Research On Reverse Time Migration Imaging Of Three-dimensional Multi-offset Ground Penetrating Radar Data

As an important shallow geophysical exploration method,ground penetrating radar(GPR)has been widely used in many fields,such as geological survey,engineering investigation and environmental monitoring,due to its advantages of high efficiency,high precision,nondestructive detection and real-time imaging.In recent years,the increasingly refined detection of underground shallow structures,complex distribution of shallow structures and strong urban electromagnetic interference often make it difficult to identify and interpret the effective waves in radar profiles.Therefore,it is particularly necessary to develop efficient and high-precision data processing and imaging methods.Reverse time migration,as the most accurate migration method,has been widely used in two-dimensional GPR data imaging.However,in practical GPR detection,high frequency electromagnetic waves propagate in underground three-dimensional space,and conventional two-dimensional reverse time migration is difficult to achieve accurate imaging of three-dimensional GPR data.In addition,due to the influence of underground conductivity,high-frequency electromagnetic waves propagate in underground media and exhibit strong attenuation characteristics.Conventional reverse time migration mostly fails to compensate the attenuation energy of electromagnetic waves,so the imaging accuracy is limited.In view of the above problems,this paper studies the method of three-dimensional multi-offset GPR data reverse time migration and electromagnetic wave attenuation compensation reverse time migration.The main contents and conclusions are as follows:(1)Reverse time migration is described in detail.And the three-dimensional multi-offset GPR data reverse time migration algorithm is constructed on the basis of the finite difference time domain method and normalized cross-correlation imaging conditions.The calculation flow of reverse time migration is given,and the MATLAB program is compiled.On this basis,the influences of direct wave,filtering,migration velocity model and center frequency of the source wavelet on the accuracy of reverse time migration imaging are analyzed.The three-dimensional reverse time migration imaging of the forward modeling data of two typical geoelectric models is carried out and compared with the two-dimensional reverse time migration.The results show that the three-dimensional reverse time migration imaging location is more accurate,and the description of the underground target morphology is more consistent with its real spatial distribution.(2)The conventional data processing method and the multi-migration data velocity spectrum analysis method to construct the underground two-dimensional velocity model are introduced.The importance of conventional GPR data processing before reverse time migration for GPR is discussed with the practical examples of reverse time migration.By combining the velocity spectrum analysis results of the multi-offset data of multiple measurement points on the survey line,a more accurate subsurface two-dimensional electromagnetic velocity model is constructed.And it is applied to reverse time migration imaging to effectively improve the resolution of radar profile.(3)Based on the attenuation characteristics of high frequency electromagnetic wave propagating in underground medium,the reason of low imaging resolution in high conductivity region with conventional reverse time migration is analyzed.On this basis,by changing the electromagnetic wave reverse-time extrapolation of wave equation with the plus or minus a derivative of the conductivity of which human guarantee back propagation of electromagnetic wave time symmetry and invariance,compensate the energy attenuation of electromagnetic wave in the forward transmission,building a three-dimensional reverse-time migration based on the electromagnetic wave attenuation compensation algorithm,one of the high density resistivity method as the offset conductivity model inversion results.Numerical and physical test results show that the reverse time migration based on electromagnetic wave attenuation compensation can accurately compensate the energy attenuation during electromagnetic wave propagation.Compared with conventional reverse time migration,the high conductivity region has better imaging accuracy and higher resolution.