Detection of Sea-water Intrusion Caused by Tidal Action Using DC Resistivity Monitoring

전기비저항 모니터링을 이용한 해수침투 파악

  • Hwang, Hak-Soo (Resource Division, Korea Institute of Geology, Mining and Materials) ;
  • Lee, Sang-Kyu (Resource Division, Korea Institute of Geology, Mining and Materials) ;
  • Ko, Dong-Chan (Earth and Environmental Division, Korea Institute of Geology, Mining and Materials) ;
  • Kim, Yang-Soo (Dept. of Geology, Chungnam National University) ;
  • Park, In-Hwa (Resource Division, Korea Institute of Geology, Mining and Materials)
  • 황학수 (한국자원연구소 자원연구부) ;
  • 이상규 (한국자원연구소 자원연구부) ;
  • 고동찬 (한국자원연구소 지구환경부) ;
  • 김양수 (충남대학교 지질학과) ;
  • 박인화 (한국자원연구소 자원연구부)
  • Published : 2000.02.01

Abstract

The 1 $km^2$ area studied is located in Sukchun-ri, Hwasung-koon, the southern part of Kyeonggi-do. Even though this site has been known as a contaminated area caused by seawater intrusions, geophysical and geochemical surveys have never been carried out at the site to determine the extent of the seawater contamination and to investigate whether the seawater intrusion is in progress. The purpose of this study is to determine the extent of seawater contamination and a preferred channel of the seawater intrusion using geophysical methods such as DC resistivity surveys with Schlumberger array and a dipole-dipole array. In order to determine whether the seawater intrusion is in progress in the area, DC resistivity monitoring with Schlumberger array was performed. According to the resistivity map obtained from the inversion of the resistivity data measured with Schlumberger array, the study area is divided into two districts as relatively lowly resistive (less than 30 ohm-m) and highly resistive (more than 30 ohm-m) areas. The distribution of the lowly resistive area is consistent with the distribution of the layer composed of clay minerals, and the resistivity of this layer decreases slowly as approaching to the old seashore. Hydrogeological analysis shows that the clay layer within a distance of about 200 m from the seashore has been already contaminated by sea-water and its electric conductivity is 8 times higher than that of the sand layer covered by the clay layer. According to the results of the 2-dimensional DC resistivity surveys with a dipole-dipole array, there are two preferred channels of the seawater intrusion in the site, and both the channels are in the NW-SE direction from the old seashore. The lowly resistive zone in the southern channel extends to a depth of 80 m. The DC resistivity monitoring with Schlumberger array was carried out along the preferred channel which has the low resistivity Bone (fracture zone) that extended to a depth of 80 m. The time series of apparent resistivity, measured at a distance of 260 m from the old coast line, fluctuates with a period of 12 hours. From these observations, it can be concluded that the seawater intrusion caused by tidal action is still in progress along the fractured zone interpreted by the DC resistivity surveys with a dipole-dipole array.