Study On Numerical Simulation And Imaging Algorithms Of Bore-hole GPR In Inhomogeneous Media

Abstract:Borehole-GPR operates in a single borehole or between two boreholes. But it also has the advantages that conventional surface GPR has. The hypothesis of majority of GPR numerical simulations is that the medium is isotropic. As the medium is inhomogeneous, CPML boundary conditions are used for borehole GPR numerical simulation. By derivation of the formulas, CPML boundary conditions have been tested. Both single-hole and cross-hole detection numerical simulation in inhomogeneous media were carried out. Comparing with numerical simulation in homogeneous media, it is obvious that the method works well for this simulation.For the borehole radar antenna has to be treated as a line source instead of a point source, formula derivatives and numerical simulation of borehole GPR were carried out. To simulate transmission, current distribution in the antenna was obtained by convolving the excitation voltage pulse with a certain parameter, and then the E field of the region between two boreholes was obtained. In terms of reception, I calculated it by convolution of the electric field generated by the receiving antenna with a certain parameter, and then multiply the result by impedance to obtain the receiving load voltage.First-arrival travel-time at large transmitter-receive angles is often difficult and inaccurate to pick by using common tools because of low S/N in the data. So I introduce the new travel-time picking scheme. First, I normalize each trace by signal maximum, and sort data into common-ray-angle gathers. Then, we align traces in each common-ray-angle gather by cross correlating with either:trace having highest signal-to-noise ratio, determine mean trace for each angle gather. Finally, I pick first-arrival travel-time on each mean trace, cross-correlate each trace in the dataset with appropriate reference waveform for that angle, and determine first-arrival travel-time.Combing the results of the numerical simulation and first-arrival travel-time based on the tomography theory, the apparent velocity of first-arrival wave in isotropic and inhomogeneous media was calculated. When the angle and the distance between transmitter and receiver are relative small and close, the apparent velocity is relative accurate in the calculation, but errors increase, reach its peak and then decrease as the size of the angle becomes large. Regardless of relative accuracy, the apparent velocity is, actually, larger than real velocity. Accordingly, an hypothesis about the path is proposed, the theoretical value of the hypothetical path is compared to the result from the simulation. It is proved that the first-arrival energy travels from the center along the transmitter antenna, and then it is received at the opposite tips of receive antenna, travels along the receive antenna from the tip to the center. So travel-time correction must be executed.In the test of synthetic data and field RAMAC data, the approach improves the results.Comparing with velocity tomography derived from common technology, the new velocity tomography is more accurate, more clearly, and more helpful to distinguish the target under the ground.