直光纤及螺旋缠绕光纤三维地震响应机制

Three⁃dimensional seismic response mechanism of straight and helically wound optical fibers

分布式光纤(DAS)测量轴向应变或应变率,其中直光纤接收的是单分量信号,对垂直于光纤入射的波的响应不敏感,螺旋缠绕光纤可接收多分量信号,可以解决上述响应不敏感问题。首先,分析了直光纤及缠绕角分别为35.3°、54.7°的螺旋缠绕光纤的P波、SV及SH波的理论轴向应变率响应;其次,利用三维弹性有限差分模拟了轴向应变率响应;然后,在地表、水平井、竖直井布设DAS,利用均匀模型、双层模型及西南页岩气模型模拟直光纤、缠绕角分别为35.3°、54.7°的螺旋缠绕光纤的地震响应,并将DAS轴向应变率响应与常规检波器的速度z分量、压力分量对比。结果表明:①直光纤接收单分量信息,在垂直于光纤方向接收的波的响应较弱,而螺旋缠绕光纤接收多分量信息,因此在垂直于光纤方向接收的波的响应较强;②缠绕角为35.3°的螺旋缠绕光纤地震记录中无S波响应,其波形与常规检波器压力分量相似,在成像中可以利用声波方程成像。

Distributed Acoustic Sensing(DAS)measures axial strain or strain rate,and straight optical fibers receive a single component signal and are insensitive to the response of waves incident perpendicularly to the fiber.Helically wound optical fibers can receive multi‐component signals,which can solve the problem of insensitive response.Firstly,the theoretical axial strain rate responses of P‐wave,SV,and SH waves of straight optical fibers and helically wound optical fibers with winding angles of 35.3°and 54.7°are analyzed.Secondly,the axial strain rate response is simulated using the three‐dimensional elastic finite difference method.Finally,DAS is deployed on the surface,horizontal wells,and vertical wells,and the seismic responses of straight optical fibers and helically wound optical fibers with winding angles of 35.3°and 54.7°are simulated using uniform models,double‐layer models,and southwest shale gas models.The axial strain rate response of DAS is compared with the velocity z component and pressure component of conventional detectors.The results show that:①Straight optical fibers receive single component information,but the response of waves received in the direction perpendicular to the optical fiber is weak,while helically wound optical fibers receive multi‐component information,so the response of waves received in the direction perpendicular to the optical fiber is strong;②there is no S‐wave response in the helically wound optical fiber seismic record with a winding angle of 35.3°,and its waveform is similar to the pressure component of a conventional detector.The acoustic equation can be used for imaging.