| Infiltration process of water into a small model levee was monitored at short intervals by means of a high-speed resistivity measurement tool. The tool can transmit 24 individual current signals simultaneously and measure 24 potential responses. A code division multiple access (CDMA) technique is adopted for the current signal transmission. Because of its short interval time, only 30 seconds are enough to obtain and store a total of 576 (24 x 24) dataset. It allows us to conduct high-speed time-lapse or dense 4D ERT measurements in the field. All current signals and potential response waveforms are recorded and used to calculate covariance matrix to obtain individual potential responses. We built a small, 2 m high by 6 m wide model levee, and infused water into the levee from a trench set on the crown. A total of 4 current lines and 4 potential lines were set parallel on the crown and the slope of the levee. Electrode stakes were placed at 25 cm spacing along each line, and spacing between the lines was set to 25 cm. A total of 6,912 (12 pairs x 576) data were acquired at 8 min intervals in the beginning and at 30 min to half a day in the later stage. 4D tomographic analysis successfully imaged a 4D state of infiltrated water in the levee body. Consequently, the field measurements demonstrated the effectiveness of dense 4D electric resistivity tomography for the characterization of dynamic unsaturated to field saturated permeability of levee body materials in the field. |
