【作者】 曹令敏；

【导师】 刘光鼎；

【作者基本信息】 中国地质大学（北京） ， 地球物理， 2011， 博士

【摘要】 本文首先回顾了地震层析成像技术的发展历史,详细阐述了有限差分走时计算方法和阻尼最小二乘反演方法的原理与各自的优缺点。在Vidale有限差分方法的基础上进行了改进,通过计算基块体中的射线数来对网格进行重新划分,使其适用于不规则网格参数化的反演计算。利用改进后的有限差分层析成像方法分别对云南腾冲及其邻区、日本西南九州岛弧地区的天然地震资料以及甘肃省天水——武都地区的人工地震测深资料进行反演,并对各地区的深部速度结构、构造特征与火山、地震之间的关系进行研究与讨论。云南腾冲火山及龙陵地震区的层析成像结果表明,腾冲火山区浅表层的高速异常很可能是冷却凝固后的岩浆侵入体或早期火山作用中难以挥发的高密度残留体;两侧低速异常主要是盆地沉积、火山堆沉积等因素引起的;5—20km以下为明显的低速异常,宽度在20km左右,是现今壳内岩浆活动的主要区域,是地幔岩浆源区物质上升的通道。此外,龙陵地震的震源分别位于龙陵断裂与怒江断裂一带低速与高速体的突变区域,震源之间呈低速异常,岩石强度较小;震源两侧为高速体,岩石强度大,这一横向上速度及岩石强度的极度不均,使得该区应力积累也分布不均,成为龙陵地震发震的主要原因。日本九州岛弧成像结果表明,以高速为主要特征的菲律宾海板块大角度俯冲至九州岛弧下150km深度以下,并且俯冲板块上覆60-110km的低速体是由于海洋地壳以及沉积层脱水作用引起地幔物质的部分熔融,熔融物质的上升为该区火山活动的主要原因。此外,富水流体进入弧前地幔,使得弧前地幔中的部分橄榄岩蛇纹石化,降低了地震波速度。此外,云仙岳下方低速的岩浆通道一直延伸至100km,樱岛下的低速异常也十分显著,同时在莫霍面深度附近还伴有大量低频地震活动,说明该火山比较活跃,将来有喷发的可能。西秦岭天水-武都地区层析成像结果显示,该区基底界面起伏较大,断裂丰富且复杂,多呈直立产状,延伸较大,多数断层切割基底。礼县——西和之间的低速异常反映了沉积盆地的结构特征,结晶基底深度在5km左右。武山以东速度等值线横向变化明显,表现为褶皱。成县盆地速度变化平缓且速度值相对较高,基底埋深较浅,深度不到3km。礼县以西的南北向断裂为一条上涌低速异常带,以往研究证实其为南北向超壳深断裂,并与青藏高原东北缘出露的地幔岩石包体有关。研究区浅部构造走向近东西向,与秦岭北缘断裂带的影响有关;深部存在北东向或北北东向构造,与天水地区的地震发育有关。历史上天水地区发生的8级地震震中位于断层3上,反映出大震地壳上部复杂的速度特征与断层结构。最后,根据金属矿的地质特征,通过分析有限差分层析成像方法的理论基础、特点及优势,认为层析成像方法在金属矿勘探中具有一定的可行性。本文对有限差分层析成像程序进行了改进,使其适用于金属矿地区起伏地表的激发与接收,并通过金属矿复杂速度模型进行理论验证。结果表明有限差分层析成像对地层或岩石等横向不均匀介质具有很好的恢复能力,给金属矿深部找矿的研究提供了新的技术发展思路。更多还原

【Abstract】 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.更多还原