分布式光纤声波传感系统在近地表成像中的应用Ⅱ:背景噪声成像

Distributed acoustic sensing for imaging shallow structure Ⅱ:Ambient noise tomography

随着城市化的发展,城市地球物理日益成为地球物理研究的重要方向,地震成像是构建城市地下空间三维/四维图像的重要手段,但面临观测成本高的困难.近年来国际上新发展的分布式光纤声波传感器作为高密度地震观测系统已经在地震层析成像方面得到了应用,在提高成像分辨率的同时,又降低了观测成本.本研究使用国产分布式光纤声波传感器开展了观测实验,利用480m埋地光缆记录了13h背景噪声,计算得到噪声互相关函数,获得了高频Rayleigh面波信号.采用多道面波分析方法提取相速度频散曲线,其结果与传统检波器记录和主动源结果较为一致.采用遗传算法反演得到了研究区内二维S波速度剖面,获得了下方沉积物横向变化特征.通过本次实验,初步验证了国产设备开展地震背景噪声成像研究、构建地下浅层结构模型的可行性.

Distributed acoustic sensing system is a low-cost dense seismic acquisition system,which helps in improving resolution of seismic tomography.A field trial was conducted in Beijing National Earth Observatory with a DAS interrogator made in China.The noise cross-correlation functions(NCFs)were computed with 13-hour ambient noise records and high frequency Rayleigh wave signals clearly emerge on the NCFs.Phase velocities were obtained with themultiple-channel analysis of surface wave method and it agrees well with results obtained with geophone dataset and active source dataset.The dispersion curves were used to invert for the 2D shallow structure beneath the fiber-optic that shows strong lateral heterogeneities.This experiment preliminarily validates the feasibility of the domestic equipment for ambient noise tomography research.