Ground Truth Verification Of An Integrated Geophysical Investigation For The Assessment Of An Earthen Levee

Abstract

We proposed an integrated geophysical technique to assess the vulnerability of earthen levees to flooding and earthquakes. The geophysical technique mainly consists of multi-channel surface wave dispersion measurements (MASW) using Land Streamer, capacitively-coupled resistivity (CCR) measurements, and additional multi-frequency electromagnetic (EM) survey. The effective performance of field measurements using these methods is assured because sensors are not fixed on the levee surface. The geophysical properties evaluated by these methods, resistivity and S-wave velocity, are essential to evaluate the permeability and stiffness of the levee body. We also demonstrated the usefulness of crossplots of the measured data as criteria for the assessment of levee conditions. However, the collection of ground truth data is imperative for the validation of the criteria. Consequently, we then conducted detailed field measurements at an open-cut of an earthen levee on which the integrated geophysical survey had been applied. The investigated levee, which runs along the right bank of Chikuma River in central Japan, was damaged by a high water event in 2006. In addition, open-cut work was planned within the surveyed line to replace an old sluice. We therefore could observe the internal structure of the levee and take samples of levee materials for grain size analysis at the open-cut. In conclusion, the mapped resistivity profile was concordant with the surface survey results. Moreover, a clear relationship was recognized between grain size and measured resistivity. We interpreted the high resistivity anomalies in the levee body detected by means of surface geophysical survey as potential high-permeability zones. Actually, seepage was observed along the rear slope of the levee just in anomaly zones delineated by the integrated geophysical investigation.