitude (A), apparent energy ( E # apparent accumulated energy (SE) , and apparent energy rate ( [ER] : are similarly dependent on sensitivity and the signal-to-noise ratio b are also dependent on the frequency response of the overall monitorii system and the frequency spectra of the microseismims themselves. ...
... 3) Load on the specimen was increased incrementally every 40 min up to specimen failure. 4) Between load changes, the resulting axial and transverse creep strain, load fluctuation, and microseismic activity were recorded. A timing marker imposed simultaneously on the two strip chart recorders l and the magnetic tape recorder served to synchronize all recorded datai. ...
Since 1964, the Dept. of Mining at the Pennsylvania State Univ. has been conducting extensive studies associated with the phenomena of microseismic activity and inelastic behavior in geologic materials. The program to date has involved the following stages: (1) study of the microseismic noise and energy rates for selected geologic materials under incremental, uniaxial tension; (2) investigation of the frequency spectra of microseismic activity of selected geologic materials under incremental, uniaxial tension; (3) investigation of the time-dependent inelastic deformation of geologic materials and the analysis of such deformation in terms of mechanical models; and (4) simultaneous study of microseismic activity and time-dependent inelastic deformation in geologic materials under incremental, uniaxial compression. Details of the research involved in the fourth stage of the program constitute the subject of this study. Experimental data are presented on both the microseismic activity and the inelastic behavior of 3 rock types (sandstone, limestone, and granite). The inelastic parameters associated with the axial data are computed and are presented in tabular form. (43 refs.)
... Because frequency content in an AE is determined by the wave source, studying the dominant frequency characteristics provides significant controls on investigations of microscopic fractures. Some of the pioneering work that investigated the dominant frequency characteristics of rocks was performed based on spectral analysis (e.g., Chugh et al. 1968;Armstrong 1969). Later, numerous relevant studies were conducted that evaluated the dominant frequency characteristics of AE waveforms (e.g., Stephens and Pollock 1971;Fleischmann et al. 1975;Woodward 1976;Kranz 1979;Sondergeld et al. 1980;Read et al. 1995;Xu et al. 2010). ...
... The frequency characters of Acoustic Emission (AE) have been studied by some experts, and some results have been obtained about dry rock samples with loading [1][2][3][4][5]. Y.P. Chugh presents that the spectrum character is expressed by two main frequency bands which are 0 kHz-6 kHz and 10 kHz-15 kHz, or is expressed by main energy band [6]. While R.M. Koerner finds that the main frequency interval is between 0 kHz-50 kHz, and it has the trend of shifting to the low frequency with adding stress [7]. ...
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In order to predict the rock rupture and instability, acoustic emission signals of natural sandstones and hydrous sandstones are analyzed through the Fast Fourier Transform and the Welch method. It is found that the acoustic emission signals of the hydrous sandstones have narrower range of frequency spectrum and lower average dominant frequencies than the natural sandstones. The dominant frequencies of the acoustic emission signals of the hydrous sandstones reduce more dramatically. In addition, two types of Welch power spectrum of the acoustic emission signals in terms of the shape are presented in this paper, which are named Type A and Type B. The Type B Welch power spectrum emerges when the acoustic emission occurs frequently, in which a disaster tends to appear. Time baselines are selected when relative stronger cracks and crack coalescence happens. We find that the greater signals energy at the time baseline, the higher probability of occurrence of Type B spectrum appears before the time baseline. Also, the probabilities of Type B before the corresponding the baseline time of rock last rupture in hydrous sandstones are higher than that of natural sandstones.
Microseismic Techniques — Basic and Applied Research. The phenomenon of microseismic activity appears to provide the basis for one of the most useful tools presently available in the field of rock mechanics. During the last six years the Rock Mechanics Laboratory at The Pennsylvania State University has been involved in studies related to this phenomenon, and the present paper describes a number of the basic and applied research studies undertaken. Emphasis however will be on the applied aspects of these studies, including the use of microseismic activity in the laboratory to define failure in pressurized gas storage reservoir models, and in the field to study underground gas storage reservoir stability and to evaluate coal mine strata control techniques. A mobile laboratory has been under development in conjunction with the two field programs and a brief description of this laboratory is included.
During the past four years experimental and theoretical studies of the fracture and flow of rock have provided important new insights into the dynamics of geologic processes in the crust and upper mantle. In addition, intensive new observational studies, particularly along the San Andreas fault in California, have yielded new information on the mechanism of earthquakes and faulting. Of particular interest are: (1) work on the friction characteristics of rock under crustal temperature and pressure conditions and its application to the mechanism of faulting; (2) use of laboratory experiments as scale models of seismicity with applications to earthquake statistics; (3) recognition and measurement of fault creep over extensive portions of the San Andreas fault system; (4) use of geologic, geodetic, and seismic data for the determination of rates of movement and strain accumulation on the San Andreas fault; (5) observations and experiments with earthquake triggering; (6) studies of the creep of rock at high temperatures and pressures.
Acoustic emission generated during the uniaxial compression of brittle rocks were counted after band-pass filtering. Throughout the whole process from application of load to failure, the count rate monitored through a low pass window was compared statistically with the monitored through a high passs window. The emission rate monitored through a low pas window increases more rapidly than that monitored through a high pass window as rock approaches failure. Two possible explanations for this effect are (1) generation of larger cracks (or coalescence of cracks into larger sizes) and (2) the relative attenuation of higher-frequency wave components. This finding is consistent with past observations, i.e., the relative number of large-amplitude events increases (i.e., a decrease in b value) as rock approaches failure. We suggest that larger cracks tend to generate events with larger amplitudes and containing lower-frequency components. The frequency characteristics of acoustic emissions are used to characterize the microfracturing processes leading to failure. The beginning of a sequence of acoustic emission activity is discussed in relation to the stress-strain curve and the onset of dilatancy. The present observations may be utilized in understanding structural instability of a region where rockbursts or earthquakes can potentially occur. If a sizable microseismic population is observed through sensors placed near the fault, then a potential exists for predicting major earthquakes.
Fracture is perhaps the most important topic in rock mechanics today, primarily because it is the dominant mechanism of rock failure at the relatively low pressures and temperatures at shallow depths in the earth's crust and because existing fractures strongly influence the physical and mechanical behavior of the rock mass. Fracture considerations therefore pervade all of man's interests dealing with exploration and production of natural resources, engineering projects under or on the earth's surface, and the prediction, modification, and control of earthquakes. For example, (1) estimates indicate that about 5% of the energy generated in the United States is consumed in fracturing rocks [Lewis, 1966]; (2) in the petroleum industry an understanding of fracture is essential in relation to `fracture porosity' reservoirs, hydraulic fracturing, secondary recovery programs, structural inferences from existing fractures, underground storage of gas, and drilling technology; (3) in certain geothermal projects, fractures serve both as subsurface permeability channels and as surfaces to heat circulating waters; (4) the in situ `mining' of oil shale or of the gasification of coal is primarily a matter of man's ability to fracture the rock mass under controlled conditions; (5) in mining and tunneling operations, fractures influence the stability of the openings as well as the extraction of rock or ore; and (6) recent developments in earthquake research have focused attention on fracture (dilatancy) as the source of a host of premonitory events from which it may someday be possible to predict earthquakes [Nur, 1972; Whitcomb et al., 1973; Scholz et al., 1973]. Accordingly, there is ample mission orientation for even the most basic research designed to gain a better understanding of fracture processes in rock and of the mechanical behavior of rock masses containing fractures.
Tensile tests were conducted on cylindrical rock specimens, cemented to end-caps attached to nontwist cables. In other tests, the rock specimens were bonded to platens cycled between tension and compression. The results indicated that the initial tangent modulus of rock is similar, both in uniaxial tension and in compression, but in tension the modulus decreases with increase of applied load; whereas, in compression, the modulus increases up to the stage of incipient failure. The value of the 50% modulus in compression is usually greater than the value in tension. At very low stresses, the Poisson ratio of rock in tension can be greater than 0.5, but after load cycling, the ratio is around or less than 0.1. (17 refs.)
A review of the application of emission analysis to evaluate materials properties and defect structure is presented. Topics discussed include fracture toughness and crack propagation, fatigue, plastic deformation, and creep processes in metals, composites, and rock materials.
The nature and sequence of deformation in the region of the maximum stress were investigated in specimens deformed in uniaxial stress compression at room temperature and at constant strain rates from 10−4/sec to 103/sec. Microscopic observations show that no visible fracture or other evidence of deformation takes place until > 99% of the average maximum stress at a strain rate of 10−4/sec is attained by the granite. Detailed examination of an incipient shear fracture in a granite specimen suggests that the fracture is formed by the coalescence of links between en echelon inclined grain boundaries and cleavages. Photographs taken during loading in the split-Hopkinson bar device illustrate that macroscopic shear fractures develop before extension fractures in the sequence of events leading to gross failure of the specimens.
Acoustic emissions generated during creep of rocks in the brittle regime were counted after band-pass filtering. The count rate detected through a low pass window was compared statistically with that detected through a high pass window throughout the entire process—from application of a constant load to failure. The emission rate detected through a low pass window increased more rapidly than that detected through a high pass window as failure was approached. Two possible explanations for this effect are (1) coalescence of cracks into larger sizes and (2) the relative attenuation of higher-frequency wave components. The cumulative number, N, of emission events during creep is represented by (1) N = A log (t+1) + N0 (0⩽t⩽tb) and (2) N = A log (t+1)+B(t−tb)β + N0 (tb<t⩽tc), where A, B, β and N0 are constants, t is time, tb is the time when the observed cumulative number begins to deviate from equation (1), and tc is the time when the cumulative number begins to deviate from equation (2). It is concluded that crack-interaction cracking begins to occur at t = tb, and that the critical crack density has been achieved at t = tc. A theoretical explanation for equation (1) can be given on the basis of reaction rate theory. A correlation between the cumulative number of emission events and inelastic volumetric creep strain is examined. The correlation is found to be fairly good in the primary and secondary phases; however, it is poor in the tertiary phase. The non-proportionality of the cumulative number of emission events to the inelastic volumetric strain in the tertiary phase is discussed.
The author discusses the significance and measurement of the frequency and shape of acoustic emissions. He considers theoretical descriptions of acoustic emission and compares them with experimental evidence. He considers the implications of this understanding of the nature of acoustic emission and illustrates these implications by a description of a programme of investigation at AERE Harwell. This programme added to our knowledge of acoustic emission and the author considers the results in the light of other experiments. He concludes that spectral analysis could complement the use of amplitude distribution and ring-down counting but not enough is yet known of the general meaning of the spectrum of an acoustic emission.
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