Assessment of Seepage in an Embankment Dam Using Very Low Frequency Electromagnetic and Geoelectrical Methods

Abstract

Geoephysical surveys involving Very Low Frequency Electromagnetic (VLF-EM), Vertical Electrical Sounding (VES) and 2D resistivity imaging were conducted along the embankment of Asejire dam to detect potential seepage zones and assess the integrity of the dam. 750 VLF-EM measurements were made at 10 m station interval using the VLF-EM Equipment. 24 Schlumberger VES were conducted at 20 m interval using resistivity meter and its accessories. The current electrode spacing (AB/2) was varied from 1 m to 100 m. The 2D resistivity profiling employed the dipole-dipole configuration with electrode spacing, a = 20 m and expansion factor, n = 1 - 5. The VLF-EM data were processed and modelled using Fraser Filtering and Karous-Hjelt software to delineate subsurface zones of varying conductivities suggesting anomalous seepage. The VES data were quantitatively interpreted using the partial curve matching technique and 1D resistivity inversion algorithm while the dipole-dipole data were inverted using 2D resistivity inversion procedure. The VLF-EM inverted sections revealed prominently conductive zones indicating anomalous seepage zones beneath the dam embankment. The relatively less conductive zones possibly indicate reduced seepage. The results of VES interpretation revealed three geoelectric layers beneath the dam embankment representing the caprock, core and bedrock. The 2D inverted resistivity sections delineated zones with anomalously low resistivity generally less than 10 Ωm, indicating anomalous seepage, beneath the embankment. This study has demonstrated the effectiveness of combining the VLF and geoelectrical methods for delineating anomalous seepages in the assessment of dam safety. The anomalously low resistivity/high conductive zones identified beneath the dam embankment are suspected anomalous seepage zones which can threaten the integrity of the dam. Routine monitoring and remedial measures are therefore recommended to forestall the failure of the dam.