The Research On All-optical Ultrasonic Imaging Technology Of Seismic Physical Models

The ultrasonic imaging technology of seismic physical model was a common experimental simulation technology in the seismic wave exploration.Seismic physical models were the simulation structures of resource formation reduced by the scale of 1:10²?1:10⁷,which were regarded as a method to study the transmission law of seismic wave and its theoretical deduction.Aiming at the existing problems of the electrical transducers in the conventional ultrasonic imaging,such as large volume,susceptible to electromagnetic interference and low scanning efficiency,the paper proposed ultrasonic excitation methods and fiber-optic ultrasonic receiver combined with the advantages of all-optical detection?broadband response,high spatial resolution,high sensitivity,etc.?.And a unique ultrasonic imaging technology of seismic physical model was proposed.The laser ultrasonic excitation law of tungsten disulfide?WS₂?films was studied to apply it to the ultrasound imaging of seismic physical model.An acoustic coupling cone-based fiber-optic Mach-Zehnder interference ultrasonic sensor,made by fiber microstructure processing technology,was proposed,which was of great significance to improving the quality and detection resolution of ultrasound and enhancing the sensitivity of sensors.During the research of WS₂ films-based transducer,WS₂ films,fabricated by liquid exfoliation technique,were characterized by X-ray diffraction?XRD?,Raman spectroscopy and ultraviolet-visible absorption spectroscopy.And the two key factors affecting the photoacoustic generation efficiency of WS₂,i.e.,laser power and the amount of WS₂during films formation,were experimentally demonstrated.Experiments showed that the increasing volume of WS₂ dispersion and laser power increased the photoacoustic output of WS₂ films.The study provided an excellent ultrasonic source for all-optical ultrasonic imaging of seismic physical model and improved the efficiency of ultrasonic imaging.For the design of fiber-optic ultrasonic sensor,an acoustic coupling cone fiber-optic Mach-Zehnder interference ultrasonic sensor was proposed by the principle of the core-mode-field mismatch between a thin-core fiber?TCF?and a single-mode fiber?SMF?.In the sensor,the fiber microstructure processing technology was utilized to modulate the refractive index of the TCF to increase the coupling efficiency of light waves,combined with the acoustic focusing characteristics of an acoustic coupling cone,so as to further enhance the ultrasonic response sensitivity of the sensor.The signal-to-noise ratio of the optical fiber sensor for 5 MHz ultrasonic pulse was 63.72 d B.And the sensor had a great stability.Due to the outstanding sensing of high frequency ultrasonic,the optical fiber sensor could improve the spatial resolution of ultrasonic imaging.And it was suitable for all-optical ultrasonic imaging system of seismic physical model.The advantages of the fiber-optic ultrasonic sensing technology and the laser ultrasonic excitation technology were combined in the all-optical ultrasonic imaging.The all-optical ultrasonic imaging technology of two kinds of seismic physical models?the arch bridge and the square hole?was realized by attaching WS₂ films material to the model surface,using the pulsed laser to generate ultrasound,and utilizing the proposed fiber ultrasonic sensor to obtain ultrasonic echo signals carrying the information of model structure.