9. Shallow/Near-Surface Structural Applications
A Microgravity Survey for Mapping Karstic Cavities at Muan, Korea
Yeong-Sue Park, Hyeongrae Rim, Mutaek Lim, Sung Bon Koo and Young-Chal Lee
Korea Institute of Geoscience and Mineral Resources, Korea.
Abstract
A microgravity survey was executed at Deokbo rice field in Muan county for mapping karstic cavities, which were developed in limestone interbedded in paleozoic schists. Sinkhole collapses have been reported several times in downtown Muan. The site is worried about subsidence due to excessive pumping of groundwater for agricultural irrigation. The data were collected using a Scintrex CG-3 gravimeter at about 800 stations by 5m interval along 14 traverse and 3 cross profile lines. The stations were determined by Total Station and DGPS to an accuracy of 3mm. The data were corrected for base station offset, earth tide, free-air, Bouguer, and terrain effect calculated by 30m DEM to give Bouguer gravity. Residual gravity anomaly map by third-order polynomial fitting indicated mass depletion regions which were associated with cavities. The drilling confirmed cavities in the negative gravity anomaly zones. The position of cavities which caused negative anomaly was determined by Euler deconvolution. The density distribution beneath the profiles was drawn by two dimensional inversion based on the minimum support stabilizing functional, which generated more focused images of density discontinuities. We also imaged three dimensional density distribution by growing body inversion method. Gravity inversion has inherent non-uniqueness. It becomes much severer in case of three dimensional high resolution imaging, because of insufficient knowledge of the field compared with the number of unknown parameters. In order to reduce non-uniqueness, we devised an inversion scheme utilizing Euler deconvolution as a priori information. The essential point of the scheme is to reduce the non-uniqueness by restricting the model space with help of Euler deconvolution, which pointed plausible locations of anomaly sources. The inversion carried out systematic exploration of the growing body process only in the space within a certain radius around the Euler solutions. The scheme applied to semi-gridded data to image three dimensional subsurface density distribution. It showed that the cavities were dissolved, enlarged and connected into a network cavity system.
Last modified: Tue May 09 12:09:40 2006