Repeated Resistivity and GPR Surveys for the Monitoring of Water Content and Temperature in the Unsaturated Zone

Repeated resistivity and GPR surveys were carried out in the KR-1 groundwater observation site, North-Kanto, Japan, where the measurements of near-surface water content and temperature have been conducted with the meteorological observation. The purposes of these repeated surveys were to detect the changes of hydrogeological environment in the unsaturated zone. Twenty-five electrodes were placed at 1 m intervals along a 24-m long line that crossed immediately near the soil moisture and temperature meters. The volumetric water content was measured every 10 minutes at ten depths from 0.6 to 10 m. The soil temperature was measured at depths of 0.6, 3, 10 m. Using dipole-dipole and Wenner electrode arrays, resistivity data were collected for every month from August 2006 to August 2007. A 2-D resistivity section was obtained for each resistivity data set and the seasonal resistivity changes were recognized. GPR surveys were carried out in December 2006, January 2007, February 2007 and July 2007. In these surveys, 3-D GPR data were acquired with a 350-MHz GPR system in the area of 16 m - 17 m around the soil moisture and temperature meters. 2-D GPR profile data for deep depth were collected with a continuous wave (5 to 150-MHz) GPR system along 53-m long and 55-m long lines. At several points, wide angle measurements were done to estimate the EM wave velocity. We compared the changes of resistivity and GPR profiles with those of precipitation, volumetric water content, and soil temperature. The resistivity near the surface increased from summer season to winter season and then decreased. However the changes of EM wave velocity and water content were relatively small. It seems that the change of resistivity was most influenced by the change of soil temperature.