Journal of the Korean Society of Mineral and Energy Resources Engineers ISSN:2288-0291(Print) 2288-2790(Online) 한국자원공학회지

We applied a nonlinear ray tomography using GPR(Ground Penetrating Radar) data in order to image the inside of the base part of stone pagodas non-destructively. Nonlinear ray tomography is a method that reconstructs the image of the velocity structure from the first arrival time data. In forward calculation of raypaths and traveltimes, the fastest paths calculated in polar coordinates were used with Fermat\'s principle and the principle of reciprocity. In inversion problem, we adapted the iterative backprojection method which is known to be able to image the smoothly varying medium or a complex structure. We made a model consisting of four different inner stones filled with soil as a model of the base part of a stone pagoda. From the results of the model studies, we analyzed the sizes and the positions of the inner stones accurately, whereas we could not distinguish the exact shapes of the inner stones. We obtained the similar results from a model with connected three stones and one stone. Also a diagonal air layer model and two horizontal and vertical air layers model were chosen and the resulting images revealed the dimension and the direction of the air layer quite well. Therefore we concluded that this tomography technique gives the information about existence of void region that affects mainly the safety of the base part of the stone pagoda. In actual GPR survey on Jeongnimsaji five-story stone pagoda, we applied nonlinear ray tomography and got inner image similar to the migrated image.