Application of Integrated Geophysical Methods to Investigate the Cause of Ground Subsidence of a Highly Civilized Area

Abstract

ABSTRACT Ground subsidence has occurred in the downtown of Muan-eup in Korea. Integrated geophysical survey, including two-dimensional (2-D) resistivity, CSMT (controlled source magnetotelluric), magnetic, borehole logging, and resistivity tomography, has been conducted to investigate the cause of the subsidence and ground condition. Since the target area is in the city downtown, there were not enough spaces for surface geophysical methods. To get regional geology and to facilitate the detailed geophysical interpretation in the survey area, 2-D resistivity, CSMT and magnetic surveys have been applied in the outer region of the downtown. From these results, we could accurately define the Gwangju fault system and estimate the geologic condition in the downtown. For the detailed survey of the downtown area, resistivity tomography and borehole logging data have been acquired using a few tens of densely located boreholes. Among these survey results, borehole logging data provided a guide to classify the rock type and to define the geologic boundary of granite and limestone formations. From the resistivity tomograms of 42 sections, which are densely located enough to be interpreted in a three-dimensional manner, we could delineate the possible weak zones or cavities in the limestone formations. In particular, resistivity tomograms in the subsided area showed the real images of ground subsidence. The map of hazardous zone has been derived from the joint interpretation of these survey results and we could provide the possible reinforcement strategy in this area.