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Acoustic Characteristics of Gas-related Structures in the Upper Sedimentary Layer of the Ulleung Basin, East Sea

동해 울릉분지 퇴적층 상부에 존재하는 가스관련 퇴적구조의 음향 특성연구

  • Park, Hyun-Tak (Department of Oceanography and Ocean Environmental Sciences, Chungnam National University) ;
  • Yoo, Dong-Geun (Petroleum and Marine Research Division, Korea Institute of Geoscience and Mineral Resources (KIGAM)) ;
  • Han, Hyuk-Soo (Korea Seabed Information (KOSBI) Corporation) ;
  • Lee, Jeong-Min (Korea Seabed Information (KOSBI) Corporation) ;
  • Park, Soo-Chul (Department of Oceanography and Ocean Environmental Sciences, Chungnam National University)
  • 박현탁 (충남대학교 해양환경과학과) ;
  • 유동근 (한국지질자원연구원 석유해저연구본부) ;
  • 한혁수 ((주)한국해저정보) ;
  • 이정민 ((주)한국해저정보) ;
  • 박수철 (충남대학교 해양환경과학과)
  • Received : 2012.09.04
  • Accepted : 2012.10.11
  • Published : 2012.10.28

Abstract

The upper sedimentary layer of the Ulleung Basin in the East Sea shows stacked mass-flow deposits such as slide/slump deposits in the upper slope, debris-flow deposits in the middle and lower slope, and turbidites in the basin plain. Shallow gases or gas hydrates are also reported in many area of the Ulleung Basin, which are very important in terms of marine resources, environmental changes, and geohazard. This paper aims at studying acoustic characteristics and distribution pattern of gas-related structures such as acoustic column, enhanced reflector, dome structure, pockmark, and gas seepage in the upper sedimentary layer, by analysing high-resolution chirp profiles. Acoustic column shows a transparent pillar shape in the sedimentary layer and mainly occurs in the basin plain. Enhanced reflector is characterized by an increased amplitude and laterally extended to several tens up kilometers. Dome structure is characterized by an upward convex feature at the seabed, and mainly occurs in the lower slope. The pockmark shows a small crater-like feature and usually occurs in the middle and lower slope. Gas seepage is commonly found in the middle slope of the southern Ulleung Basin. These gas-related structures seem to be mainly caused by gas migration and escape in the sedimentary layer. The distribution pattern of the gas-related structures indicates that formation of these structures in the Ulleung Basin is controlled not only by sedimentary facies in upper sedimentary layer but also by gas-solubility changes depending on water depth. Especially, it is interpreted that the chaotic and discontinuous sedimentary structures of debris-flow deposits cause the facilitation of gas migration, whereas the continuous sedimentary layers of turbidites restrict the vertical migration of gases.

동해 울릉분지의 천부퇴적층은 상부사면에는 함몰사태와 미끄럼사태 퇴적체가 분포하며, 중부와 하부사면에는 쇄설류 퇴적체가 우세하고 분지평원에는 저탁류/반원양성 퇴적체가 분포한다. 기존연구에 의하면 울릉분지에서 가스 또는 가스하이드레이트의 부존이 확인되었으며, 이들에 대한 연구는 해양자원, 환경변화 그리고 지질재해적인 측면에서 매우 중요하다. 본 논문에서는 고해상도 탄성파 자료를 분석하여 울릉분지에 존재하는 천부가스관련 구조들(음향공백기둥, 증폭반사면, 돔구조, 폭마크, 가스유출구조)의 음향특성 및 분포특성을 파악하였다. 음향공백기둥은 퇴적층 내에서 투명한 기둥형태를 보이며 주로 분지평원에서 나타난다. 강한 진폭특성을 보이는 증폭반사면은 퇴적층 내에서 층리를 따라 수평적으로 수십 km 이상 연장되어 나타난다. 또한, 돔구조는 가스가 퇴적층의 공극을 채워 해저면이 부풀어 오른 형태를 보이며, 하부사면에서 주로 확인된다. 폭마크는 해저면이 움푹 파인 형태로 중부와 하부사면에서 분포한다. 가스유출구조는 주변 퇴적층에 비해 매우 약한 반사강도를 보이며, 중부사면에서 우세하게 나타난다. 이러한 가스관련 구조들은 퇴적층 내에서 가스가 수평 수직적으로 이동하여 수층으로 방출되는 과정 중에 형성된다. 또한, 가스관련 구조들의 분포양상은 상부퇴적층의 퇴적상과 수심에 따른 가스의 용해도 차이에 의해 조절된 것으로 사료된다. 특히, 울릉분지 사면지역의 쇄설류 퇴적체는 무질서하고 불연속적인 퇴적구조를 보이며, 이는 가스의 이동을 용이하게 한다. 반면, 분지의 저탁류 퇴적체는 수평층리가 잘 발달되어 있어, 가스의 수직이동을 제한하는 것으로 해석된다.

Keywords

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