【作者】 刘辉；

【导师】 李静；

【作者基本信息】 吉林大学 ， 地球探测与信息技术， 2022， 硕士

【摘要】 分布式光纤声波传感系统(Distributed Acoustic Sensing,DAS)是一种近年来迅速发展的新型地震观测系统。DAS技术基于光纤内的瑞利散射效应实时感知外界振动引起的光纤局部应变,通过提取不同时刻的背向散射信号并解调得到相位与应变变化实现光纤轴向应变测量。光纤的每一小段可以等效为一个单分量应变仪,从而实现分布式测量。与传统地震检波器相比,DAS具有耐受恶劣环境,易实现大区域高密度观测等优点,广泛应用于信号识别、目标分类、环境活动的实时监测以及借鉴常规地震数据处理技术,实现地下结构成像和速度反演。在速度结构反演成像方面,由于DAS技术只接收来自光纤轴向应变率响应,数据信噪比较低,在结构成像和速度反演方面主要采用常规的面波一维频散反演方法,难以获得可靠、高精度的速度成像结果。另外,对于光纤信号的传播机制、响应敏感度、标距长度、散射方向性等理论基础还缺乏完整分析。针对上述问题,本文首先基于平面波质点位移—速度公式以及速度与应变率关系,开展DAS信号振幅及灵敏度响应研究,对比传统地震检波器响应,分析标距长度(gauge length)、波长、不同方位角等对光纤信号响应的影响,为实测DAS数据采集及处理提供理论指导。然后基于波动方程有限差分数值模拟方法,结合速度与应变率关系实现了DAS信号的弹性波有限差分数值模拟。以传统检波器弹性波数值模拟结果为参考,对比两者的波形以及面波频散差异,为后续反演成像提供理论基础。最后基于波动方程反演理论,推导实现了直接基于应变率的DAS数据波动方程反演成像理论并开展了DAS数据全波形反演(Full waveform inverision)方法和波动方程骨架反演(Skeletontized inversion)方法研究,通过典型模型和实测DAS数据验证了本文提出的方法可适用于不同尺度横波速度结构并获得高精度的成像效果,为DAS技术在地下结构速度成像方面的应用提供了可靠的技术手段。更多还原

【Abstract】 Distributed Acoustic Sensing(DAS)is a new seismic observation system that has developed rapidly.The DAS technology senses the fiber deformation caused by external vibration in real-time based on the Rayleigh scattering effect in the fiber.By extracting the backscattered signals at different times and demodulating the phase and strain changes,the axial strain measurement of the fiber is realized.Each small section of the optical fiber can be equivalent to a single-component strain gauge to realize distributed measurement.Compared with traditional geophones,DAS is resistant to harsh environments and easy to achieve high-density observations in large areas.Imaging and velocity inversion.Regarding velocity structure inversion imaging,since DAS technology only receives the axial strain rate response from the optical fiber,the data signal-to-noise ratio is low.Reliable,high-precision velocity imaging results are challenging to obtain.In addition,the theoretical basis of fiber signal propagation mechanism,response sensitivity,gauge length,and scattering directionality is still lacking incomplete analysis.Given the above problems,this paper first analyzes the DAS signal amplitude and sensitivity response based on the plane wave-particle displacement-velocity formula and the relationship between velocity and strain rate,compares traditional geophones’ response,and analyses the response gauge length,wavelength,and different azimuth angles.It provides theoretical guidance for the acquisition and processing of measured DAS data.Then,based on the finite-difference numerical simulation method of the wave equation,combined with the relationship between velocity and strain rate,the DAS signal’s elastic wave finite-difference numerical simulation is realized.Taking the traditional geophone simulation results as a reference,comparing the DAS and geophone data’s waveform and surface wave dispersion difference provides a theoretical basis for subsequent inversion imaging.Finally,the DAS wave equation inversion theory directly based on the strain rate is deduced and realized.The DAS fullwaveform inversion(FWI)method and wave-equation dispersion inversion are studied.The typical model and measured DAS data are used to verify this paper.The proposed method can obtain the S-wave velocity structure imaging at different scales,providing a reliable image method for applying DAS technology.更多还原