FBTS 法によるコンクリート中の空洞探査について検討した．入射電流源によって再構成画像の精度が大きく異なることを示し，低周波のパルスの必要性を明らかにした．空洞の深さや形状によって，パルスの形状やアンテナの配置を変更することおよび実測データを用いた空洞の推定が今後の課題である．%Ground penetrating radar has been attracting interest in recent years for purpose of detecting pipes, sewage lines, tunnels and other subsurface spaces in situations calling for nondestructive exploration. We proposed a forward-backward time-stepping (FBTS) algorithm for reconstructing electrical property distributions of buried cylindrical objects. Until now, though the upper part of a underground cavity was able to be detected, the size of the cavity was not able to be estimated. In this paper, the detection of the cavity in the concrete structure is performed by using FBTS method. The effect of the incident pulse on reconstruct image is discussed. It is shown that the size of a cavity is able to estimate by the use of the pulse with a low center frequency.
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