Induced Field Rotation Interference From Rough Interfaces And Its Effects On Full-Polarimetric Ground Penetrating Radar Data

Full polarimetric ground penetrating radar(FP-GPR)is a nondestructive detection method that uses high-frequency electromagnetic waves of different polarization modes to detect the targets in subsurface.Compared with the traditional GPR,which only uses one pair of antennas for data acquisition and can only obtain single polarization data of the target,FP-GPR uses an antenna array with four different polarization modes to transmit and receive electromagnetic waves of different polarimetric modes,and can describe the polarization scattering characteristics of the target completely.When electromagnetic(EM)waves propagate through interfaces with different dielectric constants,induced field rotation(IFR)will occur.IFRs are related to the shape of targets,based on which the identifications can be performed for different targets.However,the IFRs occur not only on the surface of targets but also when the EM waves propagate through the ground surface or other subsurface interfaces.This polarization interference generated at the interfaces between antennas and targets hinders the FP-GPR from obtaining high-quality target scattering matrix from the complex shallow subsurface,and affects the application of polarization information to the improvement of the detection accuracy.If we can obtain more accurate information of the subsurface targets,the geometric and structural attributes of the target can be analyzed more accurately and the accuracy of analysis can be improved.This paper firstly studies the IFR effect of rough surface on high-frequency EM waves.By applying the transmission and scattering coefficients of the EM wave on the rough surface based on the small perturbation method(SPM),the mathematical relationship between the IFR from the rough surface and the measured data of the FP-GPR is established,and the calibration formula of IFR interference from rough surface is derived.The IFR coefficient C is proposed to describe the transmission scattering characteristics of polarized EM waves on rough interfaces,and the effects of different parameters of the rough surface,the medium,and the FP-GPR measurement on IFR coefficient C and the measured FP-GPR data are also studied.The results show that the parameters of the rough interface,such as the root mean square slope and correlation length,can affect the values of the four elements in the measured scattering matrix of the FP-GPR,but they have the same effects on the four elements.Therefore,the matrix and polarization analysis results will not be changed by the the rough interface parameters.The results also indicate that the incident angle of the EM wave on the rough interface and the relative permittivity of the media at two sides of the interface are the two main factors affecting the measurement data of the FP-GPR under rough surface conditions,and the impact of the incident angle is greater than that of the relative permittivity.For the H-Alpha decomposition results of three typical targets,the flat and dihedral are little influenced by IFR interference,but the cylinder such as the underground pipe is seriously affected.Simultaneously,a template for discriminating whether the calibration for H-Alpha decomposition is necessary is established.Both numerical and experimental tests validate the conclusions.A novel application strategy of H-Alpha decomposition is proposed,which has taken IFR from the rough surface into considerations.Finally,the calibration formula of IFR is applied to the field underground pipe detection,which greatly improves the accuracy of detection.China's first Mars probe Tianwen-1 carrying the Zhurong rover has successfully landed on the pre-selected area of the southern Utopia Planitia of Mars on May 15,2021.This mission focus on two scientific problems regarding the environment in which life existed,and the origin and evolution of Mars and the solar system.Therefore,the Zhurong rover is equipped with a full-polarimetric Mars rover penetrating radar(FP-Ro Pe R)system,aiming to map the morphology and geological structure of Mars and investigate the regolith characteristics and water ice distribution.This mission uses polarimetric technology to detect buried water ice,and its radar data will inevitably be affected by the rough surface and subsurface interfaces of Mars.Therefore,we study the effect of the induced field rotation on the FP-Ro Pe R data to provide support for water ice detection.The effects of IFR from multilayered rough interfaces on high-frequency EM waves were studied firstly.The mathematical relationship between the IFR on the rough surface and the measured FP-GPR data is extended to the case of multi-layered rough interfaces.The effects of different medium parameters and different measurement parameters of FP-GPR on the IFR are studied.The results show that,for the FP-Ro Pe R signals of Tianwen-1 Mars rover,the IFR from rough interfaces will result in the imbalance between two co-polarized Ro Pe R data,which will be stronger in the medium with greater relative permittivity and will decrease with the increasing of depth,and depth plays a more important role than the number of rough interfaces for the FP-Ro Pe R system,which will make a greater error in the shallow region(0–1 m),and the relative error in the regions of depths greater than 1 m can be guaranteed to be under 10%.Subsequently,the calculation formulas of the underground circular polarization ratio(CPR)(an important parameter for water ice detection)obtained by using the FP-Ro Pe R is derived,and the effects of different medium parameters on the IFR are studied.The results show that the CPR will be smaller than the real value when|S_HH|is larger than|S_VV|and|S_HV|or|S_HV|is larger than|S_HH|and|S_VV|,the CPR will be larger than the real value when|S_VV|is larger than|S_HH|and|S_HV|.The correctness of the results are verified by the forward simulation of multiple models,which is of vital importance to the detection of water ice by the Tianwen-1 Mars rover.