Influence of capillary fringe on the groundwater survey using ground-penetrating radar

Abstract

Ground-penetrating radar (GPR) is well known as one of the best tools to detect groundwater surface. However, very few people take into account the effect of a capillary fringe over water table with groundwater survey. Water contents vary from 100 percent to several percent in this capillary fringe. The behavior of contaminants is highly influenced by this transition zone. This also has an impact on the design of detailed exploration such as a time-lapse survey. In this study, we took a stepwise approach to examine the influence of the capillary fringe. First, the response of electromagnetic waves reflected from water table was examined in a small scale model in our laboratory. The result indicates that multiple reflections are sometimes detected around the water table and the capillary fringe. It also proved that the wave event which is considered to be reflected on the water table was actually emerged from the transition zone of water content inside the capillary fringe. The waveform of this reflection is not sharp compared with other reflections. This is because the dielectric constant gradually increases in the capillary fringe. In the second step, a field survey was applied to the edge of Kurobe alluvial fan along the seashore. Our instrument having real-time kinematic GPS system enabled us the simple and rapid survey in the wild field with complicated surface roughness. Observed reflections are unshapely detected as the similar shape shown in the scale model experiment. Finally it is concluded that the influence of the capillary fringe should be taken into account the analysis of the detailed groundwater survey.