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Research Article

Distribution of Resistivity Zones Near Nari Caldera, Ulleung-do, Korea, Inferred from Modified Dipole Arrays

변형 쌍극자배열법을 적용한 울릉도 나리 칼데라 주변 조면안산암 지역의 비저항분포 특성 분석

Ki-Beom Kim1
,
Man-Il Kim2*
김 기범1
,
김 만일2*
1Adjunct Professor, Department of Earth and Environmental Sciences, Andong National University
2Principal Researcher, Forest Engineering Research Institute, National Forestry Cooperative Federation
1안동대학교 지구환경과학과 겸임교수
2산림조합중앙회 산림공학연구소 책임연구원
*Corresponding Author
30 September 2019. pp. 223-236
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Abstract

Resistivity surveys can identify the distribution of geological units and structures (including fragmented fault zones), the extent of weathered and modified geological strata, and the characteristics of groundwater. This study aims to analyze the underground sedimentary layers and geological structures near the Nari and Albong Basins of Ulleung-do, Korea, focusing on six survey lines to identify the spatial trends in subsurface resistivity. A modified dipole array method (D method) was employed, combining resistivity results obtained by existing dipole array methods (A and C methods). The modified method provides optimal analysis of the cross-section of underground resistivity, and shows a clear boundary between a low-resistivity zone (≤500 Ω ‧ m) of sedimentary layers and weak zones, and a high-resistivity zone (≥5,000 Ω ‧ m) of volcanic rock (trachyandesite). The estimated average thickness of the sedimentary layers is 50~100 m for the Albong Basin and 100~200 m for the Nari Basin. An anomaly zone, different from the weak zone in the bedrock, is identified as a caldera fault, and the low-resistivity zone extends from the surface down to the lowest survey depths.

Keywords
modified dipole array method
low-resistivity zone
sedimentary layer
trachyandesite
caldera

전기비저항탐사는 지질 분포나 단층 파쇄대의 유무 등의 지질 구조, 지층의 풍화 및 변질 정도, 지하수 등을 파악하기 위하여 광범위하게 활용되는 탐사기법이다. 본 연구에서는 지하 전기비저항의 분포특성을 분석하기 위하여 총 6개 측선을 중심으로 울릉도 나리분지와 알봉분지 주변 지하 칼데라 퇴적층과 지질구조를 파악하기 위하여 기존의 쌍극자배열법(A 방법과 C 방법)의 탐사결과와 함께 변형된 쌍극자배열법(D 방법)을 적용하여 연구를 수행하였다. 본 연구결과, 변형된 쌍극자배열법을 적용하여 최적의 지하 비저항 분포단면 분석을 통해 퇴적층과 연약대가 분포하는 구간인 500 ohm-m 이하의 저비저항대와 화산암체인 조면안산암질 암류가 분포하는 5,000 ohm-m 이상의 고비저항대의 경계를 명확하게 파악할 수 있었다. 알봉분지의 퇴적층은 평균 50~100m 내외, 나리분지의 퇴적층은 평균 약 100~200m 내외의 두께로 분포하는 것으로 추정되며, 칼데라 단층으로 추정되는 이상대는 지표부근에서 탐사심도 하부까지 500 ohm-m 이하의 저비저항대가 수직으로 연장성을 가지면서 암반 내 연약대와 구분되는 것으로 분석되었다.

키워드
변형 쌍극자배열법
저비저항대
퇴적층
조면안산암
칼데라
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10.1071/EG11005
Information
  • Publisher :Korean Society of Engineering Geology
  • Publisher(Ko) :대한지질공학회
  • Journal Title :The Journal of Engineering Geology
  • Journal Title(Ko) :지질공학
  • Volume : 29
  • No :3
  • Pages :223-236
  • Received Date : 2019-08-06
  • Accepted Date : 2019-08-16
  • DOI :https://doi.org/10.9720/kseg.2019.3.223
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