Study On Leakage Monitoring Model Of Impermeable Layer Based On Ultrasonic Scattering Characteristics

The implementation of GB 16889-2008 has played a guiding and promoting role in the construction of our standardization,standardization and internationalization of landfill.At the same time,with people’s pursuit of high quality life,the amount of household waste was increasing year by year.As one of the main ways of waste disposal,landfill environmental protection operation has become the focus of supervision.The existing landfill usually uses HDPE film for seepage prevention.Because manual welding is not standard and other reasons,resulting in the landfill before operation of HDPE film damage.If not timely positioning and repair,this case will inevitably lead to leachate pollution of groundwater.In this thesis,the propagation characteristics of Lamb waves in HDPE films were analyzed based on the elastic dynamics theory.The Rayleigh-Lamb frequency equation was obtained and the relationship between frequency and wave number of ultrasonic Lamb waves in HDPE films was established.The group velocity dispersion curve and phase velocity dispersion curve of HDPE film were solved,which provides a basis for the selection of excitation signal in the follow-up detection.Then COMSOL was used to simulate Lamb wave conduction in HDPE film,and the scattering and amplitude attenuation phenomenon of Lamb wave through the damaged position were verified.Then the spacing of excitation signal and piezoelectric sensor was determined according to the Lamb wave dispersion curve.The characteristics of energy attenuation was caused by ultrasonic scattering through leakage position.The energy density parameters with and without leakage were extracted by wavelet analysis,and the damage index was defined according to the parameters.Combined with the improved damage imaging algorithm,the damaged location model of HDPE film was established.Secondly,a signal noise reduction model based on the improved MP algorithm was established.This model solve the problem of flooding damage characteristic information caused by strong background noise and weak echo signal in HDPE film damage detection.In this model,the noise reduction problem of ultrasonic Lamb wave detection signals was transformed into an objective function optimization problem.The model can complete the reconstruction of Lamb wave echo signal and realize signal noise reduction by establishing the super complete dictionary,determining the objective function and superimposing the optimal matching atom.In order to verify the effectiveness of the noise reduction model,the simulation signal containing noise was established and the noise reduction performance is analyzed.Finally,according to the above theories,the ultrasonic Lamb wave detection system of HDPE film was built.The hardware design was carried out for ultrasonic transmitting and receiving module,gain amplification module,high-speed A/D conversion module and control chip.Lab VIEW was used to design the login interface,data acquisition module,data storage and playback module,USB data transmission module and waveform display module.According to the built software and hardware system,design and complete the test.The experimental results showed that the collected ultrasonic Lamb wave echo signals can achieve better positioning accuracy after being processed by the noise reduction model.By comparing the damage index DI_p defined by noise reduction signal with the damage index DI defined by wavelet analysis method,it was found that DI_p was more sensitive to damage point identification around the sensing path.In the case of single damage point and double damage point,the maximum deviation was 2.6mm in the case of single damage point and 6.4mm in the case of double damage point.The positioning accuracy of the smaller wave analysis method was improved by 68.2% and 54.0%.