盆地沉积结构对地震波初至走时的影响  

Influence of the basin sedimentary structure on first-arrival traveltime for seismic wave

作　　者：李佳欣 王赟[1] 杨春[1] LI JiaXin;WANG Yun;YANG Chun(MWMC Group,State Key Laboratory of Geological Processes and Mineral Resources,China University of Geosciences,Beijing 100083,China)

机构地区：[1]中国地质大学(北京),地质过程与矿产资源国家重点实验室,多波多分量研究组,北京100083

出　　处：《地球物理学报》2022年第9期3554-3568,共15页Chinese Journal of Geophysics

基　　金：国家自然科学基金项目(U1839208,62127815,42150004)资助。

摘　　要：相对于天然地震的低频长波长特征,上地壳广泛存在的盆地沉积层厚度相对较薄,在研究壳幔结构或地震定位时往往做各向同性或VTI层等效的简化.这种简化的适用条件与误差是本文讨论的主要问题.以塔里木盆地为例,利用塔中某测井数据建立盆地二维水平层状互层模型;以此为参考,可进一步得到简化的各向同性等效和VTI近似两种单层模型.通过计算两种简化模型相对参考模型的初至走时误差,可以发现:(1)当震源处于沉积层内和沉积层下方时,两类简化模型得到的走时呈现完全不同的变化规律;(2)当震源处于沉积层内时,简化模型的走时相对误差均随震中距的增加快速增大,各向同性等效的误差极值在震源深度6km、震中距100km处可达60%左右,而VTI近似约为45%;(3)当震源处于沉积层下方时,区域震条件下各向同性等效与VTI近似的误差均未超过8%,且各向同性等效的精度优于VTI近似.因此,当利用盆地内观测数据定位不同深度、不同震中距区域地震时,宜采用不同的上地壳沉积层简化模型以保证合理的精度.Compared to the long-wavelength characteristics of earthquakes in low frequency,the thickness of basin sedimentary layers that are widespread in the upper crust is relatively thin,in which case the sedimentary structure in a basin is often simplified by an isotropic or VTI layer when studying the crust-mantle structure or seismic localization.The application conditions and errors under the simplification are the main issues discussed in this paper.Taking the Tarim basin as an example,a two-dimensional horizontal layered interstratification model of the basin is established using data from a well logging in the central Tarim basin.Referring to this model,two further single-layer models,the simplified isotropic equivalent and the VTI approximation models,can be obtained.By calculating the first-arrival traveltime errors of the two simplified models corresponding to the reference model,it can be found that:(1)the traveltime obtained from the two simplified models show completely different patterns of variation when the source is within and below the sedimentary layers;(2)when the source is within the sedimentary layers,the relative traveltime errors of the simplified models both increase rapidly with the increasing epicenter distance,and the extreme value of the isotropic equivalent error is up to 60%,while the error of the VTI approximation is about 45%at the source depth of 6 km with the epicenter distance of 100 km;(3)when the source is below the sedimentary layers,the errors of the isotropic equivalent and the VTI approximation do not exceed 8%under regional seismic condition,and the accuracy of the isotropic equivalent is higher than that of the VTI approximation.Therefore,when the observation data in the basin are used to locate regional earthquakes with different depths and epicentral distances,it is appropriate to use different simplified models of the upper crustal sedimentary layers to ensure a reasonable accuracy.

关 键 词：长波长 盆地 地震定位 各向同性 VTI 

分 类 号：P315[天文地球—地震学] P631[天文地球—固体地球物理学]