• Title/Summary/Keyword: gassy sediments

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Acoustic Properties of Gassy Sediments: Preliminary Result of Jinhae Bay, Korea (가스함유퇴적물의 음향특성: 한국 진해만의 예비결과)

  • Kim, Gil-Young;Kim, Dae-Choul;Yeo, Jung-Yoon;Yoo, Dong-Geun
    • The Journal of the Acoustical Society of Korea
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    • v.26 no.1E
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    • pp.33-38
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    • 2007
  • Compressional wave velocity and shear wave velocity were measured for gassy sediments collected from Jinhae Bay, Korea. To distinguish inhomogeneities of gassy sediments, Computed Tomography (CT) was carried out for gassy sediment using CT Scanner. The cored sediments are composed of homogeneous and soft mud (greater than $8{\Phi}$ in mean grain size) containing clay content more than 50%. In depth interval of gassy sediments, compressional wave velocity is significantly decreased from 1480m/s to 1360m/s, indicating that the gas greatly affects compressional wave velocity due to a gas and/or degassing cracks. Shear wave velocity shows a slight increasing pattern from ${\sim}55\;m/s$ in the upper part of the core to ${\sim}58\;m/s$ at 320 cm depth, and then decreases to ${\sim}54\;m/s$ in the lower part of the core containing a small amount of gas. But shear wave velocity in the gassy sediments is slightly greater than that of non-gassy sediments in the upper part of the core. Thus, the Vp/Vs ratio is decreased (from 30 to 25) in gas charged zone. The Vp/Vs ratio is well correlated with shear wave velocity, but no correlation with compressional wave velocity. This suggests that low concentrations of gas have little affects on shear wave velocity. By CT images, the gas in the sediments is mostly concentrated around inner edge of core liner due to a long duration after sediment collection.

Characteristics of Velocity and Electrical Resistivity in Gassy Sediments Results of Mudbelt Sediments in the Southeastern Inner Shelf of Korea (가스함유퇴적물에서의 음파전달속도 및 전기비저항 특성: 한국남동해역 이토대 퇴적물의 분석결과)

  • Kim, Dae-Choul;Park, Soo-Chul;Seo, Young-Kyo
    • The Sea:JOURNAL OF THE KOREAN SOCIETY OF OCEANOGRAPHY
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    • v.6 no.4
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    • pp.249-258
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    • 2001
  • Compressional wave velocity and electrical resistivity of muddy sediments in the southeastern inner shelf of Korea were studied using nine piston core samples. The acoustic and physical properties were measured with 10 cm depth interval. Sediment structures were examined by x-radiographs of the cored sediments. Subbottom profiles were obtained by a high-resolution acoustic subbottom profiler. Acoustic turbid layers are clearly seen on the profiles, and x-radiographs of the sediments showed degassying structures formed by gas escaping. On the basis of x-radiographic images, velocities, electrical resistivities and physical properties, the sediments are divided into gassy and non-gassy sediments. The presence of gas and degassying structures result in a marked variation in velocity and electrical resistivity. It can be concluded that velocity and electrical resistivity arep arameter to recognize gassy sediment. The velocity is important parameter to indicate gassy sediment.

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Estimation of Nonlinear Parameter in Water - saturated Sandy Sediment by using Difference Frequency Acoustic wave (수중 모래 퇴적물에서 차 주파수 음파를 이용한 비선형 변수 추정)

  • Kim Byoung-Nam;Lee Kang Il;Yoon Suk Wang;Choi Bok Kyung
    • Proceedings of the Acoustical Society of Korea Conference
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    • autumn
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    • pp.429-432
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    • 2004
  • Nonlinear acoustic responses of water-saturated sediments are very important to understand nonlinear phenomena of gassy ocean sediments. Especially, the second harmonic, the sum and the difference frequency acoustic waves in water-saturated sediments can provide practical criteria to estimate the nonlinear parameter of gassy sediments. In this paper, the difference frequency acoustic wave in water-saturated sandy sediment was observed in a water tank experiment with a pulse transmission technique. Its pressure level was 12 dB higher than the background noise level at a maximum undistorted driving pressure of source acoustic transducer. The experimental results were compared with a theoretical estimation of the parametric acoustic array. The nonlinear parameter of water-saturated sandy sediment was also estimated as 73 with their comparison. This value can be utilized as the background information to estimate gas void fraction in the water-saturated gassy sandy sediment.

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The Consolidation Behaviour of Muddy Soil Containing Gas Bubbles. (Gas를 함유한 점성토의 압밀특성에 관한 연구)

  • 김수삼
    • Geotechnical Engineering
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    • v.5 no.4
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    • pp.47-60
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    • 1989
  • Soft offshore sediments quite frequently contain undissolved gas, probably methane pro- duced biogenically. The presence of gas bubbles can have a significant effect on the engineering behaviours of the seabed. One of the main difference between saturated and gassy soils is that the undrained response is not incompressible, and without volume change, may be assumed for a fully saturated soil. This paper describes the basic experimental work to further understanding of a gassy soil. The test has been performed for a gassy soil under undrained and drained conditions. It was confirmed that the gas inclusions deformed due to changes in the total stress on the sample and also the pore gas pressure response to change in total stress. but not directly to those in pore water pressure. And the test which applied the repeated load under undrained state also showed the similar behaviour as the simple load.

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3.5kHz seismic images of the gas-charged shallow sediment at Kwangyang Bay and the Yeosu Sound on the southern coast of Korea (광양만과 여수해만의 가스함유 표층퇴적물의 3.5kHz 탄성파 영상)

  • 오진용
    • Economic and Environmental Geology
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    • v.33 no.3
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    • pp.239-246
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    • 2000
  • The 3.5kHz sub-bottom profiling was carried out over both Gwangyang Bay and the Yeo Sound . High -resolution digital images of uppermost sediment layers are obtained from the field data which were originally recorded in analog mode. Most prominent feature along the acoustic profiles is the chaotic reflections which imply the presence of shallow gas within the silty sediments. In the western part of Gwangyang Bay, the gas-charged sediments are assoicated with the acoustic turbidity of the blanket type. Across the Seomjin Delta in the eastern part of Gwangyang Bay, the gas-charged seismic facies are observed just beneath the sea bottom. In the western Yeoul Sound , the gassy seiments occur widely , whereas it is rare in the eastern counterpart with the <30-m-deep channel. We postulate that this gas was biogenetically produced within the organic-rich deposits.

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