Time-lapse Cross-borehole Electrical Resistivity Tomography Monitoring in an Urban Tunnel Site

This paper describes how wells drilled during the geotechnical investigation of an urban tunnel into Holocene delta sediments can be employed to complete the geological information and to monitoring drilling processes with low cost experimental electrodes and cables for single-hole (SHERT) and cross-hole (CHERT) electric resistance tomography measurements. A low cost experimental electrodes and specific cables for SHERT and CHERT measurements were produced and fixed during PVC tube equipment as potential/current electrodes every 1 m around the PVC pipe directly. The high electrical conductivity of the sediments of the test site made it difficult to obtain a very large contrast between the tunnel shaft and the background, but inversion of resistance ratios allows to track time variations of resistivity as small as a few tens percent with respect to the background value. NG borelog and SHERT have been efficient to detect minor changes in clay content, and they can be used to refine the available geological information.