Research On Laser Ultrasonic Imaging Technology Of Seismic Physical Models Based On Metallic Thin Films

China has a vast territory and vast territory,with rich reservoir and complex structures.Under the new normal of national economic and social development of energy conservation,emission reduction and sustainable growth,people are driven to pay more attention to the detailed analysis,in-depth understanding and accurate grasp of geophysical information;Work harder to explore the new characteristics,new laws,and new storage of stratum energy structure.In the study of formation information imaging,conventional electrical imaging means have certain limitations,it is easy to be interfered by electromagnetic waves,is not flexible and occupies a large space.In the way of laser ultrasonic imaging,laser has the advantage of flexibility and control.The optical fiber structure has the advantages of small size,light weight,anti-electromagnetic interference,and better sensitivity than electrical sensors.Therefore,the study of laser ultrasound seismic physical model imaging technology has important significance.In this paper,the seismic physical model imaging system uses a high-power short-pulse laser as the excitation source,combined with metal film photoacoustic functional materials,to generate high-quality ultrasonic signals on the surface of the seismic physical model.Its main technical advantages are: non-contact excitation of different modes of ultrasonic waves(longitudinal waves,transverse waves,surface waves)on the surface of the model to be tested,and flexible adjustment of laser sound sources(points,lines,rings,etc.);Advanced functional materials increase the laser damage threshold of the model surface while enhancing the acoustic excitation to realize non-contact and non-destructive testing.It is a new technology for the ultrasonic excitation of seismic physical models.The experiment uses laser ultrasound technology combined with pulsed laser to irradiate the sample to perform seismic physical model imaging.In order to achieve the purpose of enhancing the laser ultrasonic signal,a layer of metal functional material film is attached to the surface of the model.The sound pressure is linearly related to the intensity of laser radiation.The excited ultrasonic waves are transmitted to the seismic physical model,and a standard piezoelectric ceramic transducer(OLYMPUS NDT)is used to collect the echo signal reflected by the internal structure.Finally,the time inversion algorithm of seismic physical model imaging is used to reconstruct the data of the model.Then,a Fabry-Perot optical fiber ultrasonic sensor was designed to improve the defect of the piezoelectric ceramic transducer as the receiving end.The transducer was used to excite ultrasonic waves of different frequencies and different energy levels to test the sensor's response to ultrasonic waves.Experimental results show that the sensor has a good performance for ultrasonic detection,and it can be used as the receiving end of an imaging system in future research,in order to realize an all-optical laser ultrasonic seismic physical model imaging system.In this paper,four metal thin film materials(gold,silver,copper and aluminum)are mainly selected for the research of laser ultrasonic seismic physical model imaging.The photoacoustic response signal response of different materials was tested and characterized,and the different thickness of the same material and the same thickness of different materials were compared with the experiment,aiming to solve the problem of low photoacoustic conversion efficiency and photoacoustic signal in the research of seismic physical model laser ultrasonic imaging technology.Weak and other issues.The gold nano-film material with good photoacoustic effect is combined with the seismic physical model.The nano-scale film interacts with the laser to generate acoustic waves,and the propagation process within the seismic physical model is further simulated and analyzed,and the simulation data is obtained.Then compare with the experimental data processing results continuously optimize and improve.The research content of this paper has important scientific significance and practical application value for the research of laser ultrasound imaging.