Lilong Zou

Calibration With High-Order Terms of Polarimetric GPR

To provide meaningful data, polarimetric ground penetrating radar (GPR) requires accurate calibration of channel imbalance and crosstalk not only in the amplitude term but also in the phase term. Currently, calibration techniques usually ignore higher-order terms to simplify equations. These techniques, though very easy to perform, provide less accurate calibration results for the crosstalk. To improve the accuracy of calibration, we have derived a new mathematical formulation to calibrate a polarimetric GPR system. We measured several scattering matrices to obtain the necessary calibration parameters. The calibration technique was tested from measurements conducted on a 22.5° oriented dihedral corner reflector from 1 to 6 GHz with a step of 0.1 GHz. The calibrated results are found to be consistent with the theoretically values, especially for the crosstalk.

Developing a novel full-polarimetric GPR technology

Generally GPR transmits and receives radio waves with a single polarization using two parallel antennas. But it is possible to improve the GPR ability of discrimination and imaging of subsurface targets by analyzing the backscattered wave with a variety of polarizations. So we are developing a full-polarimetric GPR system, including PC, network analyzer, rectangular coordinates robot, switch driver, and polarimetric antenna array. Polarimetric antenna array is used to transmit and receive both co-polarimetric and cross-polarimetric signals. Currently polarimetric GPR do not execute precise calibration. But good calibration can improve the classification ability of subsurface targets. So we introduced the calibration technique into the polarimetric GPR, and derived a calibration formula.

Developing calibration technology for full-polarimetric GPR

Polarimetric GPR requires accurate calibration of channel imbalance and crosstalk not only in the amplitude term but also in the phase term. Currently, there have some calibration techniques. Though these techniques are very easy to perform, they provide less accurate calibration results for the crosstalk. To improve on the accuracy of calibration, we have developed a mathematical formulation to calibrate polarimetric GPR data. We measured several scattering matrices to obtain the necessary calibration parameters. The calibration technique was tested from measurements conducted on dihedral corner reflector.

3D velocity model and ray tracing of antenna array GPR

Migration is an important signal processing method that can improve signal-clutter ratio and reconstruct subsurface image. Diffraction stacking migration and Kirchhoff migration sum amplitudes along the migration trajectory, which generally is hyperbolic. But when the ground surface varies acutely, the migration trajectory is not hyperbolic. To computer the migration trajectory need the technique of ray tracing. We introduce a method of ray tracing based on 3D velocity model. Firstly, we build the 3D velocity model depending on the estimation of both ground surface topography and velocities. Then we compute the travel time between transmitter, receiver and each subsurface scattering point, and search the propagation ray depending on the Fermats principle. The method is tested by an experiment data acquired by the stepped-frequency (SF) CMP antenna GPR system. The target is a metal ball that is buried under a sand mound. A nice result of ray tracing is shown in the case.

Propagation characteristics of joint physical simulation of both electromagnetic wave and ultrasonic wave in fractured media model

Fractured reservoir plays an important role in exploitation of the oil-gas recourses especially shale gas, at the same time the fractures have an important influence on the dams, bridges, highways and other builds. So fracture detection and its attribute evaluation is an important aim on study of exploration geophysics. Both electromagnetic wave and elastic wave have a good effect on fracture detection and its attribute evaluation. Joint exploration and inversion is an effective way to improve the accuracy of exploration. But we needs further study on the propagation response characteristics of electromagnetic wave and elastic wave. Physical simulation is an intuitionistic and effective method in study two kinds of wave-field propagation response characteristics in fractured media. So we carry out the combining electromagnetic wave and ultrasonic wave physical simulation experiment on fractured medium model. We have established a three-dimensional physical model with a series of tiny parallel fractures in the vertical. And we have explored this model using by ground penetrating radar, and collected electromagnetic sounding data of three-dimensional co-polarization and cross-polarization, and analysed polarization characteristics. At the same time we have explored this model using three-component transducer, and collected elastic wave data of three-dimensional, and analysed shear wave propagation characteristics. By making a study of the change of co-polarization and cross-polarization response amplitude values and the phenomenon of S-wave splitting in elastic wave, we can judge the direction and density of cracks and other properties in it. By study of the two kinds of wave-field physical simulation, we can lay a solid foundation for joint inversion of electromagnetic wave and ultrasonic wave in the future.

Study on the effect of LNAPL pollution to soil complex permittivity

According to the principle of reflection of electromagnetic waves, the authors use the open-ended coaxial probe method which operate the high frequency electromagnetic wave (200 MHz-8.5 GHz) to measure the dielectric constant of sand saturated with water and LNAPL (light non-aqueous phase liquid) under the laboratory conditions. The experimental results show the real part of the complex permittivity increases with LNAPL volume increasing and the imaginary part changes very little. However, the variation of the complex permittivity is not obvious after the sand is saturated. When the quartz sand contains water, the real and the imaginary parts of the complex permittivity increase with the volumetric content of LNAPL increasing. After some hours, both the real and imaginary parts of complex permittivity of the sand partly saturated with water and LNAPL have changed. In the study frequency range, the real part of complex permittivity decreases gradually with the frequency of electromagnetic wave elevation, the changes of the imaginary part is different. The experimental results show that the change of the mixture structure due to the LNAPL replacing the air in the pore and the absorption of the LNAPL, water and medium changes the ionic polarizability and further changes the dielectric properties of the mixture.

Polarimetric GPR Vivaldi Antenna Array and Experiments

Conventional GPR use a single antenna mode for transmission and reception, resulting in a response only to Co-Polarization signal. However, we expect that more polarmetric information can be obtained if CrossPolarization signal is also measured. So we developed a polarimetric antenna array using Vivaldi antenna elements, due to their wide bandwidth and nondispersion nature. The paper describes design and realization of the polarimetric antenna array accommodated on the full-polarimetric GPR system. Two groups of experiment have been performance. The first group is concerned with the standard dihedral which is made of two orthogonal conducting plates as the targets to prove the availability and accuracy of the polarimetric Vivaldi antenna array. The second group is using a metallic cylinder as the target. The results of experiment are shown in this presentation that CoPolarization and Cross-Polarization information can be obtained for each transmission and help us to identify target attribute such as direction.