The development of underground structure estimation system using the electromagnetic wave

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Underground exploration technology using the electromagnetic wave is divided roughly into 2 types. One is the ground penetrating radar (GPR). GPR has been attracting interest for purpose of detecting pipes (communication cables, gas pipes, electricity cables), sewage lines, tunnels and other subsurface spaces in situations calling for nondestructive exploration and evaluation. The other is the borehole radar. The borehole radar has been attracting interest for purpose of detecting the crack of the rock and the cavity, evaluating the stratum. In recent years, not only the locations and sizes but also the electrical property (permittivity and conductivity) distributions of underground objects are of interest. A new inversion algorithm for reconstructing electrical property distributions of underground cylindrical objects by means of the measured time-domain field data is presented. Main results of achieving the term of this project are as follows :(1) FBTS method proposed for reconstructions of dielectric objects in free space is improved in order to apply to the underground exploration.(2) Numerical simulations show that improved FBTS method can estimate the electric properties of underground structure.(3) As the calculation time of FBTS method is very much lengthened for widening a search region, the speedup of the computation speed was carried out by combining the synthetic aperture processing with the FBTS method.(4) The Filtered FBTS method was proposed in order to enable the reconstruction from the data with large noise.(5) However, the expansion of reconstruction area and existence of large noise degrade the quality of reconstructed image and require long CPU time. In order to improve them, we propose a new reconstruction algorithm that combined Filtered FBTS algorithm with multigrid.