Subsurface cavity detection in a karst environment using electrical resistivity (er): a case study from yongweol-ri, South Korea

Karst voids were encountered during road construction in Yongweol-ri, South Korea Attempts to determine the extent of the karst voids through drilling was expensive, time-consuming and too dangerous An electrical resistivity survey was thus carried out to investigate current subsurface geology beneath the proposed road network This investigation was aimed at imaging karstic voids and detecting areas prone to ground subsidence through the collapse of cavities beneath a road segment overlying such features A numerical modelling study preceded the field survey to determine whether the electrical resistivity method could identify such features The field data set consisted of eleven electric profiles acquired using dipoledipole array; electrical resistivity profiles (100 m and 300 m length) were measured using a dipole-dipole electrode configuration and electrode spacing was kept at 5 m to ensure good coverage of the study area The resistivity data was inverted using 2D and 3D algorithms based on the least squares smoothness constraint technique The inverted resistivity images provide a clear view of weathered soils, the distribution of weak areas or karst voids and bed rock Several low resistivity areas were identified and the subsequent drilling of such anomalous areas led to the discovery of several weak zones or clayfilled underground cavities; the drilling results had excellent correlation with the resistivity images The electrical resistivity imaging result proved precise and extremely efficient in delineating the karstic void investigation and should be taken into account when choosing an investigation technique to be used at complex geological sites A remedial action plan involving consolidation grouting work has been suggested prior to road construction