| Infrasound(the frequency of the sound is less than 20 Hz) has the characteristics of low frequency, easy penetration, low attenuation, and it is not easy to be absorbed, it can spread within a far distance in the atmosphere, and the signal is more suitable for long distance measurement, researches and applications have been carried out in the disasters monitoring field of nuclear explosion, volcanic eruption, debris flow and others. It has achieved great effect especially in the aspect of active monitoring(volcanic eruptions, the gas pipeline leakage and others), however, the wavelength of the infrasound is longer, the diffraction and avoidance usually happened when it encountered with the obstacles, so the location has a great difficulty to the monitoring for passive disaster, the further step research and analysis are still needed. And so far, the mechanism of infrasound is still a mystery, it is necessary to carry out in-depth research and analysis on it from different perspectives, this is also the goal and direction for the workers of infrasound researches.The disaster of infrasound is generated by the airwaves caused in the late pregnant and the occurrence and development of disasters. The disaster of infrasound can be divided into several kinds, one is triggered in the late pregnant of disaster before it happened, for example, the pre-slip before the earthquake and mine earthquake,the long period wave caused by the micro rupture made by the extension, named infrasound before the disaster; the other is the infrasound generated by the strong airwaves caused by the occurrence and development stage of the disaster, named the infrasound in disaster, such as the nuclear explosion infrasound, fireball infrasound and others, it can provide help for the work of identification and verification through the monitoring and positioning research. There is a special class in the infrasound in disaster, such as the debris flow infrasound, the typhoon infrasound and others, the spread speed of the infrasound(about 340 m/s) is larger than the speed of debris flow(about 10 m/s), the typhoon(12grade, 33 m/s) and other disasters, it can also achieve the purpose of warning when it monitored the infrasound in disaster. Therefore, this kind of infrasound in disaster and infrasound before disaster are all the warning infrasound.Infrasound monitoring is mainly IMS international infrasound monitoring stations. The distance between stations is two thousand kilometers, and is too sparse, signals are difficult to be captured by two or more infrasound monitoring stations, and the signal received is the local signal, signal source is not clear, it is difficult to used to fix the position of the signal and study the relationship, and even if some signal source is clear, but are interfered by local and outside noise, the signals are very weak. On the basis of the noise, the judgment of the signal is a difficulty. At present, many countries actively create their own infrasound station monitoring system, but are relatively scattered, does not form a scale effect, and are lack of historical data of the infrasound monitoring. Problems of positioning the passive monitoring and the problems of monitoring signal not easy to identify ultimately are the problems of sound source. The international and the domestic scholars tried to research from the aspects of propagation path and the receiving part, but had little effect. We carry out the research work from the three aspects theoretical research, physical experiment and numerical simulation, and find the riddle of sound source mechanism. Based on it, we start from the aspects of numerical simulation, and accumulate experience for subsequent physical experiment and theoretical research. This article uses the finite element method to do the numerical simulation research for the source of explosion, mine earthquake and earthquake. Explosion is designed to simulate the instantaneous process of rock-burst. Mine earthquake is designed to simulate the micro rupture process prior to the earthquake and rupture process of earthquake. Set Wenchuan earthquake as an example, base on the actual situation to simulate the infrasound caused by the geological changes at the end of earthquake. The simulation includes the infrasound before disaster and the infrasound in the disaster. Use the simulation to study the mechanism and influence factor of the infrasound, from the aspects of mechanism, analyze and compare the common and the particularities of the infrasound before disaster and in disaster. Analyze and compare the waveform, amplitude, time domain and frequency domain, energy density and other characteristics. This work mainly includes the following six aspects:1. Based on the acoustic wave equation, we consider the boundary conditions of the coupling of geological body and interface between troposphere atmosphere, and make the geological body and atmospheric troposphere discrete by the finite element to make the matrix equation, derive motion differential equation of geological body and atmospheric troposphere coupling.2. Study the composition and structure features of infrasound monitoring collection system and the current mainstream sensor for infrasound band and its disposal ways, compare and analyze normal signal, the jamming signal and abnormal signal of infrasound.3. We use the finite element method to simulate the infrasound of the chemical explosion process, analyze amplitude, frequency, waveform generated by air and rock in the process of explosion. And change the loading time, the shape, size of explosive, the parameters and variables in equation of state, the state variables to change the influence of the amplitude and frequency of the sound waves.4. We use the finite element software ANSYS/LS-DYNA to simulate infrasound before disaster of the micro fracture process and the infrasound in disaster of the rock burst. Analyze frequency, waveform, Wigner-Ville distribution of both infrasound, and look for the common and the peculiarities of the infrasound before disaster and infrasound in disaster.5. We use the geologic features of the Longmen Mountain fault zone, and study the geologic body movements before Wenchuan earthquake on May 12, 2008 by the finite element method and simulate infrasound before earthquake at the end of the imminent earthquake preparation. Analyze the features such as amplitude, frequency, waveform, and are compared with the actual monitoring earthquake infrasound abnormal signal in the area.6. By studying the relationship of the micro fracture and damage and the relationship between damage and acoustic emission, it shows that acoustic emission and rock damage has consistency, rock acoustic emission is external manifestation of internal damage. Again from the perspective of crack growth, the paper analyzes the acoustic emission that consists of long period of low frequency sound, and close to the advantage of burst acoustic emission frequency will be concentrated in the low frequency part. |
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