Application of Two Dimensional Electrical Resistivity Tomography Survey for Delineating Cavities and Flowpath in Consolidated and Unconsolidated area of Pokhara Valley, Western Nepal

Pokhara valley represents an intermontane fluvial basin spread around the midstream of the Seti river in the Lesser Himalaya of Nepal. Two-Dimensional Electrical Resistivity Tomography (2D-ERT) Survey was conducted in unconsolidated area (Armala) and consolidated area (Mahendra Cave) to investigate subsurface anomalies for the detection of subsurface cavities, depressions, sinkholes, joint/fractures, groundwater flowpath and areas prone to sinkholes formation/subsidence.

2D-ERT survey was carried out using Multi-electrode Resistivity meter at 65 profiles and 12 profiles in unconsolidated and consolidated area respectively. Results shows resistivity value ranging from 30Ωm-14000Ωm representing subsurface variations with undulations in depositional pattern comprising of clay, clayey silt, silt, sand, rock fragments, conglomerates and limestone. Cavities were encountered in clayey-silt layer and gravel layer with size ranging from 1m-2m to 10m-12m in depth and 2m-10m in diameter either empty or water filled depending on locations in Armala area. Fifteen cavities, three major and four minor groundwater flow paths were detected which has been later confirmed by tracer test, formation of new sinkholes and during excavation for construction. Major flow path was detected at a depth of 7m in Armala area. Smaller cavities were encountered at a depth of 3-5m in Mahendra Cave area. The main cavity (Mahendra Cave) and its westward branching have been encountered at a depth of 10m from the surface.

The study demonstrates that ERT survey can effectively reflect and differentiate subsurface strata, caves and cavities, flow channels which will help in determining the level of geo-hazards in the area for proper planning.