Ultrasonic Imaging Of Seismic Physical Model Using Fiber Grating Technology

Seismic physical model ultrasonic imaging technology is an important method of getting the surface or internal reflected acoustic signal of the physical model by using the high sensitive seismic wave detecting technology, and obtains the profile images by inversion. Aiming at the existing problems in seismic physical model ultrasound imaging at home and abroad, such as low spatial resolution and scanning efficiency, Fiber Bragg Grating (FBG) based ultrasonic sensor has been proposed with high responding sensitivity, the seismic wave detecting and seismic physical model imaging mechansim have been systematically studied theoretically and experimentally, which is of great significance in improving the sensitivity of ultrasonic detecting effectively, reducing the sensor's size, enhancing the multiplexing capability, and improving the imaging quality. Three kinds of fiber ultrasonic sensors based on diffient sensing mechanism have been proposed for different seismic physical model scanning in this thesis.An ultrasonic sensor based on Phase-Shift Fiber Bragg Grating (PS-FBG) has been proposed. The ultrasonic wave has been coupled into the fiber via the end face, and then transmitted to the sensing area. Edge filter method has been implemented to transfer the light signal of the modified fiber grating spectrum to the electric signal via the photoelectric detector, and the reflected ultrasonic pulse signal (300KHz) from different interface of the seismic physical model has been obtained. The proposed fiber ultrasonic sensor suffered from low sensitivity. However, considering the compact size, it is suitable for the high spatial resolution imaging of small seismic physical model.An ultrasonic sensor based on Tilted Fiber Bragg Grating (TFBG) has been proposed. Compared to the former design, the coupling efficiency of this structure has been enlarge widely because of the unique encapsulation method, which finally results in the high sensitivity responding to the ultrasonic signal (300KHz) by edge filter method. In addition, the oblique encapsulation has not only enlarged the sensor's mechanical strength, but also makes the sensor directional, which can be used for direction detection of the unknown ultrasound source.An ultrasonic sensor based on Fiber Bragg Grating (FBG) has been proposed. An aluminum cone has been used to couple the ultrasound to the fiber, which increases the coupling efficiency by enlarge the coupling interface. High responding sensitivity detection for the ultrasonic signal (300KHz) has been achieved by using the edge filter method. And all-metal encapsulation structure makes the fiber sensor sturdy and durable. The proposed sensor is capable of detecting the source direction-known ultrasonic signal (the ultrasound can only be coupled to the sensing area form the end-face of the aluminum cone), which makes it suitable for seismic physical model ultrasonic imaging.