| The seismic wave carries a wealth of information about the physical properties of the subsurface when propagates in the earth medium. It is the most important research tool for us to get knowledge of the Earth’s interior structure, material composition and state, and so on. Through studied on the seismic waves excited by earthquakes, the human being has made tremendous progress in understanding the global distribution of earthquakes, the spherical structure, geodynamic process, material composition, lateral heterogeneity, etc, about the Earth’s interior, which sketches out for us a relatively complete three-dimensional static image of the overall Earth’s structure. However, it is still not meeting the requirements, when faced with serious earthquake disasters, strong volcanic eruptions and other issues, what we needs is not only the static images of the Earth’s interior, also to understand and master the process of change in the Earth’s interior.Now many scholars used the repeated earthquake, noise and artificial source to study the dynamics monitoring of the subsurface in domestic and foreign. As the low frequency of earthquake occurrence, especially the low accuracy in the depth of the earthquake location, and the occurrence time and location of the repeat earthquake can not be controlled, so it is not suitable for carry out monitoring in long-term. The energy of the noise source is very weak, needed a long time to stack for get reliable measurements. The resolutions and precisions of passive source monitoring are limited by the spatial and temporal distributions of source, but the artificial sources with precise timing, location and characteristics can be measured, and likely to achieve highly repeatable. In addition, the observation system can achieve high accuracy detection of changes in travel time with flexible and intensive, expected to compensate the lack of the natural source in accuracy with the precise time and location. With the development of the artificial source detection technology, monitoring subsurface changes with the seismic wave generated by repeatable artificial source has become another goal to pursue.In this dissertation, combined with the need of dynamic monitoring subsurface on regional scale, we carried out research focus on the key issues of the dynamic monitoring technology with active source, which are the signal to noise ratio (SNR), repeatability of the seismic source and accuracy in measure velocity change. Based on comprehensive analysis and summary of the existing theoretical research achievements and experiment results, we adopted fixed stations monitored for earthquakes and portable seismic station with flexible and intensive to receive signal, conducted comparative analysis on several high-power artificial source and select two sources representative for continuous and pulse sources, respectively. We constructed the corresponding high-performance dynamic monitoring system with Accurately Controlled Routinely Operated Seismic Source (ACROSS) and large volume airgun, and studied on field experiments. We observed some information of velocity change, obtained some meaningful results, and discussed the reasons for seismic-wave velocity changes, the experimental measurement accuracy, the problems in the method that need further refinement and study, and practical applications of the exploration for seismic-wave velocity changes in the medium. The main researches of this dissertation can be divided into the following four aspects.First, the detecting principle used active source on regional scale. We introduced the principle and method of dynamic monitoring technology with active source, developed the precise measurement method in velocity changes contrary to the low precision used the traditional methods, and took the observed atmospheric or tidal change by theoretical calculation as a continuous calibrated and test signal. On the aspect of building up the active monitoring system, we take the hand from the signal excitation, propagation, reception, signal analysis and other aspects, given out the technical requirements for carry out dynamic monitoring with artificial source on regional scale, and conducted matching analysis between the active source excitation and signal reception. Considering the existing observation mode for earthquake, we used fixed station and portable seismic station, combined network with mobile survey line mode. With the seismic source, we carry out comparative analysis the existing high-power (excitation energy can exceed106J) artificial source characteristics, and select two sources representative for continuous source and pulse source, respectively, namely ACROSS and large volume airgun sources.Second, the characteristics and signal detection method of ACROSS. According to10tons ACROSS produced by Beijing Gangzhen Mechanical and Electronic Technology Company, we give a comprehensive introduce its development process, working principle, source characteristics, and build up a high-performance dynamic monitoring system, which was widely used to detect and monitor the velocity change in subsurface, with its simple structure, easy operation and maintenance. The ACROSS signal has the characteristic of high repeatability and low frequency rang from2Hz to10Hz, and there is a relationship of square times between the energy and scan frequency. On signal detection, the cross-coherence and deconvolution method are more suitable than the cross-correlation and short-window correlation with comprehensive analysis from repeatability, travel time profile, spectral characteristics, SNR, and can remove the effect of the source characteristic. The dynamic monitoring system has high detection precision with ACROSS, and observed velocity changes have a relationship of positive correlation with atmospheric pressure.Third, the monitoring and analysis on coseismic velocity change in Wenchuan aftershock. After the Wenchuan earthquake, combined with Wenchuan Fault Scientific Drilling (WFSD) Project, we carried out continuous monitoring across the range-front fault zone of Longmenshan fault using10tons ACROSS, and adjacent to WFSD. We observed travel time delay of～5-9ms associated with a Ms5.6earthquake from the seismic stations on hanging wall of the Jiangyou-Guanxian fault, while from stations on the footwall, they did not observe any travel time delay larger than measuring error. We in this study carry out forward modeling by using two-dimensional Spectral Element Method to further investigate the amplitude and spatial distribution of observed velocity change. The model is well constrained by results from seismic reflection and WFSD coring. Our model strongly suggests that the observed coseismic velocity change is localized within the fault zone with width～120m rather by dynamic strong ground shaking, and a velocity decrease of～2.0%within the fault zone is required to fit the observed travel time delay distribution, which coincides with rock mechanical experiment and theoretical modeling.Fourth, the active monitoring experiment on regional scale used large volume airgun source. Based on the large volume airgun source, we designed, processed and allocated relative equipments, and constructed a high-performance active monitoring system with large volume airgun source. Cooperation with a number of units, carried out experiment with inland reservoirs and artificial water body in Hebei, Yunnan and Xinjiang, and conducted a feasibility and effectiveness analysis with dynamic monitoring the stress state and its change used airgun from detection distance and precision. The results showed that:The large volume airgun source has power of energy, the characteristic of low frequency, high-repeatability, can stimulate strong S wave, and so on. The active monitoring system has long detection distance at380km also can detect signal by stack, high detection precision, and have observed the continuous change caused by the Tide. It can be applied to carry out the active monitoring of subsurface and4D seismology on regional scale, which are provide a new technology to study on the medium properties, crustal stress distribution and dynamic variety. |
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