Research And Application On Seismic Tomography And Seismic Location Technique

Since the70s,seismologist began to study seismic tomography which propeledthe seismic research to a more vast world and broad field. As a comprehensivediscipline, seismic tomography involves in the fields of seismology, algorithmanalysis, graph and image processing and Computer Science. In recent years,domestic and international seismologists using seismic tomography technology hasmade a lot of important achievements, such as deep structure of orogenic belts,tectonic dynamics, mechanism of natural earthquake, volcanic activity, earth’s deepmantle thermal convection, as well as the study of mantle plume and mid-ocean ridge.Microseism means small earthquake of rock rupture or fluid perturbationproduce. In the broadest sense, Microquakes can be divided into engineeringproduction micro earthquakes and naturally micro seismic. Seismic technique has animportant role in mine, seismic technology, underground engineering construction andsafety protection. So study volcanic earthquake disaster has a very wide application.The study of microseism in China is still in the primary stage, but as the pace of ourindependent development of resources speeds up and the rapid development of naturalscience, microseism technology will certainly has a broad application prospect in thefields, such as metal mines, oil fields, volcanoes and volcanic earthquake monitoringand so on.First, for microseism locaion algorithm, we theoretically use numericalsimulation method to do forward modeling and inversion tests. Forward: establish atwo-layer geological model. The upper layer is isotropic elastic medium, which isused to simulate overlying stratum of fracturing formation; the under layer withbipolar anisotropic medium induced by cracks, is used for simulate hydraulic fracturing formation, and carries on the analysis of the characteristics of seismicwaves in two-phase anisotropy. It is the reason fracturing formation fracture induceanisotropy and wave type conversion phenomena occur on formation interface, thatcause complex wave field characteristics when single microseism begins. When wedo artificial fracturing on formation, there will be a series of cracks in the direction ofmain stress and secondary stress affected by the press changes. Cracks inducemicroseism, at the same time cracks contributed to the anisotropic characteristics offracturing formation. Seismic wave of microseism has complex characteristics instratum. Two microseisms at different positions is easy aliasing, when twomicroseismic time interval is relatively short. We need to carefully identify to ensurethe correctness and accuracy of positioning results using the P wave pick up to get thelocation of the focus. Inversion: according to the wave datas from stations, we get theideal experiment results which confirmed the feasibility of the method by havingaccurate P wave arrival time and using microseism locating method to analyse thevirtual focal position.Second, using the microseism locating method to do application experiment wasin Jiapigou gold mine. Experiment site was selected in the gold mine of Sandao,which is one of most largest output place of the Jiapigou gold mine. Blastingoperation here is frequent, and exploition is deep. So it is suitable for makingmicroseism location. Moreover, since the mining area belongs to an old mining areaof rock fracture, it prones to collapse and crack. Therefore, it really has practicalsignificance to monitor microseism here to prevent the occuring of the major minedisaster and reducing damages. Experimental results show that the microseismlocation consistent with actual underground blasting, and can better monitor andidentify seismic of rock burst, can in some degree shows the situation of undergroundcracks, and the development direction of underground rock failure. Analyzed anddiscussed the error s in process of calculation.Finally, combining the P wave traveltime data from new wide-band seismicstations along the changbai mountain and earthquake recorded seismic network ofJilin province with tomographic imaging method to study the3-D velocity structure ofthe Changbai mountain and its adjacent area to40km deep crust and the top of uppermantle.Results show that the velocity structure of Changbai mountain Tianchi crater andits surrounding areas presents heterogeneity changes.0~5km below the volcano has obvious high-velocity anomaly of P wave,0~40km has low-velocity anomaly, and itsspeed is reduced by2%~3%compared with medium around. P wave low-velocityanomaly has close relationship with the higher temperature of underground medium.Accordingly we can speculated that the crater below10~40km has magma chamber.It also provided evidence that below the Changbaishan there may be volcanic magmasystem.In addition this paper speculated that magma chamber location is not under thechangbai mountain directly, but is in the southwest of the crater. Magma chamber isclose to the direction of the north east directionn in10to20km, and north-south inthe20~40km. So we have a deeper understanding of the distribution of magmachamber that the existence and distribution of it related to volcanic activity and thegeological structure of that area. It provides an evidence for further explanation andunderstanding of volcanic activity.