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Ground-penetrating radar (GPR) can detect the distribution of volumetric water content under the ground to analyze the propagation velocity of electromagnetic waves. However, the level of water table in standard soil layers including many scatterers is difficult to be measured by using GPR, because the amplitude of signals from a moisture transition zone is frequently smaller than that from stones. We evaluate time-shift analysis for time-lapse GPR records acquired from layers with different distributions of water content including strong events from scatterers to detect events of relationship with ground water. The reflection event from the transition zone of water content is sometimes difficult to detect on a survey section of GPR and also it is sometimes difficult to detect fluctuations of water table using the survey section subtracted between two time-lapse records. However, the time-shift analysis can detect slight changes caused by shift of the depth of a transition zone, if other processings cannot be applied well. Upper events of the transition zone move to shallower record time and lower events move to deeper time when a water table moves to a shallower area, when time-lapse GPR records with many scatterers are applied with the time-shift analysis. We demonstrate such characteristic phenomena using FDTD numerical simulations and discuss the analysis results applied for time-lapse records acquired in an actual field.

We conducted a detailed near surface geophysical survey at Mashiki Town, Kumamoto Prefecture, western Japan, where was severely damaged by the 2016 Kumamoto twin earthquakes of magnitudes 7.3 as the mainshock and 6.4 as a foreshock. The near surface survey comprised "Hybrid Surface Wave Survey" (HSWS), capacitively coupled resistivity (CCR) measurement using OhmMapper, and GPR measurements using Utility Scan DF or 350 HS tools. A total of 5 short survey lines were set to intersect a branched surface rupture, or placed in the downtown area of Mashiki Town. The purposes of the survey were to assess the usefulness of the above geophysical methods for the delineation of near-surface conditions of such earthen structures as levees and road embankments attacked by strong earthquakes, and to provide high-resolution subsurface profiles of the sites where surface structures were sporadically damaged. Notable results of our study were as follows. First, GPR successfully imaged detailed structures on the surficial zones form 2 m to in case to 5 m in depth. It was characteristic that a number of step-like sharp dislocations were identified around the surface rupture. Layered resistivity structures, concordant with inferred geological structure of the area, were imaged by CCR surveys up to 10 m in depth. HSWS, recently proposed by the authors, reconstructed S-wave structures up to 40 m in depth. In addition, HSWS records were processed through an ordinary seismic reflection data processing flow. As a result, reverse faulting structure was clearly imaged in concordance with the other near-surface survey results.

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