| High-resolution time-lapse electrical resistivity tomography (ERT) measurements were conducted to monitor underseepage beneath a model levee, 2 m high, 12 m wide at the bottom. A small but high-permeable channel had been buried beneath the model levee. A total of 13 m3 or 26 m3 water was poured separately into a seepage pond dug at the foot of the levee. Dense 2D or 3D electrode arrays, set to interest the levee just above the buried channel, were utilized to monitor groundwater infiltration through the channel. Supplementary seismic measurements were also carried out at the model levee. The first test, conducted in March 2018, revealed unexpectedly high permeability characteristics of the channel. It took only three and a half hours to observe water seepage at a foot zone of the opposite side of the levee. At the second test, surveyed in March 2019, we increased the pouring water to 26 m3 and shortened monitoring intervals from 15 minutes to 6 minutes. As a result, underseepage water migration was clearly imaged on the ERT sections. Hybrid surface wave survey and SH-wave seismic tomography results also identified water infiltration along the channel as S-wave velocity decrease zone. The field tests demonstrated the effectiveness of high-resolution, both in space and in time, geophysical measurements for the delineation of potential underseepage zone and its migration through a specific channel. |
