Surface-to-Borehole and Cross-hole ERT Tomography in a Test-Bed of South Korea

In a 57.5 m by 60 m test-bed in South Korea, surface-to-borehole and cross-hole electrical resistivity tomography (ERT) was performed between five boreholes (BH-1 ~ BH-5) in order to delineate the distribution of weathered rock; surface ERT and seismic refraction survey were also made in the site together with several geophysical well logging and cross-hole seismic tomography. Since the test-bed has steep sides at west, south and north directions, tomography data can be distorted by small-plateau-like topography. Near the slope at the west side, BH-1 and BH-2 lie while at the south side BH-3. BH-5 lies at the center of the rectangle, which the other four boreholes make. Before making inversion of surface-to-borehole or cross-hole ERT data, we first make analyze effects of plateau-like topography on ERT data through numerical tests for a similar model of the test-bed to confirm the topography produces very resistivity artifacts. Based on the analysis, we first corrected the distorted ERT data by the topography, and inverted the corrected ERT data. Subsequently, we directly inverted the distorted ERT data with the consideration of the plateau-like topography; we included the topography in the forward simulation while excluded in inversion blocks. By comparing both inversion results properly identified the water-saturated weathered rock layer, which agrees with well logging data.

Acknowledgements

This work was supported by Korea Ministry of Environment as The SEM projects (No. 2018002440005) and the Institute for Korea Spent Nuclear Fuel (iKSNF) and Korea Foundation of Nuclear Safety(KOFONS) grant funded by the Korea government(Nuclear Safety and Security Commission, NSSC)(No. 2109092-0121-WT112).