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An artificial groundwater infiltration experiment was conducted in Nagaoka city in Japan, and time-lapse cross-borehole ground-penetrating radar (GPR) data were collected to monitor the infiltration process in the vadose zone using zero-offset profiling (ZOP) mode. The downward migration of induced water was observed as a variation of GPR travel times, which can be transformed into dielectric constant and further converted to volumetric water content. In this paper, we present an effective approach to extract accurate information about the hydrogeologic process in the vadose zone from ZOP data. This approach is based on a least squares inversion method using singular-value decomposition, in which a finite-difference time-domain forward modeling is used for computing electromagnetic wave fields on 2-D cylindrical coordinates. The inversion approach is validated using a synthetic example and applied to the field data. We can successfully estimate the variation of soil water content during infiltration in the Nagaoka site from the reconstructed dielectric constant models. The inversion results show that the saturation information is useful to assess hydrogeologic properties of the test soil zone.