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

• Geophysics and Geophysical Exploration
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• v.23 no.4
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• pp.230-242
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• 2020

We performed a magnetotelluric (MT) survey to delineate the geological structures below the depth of 20 km in the Gyeongju area where an earthquake with a magnitude of 5.8 occurred in September 2016. The measured MT data were severely distorted by electrical noise caused by subways, power lines, factories, houses, and farmlands, and by vehicle noise from passing trains and large trucks. Using machine-learning methods, we classified the MT time series data obtained near the railway and highway into two groups according to the inclusion of traffic noise. We applied three schemes, stochastic gradient descent, support vector machine, and random forest, to the time series data for the highspeed train noise. We formulated three datasets, Hx, Hy, and Hx & Hy, for the time series data of the large truck noise and applied the random forest method to each dataset. To evaluate the effect of removing the traffic noise, we compared the time series data, amplitude spectra, and apparent resistivity curves before and after removing the traffic noise from the time series data. We also examined the frequency range affected by traffic noise and whether artifact noise occurred during the traffic noise removal process as a result of the residual difference.

• The Journal of Engineering Geology
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• v.11 no.1
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• pp.1-11
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• 2001

The geological characteristics of Korea are that we can encounter the rock layer only after 10m of excavation, methods to presume the rock pressure distribution of the rock layer is urgently needed. When using the existing empiric science of Terzaghi-Peck, Tschebotarioff to measure the rock pressure of the rock layer, underestimate the real strength because of the cohesion is ignored. Therefore calculating the horizontal sliding force of wedge block, which includes the dips and shear strength of discontinuities and surcharge load etc., think to be to getting a closer rock stress of the real rock pressure acting upon the earth structure in rock mass. This research use Coulomb soil pressure theory assuming that the backfill soil will yield wedge failure when it has cohesion, applying Prakash-Saran(l963), and then it uses equilibrium of force and shear strength $\tau$=c+$\sigma$tan $\Phi$ of the cliscontinuities. Analyzing shear strength and dips of cliscontinuities using calculated theory according to the status of discontinuities aperture, we were able to find out that because the cohesion and friction angle of the rock layer itself is large enough, how the dip directions and dips facing the excavation face is the only factor deciding whether or not the rock stress is applied. The evaluated theory of this research should be strictly estimated, so that the many parameters such as c, $\Phi$value, types and structures of rock class, excessive lateral pressure, dynamic load, earthquake, needed later when calculating shear strength of discontinuities and especially the ground water effect acting on rock layer should be coumpted with many measuring data achieve at the insite to study the application.

• The Korean Journal of Petroleum Geology
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• v.9 no.1_2 s.10
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• pp.16-23
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• 2001

The study area in the East Sea is located on the northeastern margins of the Ulleung Basin near the Kita-Yamato Bank. The research area provides the important clue to the development of Miocene basins which are characterized by the normal faults and volcanic activities related to rifting in the continental crust. Kita-Yamato Bank is a small sediment-filled graben which was formed by failed rifting in the Early Miocene. The basins rapidly vary the bathymetry, depth of acoustic basement and thickness of sedimentary layer. The tension in the study area caused the extensional lithospheric deformation before/during the Early Miocene. In consequence, tectonic forces resulted in the depression or subsidence of basement from continental rifting in the Kita-Yamato Bank followed by the opening of the Ulleung Basin, and caused the onset of graben or half-graben structure bounded by large blocked syn-rift faults. Afterward no significant tectonic deformation exists, with the consequence that post-rift normal faults with small heave were formed and reactivated by the resultant forces such as tectonic subsidence, sediment loading and volcanic activity. The Cenozoic sediment layer has a maximum thickness of 1.0 s along the center of the graben or half-graben, which overlies the consolidated acoustic basement. Seismic units V and IV supposed to be syn-rift sedimentary rocks are deformed by both the volcanic activities and numerous basement-involved normal faults induced from extension. In the uppermost layer, slump scars resulted from the slope failure are recognized.

• Journal of the Korean earth science society
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• v.21 no.3
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• pp.302-314
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• 2000

Keumseong area in the southern part of the Jecheon city, the Ogcheon Belt, consists of Precambrian Dangdusan Metamorphic Complex, Dori Formation of the Choseon Supergroup, and Jurassic Jecheon Granite. The Dangdusan Metamorphic Complex consists of quartz schist, mica schist. quartzite and pegmatite. The Dori Formation is composed of mainly laminated limestone. The rocks in the study area have been undergone at least three phases of deformations since Paleozoic period. The Dangdusan Metamorphic Complex is outcrop at three areas in the study area, which are exposed along the faults and occurred as inlier within the Dori Formation. Previous authors interpreted the uplift of the Dangdusan Metamorphic Complex by the Dangdusan Fault, but we could not find any evidences related to the Dangdusan Fault. Thus, we interpret the uplift of the Dangdusan Metamorphic Complex due to the D$_2$ Weolgulri and Dangdusan thrusts and post-D$_2$ Jungbodeul, Kokyo and Jungjeonri faults. The uplift of the Busan Metamorphic Complex to the west of the study area was interpreted by ductile deformation. However, the Dangdusan Metamorphic Complex is formed by brittle thrusts and faults in this study. According to deformation sequence, the characters of deformations in the Choseon and Ogcheon suprergroups had been changed from ductile to brittle deformations through the time. Therefore, we interpret the Dangdusan Metamorphic Complex is exposed later than the Busan Metamorphic Complex.

• Journal of the Korean earth science society
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• v.34 no.6
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• pp.492-506
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• 2013

The historical accounts and materials about the eruption of Mt. Baekdusan as observed by the geological survey is now showing some signs of waking from a long slumber. As a response of the volcanic eruption of Mt. Baekdusan, water release may occur from the stored water in Lake Cheonjii caldera. The volcanic flood is crucial in that it has huge potential energy that can destruct all kinds of man-made structures and that its velocity can reach up to 100 km $hr^{-1}$ to cover hundreds of kilometers of downstream of Lake Cheonji. The ultimate goal of the study is to estimate the level of damage caused by the volcanic flood of Lake Cheon-Ji caldera. As a preliminary study a scenario-based numerical analysis is performed to build hydrographs as a function of time. The analysis is performed for each scenario (breach, magma uplift, combination of uplift and breach, formation of precipitation etc.) and the parameters to require a model structure is chosen on the basis of the historic records of other volcanos. This study only considers the amount of water at the rim site as a function of time for the estimation whereas the downstream routing process is not considered in this study.

• Tunnel and Underground Space
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• v.18 no.1
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• pp.80-89
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• 2008

The influence of in-situ rock stress on the stability of an underground rock structure increases as the construction depth become deeper and the scale of a rock structure become larger. In general, hydraulic fracturing stress measurement has been performed in the surface boreholes of the target area at the design stage of an underground structure. However, for some areas where the high horizontal stresses were observed or where the overstressed conditions caused by topographical and geological factors are expected, it is desirable to conduct additional in-situ stress measurement in the underground construction site to obtain more detailed stress information for ensuring the stability of a rock structure and the propriety of current design. The study area was a construction site for the additional underground oil storage facility located in the south-east part of OO city, Jeollanam-do. Previous detailed site investigation prior to the design of underground structures revealed that the excessive horizontal stress field with the horizontal stress ratio(K) greater than 3.0 was observed in the construction area. In this study, a total of 13 hydraulic fracturing stress measurements was conducted in two boreholes drill from the two water tunnel sites in the study area. The investigation zone was from 180 m to 300 m in depth from the surface and all of the fracture tracing works were carried out by acoustic televiewer scanning. For some testing intervals at more than 200 m ind depth from surface, the high horizontal stress components the horizontal stress ratio(K) greater than 2.50 were observed. And the overall investigation results showed a good agreement with the previously performed test.

• Tunnel and Underground Space
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• v.28 no.3
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• pp.258-276
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• 2018

The accurate and quantitative ground information on the hydraulic conductivity characteristics of rock mass is one of the key factors for evaluation of the hydro-geological behaviour of rock mass around an excavated opening under high water pressure. For tunnel and rock structures in seabed, where the sea acts as an infinite source of water, its importance become greater with increasing construction depth below sea level. In this study, to improve the problems related with poor system configuration and incorrect data acquisition of previous hydraulic packer testing equipment, we newly developed an integrated main frame and 30 bar level waterproof downhole sonde apparatus, which were optimized for deep hydraulic packer test in seabed rock mass. Integration of individual test equipment into one frame allows safe and efficient field testing work on a narrow offshore drilling platform. For the integrated type main frame, it is possible to make precise stepwise control of downhole net injection pressure at intervals of $2.0kg_f/cm^2$ or less with dual hydraulic oil volume controller. To ensure the system performance and the operational stability of the prototype mainframe and downhole sonde apparatus, the field feasibility tests were completed in two research boreholes, and using the developed apparatus, the REV(Representative Elementary Volume) scale deep hydraulic packer tests were successfully carried out at a borehole located in the basalt region, Jeju. In this paper, the characteristics of the new testing apparatus are briefly introduced and also some results from the laboratory and in-situ performance tests are shown.

• Economic and Environmental Geology
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• v.14 no.3
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• pp.111-122
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• 1981

This study is a model analysis for an effective application of the geophysical prospecting to the investigation of geological structures or useful resources, and the purpose of it is to research a property of the electrical resistivity prospecting, especially by using a Three-Point electrode array method. In using the Three-Point electrode array method, it is theoretically assumed to choose the infinite for a distance between the two current electrodes, however it is impossible in applying to the practical field prospecting. Therefore this study was conducted for determination and presentation of a minimum appropriate distance between the two current electrodes by making a study on prospecting effect in the variation of distance between both the electrodes. In case that the ratios of the distance between the two current electrodes to that between the two potential electrodes are respectively chosen for 40, 400, 5,000, the experimental data of this study showed that the minimum appropriate distance between the two current electrodes is forty times as much as that between two potential electrodes. In order to make clear a problem about prospecting depth which is essential to the data processing, it had been chosen equally to the distance between two potential electrodes. As a result of it, it was shown that the anomaly is appeared along the position of an assumed ore body. Consequently it was found out that the prospecting depth of the Three-Point electrode array method is the same as the distance between the two potential electrodes. From the model experiment on the sheeting ore body(or linear structure) of horizontal, dipping of $30^{\circ}$, $60^{\circ}$ and vertical on the basis of above experimental condition, it was found out that the position and dip of assumed ore body could be inferred from the aspects of the equiresistivity curve. In consequence of performing out the simultaneous Normal and Reversal electrode movement, it was shown that the electrode movement of the Reversal forms the anomaly more clearly than that of Normal when the sheeting ore body is situated obliquely, therefore it could be ascertained that the electrode movement have to be performed simultaneously in the manner of Normal and Reversal. It was also exhibited that the aspect of the equiresistivity curve forms symmetrically when an assumed ore body (or linear structure) is situated horizontally or vertically, that is, symmetrically, and moreover that the aspect of the equiresistivity curve forms unsymmetrically when an assumed ore body (or linear structure) is situated obliquely. On the basis of these experimental analysis it is thought that it can be inferred from the aspect of equiresistivity curve whether an assumed ore body is obliquely situated or not.

• The Journal of Engineering Geology
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• v.30 no.3
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• pp.289-302
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• 2020

To determine the shallow subsurface structure and sliding surface of land creeping in 2016 at Hadong-gun, Gyeongsangnam-do, geophysical surveys (electric resistivity, and refraction seismic methods, borehole televiewer) and slope stability analysis were conducted. The subsurface structure delineated with borehole lithologies and seismic velocity structures provided the information that the sediment layer on the top of the slope was rather as thick as 20 m and the underlying weathered rock (anorthosite) was thinner than 1 m. Based on the tension cracks observed during the geological mapping, televiewer scanning was performed at the borehole BH-2 and detected the intensive fracture zones at the ground-water level, associated with the slip weak zones mapped in dipole-dipole electrical resistivity section. Downslope sliding and slightly upward pushing at the apex of high resistive bedrock explains the curved slip plane of the land creeping. Such a convex structure might play a role of natural toe abutment for preventing the downward development of slip weak zones. In slope stability analysis, the safety factors of the slip weak zone are calculated with varying the groundwater levels for dry and rainy seasons and the downslope is founded to be unstable with safety factor of 0.89 due to fully saturated material in rainy season.