Characterization Of Settlement Process And Thickness Detection Of Reclaimed Soil Overburden Based On Ground Penetrating Radar

The exploitation and utilization of mineral resources is the basic source of human production and life.However,due to long-term,large-scale and high-intensity resource exploitation,many environmental problems such as farmland destruction have been caused.As an important means of land restoration in the mining area,land reclamation has made remarkable achievements in the field of environmental restoration in the mining area.However,the subsequent soil layer settlement problem seriously affects the change of the overburden thickness and the stability of the soil layer.Therefore,it is of great significance to characterize the settlement process of the soil layer quickly,accurately and nondestructive and to determine the change of the overburden thickness.As a new detection technology,ground penetrating radar(GPR)has the advantages of continuous detection in a large range,fast and nondestructive in the detection of soil structure,making up for the shortcomings of traditional methods.In order to explore the feasibility of ground penetrating radar in characterizing the soil layer settlement process and determining the overburden thickness,this paper,through field tests,uses ground penetrating radar as the data acquisition platform,and combines Hilbert Huang transform,short-time Fourier transform and other signal transformation methods to explore the changes of different characteristics of ground penetrating radar signals with the settlement process,thus realizing the characterization of the soil layer settlement process;And by building the relationship model between characteristic information and dielectric constant,combined with the time domain information of specific layered location,the thickness of overburden layer can be determined.The main conclusions in this paper are as follows:(1)The original ground penetrating radar signal contains a variety of characteristic parameters,mainly including energy,amplitude and frequency.Under different overburden thicknesses,the fluctuation trend of the signal is basically consistent,but the energy,amplitude and frequency information will be different.The original signal of the ground penetrating radar can reflect the overall attenuation trend of the electromagnetic wave when it propagates in the soil layer.The original signal of the ground penetrating radar is processed and analyzed through Hilbert transform and Fourier transform respectively.With the increase of the thickness of the covering soil,the initial energy peak value gradually rises,the takeoff position advances,and the peak frequency moves forward;The radar signal after gain processing can reflect the amplitude and frequency information at a specific location.After processing the gain signal through two time-frequency analysis methods,namely,short-time Fourier transform and Hilbert Huang transform,it is found that with the increase of the overburden thickness,the amplitude information at the interface between the overburden layer and the filling layer gradually increases,and the frequency component is mainly stable between 300-400 MHz and gradually decreases.(2)The change of characteristic parameters of ground penetrating radar signal in time series can characterize the settlement process of overburden.The settlement process of the overburden layer is mainly divided into two stages: external load settlement and self consolidation settlement.The change of radar signal characteristic parameters under different overburden thickness is basically the same.In the external load settlement stage,with the passage of time,the overall attenuation degree of the signal gradually increases,the initial amplitude value decreases,and the peak frequency component moves forward.The minimum value is about 250 MHz.The frequency component at the interface mainly decreases in this stage,and the amplitude intensity decreases;In the self consolidation settlement stage,with the discharge of water in the soil,the attenuation degree of the signal on the whole weakens,the peak frequency moves backward,gradually increases to about 400 MHz,and then remains stable.The frequency change rule at the interface is the same as it is,and the amplitude intensity rises slightly,and then remains stable.According to the actual soil layer thickness change curve,the frequency information of the signal in the overall and local positions keeps the same change trend with that of the signal;In addition,according to the change law of radar signal,the soil is mainly subject to instantaneous settlement within 30 days after reclamation and consolidation within 15 days,indicating that the change of radar signal characteristic parameters can provide a reference for the characterization of soil layer settlement process.(3)Hilbert Huang transform has good applicability in soil layer thickness detection.According to the Hilbert spectrum after signal processing,on the one hand,a clear time domain of the overburden layer is obtained by means of time energy information.Under different overburden thicknesses,the time domain position at the interface moves backward with the increase of overburden thickness;On the other hand,the instantaneous frequency information provided by Hilbert spectrum eliminates the high-frequency and low-frequency signals in the original signal,and provides accurate frequency change information.There is a good correlation between the average value of the instantaneous frequency and the relative dielectric constant.Among the relationship models established by multiple regression methods,the polynomial regression method has the best modeling effect,with a modeling R2 of0.8870;In the final overburden layer thickness detection,with the increase of the overburden layer thickness,the detection accuracy will decline slightly,and the average relative error of the overall detection accuracy is about 3%.