| Combined GPR and integrated geophysical surveys were conducted at an underseepage site where leakage had occurred accompanied with rise in river water level not only at the foot of a levee but also at the hinterland about 60 m far from the levee. GPR survey, with aid of high-precision VRS-GNSS positioning system, densely covered a paddy field of 60 m wide by 100 m long in the hinterland, and successfully identified an irregular shape of the bottom of surface soils at shallow depths from 30 cm to 3 m. 2D high-resolution surface wave and DC resistivity surveys along a total of 6 lines delineated the very shallow surface structure up to 4 m and clearly identified an underlying layer having high Vs and high resistivity. The depths and shape of the boundary were quite concordant with each other. We then reconstructed a 3D shallow surface model based on the geophysical data and interpreted it in combination with the surface leakage information. As a result, distribution of leakage prone zones in the hinterland matched well with the portion where the thickness of surface soils was very thin or high permeable substrata were underlain at very shallow depths. The identified substrata were interpreted as chute bar or crevasse splay deposits which might have high potentiality in underseepage. The field surveys demonstrated the effectiveness of detailed geophysical methods for locating potential underseepage zone in the hinterland in 2D or 3D. |
