Gwang-Soo Lee

Physical and Acoustic Properties of Gas-bearing Sediments in Jinhae Bay, the South Sea of Korea

High-resolution seismic survey and sediment core sampling were conducted to investigate acoustic characteristics of gas-bearing sediments in Jinhae Bay, the southeast of Korea. The sediment in Jinhae Bay is mostly homogenous mud deposited after the Holocene transgression. Along with the 410 km of chirp seismic profiling, five piston core samples were collected on the track lines. Gassy sediments are common and occur widely in the bay. Core samples were analyzed for sediment texture, physical properties (porosity, water content, bulk density, and grain density), acoustic properties (compressional wave velocity and attenuation), and electrical resistivity. X-radiograph image analysis was also performed to observe the shape of degassing cracks. There is no significant downcore variation on physical and sediment textures regardless of existence of gas bubbles. However, compressional wave velocity dramatically decreases from average 1480 to 1380∼739 m/s for the cores that penetrate the gas-bearing zones. This is probably due to degassying cracks that developed by escaping gases and free gas bubbles that are still trapped in the cores. Electrical resistivity is the only geotechnical property that increases in the gas-bearing zone where compressional wave velocity abruptly decreases. This indicates the possibility of using both electrical resistivity as an index variable as well as to compressional wave velocity to identify gassy sediment microstructure because there are little changes in texture and composition of sediment.

Seafloor Morphology and Surface Sediment Distribution of the Southwestern Part of the Ulleung Basin, East Sea

Multi-beam echosounder data and grain size analysis data of surface sediment were acquired and analyzed in order to investigate the shelf-to-slope morphology, geological character, and their geological controlling factors in the southwestern margin of the Ulleung Basin. According to the morphological character, the continental shelf can be divided into two parts: (1) shallow (~100 m) and steep (0.5o ) inner shelf, (2) deep (100-300 m) and gentle (0.2 o ) outer shelf. The continental slope is featured with eight distinct topographic depressions of various spatial dimension (~121 km 2 in area) and head wall gradient (~24.3 o ). They are developed adjacent to each other and presumably formed by submarine landslides which have recurred under the strong influences of earthquakes and eustatic sea-level change. The inner continental shelf and the continental slope are dominated by fine-grained sediment, whereas the outer continental shelf is dominated by coarse-grained sediment. The surface sediment distribution seems dominantly influenced by eustatic sea-level change. The outer continental shelf is mostly covered by coarse relict sediment deposited during lowstand sea-level, while the inner shelf is covered with recent sediment during highstand sea-level. The surface of the continental slope is covered with fine-grained sediments which were supplied by hemipelagic advection process.

Implementation of Acoustic Properties Measurement System Based on LabVIEW Using PXI for Marine Sediment

기존의 자동음파전달속도 측정시스템을 이용하여 해양퇴적물의 음향특성을 측정하는 경우 최초도달신호시간의 결정에 발생하는 오류와 측정 절차의 번거로움 등의 몇 가지 문제점들이 존재한다. 이러한 문제점을 개선하기 위하여 PXI모듈을 이용한 LabVIEW기반의 측정 시스템을 개발하였다. 새로운 측정시스템을 검증하기 위해 동일한 시료와 실험환경에서 기존 시스템과 병행하여 측정하였다. 그 결과 1~2%의 음파전달속도 측정오차를 보였으며 음파감쇠 역시 기존 시스템과 유사한 결과 값을 도출하였다. 새로이 개발된 PXI(PCI eXtentions for Instrumentation)를 이용한 LabVIEW기반 음향특성 측정시스템은 퇴적물의 음향특성을 보다 신속하고 효율적으로 측정할 수 있고 측정 자료뿐만 아니라 분석된 신호의 원시자료에 대해서도 지속적인 데이터베이스를 구축할 수 있다.Abstract − A previous velocity measurement system for marine sediment had several problems such as the errorsoccurred when picking first arrival time and the inconvenient measurement procedure. In order to resolve theseproblems, we developed a new acoustic properties measurement system by using PXI (PCI eXtentions for Instru-mentation) module based on LabVIEW. To verify the new system, we measured the velocity and attenuation ofsediment using the new system in a parallel with the previous system under the same experimental environment.The result of measurement showed 1~2% margin of error for the velocity as well as similar attenuation values. Weconcluded that the new system can efficiently measure the acoustic properties of marine sediment. It also has anadvantage to construct the database of acoustic data and raw signal.Keywords: LabVIEW, PXI, Acoustic properties measurement system(음향특성 측정시스템), Marine sediment(해양퇴적물), Velocity(음파전달속도), Attenuation(감쇠)

Seismic stratigraphic reconstruction of late Pleistocene lowstand wedges on the shelf margin and trough region of the Korea Strait: a review

This paper is a review of stratigraphic architectures and evolution history of late Pleistocene lowstand deposits on the outer shelf of Korea Strait. Located between the southeastern tip of the Korean Peninsula and the Tsushima Island, the Korea Strait is a seaway connecting the East Sea with the East China Sea. Detailed analysis of high-resolution seismic profiles and sediment data give a good evidence for the existence of three superimposed wedge-shaped sedimentary units that thicken seaward. The individual sedimentary wedges are characterized by seaward dipping and complex sigmoid-oblique prograding reflection configurations with some chaotic or hummocky reflectors. On the basis of distribution and external geometry, these sedimentary wedges are interpreted as a “shelf-margin or shelf-edge delta”, mainly related to falling and lowstand sea level. Occurrence of this delta type is a common constitute of the Quaternary shelf in the Korea Strait. Although the entire units have not been fully dated, it is believed that these wedges were deposited during late Pleistocene sea-level lowstands. Repeated falls and lowstands of sea level (isotopic stages 8, 6 and 2/4) resulted in the formation of lowstand prograding wedges along the present shelf margin and trough region, having been separated from the inner shelf deposits. During subsequent episodes of sea-level rise (isotopic stages 8/7, 6/5, and 2/1), the upper part of sedimentary wedges was eroded, and thin covered by thin transgressive sands. Consequently, the late Pleistocene sequences on the shelf margin and trough region in this area mainly consist of a succession of stacked lowstand prograding wedges formed during the regression and lowstand of sea level.

A multi-factor approach for process-based seabed characterization: example from the northeastern continental margin of the Korean peninsula (East Sea)

This study investigates sediment transport and depositional processes from a newly collected dataset comprising sub-bottom chirp profiles, multibeam bathymetry, and sediment cores from the northeastern continental margin of Korea in the East Sea (Japan Sea). Twelve echo-types and eleven sedimentary facies have been defined and interpreted as deposits formed by shallow-marine, hemipelagic sedimentation, bottom current, and mass-movement processes. Hemipelagic sedimentation, which is acoustically characterized by undisturbed layered sediments, appears to have been the primary sedimentary process throughout the study area. The inner and outer continental shelf (<150 m water depth) have been influenced by shallow-marine sedimentary processes. Two slope-parallel canyons, 0.2–2 km wide and up to 30 km long, appear to have acted as possible conduits for turbidity currents from the shallower shelf into the deep basins. Bottom current deposits, expressed as erosional moats immediately below topographic highs, are prevalent on the southern lower slope at water depths of 400–450 m. Mass-movements (i.e., slides/slumps, debris flow deposits) consisting of chaotic facies characterize the lower slope and represent one of the most important sedimentary processes in the study area. Piston cores confirm the presence of mass-transport deposits (MTDs) that are characterized by mud clasts of variable size, shape, and color. Multibeam bathymetry shows that large-scale MTDs are chiefly initiated on the lower slope (400–600 m) with gradients up to 3° and where they produce scarps on the order of 100 m in height. Sandy MTDs also occur on the upper continental slope adjacent to the seaward edge of the shelf terrace. Earthquakes associated with tectonic activity and the development of fluid overpressure is considered as the main conditioning factor for destabilizing the slope sediments. Overall, the sedimentary processes show typical characteristics of a fine-grained clastic slope apron and change down-slope and differ within each physiographic province. Furthermore, the influence of geological inheritance (i.e., structural folds and faults) on geomorphology and sediment facies development is an important additional factor on the lower slopes. Together, these factors provide a rational basis for continental margin seabed characterization.

Correcting the Sound Velocity of the Sediments in the Southwestern Part of the East Sea, Korea

To investigate the in-situ sound velocity of sediment in the southwestern part of the East Sea, the laboratory sound velocity was measured using the pulse transmission technique. The sediment sound velocity measured in laboratory was corrected to in-situ sound velocity based on the seafloor temperature, seawater sound velocity, Kim et al. (2004) model, and Hamilton (1980) model. The distribution of the corrected in-situ sound velocity applying Kim et al. (2004) and Hamilton (1980) models reflects the characteristics of sediments of the study area and shows a similar distribution pattern. The correction for in-situ sound velocity was mostly influenced by seafloor temperature. Then, correction of sound velocity using seafloor sediment temperature data should be accomplished for conversion of laboratory data to in-situ sound velocity.

Geoacoustic Properties of Marine Sediment Adjacent the Southwestern Taean Peninsula, the Yellow Sea

− Physical and acoustic properties of sediment on the southwestern Taean Penisula, the Yellow Sea, were studied using eight piston cores. The sediments in the study area are largely composed of sand which has been deposited with sea-level change after LGM(Last Glacial Maximum). After the sea-level rise, fine-grained sediments discharged from Keum River and neighboring coast area were deposited as muddy sand or sandy mud. Results of these sedimentary environment in this area, the texture of sediments are different from place to place with variable horizontal and vertical distribution of physical and acoustic properties. Correlations among the physical, geoacoustic properties, and mean grain size show slight deviations from those of the South Sea in spite of similar pattern. This is probably due to the differences in sedimentary environment, mineral composition, and measurement system.

Study of Sedimentary Deposits using High Resolution Seismic data in Suyeong Bay, Busan

High resolution seismic profiles (chirp and sparker system) were analyzed for the interpretation of nearshore sedimentary environments of Suyeong Bay, Busan. The sedimentary sequence is classified into three seismic units (SU1a, SU1b, and SU2), overlying acoustic basement, and each units can be defined as erosional and disconformable strata. The lowermost SU1a is characterized by the acoustically parallel and prolonged inner reflections, compared with the upper SU1b displays irregular internal reflectors. The uppermost unit, SU2, is acoustically transparent. The acoustic basement is incised with channels, probably due to the active erosion during the early period of transgression. The acoustic basement deepens eastward in the study area, suggesting primary association with the Suyeong River. The upper SU1a and SU1b units constitute lowland-fill strata. SU2 is widely distributed over the study area. High resolution seismic profiles of Suyeong Bay provide significant information crucial to the interpretation of sedimentary environmental history, which is closely related to the sea level change, estuarine environment and influx of terrestrial sediments from the adjacent rivers.