• Journal of the Korean earth science society
• /
• v.28 no.1
• /
• pp.64-74
• /
• 2007

The marine seismic prospecting using a research vessel in the shallow sea near the coastal area has certain limits according to the water depth and survey environment. Also, for the electrical resistivity survey at seashore area, one may need a specially designed high-voltage source to penetrate the very conductive surface layer. Therefore, we have conducted a feasibility study on the application of magnetotelluric method (MT), a passive geophysical method, on investigating of shallow marine environment geology. Our study involves both theoretical modeling and field survey at the tidal flat area which represent the very shallow marine environment. We have applied the audio-frequency magnetotelluric (AMT) method to the intertidal deposits of Gunhung Bay, west coast of Korea, and analysed the field data both qualitatively and quantitatively to investigate the morphology and sedimentary stratigraphy of the tidal flat. The inversion of AMT data well reveals the upper sedimentary layer of Holocene intertidal sediments having a range of 13-20 m thickness and the erosional patterns at the unconformable contact boundary. However, the AMT inversion results tend to overestimate the depth of basement (30-50 m) when compared with the seismic section (27-33 m). Since MT responses are not significantly sensitive to the resistivity of middle layer or the depth of basement, the AMT inversion result for basement may have to be adjusted using the comparison with other geophysical information like seismic section or logging data if possible. But, the AMT method can be an effective alternative choice for investigating the seashore area to get important basic informations such as the depositional environment of the tidal flat, sea-water intrusion and the basement structure near the sea shore.

• Journal of the korean society of oceanography
• /
• v.36 no.1
• /
• pp.1-8
• /
• 2001

An in-situ four-electrode contact resistivity probe system was designed, and field-tested in submarine sediments. Seismic survey was also performed to support and compare the results of electric survey. The probe was designed to be driven to selected depths below the seafloor using a Vibracore system. The four insulated electrodes were, spaced equidistant across the wedge, were extended beyond the probe tip to minimize effects of sediment disturbance by the wedge insertion. In-situ measurements of resistivity were recorded on board by precision electronic equipment consisting of signal generators and processors, and by temperature-monitoring systems. Overall limits of Uncertainty at respective depths below the seafloor are up to ${\pm}$10 of the measured values. Best estimates of conductivity are considered to be ${\pm}$3 percent of the reported values. Resistivity measurements were made at six sites in carbonate sediments to a maximum depth of penetration of about 5 m. Average values of conductivity range between 0.88 and 1.21 mho/m. The results show the seabed is composed of alternating layers of relatively high-conductivity material (0.8 to 1.4 mho/m) in thicknesses of more or less one meter and layers about 30 cm thick having relatively low conductivities (0.4 to 0.8 mho/m).

• Journal of the Korean Geotechnical Society
• /
• v.28 no.5
• /
• pp.109-120
• /
• 2012

In marine site surveys, drilling boreholes are restrictively conducted, while geophysical surveys are used with ease. To overcome the limitations of marine site survey, various site survey data should be collected and analysed by adopting complementary pros and cons. In this study, the integration assessment was established to digitize 2D geo-layer based on the overlapping of a few borehole data to seismic refraction tomography and to determine 3D geo-information based on the kriging for the design of offshore-foundation. And the overlapping method was proposed considering spatial variability of the tomography and separation distance from borehole position to determine the 2D geo-layer. Finally, the integration assessment was applied to offshore wind-turbine site in Jeju-do, and its applicability was verified based on the cross-validation.

• The Korean Journal of Petroleum Geology
• /
• v.7 no.1_2 s.8
• /
• pp.19-27
• /
• 1999

Korea, an energy-resources-poor country, imports $100{\%}$ of its, oil and, natural gas supply, which accounts for the greater part of its total primary requirements. One of the important task of the government is diversification of available energy resources such as oil and natural gas. Natural gas hydrate, which is non-conventional types of natural gas, distributes worldwide, especially in marine and permafrost. It would become a target of natural gas resources in the near future. Especially sigrificant amount of hydrates are expected to be located in the East Sea around Korea Peninsular. This paper describes about a multi-year overall project framework of basic research and technological development of natural gas hydrate in Korea focused on the interpretation of the seismic survey, the characteristics and physical properties of the natural gas hydrate, and the utilizable technology of natural gas hydrates from the status of research and development of the world.

• Geophysics and Geophysical Exploration
• /
• v.12 no.3
• /
• pp.255-262
• /
• 2009

Conventionally, vertical-seismic-profiling (VSP) survey that provides high-resolution information has mainly performed to obtain the exact depth of the gas hydrate-bearing sediment, which is one of the key factors in the development of the gas hydrate. In this study, we extracted interval velocities and created corridor stacks from the first domestic zero-offset VSP data, which were acquired with three component receivers at UBGH09 borehole in Ulleung Basin where gas hydrate exists. Then we compared the corridor stacks with a CMP stacked section from surface seismic data. First of all, we converted the signals recorded with three component receivers to true vertical and horizontal components by phase rotation, and divided the data into direct waves and reflected waves by wavefield separation processing. The trend of the interval velocity extracted from the zero-offset VSP was similar to that of the sonic log obtained at the same borehole. Because the interval velocity of the gas hydrate-bearing sediment above the BSR was high, and it decreased suddenly through the BSR, we could infer that free gas is accumulated below the BSR. The results of comparing the corridor stacks to the CMP stacked section of the surface seismic data showed that most reflection events agreed well with those in the surface CMP stacked section and that the phase-rotated VSP data corresponded better with the surface seismic data than the VSP data without phase rotation. In addition, by comparing a corridor stack produced from the transverse component with the CMP stacked section of the surface seismic data, we could identify PS mode-converted reflections in the CMP stacked section.

• The Journal of Engineering Geology
• /
• v.28 no.2
• /
• pp.161-174
• /
• 2018

$CO_2$ storage site for small-scale demonstration has been investigated in Yeongil Bay, Pohang, SE Korea, using seismic survey and exploration well data. We found a potential storage formation consisting mainly of conglomerate and sandstone. The storage formation unconformably overlies volcanic basement rocks that are located in a depth from 650 to 950 m (below sea level). The depth of the storage formation is suitable for injecting supercritical $CO_2$ in the Pohang Basin. The average thickness of the storage formation is about 123 m, which possibly provides sufficient capacity at the level of small-scale storage demonstration. The overlying fine-grained deposits consist mainly of marine hemipelagic muds and interlayered turbidite sands. The overlying formation is considered as a good seal rock that is over 600 m thick and widely distributed in the onshore and offshore portions of the basin. NNE-trending faults found in the study area likely formed at basement level, probably not continue to seafloor. Such faults are interpreted as syndepositional faults involved to the basin initiation. This study reveals that the offshore area of the Pohang Basin contains deep geological formations suitable for small-scale $CO_2$ storage demonstration.

• Geophysics and Geophysical Exploration
• /
• v.11 no.1
• /
• pp.26-33
• /
• 2008

The analysis of wide-angle seismic reflection and refraction data plays an important role in lithospheric-scale crustal structure study. However, it is extremely difficult to develop an appropriate velocity structure model directly from the observed data, and we have to improve the structure model step by step, because the crustal structure analysis is an intrinsically non-linear problem. There are several subjective processes in wide-angle crustal structure modelling, such as phase identification and trial-and-error forward modelling. Because these subjective processes in wide-angle data analysis reduce the uniqueness and credibility of the resultant models, it is important to reduce subjectivity in the analysis procedure. From this point of view, we describe two software tools, PASTEUP and MODELING, to be used for developing crustal structure models. PASTEUP is an interactive application that facilitates the plotting of record sections, analysis of wide-angle seismic data, and picking of phases. PASTEUP is equipped with various filters and analysis functions to enhance signal-to-noise ratio and to help phase identification. MODELING is an interactive application for editing velocity models, and ray-tracing. Synthetic traveltimes computed by the MODELING application can be directly compared with the observed waveforms in the PASTEUP application. This reduces subjectivity in crustal structure modelling because traveltime picking, which is one of the most subjective process in the crustal structure analysis, is not required. MODELING can convert an editable layered structure model into two-way traveltimes which can be compared with time-sections of Multi Channel Seismic (MCS) reflection data. Direct comparison between the structure model of wide-angle data with the reflection data will give the model more credibility. In addition, both PASTEUP and MODELING are efficient tools for handling a large dataset. These software tools help us develop more plausible lithospheric-scale structure models using wide-angle seismic data.

• Journal of the Korean Geophysical Society
• /
• v.3 no.4
• /
• pp.215-226
• /
• 2000

In order to map the acoustic basement and to locate fracture zones in the Galgok fault, seismic refraction data were acquired near the Chonbuk ranch in Gyeongju. Along three profiles of 72m(Line 1), 72m(Line 2), and 36m(Line 3) long, seismic signals were generated by a 5kg hammer. The refraction data were collected by employing twelve 8 Hz geophones at an interval of 3m and recording time of 192ms at a sampling rate of 0.2ms. The data are interpreted using GRM method. The top layer (Layer 1) is characterized as the velocity of approximately250 m/s and thickness of approximately 2.1m. This layer is regarded as a soil layer. Underneath Layer 1 lies unconsolidated layer (Layer 2) whose refraction velocity is determined to be $1,030{\sim}1,400m/s$. Layer 2 is approximately 4.6m thick and is regarded as a Quaternary gravel layer. The third layer (Layer 3) has the mean refraction velocity of $2,100{\sim}2,200m/s$ and is interpreted to be the acoustic basement. In some parts of Lines 1 and 3, the difference in depth to the top of Layer 2 is greater than 20 cm indicating the possibility of existence of Quaternary faults. Along Line 3 and the eastern part of Line 1, refracted energy from the acoustic basement was not recorded. This may highly indicate that a relatively large scale fault exists under the western part of Line 1.

• Economic and Environmental Geology
• /
• v.44 no.3
• /
• pp.229-238
• /
• 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.

• Economic and Environmental Geology
• /
• v.15 no.2
• /
• pp.65-88
• /
• 1982

In the last ten years, marine geological and geophysical survey and research were conducted by Japanese, Russian and American scientists in the East Sea of Korea (Japan Sea). Many research results were published. However, regional research of the geology of the continental margin of the Korean Peninsula was not conducted. This study has made on attempt to classify submarine strata and stratigraphic boundaries. The study has revealed characters of submarine geology and structure. Isopach maps of each identified stratigraphic unit have been constructed as the results of this study. The study was conducted on the basis of analyses of marine seismic surveys carried out in the continental margin of the East Sea between Kangneung and Pohang. Three depositional basins were identified in the study area and they were named as, Mukho Basin, Hupo Basin and Pohang Basin. The Mukho Basin is developed in continental slope and shelf in the area between Kangneung and Samcheog. Quaternary and Pliocene sediments attain a maximum thickness of 900 m. Basement rocks are interpreted as granite and gneiss. They are correlated with granite-gneiss of the Taebaecksan Series of Pre-cambrian age and the Daebo granite of Jurassic age. The Hupo Basin is developed in the continental shelf between Uljin and Youngdeok. Quaternary and Pliocene sediments attain a maximum thickness of 600 m. Basement rocks were interpreted as granite and gneiss and they are correlated with metamorphic rocks of Pre-cambrian age and the Daebo granites, comprising the Ryongnam Massif. The Pohang Basin is developed in the area between Pohang and Gangu. This basin contains Miocene and older sediments. Basement rocks are not shown. Many faults are developed within the continental shelf and slope. These faults strike parallel with the coast line. A north-south direction is predominant in the southern study area. However, in the northern study area the faults strike north, and north-west. The faults are parallel to each other and are step faults down-thrown to the east or west, forming horst and graben structures which develop into sedimentary basins. Such faults caused the development of submarine banks along the boundary between the continental shelf and slope. This bank has acted as a barrier for deposition in the Hupo Basin. Paleozoic sedimentary rocks distributed widely in the adjacent land area are absent in the Mukho Basin. This suggests that the area of the basin was situated above the sea level until the Pliocene time. The study area contains Pliocene sediments in general. These sediments overlie the basement complex composed of metamorphic rocks, granites, Cretaceous (Kyongsang System) sedimentary rocks and Miocene sedimentary rocks. These facts lead to a conclusion that the continental shelf and slope of the study area were developed as a result of displacements along faults oriented parallel to the present coast line in the post Miocene time.