Infiltration process of water into the surficial vadose zone of a levee system was monitored at short intervals by means of a newly developed high-speed resistivity measurement tool. The tool can transmit 24 individual current signals simultaneously and measure 24 potential responses. A code division multiple transmission technique is adopted for the current signal transmission. Because of its short interval time, only 30 seconds to obtain and store a total of 576 (24 x 24) dataset, we can conduct high-speed time-lapse or dense 4D resistivity measurements in the field. The tool has been used for monitoring infiltration process of surface water into the vadose zone of actual levee. We set a total of 4 current lines and 4 potential lines parallel on the slope of a levee, and infused water into the vadose zone from a small ditch dug on the levee crest. Electrode stakes were placed at 25 cm spacing along each line, and spacing between the lines was set to 50 cm. A total of 6,912 (12 pairs x 576) data were acquired at 5 min intervals in the beginning and at 30 min later. 4D tomographic analysis, applied to the dataset composed of a total of 40 stages during the infiltration across two days, successfully imaged the extent of infiltrated water in the vadose zone. Consequently, our field measurements demonstrated the usefulness of dense 4D electric resistivity tomography for the characterization of dynamic unsaturated permeability of the vadose zone of levee systems in the field. |