Integrated geophysical surveys for the safety evaluation of a ground subsidence zone in a small city

Ground subsidence occurred in the centre of a small city in South Korea. In order to investigate the cause of the geological hazards and to estimate the ground safety, we carried out integrated geophysical surveys comprising two-dimensional (2D) resistivity, controlled source magnetotelluric (CSMT), magnetic, ground penetrating radar, geophysical well logging and crosshole electrical resistivity tomography (ERT) surveys. Since the target area is located in the downtown area, surface geophysical methods could not be applied systematically. To understand regional geology and to facilitate the interpretation of detailed geophysical surveys in the target area, 2D resistivity, CSMT and magnetic surveys were conducted outside the downtown area. From these results, we could define the regional structure and successively infer the geologic condition in the city centre as well. Among the geophysical techniques applied for the detailed investigation in the main target area, crosshole ERT and geophysical well logging played the most important role. For the efficient ERT field work in the busiest quarter of the city, we devised a new electrode array, modified pole-dipole array, and proved that the proposed array is efficient particularly in the area where installing a remote electrode is nearly impossible. The distribution of cavities and weak zones was interpreted by careful examination of the resistivity tomograms and geophysical logging results. Based on the distribution of cavities interpreted in a 3D manner, numerical analyses of rock engineering were further carried out and geologic hazard maps were presented. Through this comprehensive approach comprising geophysics and rock engineering, shallow limestone cavities were found to be the main cause of the ground subsidence and the excessive pumping of groundwater might trigger or accelerate the geological hazard. Reinforcement works have been carried out based on the results of these geophysical and rock engineering investigations.