Experimental Study on the Microwave Monitoring of Rock Stress and Fracture

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1 PIERS ONLINE, VOL. 6, NO. 1, Experimental Study on the Microwave Monitoring of Rock Stress and Fracture Zhongyin Xu 1, Shanjun Liu 1, Lixin Wu 1, 2, and Zhe Feng 1 1 Institute for Geo-informatics & Digital Mine Research, Northeastern University, Shenyang, China 2 Academe of Disaster Reduction and Emergency Management, Beijing Normal University Beijing, China Abstract Recent years many authors reported thermal anomaly change around epicenter before earthquake. Some researchers tried to predict the earthquake based on the analysis on the temperature anomaly feature by using satellite thermal infrared data. However the infrared radiation can not pass through the thick-layer cloud, so as to hardly achieve the earthquake monitoring. Microwave is also a kind of thermal radiation and some scholars have found microwave radiation anomaly appearing before earthquake. To explore the feature of microwave emission anomaly before earthquake, a group of physical simulation experiments were carried out. In the experiment, a hydraulic loader (YGA-3), whose maximum vertical load is 3 kn, was applied as loading machine for the granite samples. A microwave radiometer was used for microwave emission detection in the loading process of rock. Its detection wavelength is 8 mm, temperature sensibility.1 C. The experimental results showed that microwave emission energy changed along with the change of stress. The microwave emission energy varied little in the beginning loading stage, increased in the elastic loading stage, decreased in the plastic loading stage and increased again in the fracturing stage. The AIRT has the close relationship with the load, but the brightness temperature of microwave emission has the close relationship with the AE energy. The microwave emission precursor for rock failure is more obvious than the AIRT precursor. These experiment results indicated that it was possible to use the microwave remote sensing for monitoring the stress variation and fracturing of rock masses, including earthquakes. 1. INTRODUCTION Remote sensing is becoming an effective method to monitor the anomalous changes associated with earthquake. Gorny et al. [1] first used the remote sensing data (NOAA/AVHRR) to indicate seismic activity of middle Asia region, moreover preliminarily explored the idea and the method to predict earthquake. Afterwards, many authors reported thermal anomaly change around epicenter before earthquake [2 6]. Some researchers tried to predict the earthquake based on the analysis on the temperature anomaly feature by using satellite thermal infrared data [7]. However, infrared radiation can not pass through the thick-layer cloud, so as to hardly achieve the earthquake monitoring in the cloud days. Microwave is also a kind of thermal radiation, but its can pass though the cloud, fog and rain. Some scholars have found microwave radiation anomaly appearing before earthquake [8], and some experimental results have verified that microwave signal changes with the loading of rock samples [9, 1]. To explore further the feature of microwave emission anomaly before earthquake, a group of physical simulation experiments were carried out in this paper. And the relationship between load and microwave emission was studied. 2. MICROWAVE EMISSION DETECTION EXPERIMENTAL FOR LOADING ROCK 2.1. Experimental Design The typical crustal rock, granite was processed as standard cylindrical samples of diameter 5 mm and length 1 mm. A hydraulic loader (YGA-3), whose maximum vertical load is 3 kn, was applied as loading machine for the granite samples, meanwhile the vertical and horizontal strains were measured. A microwave radiometer was used for microwave emission detection in the loading process of rock. Its detection wavelength is 8 mm, temperature sensibility.1 C. A modern TIR imaging system, VarioSCAN 321ST with temperature precision.3 C and image resolution pixel was applied for the TIR radiation detection and image recording. The TIR imaging system was aligned in level with the rock sample and approximately 1 m away from it. Besides, an acoustic emission (AE) monitoring instrument was applied to detect the AE signal in the loading

PIERS ONLINE, VOL. 6, NO. 1, 21 62 process of rock samples. The loading was displacement controlled at the rate.3 mm/min.

2 PIERS ONLINE, VOL. 6, NO. 1, process of rock samples. The loading was displacement controlled at the rate.3 mm/min. In order to minimize environmental effects on the detection of TIR radiation from the rock surface, a aluminum box was used for enclosing both the rock sample and the load platform. The photo 1 shows the experimental scene Experimental Results The Variation Feature of Load and Strain Figures 1(a), (b), (c) and (d) are respectively the load-time curve, vertical strain-time curve, horizontal strain-time curve and AE count-time curve of the rock sample in the loading process. These curves show that the variation process of load and strains can be divided into four stages: (1) Initial loading stage ( 5 s). In the stage, load increased slowly but the vertical strain increased quickly. The original holes and cracks within the rock were compressed, so the rock sample shrunk. Photo 1 Experimental scene (left) and the sample (right) Load/k N I II III IV (a) Vertical Strain/ I II III IV (b) Horizontal Strain/ (c) AE Count I II III IV I II III IV (d) Figure 1: Detection result of several instruments in the loading process of the rock sample. (a) Load-time curve, (b) vertical strain-time curve, (c) horizontal strain-time curve, (d) AE count-time curve.

3 PIERS ONLINE, VOL. 6, NO. 1, (2) Elastic loading stage (5 128 s). In the stage, the load and strains increased proportionately, and there was not new crack to be produced. Therefore, there was not AE signal appearing in this stage. (3) Crack appearing stage ( s). In the stage, the horizontal strain increases quickly due to some new cracks producing inside the rock. Therefore, there were AE signals appearing in this stage. (4) Fracturing stage (238 s ). In the stage, large numbers of cracks appeared and the increase of load became slowly. When the load increased up to peak the sample fractured suddenly. Immediately the load decrease distinctly. After that, the load increased and decreased again, and finally the sample failed The Microwave Detection Result Figure 2 is the brightness temperature-time curve of microwave emission. The figure shows following feature: (1) In the loading stage I the brightness temperature varied little with the increase of the load. (2) In the loading stage II the brightness temperature increased steadily with the increase of the load. (3) In the loading stage III the brightness temperature of microwave emission firstly stopped to increase and kept stability, then decreased. (4) In the loading stage IV the brightness temperature turn to increase again from decrease and until the failure of rock sample. If relating the Fig. 1 with Fig. 2 it can be found that in the loading stage II, the brightness temperature of microwave emission increased with the increase of load and strain, the rock is in the elastic state. When the rock developed into loading stage III from loading stage II, some new cracks began to appear and the horizontal strain increased quickly, which lead to the volume of the rock become larger and the density begin to decrease. Thus the microwave emission stopped increases and turn to decrease. The turning point A (to see Fig. 2) was the important early-precursor of rock failure. When the rock was close to facture the load increase became slow down due to large quantity of cracks appearing. When the loading enter into stage IV from loading stage III the microwave emission stopped decrease and increase again until the failure of rock sample. The turning point B was the important later-precursor for rock failure The TIR Detection Result Past research showed that the AIRT (average infrared radiation temperature) was the valid quantitative index to represent the energy of infrared radiation in the loading process of rock [11]. Fig. 3 shows the detection result of infrared imager 321ST. It can be found that from the loading stage I to loading stage III the AIRT increased with the increase of the load. They kept the similar variation step during this time. The point C was the turn point of AIRT variation. Before the point C the increase of AIRT was slower but quicker after that. The point C was the early-precursor of rock failure in TIR detection. When the loading entered into the stage IV, AIRT stopped increase and turn to decrease. The point D was the turn point of AIRT variation. When the load increased to its peak the rock sample fractured and AIRT dropped. After that AIRT waved with the load. In this stage the point D is the later-precursor of rock failure in TIR radiation. Comparing the Fig. 2 and Fig. 3 with Fig. 1 it can be found that AIRT variation is different from the variation of the microwave emission. AIRT has the close relationship with the load, but the brightness temperature of microwave emission has the close relationship with the AE energy. The microwave emission precursor is more obvious than the AIRT precursor for the prediction of rock failure. 3. DISCUSSION ON THE MECHANISM OF MICROWAVE EMISSION OF LOADED ROCK Any substance whose physical temperature is above K will radiate microwave. According to the Rayleigh-Jeans Law the microwave emission energy is in direct proportion with emissivity and physical temperature of the object. So the brightness temperature can be expressed by following formula, T B = et (1) where T B is the brightness temperature of the object, e is the emissivity which represents the emission ability of the object, and T is the physical temperature of the object.

4 PIERS ONLINE, VOL. 6, NO. 1, Brightness Temperature/ C o A B Ι ΙΙ ΙΙΙ IV Time/s Figure 2: Brightness temperature-time curve of microwave emission. AIRT Increment/ C o Ι -.2 ΙΙ ΙΙΙ IV Time/s Figure 3: AIRT increment-time curve of TIR radiation. C D Therefore the higher the emissivity and the physical temperature, the larger is the brightness temperature of object. The parameter e is mainly decided by the dielectric constant ε. For rock material the parameter ε is related to many factors, such as mineral ingredient, water contents, rock structure and temperature etc [12]. From Fig. 1, we can see that the loading process of the rock sample can be divided into four stages. In the initial loading stage I and elastic loading stage II, the increase of the load will produce the heat and cause the physical temperature increase due to the thermoelastic effect, which result in the increase of AIRT and brightness temperature of microwave emission. So the variation steps of AIRT and brightness temperature of microwave emission are coincident in the two stages. The decreases of them in initial loading stage I are mainly due to the endothermal effect induced by the pore-gas effluent of rock. When the load enters into the stage III some new cracks will appear, and the horizontal strain and the volume of the rock sample increases quickly, which will result in the density decrease. That causes the dielectric constant increase due to the increase of vacuum volume. Past experiment showed that dielectric constant was in inverse proportion with brightness temperature, i.e., the lager the ε, the lower is the brightness temperature T B. Therefore brightness temperature of microwave emission will decreases in the loading stage III. When the load enters into the stage IV large quantity of cracks appear inside the rock. The friction-heat effect happened between cracks will produce plentiful heat and cause the physical temperature increase, which results in the increase of the brightness temperature of microwave emission also. 4. CONCLUSIONS The loading of rock will cause the microwave emission change. This change can be divided into several stages due to the staged variation of the load. In initial loading stage I and elastic loading stage II the microwave emission basically is incremental with the load increase. But when the load enter into the stage III microwave emission will stop to increase and turn to decrease due to large quantity of cracks produced. The turn point is the important early-precursor of rock failure. When the load enter into the stage IV the microwave emission stop decrease and turn to increase again due to the large quantity of cracks produced. The turn point is the important later-precursor of rock failure. The experiemtal results show that AIRT has the close relationship with the load, but the brightness temperature of microwave emission has the close relationship with the AE energy. The microwave emission precursor for rock failure is more obvious than the AIRT precursor. ACKNOWLEDGMENT This work is supported by the National Natural Science Foundations of China (No ) and the Liaoning Province Technology Project (282311). REFERENCES 1. Gornyi, V. I., A. G. Salman, A. A. Tronin, and B. B. Shilin, The Earth s outgoing IR radiation as an indicator of seismic activity, Proc. Acad. Sci. USSR, Vol. 31, 67 69, 1988.

5 PIERS ONLINE, VOL. 6, NO. 1, Tronin, A. A., Satellite thermal survey: A new tool for the study of seismoactive regions, Int. J. Remote Sensing, Vol. 17, , Qiang, Z., L. Kong, and Y. Wang, Earth gas releasing, thermal infrared anomaly and seismic activity, Chinese Scinece Bulletin, Vol. 37, , Arun, K. S. and C. Swapnamita, Thermal remote sensing technique in the study of preearthquake thermal anomalies J. Ind. Geophys. Union, Vol. 9, No. 3, , Ouzounov, D. and F. Freund, Mid-infrared emission prior to strong earthquakes analyzed by remote sensing data, Advances in Space Research, Vol. 33, No. 3, , Tramutoli, V., V. Cuomob, C. Filizzola, N. Pergolab, and C. Pietrapertosa, Assessing the potential of thermal infrared satellite surveys for monitoring seismically active areas: The case of Kocaeli (Izmit) earthquake, Remote Sensing of Environment, Vol. 96, , Qiang, Z.-J. and L.-T. Du, Earthdegassing, forestfire and seismic activities, Earth Science Frontiers, Vol. 8, No. 1, , Maeda, T., Detection of microwave signals associated with rock failures in an earthquake from satellite-borne microwave radiometer data, IEEE International Geoscience and Remote Sensing Symposium, South Africa, Deng, M., Z. Fan, C. Cui, et al., The experimental study for earthquake prediction by passive microwave remote sensing, J. Infrared Millim. Wave, Vol. 14, No. 6, 41 46, Maki, K., T. Takano, E. Soma, S. Yoshida, and M. Nakatani, An experimental study of microwave emissions from compression failure of rocks, Journal of the Seismological Society of Japan, Vol. 58, No. 4, , Wu, L., S. Liu, Y. Wu, and C. Wang, Precursors for rock fracturing and failure Part II: IRR T-curve abnormalities, Int. J. Rock Mech. & Min. Sci., Vol. 43, No. 3, , Xiao, J., Study on the dielectric property of mineral and rock and its remote sensing significance, Remote Sensing of Environment China, Vol. 3, No. 2, , 1988.

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