• Title, Summary, Keyword: 울릉분지

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Origin of Sandstone Fragments Within Core Sediments Obtained from Southwestern Continental Shelf of the Ulleung Basin, East Sea (동해 울릉분지 남서부 대륙붕에서 채취된 시추퇴적물내 사암편의 기원)

  • Lee, Eui-Hyeong;Lee, Yong-Kuk;Shin, Dong-Hyeok;Huh, Sik;Kim, Seong-Ryul;Jeong, Baek-Hoon;Han, Sang-Joon;Chun, Jong-Hwa
    • The Sea:JOURNAL OF THE KOREAN SOCIETY OF OCEANOGRAPHY
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    • v.6 no.3
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    • pp.126-134
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    • 2001
  • Several angular sandstone fragments (about 7 cm in longest diameter) occur in two piston cores, obtained from the submarine trough in the northeastern part of Korea Strait. The origin of the sandstone fragments and the paleoenvironment of trough sediment could be suggested from sedimentary facies analysis of cores and identification of ostracod within sandstone fragments. Echo characteristics around two core sites in submarine trough represent the prolonged bottom echoes with diffuse or no subbottom reflectors. The cores consist of a lower bioturbated mud and an upper gravelly sand sediments with sandstone/shell fragments. The bioturbated mud sediments show low water contents (27-44%) and high shear strength (19.2->37 kPa) compared with those of Holocene sediments (60-219% and 1.0-2.7 kPa, respectively) in the inner shelf and continental slope. However, clay contents (48-56%) of the bioturbated mud sediments are similar to those of fluviatile Holocene sediments in the inner shelf. The mean grain size of gravelly sand sediments ranges from 2.3 to 3.0 ${\phi}$ and shows coarsening upward with sandstone/shell fragments. The Holocene palimpsest in the continental shelf are composed of muddy sand sediments or sandy mud sediments (mean grain size: 4.6-7.6 ${\phi}$). Those suggest that two core sediments might be formed from Paleofluvial and paleocoastal deposits during sea-level lowstand. However, sandstone fragments mainly consist of quartz grains and bioclasts, with carbonate matrix, hollow pore, and glauconite. Two extinct ostracod species, Normanicythere sp. and Kotoracythere sp., are recovered in the sand-stone fragments of core EP-7, and they continued to exist from late Pliocene to early Pleistocene in cold water environment of this area. Thus, the sandstone fragments are interpreted to be formed at the paleocoastal environment derived from the Plio-Pleistocene outcrops exposed around the submarine trough during the LGM (Last Glacial Maximum) period.

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Measurements of Dissociation Enthalpy for Simple Gas Hydrates Using High Pressure Differential Scanning Calorimetry (고압 시차 주사 열량계를 이용한 단일 객체 가스 하이드레이트의 해리 엔탈피 측정)

  • Lee, Seungmin;Park, Sungwon;Lee, Youngjun;Kim, Yunju;Lee, Ju Dong;Lee, Jaehyoung;Seo, Yongwon
    • Korean Chemical Engineering Research
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    • v.50 no.4
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    • pp.666-671
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    • 2012
  • Gas hydrates are inclusion compounds formed when small-sized guest molecules are incorporated into the well defined cages made up of hydrogen bonded water molecules. Since large masses of natural gas hydrates exist in permafrost regions or beneath deep oceans, these naturally occurring gas hydrates in the earth containing mostly $CH_4$ are regarded as future energy resources. The heat of dissociation is one of the most important thermal properties in exploiting natural gas hydrates. The accurate and direct method to measure the dissociation enthalpies of gas hydrates is to use a calorimeter. In this study, the high pressure micro DSC (Differential Scanning Calorimeter) was used to measure the dissociation enthalpies of methane, ethane, and propane hydrates. The accuracy and repeatability of the data obtained from the DSC was confirmed by measuring the dissociation enthalpy of ice. The dissociation enthalpies of methane, ethane, and propane hydrates were found to be 54.2, 73.8, and 127.7 kJ/mol-gas, respectively. For each gas hydrate, at given pressures the dissociation temperatures which were obtained in the process of enthalpy measurement were compared with three-phase (hydrate (H) - liquid water (Lw) - vapor (V)) equilibrium data in the literature and found to be in good agreement with literature values.

Plankton Community Response to Physico-Chemical Forcing in the Ulleung Basin, East Sea during Summer 2008 (2008년 하계 울릉분지에서 관측된 물리·화학적 외압에 대한 플랑크톤 군집의 반응)

  • Rho, Tae-Keun;Kim, Yun-Bae;Park, Jeong-In;Lee, Yong-Woo;Im, Dong-Hoon;Kang, Dong-Jin;Lee, Tong-Sup;Yoon, Seung-Tae;Kim, Tae-Hoon;Kwak, Jung-Hyun;Park, Hyun-Je;Jeong, Man-Ki;Chang, Kyung-Il;Kang, Chang-Keun;Suh, Hae-Lip;Park, Myung-Won
    • Ocean and Polar Research
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    • v.32 no.3
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    • pp.269-289
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    • 2010
  • In Summer 2008, a multidisciplinary survey was conducted onboard R/V Haeyang 2000 to understand plankton response to the three distinct physico-chemical settings that developed in the Ulleung Basin of the East Sea. Baseline settings of hydrographic conditions included the presence of the thin (<20 m) Tsushima Surface Water (TSW) on top of the Tsushima Middle Water (TMW). It extends from the Korea Strait to $37^{\circ}N$ along the $130^{\circ}E$ and then turns offshore and encompasses the relatively saline (T>$26^{\circ}C$, S>33.7) Ulleung Warm Eddy surface water centered at $36.5^{\circ}N$ and $131^{\circ}E$. A relatively colder and saline water mass appeared off the southeastern coast of Korea. It was accompanied by higher nutrient and chlorophyll-a concentrations, suggesting a coastal upwelling. Most of the offshore surface waters support low phytoplankton biomass (0.3 mg chl-a $m^{-3}$). A much denser phytoplankton biomass (1-2.3 mg $m^{-3}$) accumulated at the subsurface layer between 20-50 m depth. The subsurface chlorophyll-a maximum (SCM) layer was closely related to the nutricline, suggesting an active growth of phytoplankton at depth. The SCM developed at shallow depth (20-30 m) near the coast and deepened offshore (50-60 m). A fucoxanthin/zeaxanthin ratio was high in coastal waters while it was low in offshore waters, which indicated that diatoms dominate coastal waters while cyanobacteria dominate offshore waters. The community structure and biomass of phytoplanktonare closely related to nitrogen availability. Zooplankton biomass was higher in the coastal region than in the offshore region while species richness showed an opposite trend. Zooplankton community structure retained a coastal/offshore contrast. These suggest that summer hydrography is a stable structure, lasting long enough to allow a hydrography-specific plankton community to evolve.

S-wave Velocity Derivation Near the BSR Depth of the Gas-hydrate Prospect Area Using Marine Multi-component Seismic Data (해양 다성분 탄성파 자료를 이용한 가스하이드레이트 유망지역의 BSR 상하부 S파 속도 도출)

  • Kim, Byoung-Yeop;Byun, Joong-Moo
    • Economic and Environmental Geology
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    • v.44 no.3
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    • pp.229-238
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    • 2011
  • S-wave, which provides lithology and pore fluid information, plays a key role in estimating gas-hydrate saturation. In general, P- and S-wave velocities increase in the presence of gas-hydrate and the P-wave velocity decreases in the presence of free gas under the gas-hydrate layer. Whereas there are very small changes, even slightly increases, in the S-wave velocity in the free gas layer because S-wave is not affected by the pore fluid when propagating in the free gas layer. To verify those velocity properties of the BSR (bottom-simulating reflector) depth in the gas-hydrate prospect area in the Ulleung Basin, P- and S-wave velocity profiles were derived from multi-component ocean-bottom seismic data which were acquired by Korea Institute of Geoscience and Mineral Resources (KIGAM) in May 2009. OBS (ocean-bottom seismometer) hydrophone component data were modeled and inverted first through the traveltime inversion method to derive P-wave velocity and depth model of survey area. 2-D multichannel stacked data were incorporated as an initial model. Two horizontal geophone component data, then, were polarization filtered and rotated to make radial component section. Traveltimes of main S-wave events were picked and used for forward modeling incorporating Poisson's ratio. This modeling provides S-wave profiles and Poisson's ratio profiles at every OBS site. The results shows that P-wave velocities in most OBS sites decrease beneath the BSR, whereas S-wave velocities slightly increase. Consequently, Poisson's ratio decreased strongly beneath the BSR indicating the presence of a free gas layer under the BSR.

Change of Hydraulic Properties of Sand due to Fine Diatom Particle Migration (미세 Diatom 입자 이동에 의한 모래지반의 투수 특성 변화)

  • Pyo, Won-Mi;Lee, Jong-Sub;Lee, Joo Yong;Hong, Won-Taek
    • Journal of the Korean Geotechnical Society
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    • v.34 no.2
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    • pp.19-32
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    • 2018
  • During the process of gas hydrate extraction in the deep seabed, fine diatom particle migration occurs, which causes the seabed slope failure and the productivity deterioration of the gas hydrate. Therefore, a study related with the changes of the ground characteristics due to the fine particle migration is required. The objective of this study is to investigate the change of hydraulic properties of sand due to the migration of fine diatom particle in sandy soils. In order to simulate the sediments of the Ulleung basin gas hydrate in the East Sea, fifteen sand-diatom mixtures that have different diatom volume fractions (DVF) are prepared. During the falling head permeability tests, the coefficients of permeability are measured according to the DVF. In addition, for the simulation of the fine diatom particle migration, constant head permeability tests are conducted by applying the hydraulic pressures of 3 kPa, 6kPa, and 9 kPa on a specimen composed of two layers: a specimen with 50% DVF in upper layer and a specimen with 0% DVF in lower layer. Furthermore, the coefficient of permeability and the electrical resistivity of the migration zone are measured during the constant head permeability test. The falling head permeability tests show that the coefficient of permeability decreases as the DVF of the specimen increases. In addition, the gradient of the coefficient of permeability curve decreases in the DVF range of 10%~50% compared with that of 0%~10%, and increases above 50% in DVF. The result of constant head permeability tests shows that the coefficient of permeability decreases and electrical resistivity increases in the migration zone due to the fine diatom particle migration. This study demonstrates that fine diatom particle migration reduces the permeability of the soils and the behavior of the migration zone due to the fine diatom particle migration may be estimated based on the reversal relationship between the coefficient of permeability and the electrical resistivity.

Phylogeny and Conservation of the Genus Bupleurum in Northeast Asia with Special Reference to B. latissimum, Endemic to Ulleung Island in Korea (울릉도 고유종인 섬시호를 중심으로 동북아시아 시호속 식물의 계통과 보전생물학)

  • Ahn, Jin-Kab;Lee, Hee-Cheon;Kim, Chul-Hwan;Lim, Dong-Ok;Sun, Byooog-Yoon
    • Korean Journal of Environment and Ecology
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    • v.22 no.1
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    • pp.18-34
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    • 2008
  • Based on external morphology, each of five species can be classified into three groups: 1) B. falcatum group (B. falcatum, B. scorzonerifolium), 2) B. euphorbioides group (B. euphorbioides) and 3) B. longiradiatum group (B. longiradiatum, B. latissimum). B. falcatum group has cauline leaves linear or lanceolate in shape and attenuate at base and not surrounding the stem. In contrast, B. longiradiatum group and B. euphorbioides group have cauline leaves ovate, lanceolate or panduriform in shape and auriculate or cordate at base and completely surrounding the stem. The inflorescence is basically compound umbels terminated at the apex of stem. But B. euphorbioides group is small in size and pedicels are rather short and the number of the pedicel is ca. 20. On the other hand, B. longiradiatum and B. falcatum groups are large in size and their pedicels are long and the number of the pedicel is around 10. The pore of pollen aperture of B. longiradiatum and B. latissimum is partially projected or not while those of B. falcatum group and B. euphorbioides is usually remarkably projected. The number of somatic chromosomes was counted as 2n=20 in B. falcatum, 2n=12 in B. scorzonerifolium and B. longiradiatum, and 2n=16 in B. euphorbioides and B. latissimum. Although chromosome numbers of B. euphorbioides and B. latissimum are the same, the two species are not likely to relate because the karyotypes of the two species are different from each other. Based on these observations, it can be concluded that B. latissimum is most closely related to B. longiradiatum. However, molecular data indicated that the species is probably related to B. bicaule distributed in central Siberia. In terms of conservation of B. latissimum, overexploitation by human and grazing by goat are most threatened factors.

Effect of Bottom Hole Pressure and Depressurization Rate on Stability and Gas Productivity of Hydrate-bearing Sediments during Gas Production by Depressurization Method (감압법을 이용한 가스 생산 시 하이드레이트 부존 퇴적층의 지반 안정성 및 가스 생산성에 대한 시추 공저압 및 감압 속도의 영향)

  • Kim, Jung-Tae;Kang, Seok-Jun;Lee, Minhyeong;Cho, Gye-Chun
    • Journal of the Korean Geotechnical Society
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    • v.37 no.3
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    • pp.19-30
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    • 2021
  • The presence of the hydrate-bearing sediments in Ulleung Basin of South Korea has been confirmed from previous studies. Researches on gas production methods from the hydrate-bearing sediments have been conducted worldwide. As production mechanism is a complex phenomenon in which thermal, hydraulic, and mechanical phenomena occur simultaneously, it is difficult to accurately conduct the productivity and stability analysis of hydrate bearing sediments through lab-scale experiments. Thus, the importance of numerical analysis in evaluating gas productivity and stability of hydrate-bearing sediments has been emphasized. In this study, the numerical parametric analysis was conducted to investigate the effects of the bottom hole pressure and the depressurization rate on the gas productivity and stability of hydrate-bearing sediments during the depressurization method. The numerical analysis results confirmed that as the bottom hole pressure decreases, the productivity increases and the stability of sediments deteriorates. Meanwhile, it was shown that the depressurization rate did not largely affect the productivity and stability of the hydrate-bearing sediments. In addition, sensitivity analysis for gas productivity and stability of the sediments were conducted according to the depressurization rate in order to establish a production strategy that prevents sand production during gas production. As a result of the analysis, it was confirmed that controlling the depressurization rate from a low value to a high value is effective in securing the stability. Moreover, during gas production, the subsidence of sediments occurred near the production well, and ground heave occurred at the bottom of the production well due to the pressure gradient. From these results, it was concluded that both the productivity and stability analyses should be conducted in order to determine the bottom hole pressure when producing gas using the depressurization method. Additionally, the stress analysis of the production well, which is induced by the vertical displacements of sediments, should be evaluated.