ANALYSIS OF TIME-SHIFT DISTRIBUTIONS BETWEEN GPR RECORDS WITH SCATTERED EVENTS TO DETECT FLUCTUATIONS OF WATER TABLE

摘要

Ground-penetrating radar (GPR) can detect the distribution of volumetric water content under the ground to analyze the propagation velocity of electro-magnetic 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.