地震物理模型光纤光栅超声成像技术研究

发布时间：2019-02-15 11:45

【摘要】：地震物理模型超声成像是通过高灵敏地震波传感技术,有效获取物理模型表面及内部反射的超声波信号,并经反演获得物理模型剖面图像。针对国内外地震物理模型超声成像存在的空间分辨率低、扫描效率低等难点问题,本文研究了基于光纤光栅的高灵敏超声传感技术,从理论和实验系统地研究了光纤布拉格光栅用于地震波检测及地震物理模型成像的机理及技术,对有效提高超声波探测灵敏度、减小超声检波器尺寸、增强超声检波器复用能力,提高成像质量,具有十分重要意义。根据不同地震物理模型成像需求及光纤传感机理,研制了以下三种不同结构的光纤光栅超声传感器。利用相移光纤布拉格光栅制作了一种基于端面直接耦合型的超声传感器,超声波通过光纤端面耦合并沿光纤传输至光纤光栅(传感区),经超声调制后的光栅反射光谱传输至探测单元,基于光谱边带滤波技术,将光栅反射波长信息转化为强度信息,并利用光电探测器获得地震物理模型不同界面反射的超声脉冲信号(300 KHz)。该光纤超声传感器结构紧凑,适用于小型地震物理模型的高空间分辨率成像。利用倾斜光纤布拉格光栅制作了一种基于侧向直接耦合型的超声传感器,此方案中,超声波可直接耦合至光纤光栅,相比于第一种方案中光纤端面耦合超声波的方式,极大提高了超声耦合效率,从而提高光纤光栅感测超声灵敏度,配合上述相似的光谱边带滤波技术,可实现超声波(300 KHz)高灵敏探测。此外,光纤光栅斜口封装的方式,不仅可以有效改善光纤光栅机械强度,便于传感检测安装和移动,而且其非对称结构使其对不同方向超声波激励具有响应差异性,可用于未知超声源的方向识别。利用光纤布拉格光栅制作了一种基于光纤端面间接耦合型的超声传感器,此方案中,超声波通过铝锥端面耦合至光纤,可有效提高超声波耦合效率。配合上述光谱边带滤波及探测技术可实现超声波(300 KHz)的高灵敏探测。此外,由于该光纤传感器全金属的封装结构对光纤的保护使得传感器的机械强度有了极大提高,极大增加了其机械强度和稳定性。该传感器的定向性(声场仅可从铝锥端面耦合至传感区),适用于已知声源的超声波探测,同时可避免来自其它方向声波信号的干扰,适合于地震物理模型超声成像。
[Abstract]:Ultrasonic imaging of seismic physical model is an effective way to obtain the ultrasonic signals reflected from the surface and interior of the physical model by using highly sensitive seismic wave sensing technology, and the physical model profile image is obtained by inversion. Aiming at the problems of low spatial resolution and low scanning efficiency in ultrasonic imaging of seismic physical models at home and abroad, a highly sensitive ultrasonic sensing technology based on fiber Bragg grating (FBG) is studied in this paper. The mechanism and technology of fiber Bragg grating used in seismic wave detection and seismic physical model imaging are systematically studied in theory and experiment. It can effectively improve the sensitivity of ultrasonic detection and reduce the size of ultrasonic detector. It is very important to enhance the multiplexing ability of ultrasonic geophone and improve the imaging quality. According to the imaging requirements of different seismic physical models and the mechanism of fiber optic sensing, three kinds of fiber Bragg grating ultrasonic sensors with different structures are developed. A kind of ultrasonic sensor based on the direct coupling of the end face is fabricated by using the phase-shifted fiber Bragg grating. The ultrasonic wave is coupled through the end surface of the fiber and transmitted along the fiber to the fiber grating (sensing region). The reflection spectrum of the grating modulated by ultrasonic is transmitted to the detection unit. Based on the spectral sideband filtering technique, the reflected wavelength information of the grating is converted into the intensity information. The ultrasonic pulse signal (300 KHz).) reflected from different interfaces of seismic physical model was obtained by using photodetector. The fiber optic ultrasonic sensor is compact and suitable for high spatial resolution imaging of small seismic physical models. An ultrasonic sensor based on lateral direct coupling is fabricated by using tilted fiber Bragg grating. In this scheme, ultrasonic wave can be directly coupled to fiber Bragg grating, compared with the method of fiber end coupling ultrasonic wave in the first scheme. The ultrasonic coupling efficiency is greatly improved and the ultrasonic sensitivity of fiber Bragg grating is improved. Combined with the similar spectral sideband filtering technique, the ultrasonic (300 KHz) high sensitivity detection can be realized. In addition, the fiber grating oblique port encapsulation method not only can improve the mechanical strength of fiber grating effectively, and is convenient to detect, install and move, but also its asymmetric structure makes it have different response to ultrasonic excitation in different directions. It can be used for direction recognition of unknown ultrasonic source. A fiber Bragg grating (FBG) is used to fabricate an ultrasonic sensor based on the indirect coupling of fiber end face. In this scheme, the ultrasonic coupling efficiency can be improved effectively by coupling the ultrasonic wave to the fiber through the aluminum cone end. Combined with the above spectral sideband filtering and detection techniques, the high sensitivity detection of ultrasonic (300 KHz) can be realized. In addition, the mechanical strength of the fiber sensor is greatly improved, and the mechanical strength and stability of the sensor are greatly enhanced because of the protection of the fiber optic fiber by the all-metal encapsulation structure of the fiber optic sensor. The directionality of the sensor (the sound field can only be coupled from the aluminum cone to the sensing area) is suitable for ultrasonic detection of known sound sources, while avoiding interference from acoustic signals in other directions, so it is suitable for ultrasonic imaging of seismic physical models.
【学位授予单位】：西北大学
【学位级别】：硕士
【学位授予年份】：2016
【分类号】：P631.4;TB559

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