Pick should be based on a set of experimental results, this paper discusses the heterogeneity of rock specimen (micro crack and macroscopic crack) deposited on the effects of time and space distribution of acoustic emission. Comparative analysis shows that the existence of microcrack deposited before making rupture nucleation acoustic emission rate showed a rapid increase, b value short-term anomalies, which are falling and falling background, thus increasing the burst time can be forecast. The existence of the macro structures (joints, level, etc.) and b values for acoustic emission rate and the influence of micro cracks, and the macro structure has a control function on the spatial distribution of acoustic emission and acoustic emission cluster of macroscopic tectonic controls the main failure of the future. Usually this means that the existence of macro structure makes the winner of the rupture time and location prediction possible.
Micro cracks keywords acoustic emission heterogeneity macro structure
1 lead it
Since the 1960 s, the deformation of rock acoustic emission research always is one of the main research direction of experimental seismology, domestic and foreign scholars conducted a lot of research in this aspect, the content involves the acoustic emission time sequence, spatial distribution, b value and spectrum changing with time, fracture source mechanism (such as: Lockner [1], hai-kun jiang and Zhang Liu [2]). The experimental results show that the rock deformation in the spatial and temporal distribution of acoustic emission is influenced by many factors, such as rock properties, loading mode and history, confining pressure and temperature, the heterogeneity of the rock and pre-existing structures, etc. Among them
Test specimens of heterogeneity is one of the important factors affect the time and space distribution of acoustic emission. For example, early experiment studies have proposed the degree of inhomogeneous medium is the main factor that decide the size of b value [3, 4], the subsequent experiment results show that obstacle, pre-existing fault (crack) and so on also have influence to b value [5], recent studies have showed that the structure of deposited differences than mechanical condition has more important influence on the size of the b value [6, 7]. The heterogeneity of the specimen (a variety of pre-existing cracks, obstacle or inclusions, etc.), the influence of the spatial distribution of acoustic emission time series is also very obvious [6 ~ 15].
In view of the actual medium and structure of heterogeneity in the crust, studies the influence of heterogeneity on the acoustic emission characteristics, to understand the physical significance of the seismic activity image is very useful. However, due to differences in experimental conditions, laboratory equipment, etc, according to these results is difficult to discuss specimen heterogeneity on the effects of time and space distribution of acoustic emission.
Recent industrial technology research institute in Japan with the same set of experimental equipment and in the same experiment conditions, carried out the study of non-uniform acoustic emission characteristics, made some new results [16 ~ 18]. Based on these results, this paper focuses on micro and macro structures deposited the influence of the spatial and temporal distribution of acoustic emission and its significance in understanding the seismic activity image.
2 experimental methods and results
Involved in this paper, the experimental specimens including granite porphyry, granite, contain healing joint granite, shale containing quartz vein and contain healing joint granite porphyry. The specimen is a diameter of 50 mm and 100 mm length of cylinder, on both ends of the specimen surface and briquetting deployed on 20 ~ 30 diameter of 5 mm piezoelectric sensors to record sound
Launch event waveform, 6 groups in central specimens also paste the strain gauge to measure the specimen deformation. The acoustic emission signal, sampling rate through a set of 32 channels 20 MHZ, resolution of 12 bit, can record thousands of full waveform acoustic emission events per second digital observation system, the use of acoustic emission events of P wave and then consider the change of wave velocity, can be carried out on the acoustic emission source rupture location and location accuracy for 2 ~ 3 mm. In addition, there are two peak detector on the system to record the maximum amplitude of acoustic emission events, can be calculated acoustic emission value and the incidence of b. B value calculation using the maximum likelihood method, calculation window for 500 events, sliding step length for 125 events. Experiments are conducted in a liquid medium triaxial container and the stress rate loading mode, etc. The experimental equipment and methods can be found in the literature [19].
2. 1 the experimental results of granite porphyry
Granite porphyry in the main mineral composition is quartz, plagioclase, potassium feldspar, and particle size distribution range of 1 to 10 mm, the average particle size 5 mm. Microscopic observation showed that this almost without pre-existing micro crack in the rock is shown in figure 1 (d), the rock has a larger initial wave velocity (6. 15 kmPs) and wave velocity with the increase of confining pressure, slow the fact proved that [17]. Experiment is surrounded
For 60 MPa pressure, such as 6 ~ 7 MPaPmin loading specimens rupture stress rate.
Fracture stress of specimen is about 780 MPa, when stress around 450 MPa began to produce acoustic emission, but before the main rupture occurred only recorded about 2000 acoustic emission events, and in the process of dynamic fracture recorded about 2500 acoustic emission events; With 45 ° Angle in the direction of maximum principal stress strain increases linearly with time, that rupture specimens without obvious dilatancy before (figure 1 a, chart Ⅰ). The incidence of rupture occurred before the acoustic emission is always very low, with the increase of stress increased only slightly, and with the occurrence of dynamic fracture incidence of acoustic emission increased dramatically; Acoustic emission value before the burst b showed increasing trend with the increase of stress (increased by 0. 7 to 1. 0), the main failure happens quickly rose to 1. 4 is shown in figure 1 (b); Near the dynamic rupture occurred before, acoustic emission values change relatively stable b (figure 1 c). The main rupture before the acoustic emission incident random distribution in the space, the characteristics of a small cluster phenomenon has nothing to do with the final main fracture surface is shown in figure 1 (e). These phenomena show that the granite porphyry before the dynamic rupture nucleation process is very brief, rupture occurs in the form of almost impossible to predict. 

2. 2 experimental results of granite
Granite mineral composition and particle size distribution and the granite porphyry, the difference is the granite contains a large number of micro cracks, micro cracks including intergranular and transgranular crack and transgranular crack see chart Ⅰ (FIG. 2 d), the rock has a smaller initial wave velocity (4. 60 kmPs) and wave velocity increases quickly with the confining pressure increased to 60 MPa fact also proves that the existence of the micro cracks [17]. Experimental condition with the same granite porphyry.
The fracture stress of the specimen is about 500 mpa, when the stress reaches about 200 mpa began to produce acoustic emission, a large number of acoustic emission appeared after the stress of 350 mpa, the main rupture before produced more than 50000 acoustic emission events, with a large number of acoustic emission events should be relative, specimens showed obvious dilatancy (figure 2 a, possesses the Ⅰ); Acoustic emission rate increased significantly with the increase of stress, ae b value changed little in the early days, near the main rupture before shows the short-term fluctuations on the long-term decline in the background (FIG. 2 b). Especially near the dynamic
Rupture before b value by 1. 2 sharply dropped to 0. 75, accordingly, the acoustic emission rate is rising rapidly (FIG. 2 c). The main failure occurred before the acoustic emission events in space are mainly concentrated in a few scale cluster area of around 10 mm, reflecting the pre-existing crack and mineral grain boundary control of the micro fracture effect, but the cluster phenomenon, with no instructions for main failure eventually significance (figure 2) e. Obviously, the acoustic emission b value and the incidence of precursory sex change, makes rock main failure time prediction can be enhanced. 

2. 4 shale containing quartz veins of the experimental results
Shale specimens bedding development adopted in the experiment, and contains two quartz vein, the experiments of shale stratification plane and the specimens with axial (that is, the maximum principal compressive stress direction) into 25 ° to 30 ° Angle, so as to make bedding potential failure surface, and bedding plane conjugate quartz vein is the potential failure surface (figure 4 c, possesses the Ⅱ). The confining pressure of 80 mpa in the experiments, such as 3 mpapmin loading specimens rupture stress rate. Eventually formed a main failure along the bedding plane distribution, but also formed the article number of the secondary fracture, also controlled by bedding (FIG. 4 c).
The fracture stress of the specimen is about 400 mpa, acoustic emission concentration appeared before the stress close to the rupture stress rupture nucleation stage of about 20 s (figure 4 a marked "N"), its incidence rate increased rapidly until the dynamic rupture occurred (figure 4 b). At this stage, the axial stress drop, each part of the specimen are expanded, of which the most obvious expansion and quartz vein near the main failure (figure 4 b epsilon V3). Acoustic emission value b showed obvious fluctuation under the background of falling, on the contrary, the change tendency and acoustic emission rate in dynamic fracture occurs at the lowest (about 0. 7) (FIG. 4 b). The main cluster on the acoustic emission events in space in the main fracture plane and two quartz vein line (FIG. 4 d). This suggests that relatively weak bedding plane controlling the position of major rupture, whereas relatively strong quartz vein controlled rupture process (to play the role of a concave and convex body). Acoustic emission b value and the incidence of precursory sex change and expansion and precursory sex of quartz vein make possible winner rupture time prediction.
3 discussion and conclusion
The influence of the micro structure of deposited 3. 1
The experimental results show that the granite porphyry and granite have obviously different damage characteristics: granite porphyry has high rupture strength and rupture before the acoustic emission activity is very weak, acoustic emission b value increases with stress, acoustic emission source random cluster in the spatial distribution characteristics, shows that cracks produced in unpredictable ways; Acoustic emission activities before the granite fracture with stress increased rapidly, the acoustic emission b value is decreased, with the increase of stress at peak stress at the minimum, then show the drop on the background of ups and downs, acoustic emission source in spatial distribution is
Characteristic scale (~ 10 mm) clustering features of acoustic emission rate has increased dramatically and b before rupture value of short-term fluctuations can be used as a precursor to predict rupture occurred. Considering the two kinds of rock on the mineral composition and particle size distribution are exactly the same, the difference is only little granite porphyry micro cracks and granite contains a large number of micro cracks, therefore, these differences can only be attributed to the role of micro cracks pre-existing. One possible explanation is that the density of micro cracks and excluding pre-existing granite porphyry internal stress distribution is more even, could have was mainly distributed in the fracture before the acoustic emission point defect control between different minerals, so acoustic emission rate is low and the spatial distribution of random, only when the applied stress to the rupture strength of the rock, just can have more micro fracture appear at the same time. And the micro cracks of granite deposit inside micro cracks caused by stress concentration due to different scales, the stress distribution is very uneven, and the non-uniform will increase with the increase of applied stress, caused the breakdown of the large number of acoustic emission and acoustic emission rate increased; Under high stress level caused by the interaction of crack crack etc make the level of the merger, connections between large increase in the number of acoustic emission events, thus making acoustic emission value before the burst b decline trend, and b value fluctuation may interact with different area crack occurred in different time.
Liu [16], such as using the same equipment and experimental method of another kind of granite experiments show that the acoustic emission rate increases with stress before fracture, increase rate between the above two kinds of rock, and the density of micro cracks in the rock in between. Lei, etc. [20] on two different kinds of particle size distribution and crack density quite a granite experiments show that the
Characteristics of acoustic emission activity over time of basically the same. These results indicate that the micro cracks on the failure process of rock specimen has important role in the control of the existence of a large number of micro cracks can make acoustic emission before rupture nucleation rate increased rapidly, and shows the short term anomalies of b value, namely, b value under the background of the overall decline in ups and downs, so as to make the rock failure time prediction can be enhanced. The existence of microcrack also makes the fracture of the spatial distribution of acoustic emission events before by random cluster into controlled by micro cracks in the cluster, but as a result of the penetrative distribution characteristics of micro cracks, it
Kind of cluster characteristics of main rupture position, there is no instruction significance.
3. 2 the influence of the macro structure deposited
Dense granite porphyry and granite fracture containing micro cracks on the spatial distribution of acoustic emission events before all has little to do with the main burst, and acoustic emission events before contain healing joint granite burst main clustering on the space near the joint surface, and shale specimens containing quartz vein rupture acoustic emission events before the main clustering on the space in the main fracture plane and two quartz vein, persisting macro structure shows that control the space distribution of the acoustic emission and acoustic emission cluster of pre-existing structures indicate the main failure of space position in the future. From the point of time distribution, contain healing joint specimens of granite and quartz vein of shale share common features, namely, acoustic emission main cluster shorter period of time before the main rupture, acoustic emission b value under the background of falling showed obvious fluctuation and change trend is the opposite of acoustic emission rate. This means that the healing of macro joint exist that make the main rock burst forecast possible location and time. Obviously, from the point of time characteristics, pre-existing micro crack and macroscopic deposited structure of acoustic emission activity is the same. The influence of pre-existing tectonic effect on the control of the spatial distribution of acoustic emission is easy to understand, because of pre-existing tectonic deformation belt, the new micro fracture are bound to cluster with deformation concentration belt. Pre-existing tectonic characteristics of acoustic emission time distribution, the influence of possible explanation is that because of pre-existing tectonic belt within the medium intensity is relatively uniform, therefore, micro fracture (ae) event focused on the cracks within a relatively short period of time before; Due to micro fracture height clustering on the space, in which the interaction of intense, caused the fracture increased incidence of acoustic emission, a loud before launch event significantly increased the trend (b value), and acoustic emission rate and b value of volatility and pre-existing tectonic take geometry or caused by the inhomogeneous medium is concave and convex body.
3. 3 the seismological significance of experimental results
Time and space distribution characteristics of seismic activity is one of the most important parameters in earthquake prediction, but the actual earthquake cases show that seismic activity image is quite complex) before a strong earthquake [21]. So the factors affecting seismic activity image and its physical significance of earthquake prediction research is very important. From the experimental results concluded that even without the rock structure of deposited under extreme conditions, based on the seismic activity time and place, it is difficult to forecast earthquake happened in the rock penetrative structure (similar to the micro cracks) conditions, according to the b value and the incidence of earthquake precursory changes can accurately forecast the time of the earthquake, but the location prediction is difficult, in the rocks there are large-scale heterogeneous structure (fault, level, medium interface, etc.), under the condition of not only according to the incidence of b value and seismic can accurately forecast the time of the earthquake, can also according to the spatial clustering characteristics of seismic activity to predict earthquake location. Of course, the actual situation in the earth's crust is quite complex, in many cases may need to comprehensive consideration of various factors, to make more accurate judgment.
References (abbreviated)