Novel High-performance Fiber-optic Ultrasonic Sensors

In seismic exploration,efficient acquisition of geological information,scientific understanding of stratigraphic structure,and identification of reservoir distribution are important prerequisites and guarantees for the development and utilization of oil and gas resources.By simulating seismic exploration with multi-type physical models,fiber-optic ultrasonic imaging of seismic physical models can invert and predict the propagation laws and characteristics of geophysical wave fields in reservoirs under near-ideal model conditions,which act as a bridge connecting seismic exploration with theoretical calculation.Fiber-optic ultrasonic sensors are the core components to acquire the structural information of seismic physical models.Compared with the conventional piezoelectric transducers(PZTs),they have unique advantages in broadband response and long-distance signal propagation fidelity.Based on the basic structures and response mechanisms of fiber grating ultrasonic sensors and interferometric ultrasonic sensors,four new types of high-performance fiber-optic ultrasonic sensors are developed in this dissertation.Compared with our previous work,the sensor sensitivity,spatial resolution,and imaging modality have been greatly improved.Meanwhile,an experimental prototype of ultrasonic imaging of seismic physical models is built.By employing the four types of fiber-optic ultrasonic sensors,high-quality detection and imaging of a series of complex seismic physical models are realized.The contents include:(1)The fiber-optic ultrasonic sensing mechanism is theoretically expounded,including the propagation characteristics of ultrasonic waves(acoustic emission)and the basic principles of fiber-optic ultrasonic sensors(acoustic reception).The wave equations and characteristic parameters of the ultrasonic field are introduced;the longitudinal-wave acoustic field characteristics of the piezoelectric disk ultrasonic transducer are modeled and analyzed;the structure characteristics and sensing mechanisms of fiber grating ultrasonic sensors and interferometric ultrasonic sensors are described in detail.This part lays a theoretical foundation for the design and characterization of fiber-optic ultrasonic sensors.(2)In order to overcome the shortcomings of conventional direct coupling approaches between bare fibers and ultrasonic waves,such as “point” or “line”,a type of fiber grating ultrasonic sensors based on acoustic coupling cones are proposed.The acoustic coupling cone has an acoustic focusing effect that efficiently focuses the ultrasonic energy onto the sensing fiber,thus greatly increasing the sensor sensitivity.Combined with an external protective packaging structure,the mechanical strength of the sensor is largely improved.Eight high-sensitivity fiber grating ultrasonic sensors using acoustic coupling cones are developed from three aspects: different sensing elements,different coupling approaches between fibers and cones,and different cone materials.These sensors are further optimized by theoretical simulations and experimental tests.The layered,semi-cylindrical,stepped,and concave physical models are scanned using the optimized sensor.According to the time-of-flight approach,high-quality twodimensional images of these physical models are obtained.(3)Based on the essential high sensitivity of phase modulated interferometric fiberoptic sensors,a type of Fabry-Perot interferometric ultrasonic sensors based on gold films are proposed.Based on different sensing fibers(single-mode or multi-mode fibers)and ultra-thin gold films,two sensing structures are developed;the latter has a higher sensitivity with the collimation effect of the multi-mode fiber.Both of them realize the scanning imaging of layered,concave,and semi-cylindrical physical models in air.Compared with the previous chapter,this type of ultrasonic sensor is more compact and sensitive,and can even detect the weak ultrasonic waves in air.It is suitable for the ultrasonic imaging of seismic physical models in air,which greatly simplifies the imaging system.(4)Spatial resolution is the key factor determining the imaging accuracy of seismic physical models.A high spatial-resolution ultrasonic sensor based on a suspended-core fiber is proposed,which only needs to corrode a grapefruit fiber into a free suspended core.The ultra-fine suspended core can not only help to improve the spatial resolution,but also ensure a high sensor sensitivity.The sensor responds sensitively to broadband ultrasonic waves,and enables high spatial-resolution imaging of semi-cylindrical and cascaded semi-cylindrical physical models(2.5mm(300kHz)-151.05?m(5MHz)).In terms of spatial resolution,sensor sensitivity,and sensor fabrication,the suspendedcore sensor is a good alternative for ultrasonic imaging of seismic physical models.(5)Different from the conventional linear mechanical scanning approach,the endoscopic ultrasonic imaging of a pipe-type seismic physical model based on an integrated endoscopic probe is proposed.By integrating ultrasonic emission and reception,the endoscopic probe comprises a gold-film fiber-optic sensor,a PZT,and a rotating acoustic mirror,all of which are coaxially assembled to provide a 360° omnidirectional lateral scanning.The probe can be inserted into a pipe-type physical model to scan the inner wall circumferentially,so as to reconstruct the inner structures.The integrated probe provides a new method of endoscopic imaging for reconstructing the internal structural features of physical models.Moreover,the imaging system is more efficient,more stable,more compact,easier to assemble and lower in cost,which improves the acoustic performance of the fiber-optic ultrasonic detection system.(6)A system platform of “experimental prototype of fiber-optic ultrasonic imaging of seismic physical models” is built.The new experimental prototype realizes the integration,miniaturization,and instrumentation in ultrasonic imaging of seismic physical models.It also improves the correlation and stability of each part,and meets the requirements of data collection in different experimental environments.With the adaptive system,the prototype can not only realize the single-point ultrasonic detection,but also enable the high-resolution scanning imaging of multiple physical models by using an external translation stage or a DC geared micromotor.