Studies on the Geological Environment of the Nanjido Waste Disposal Site: Gravity and Magnetic Investigations

난지도 매립지 및 그 주변의 지질환경 연구: 중력 및 자력탐사

Gravity and magnetic surveys were carried out to investigate the three-dimensional configuration and characteristics of the landfills at Nanjido waste disposal site. For terrain correction and three-dimensional density inversion of gravity data an algorithm, which calculates the gravity effect of a three-dimensional body by using the solid angle method, is developed. This algorithm has been proved to give more accurate terrain correction values for the small survey area having varied topography like Nanjido site as compared with widely used methods such as Hammer's method and multiquadric equation method. Density inversion of gravity anomaly data gives very useful information about the lateral and vertical variation of the landfills, which can be used to discriminate the kinds of wastes. The average density of filled materials appears to be $1.7\;g/cm^3$ which is much higher than the value $(0.8\;g/cm^3)$ estimated by Seoul City. The lateral variation of density shows high correlation with the pattern of ongoing depression of the landfills. The northern region of the landfill no. 1, which shows low density and high depression, is closely associated with the industrial waste and sludge filled area. The magnetic anomaly data provide information about relative concentration of magnetic materials, which is also very useful to investigate characteristics of the fills. Several high positive anomaly regions on the reduced-to-pole magnetic anomaly map are appeared to be associated with the industrial waste fills, but certain industrial waste fills show low negative anomalies. This kind of magnetic information can be used in selecting drilling locations over landfills away from buried metal products during the stabilization process.