• Geophysics and Geophysical Exploration
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• v.26 no.3
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• pp.138-149
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• 2023

In geoscience and engineering the geological characteristics of sediment strata is crucial and possible if reliable borehole logging and seismic data are available. To investigate the characteristics of the shallow strata in the Korea Strait, laboratory sonic logs were obtained from deep borehole data and seismic section. In this study, we integrated and analyzed the sonic log data obtained from the drilling core (down to a depth of 200 m below the seabed) and multichannel seismic section. The correlation value was increased from 15% to 45% through time-depth conversion. An initial model of P-wave impedance was set, and the results were compared by performing model-based, band-limited, and sparse-spike inversions. The derived P-impedance distributions exhibited differences between sediment-dominant and unconsolidated layers. The P-impedance inversion process can be used as a framework for an integrated analysis of additional core logs and seismic data in the future. Furthermore, the derived P-impedance can be used to detect shallow gas-saturated regions or faults in the shallow sediment. As domestic deep drilling is being performed continuously for identifying the characteristics of carbon dioxide storage candidates and evaluating resources, the applicability of the integrated inversion will increase in the future.

• Geophysics and Geophysical Exploration
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• v.13 no.1
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• pp.61-68
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• 2010

A magnetotelluric (MT) survey has been performed to delineate deeply extended fracture systems at the geothermal field in Seokmo Island, Korea. To assist interpretation of the MT data, geological surveying and well logging of existing wells were also performed. The surface geology of the island shows Cretaceous and Jurassic granite in the north and Precambrian schist in the south. The geothermal regime has been found along the boundary between the schist and Cretaceous granite. Because of the deep circulation along the fracture system, geothermal gradient of the target area exceeds $45^{\circ}C/km$, which is much higher than the average geothermal gradient in Korea. 2D and 3D inversions of MT data clearly showed a very conductive anomaly, which is interpreted as a fracture system bearing saline water that extends at least down to 1.5 km depth and is inclined eastwards. After drilling down to the depth of 1280 m, more than 4000 tons/day of geothermal water overflowed with temperature higher than $70^{\circ}C$. This water showed very similar chemical composition and temperature to those from another existing well, so that they can be considered to have the same origin; i.e. from the same fracture system. A new geothermal project for combined heat and power generation was launched in 2009 in Seokmo Island, based on the survey. Additional geophysical investigations including MT surveys to cover a wider area, seismic reflection surveys, borehole surveys, and well logging of more than 20 existing boreholes will be conducted.

• Journal of the Mineralogical Society of Korea
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• v.17 no.1
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• pp.99-114
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• 2004

Mineralogical characteristics of fracture-filling minerals from deep borehole in the Yuseong area were studied for the radioactive waste disposal project. There are many fracture zones in the deep drill holes of the Yuseong granite, which was locally affected by the hydrothermal alteration. According to the results of hole rock analysis of drill core samples, $SiO_2$ contents are distinctly decreased, whereas $Al_2$$O_3$ and CaO contents and L.O.I. values are increased in the -90 m∼-130 m and -230 m∼-250 m zone, which is related to the formations of filling minerals. Fracture-filling minerals mainly consist of zeolite minerals (laumontite and heulandite), calcite, illite ($2M_1$ and 1Md polytypes), chlorite, epidote and kaolinite. The relative frequency of occurrence among the fracture-filling minerals is calcite zeolite mineral > illite > epidote chlorite kaolinite. Judging from the SEM observation and EPMA analysis, there is no systematic change in the texture and chemical composition of the fracture-filling minerals with depth. In the study area, low temperature hydrothermal alteration was overlapped with water-rock interactions for a long geological time through the fracture zone developed in the granite body. Therefore the further study on the origin and paragenesis of the fracture-filling minerals are required.

• Journal of the Korean earth science society
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• v.39 no.2
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• pp.154-163
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• 2018

Geoacoustic model comprises physical and acoustic properties of submarine bottom layers influencing sound transmission through sea water and underwater. This study suggested for the first time that we made a geoacoustic model of long-coring bottom layers at the SSDP-105 drilling site of the Ulsan coastal area, which is located in the southwestern inner shelf of the East Sea. The geoacoustic model of 52 m depth below seafloor with three-layer geoacoustic units was reconstructed in the coastal sedimentary strata at 79 m in water depth. The geoacoustic model was based on the data of a deep-drilled sediment core of SSDP-105 and sparker seismic profiles in the study area. For actual modeling, the geoacoustic property values of the models were compensated to in situ depth values below the sea floor using the Hamilton modeling method. We suggest that the geoacoustic model be used for geoacoustic and underwater acoustic experiments of mid- and low-frequency reflecting on the deep bottom layers in the Ulsan coastal area of the East Sea.

• Tunnel and Underground Space
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• v.34 no.4
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• pp.312-329
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• 2024

In various underground research projects such as energy storage and development and radioactive waste disposal targeting deep underground, the characteristics of permeable rock fractures that serve as major pathway of groundwater flow in deep rock aquifer are considered as an important evaluation factor in the design, construction, and operation of research facilities. In Korea, there is little research and database on the location and hydraulic characteristics of permeable rock fractures and the pattern of groundwater flow patterns that may occur between fractures in deep rock boreholes. In this paper, the hydraulic characteristics of permeable rock fractures in deep rock aquifer were evaluated through the analysis of geothermal gradient and pumping test data. First, the deep geothermal distribution was identified through temperature logging, and the geothermal gradient was obtained through linear regression analysis using temperature data by depth. In addition, the hydraulic characteristics of the fractured rock were analyzed using outflow temperature obtained from pumping tests. Ultimately, the potential location and hydraulic characteristics of permeable rock fractures, as well as groundwater flow within the boreholes, were evaluated by integrating and analyzing the geophysical logging and hydraulic testing data. The process and results of the evaluation of deep permeable rock fractures proposed in this study are expected to serve as foundational data for the successful implementation of underground research projects targeting deep rock aquifers.

• Geophysics and Geophysical Exploration
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• v.11 no.3
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• pp.204-213
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• 2008

This paper presents the analysis about the stability of the Pohang deep geothermal borehole drilled in 2006. Severe wellhole instability problems such as collapse and tight hole occurred in weak rocks while drilling. Optimal mud pressure (mud window) required to prevent instability problems during drilling is obtained from analysis on in-situ stress and rock strength. The window is bounded by vertical stress in its upper limit and by either collapse pressure or pore pressure in its lower limit. Mud window varies with different types of rocks. In the top-most semi-consolidated mudstone formation, no mud window can secure borehole stability. In some weak rock types (basic dyke and crystal tuff), the borehole pressure needs to be higher by $50{\sim}60%$ than hydrostatic pressure. That means a mud density of 1.5 g/$cm^3$ or higher should be applied during drilling in order to prevent excessive collapse around the borehole.

• Proceedings of the Korean Radioactive Waste Society Conference
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• 2008.11a
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• pp.207-208
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• 2008

• Proceedings of the Korean Radioactive Waste Society Conference
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• 2010.05a
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• pp.181-182
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• 2010

• Proceedings of the Korean Radioactive Waste Society Conference
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• 2009.11a
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• pp.233-234
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• 2009

• Proceedings of the Korean Radioactive Waste Society Conference
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• 2016.10a
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• pp.259-260
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• 2016