| The seismic-wave, as a light illuminating the Earth's interior, is the most important research tool for us to get knowledge of the Earth's interior structure, material composition and state, and so on. Over the past time period more than a century, due to the continuous progresses in observation technology, through studies of seismic-waves excited by natural 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 image of the overall Earth's structure. It is also based on the studies of the characteristics of seismic-wave propagation that the artificial seismic exploration has become an important means for understanding the energy distribution in the superficial part of the Earth and exploring the tectonic environment.However, we are now facing with the serious earthquake disasters, strong volcanic eruptions, seriously increased groundwater pollution, and excessively exploiting of the natural resources. All these require us to understand and grasp the processes of the moving and changing of the Earth's interior. These changing processes are closely related to the changes of the stress field and physical properties of the medium in the Earth's interior. The most effective means to study the stress field and the physical property changes of the Earth's medium is still the seismic waves that can penetrate through the deep interior of the Earth. Yet, the present study on the changing processes is different from the traditional seismological researches in that we have to study the subtle changes produced when the seismic-waves penetrate through the changed Earth's medium at different times to obtain information on short-term changes in the Earth's medium as a precondition to understand the Earth's medium change process. Based on the development of observation and computation techniques since the new century, it becomes possible to measure and study the short-term changes of seismic-wave velocity, which has been applied for monitoring the volcanic earthquakes, and test for changes in the physical property of the medium, and provided a lot of new information for us to study the short-term changes of the Earth. At present, it has become a new hot research issue in geophysics and an important future direction of development to explore the short-term change processes of the Earth's medium through the precise measurement of the seismic-wave velocity changes.In this dissertation, based on comprehensive summary of the existing theoretical research achievements, experiments and data processing methods, we focused on the precise measurement of seismic-wave velocity changes, conducted detailed analyses and experimental researches using exploration experiments by artificial sources, seismic waveform analyses by repeat natural earthquakes, and Green function methods from ambient noises. Through the precise measurement of seismic-wave velocity changes, we 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 three aspects:First, an artificial seismic source is an import means at present to accurately measure the seismic-wave velocity changes, because its time can be precisely controlled, and the waveforms excited have a high degree of similarity. Our research group conducted a 30-day field exploration experiment using active sources and two sets of observation systems with baseline lengths of 250 meters and 1 km respectively. Experimental results show that the correlation test of the first arrivals made the measurement accuracy of seismic-wave velocity up to10-4 to 10-5. The impulse correlation test can effectively improve the signal to noise ratio, and increase the signal detection range. The relative diurnal variation of seismic-wave velocity changes measured by coda wave interference method was 10-2 to 10-3 with a measurement accuracy of 10-4, and the seismic-wave velocity displayed a clear diurnal variation on the background with a long-term trend change. The diurnal variation showed a synchronous relationship with the diurnal variation of groundwater level in a deep well near the earthquake prediction test site, and a negative correlation relationship with the atmospheric pressure. The experimental results showed that the sensitivity of seismic-wave velocity changes caused by the stress changes was 10-6/Pa.Second, we deployed a relatively dense mobile network of digital instruments after the 2 seismic events with M6.2 and M6.1 occurred in Dayao, Yunnan province, in July and October 2003 respectively, and obtained a large number of observation data from the aftershocks. Taking advantage of these digital waveform data from the aftershocks, we developed a system for identification of repeat seismic waveforms based on distance-based clustering analysis method to identify the repeat seismic events in an aftershock sequence with different correlation coefficients as the thresholds for the aftershock sequences. For two pairs of waveform data with correlation coefficient of 0.9 from the repeat events at the fixed stations, we measured the seismic-wave velocity changes in medium using the coda-wave interference method, and did not get a result with apparent regularity of seismic-wave perturbation. This indicated that the method was much limited in application for monitoring the velocity changes due to the uncontrollable nature of the repeat earthquakes in the tempo-spatial distribution, which needs further studies.Third, it is a new direction developed in recent years in seismological researches to monitor the seismic-wave velocity changes and invert for the underground medium structure by Green functions extracted from the ambient noises. We selected data recorded continuously at three stations near the earthquake area 7 months before and after the March 21, 2008 Yingjiang M5.0 earthquake, Yunnan province, extracted Green functions from the ambient noises, and measured the Green function changes using the interference method. Results showed that anomalous changes of seismic-wave velocity occurred in the seismogenic area: some of the relative velocity changes for different paths (station to station) increased before the event, while some other decreased with the relative change amplitude up to a degree of 10-3-10-2. Using the same method, we analyzed the data continuously observed 22 months before and after the July 3, 2009 Yao'an M6.0 Earthquake in Yunnan province at three stations near the seismogenic area. The seismic-wave velocity changes in medium between different stations also displayed the similar phenomena, and the maximum amplitude of the relative changes of seismic-wave velocity may get to 1%. This result indicated that the measurement of seismic-wave velocity changes by interference of Green functions extracted from the ambient noises can be used as a new means to monitor the seismicgenic process.It is for us to obtain the short-term change information in the Earth's medium by the precise measurement of seismic-wave velocity changes in the Earth's medium, so as to understand the Earth from the static 3-D images to the time-related and dynamic 4-D images. This research work is an initial exploration for the 4-D seismological studies, and a new direction of development for the seismological studies. This kind of exploration would obtain more information on changes in the Earth's movement, deepen the Earth science researches, and meanwhile would certainly further expand the application areas of seismology. The three aspects involved in this dissertation, especially the exploration by an artificial active source, and applications of the ambient noises will play an important role in the future studies of the 4-D seismology.
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