Abstract
To make three dimensional (3-D) soil water flows easily visible, a semi-3-D resistivity monitoring method was used based on the 2-D difference inversion technique. 2-D resistivity surveys of 4 traverse survey lines were conducted continuously at a groundwater recharge test site on the Shirasu Plateau and the resistivity change ratio distributions were derived using constrained non-linear differential tomography analysis. The resistivity distributions before the recharge test reflected the geological column at the boreholes and it was assumed that the geology of this site had a layer structure. During the recharge, resistivity changes reflected changes in the degree of soil saturation at the boreholes. This shows that 2-D difference inversions can be used to image soil water flows. Low resistivity sections spread along the secondary Shirasu layer. From this it was assumed that the soil water spread horizontally along the secondary Shirasu layer. The volumetric water content at each designated depth using a neutron moisture meter and the resistivity due to difference inversion did not change in the loam layer. This may indicate the occurrence of partial flows in the loam layer. Each survey line gave a non uniform distribution for the resistivity change ratio that reflected the fact there was no uniform filling of water in the recharge area. These results indicate that multi-line 2-D resistivity surveys make it possible to monitor easily 3-D flows of soil water in shallow geological layers compared with a 3-D survey.
