利用背景噪声层析成像研究济南浅层横波速度结构

Study of the shear wave velocity structure of underground shallow layer of Jinan by ambient noise tomography

城市地球物理探测面临众多人文挑战,像城市交通、密布电网、参差楼群等,对应用不同方法进行数据处理分析提出了技术挑战,需要用创新的方式和技术进行城市探测。为探索和发展适用于城市地下空间资源开发利用的地球物理探测新技术,中国地质科学院地球深部探测中心第一次启动了城市地下空间勘查评价试点工程,并选择济南市作为试点城市,试验短周期密集台阵噪声层析技术,并取得了较好成效。获得的主要认识如下:(1)在非弥散场或非均匀噪声场源的人文干扰较为严重的城市区域,通过长时间的噪声信号的采集,可获得高信噪比的面波信号,因此密集台阵噪声成像技术适应于城市地下空间的背景岩层结构探测工作。(2)本次研究提取出了周期范围为0.2~1.5s的高信噪比面波信号,并取得与实际地质特征相一致的横波速度结构信息。(3)从横波速度结构特征来看,研究区300m以浅,主要岩性为灰岩且主要分布于研究区西侧,而侵入岩体主要分布于东侧,中间存在一个明显的分界面,指示存在一个近南北向的隐伏断裂。随着深度的增加,大面积的侵入岩体展布于深层,灰岩只在北侧局部有显现。总体来说,本项研究证明了密集台阵噪声层析方法能够适用于人文干扰严重的城市地下空间背景岩层结构探测,这一成果对认识研究区地下结构、地震防灾、工程应用具有重要指导意义。

Urban geophysical exploration often requires innovative thinking or sometimes seemingly off-thewall approaches to meet data challenges of urban environment associated with human sprawl,such as moving vehicles,dense power grids,jagged buildings and so on.Therefore,the China Deep Exploration Center(CDEC,formerly SinoProbe Plan Center)officially launched the government-lead geological survey of urban underground space resources(also named'urban underground space-exploration project')in pilot cities.One of the principal aims of the project is to develop geophysical characterization techniques that are accurate and noninvasive,and can be adapted to noisy and culturally complex urban settings.We selected Jinan city in Shandong Province as the first pilot city for both its great demand of underground infrastructure including a metro-subway system and its unique subsurface geology.In this pilot study,we seek a geophysical methodology that can meet urban challenges,such as noisy environment,large urban areas,restriction for equipment deployment,and logistics dealing with paved surfaces and roads.We applied ANT(ambient noise tomography)to a dense short-period array and obtained good results:(1)Using high-frequency waves generated by ambient noise tomography,we obtained surface waves with good signal-to-noise ratio after processing continuous ambient noise data collected from 49 broadband seismic stations for 32 days,demonstrating that ANT method is suitable for exploring background rock structures in urban underground space.(2)Using direct surface-wave tomographic method with period-dependent ray-tracing,all surfacewave dispersion data were inverted in the 0.2-1.5 speriod band simultaneously for 3 Dvariations of shearvelocity structure.The shear-velocity structure correlates well with the geological features and general lithological distribution of igneous and limestone rocks,as well as the spatial distribution of faults at depth.(3)Shear-velocity obtained from the inversion showed that the lateral and vertical velocity variation is much bigger.Specifically,in shallow regions at above 300 m depth,the lithological characters are mainly of limestone(higher shear-velocity),with much wider distribution,and of magma intrusion(highest shearvelocity relative to limestone);an obvious boundary between low and high shear-velocity indicates a concealed fault at the study area;and extensive magma intrusion occurs at greater depth whereas limestone exists only in the middle part of the north region.Overall,the study results proved that our method can be effective in helping us to better understanding local geologic structures,evaluating lithological distributions and assessing hazardous concealed active faults and their effects on springs in the future.