Research On Seismic Tomography And Its Application To Complicated Velocity Model Of The Metal Mine

As the theoretical backgrounds, the article firstly retrospects the development of seismic tomography method and the basic theory of finite difference travel time calculation method and the damping least square inversion method(DLSQR) with discussion of their advantages and disadvantages respectively. Based on Vidale's pioneering work on seismic ray tracing using finite difference method, we modify the program to calculate the number of rays crossing the basis block. The meshes are redivided to adapt to the inversion using irregular mesh parameterization technique. The modified program is applied to the study of the crustal structure of Tengchong volcanic area and Kyushu island arc area using earthquake travel-time records. To study the upper crust structure in Tianshui earthquake area (M8.0, 1654), we use the active sources observed data of Tianshui—Wudu area in Gansu province. Issues we address in this study include the relations between deep crustal velocity structure and the magmatic characteristics beneath Tengchong volocanoes, the origin of volcanoes in Kyushu and features of the faults around Tianshui M8.0 earthquake area in Gansu.The tomographic results of Tengchong volcanoes and Longling earthquakes (M7.3 & M7.4) areas show that the shallow high-velocities may correspond to condensed intrusive magma messes or non-volatile high-density remnants of early volcanic eruption. The low-velocities may represent basin deposits or partial melting volcanic rocks. A prominent low-velocity zone at depth of about 5-20km under delineates the channel through which the magma upwells. This is one of the main regions where magmatic activities in the crust happen. Furthermore, the two Longling earthquakes are located on the Longling fault and Nujiang fault, between which the geological feature is low velocity and weak rock stress state. The high-velocity zones east of the Nujiang fault and west of the Longling fault are available locations for regional stress accumulations. It leads to an uneven stress distribution across the Nujiang and Longling faults causing the two earthquakes.In the tomographic study of the volcanoes in Kyushu, the results show that the high-velocity Philippine Sea slab subducts to the 150km depth with a wide angle beneath Kyushu. The low-velocity zone at depth of 60-110km above the subducting slab maybe result from the partical melting of the mantle substance caused by dehydration of oceanic crust and sediments and serpentinization of the forearc mantle. The upwelling of melting substance is the main reason of volcanic activities. Furthermore, low-velocity magma channel extends to 100km depth beneath the Unzen Mount. The pronounced low-velocity anomalies and the high-frequency seismic activities in the Sakurajima area manifest that the volcanoes are active and likely to erupt in the future.The Tianshui—Wudu seismic profiles suggest that the interface of basement fluctuates greatly in this area. Geometry of the faults are mostly complicated with large dip angles. Most of these faults extend to great depth and cut through the basement. The low-velocity zone between Li county and Xihe county reflects the existence of Xi-li sedimentary basin, the basement of which is about 5km in depth. The pronounced changing of the velocity contour on the east of Wu Mountain is consistent with the fold region nearby. The Cheng county basin has a relatively high velocity with gently changing. The basement depth is less than 3km. The north-south fault on the west of Li county represents a low-velocity channel. Other researches have proposed that the fault is NS supracrustal fault and related to the cropping-out of mantle rock xenoliths at northeast margin of the Tibetan Plateau. On the study area, the shallow subsurface EW oriented features are influenced by the northern margin fault zone of the Qinling; while the deep NE/NNE trending structures may be related to the earthquakes in Tianshui. The epicenter of the Tianshui earthquake ( M8.0) is located on the EW oriented Fault through Li county which has complex velocity feature and structure.Finally, according to the geologic features of metal ore, after analyzing the theoretical basis, traits and advantages of finite difference tomography method, we consider that this technique has some feasibilities on the metal ore prospecting. We modify the finite difference tomography program to adapt to shooting and receiving on the fluctuant ground surface in the metal ore area and investigate the technique with complex model computing.The results illustrate that this method has great ability to renew the model with laterally varied velocity, and may bring us some new kinds of ideas on deep ore prospecting.