• The Journal of Engineering Geology
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• v.9 no.1
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• pp.45-57
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• 1999

Groundwater flow in rock mass is controlled by the fractures developed in the area. So, the conductive fractures are very important for groundwater flow in crystalline rock. This study aims to find out the characteristics of the fracture distributed in granitic rock in the northern part of the Yusong area (latitude $36^{\circ}24'18"~36^{\circ}25'08",{\;}longitude{\;}127^{\circ}21'00"~127^{\circ}23'23"$). The electric and EM surveys were carried out in the site to delineate the fracture the fracture zones. Since geophysical survey provides non-unique solution, hydraulic data and dilling log data including BHTV scanning were used as complementary data to achieve the objective of this study. Electric survey(DC) arrays used are schlumberger and dipole-dipole arrays. VLF is used for EM survey. The main charcteristics of the fracture developed in the study aera are that fractures associated with basic dyke cut corss the main fracture zone in NNW and play an important role as hydraulic barrier. In trun, groundwater table in the upstream area is lower than that downstream area.

• Proceedings of the Korean Society of Soil and Groundwater Environment Conference
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• 1998.11a
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• pp.109-113
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• 1998

본 연구에서는 전기전도도, 온도, 유속 검층을 이용하여 시추공내의 유동성 단열구조를 확인하고 각 유동성 단열구조를 통한 시추공간의 수리적 연결관계를 해석하였다. 검층은 자연상태와 인근 시추공으로부터의 양수에 의한 부정류상태에서 각각 실시되었다. 자연상태에서는 지하수 유동이 매우 느려 유동성 단열구조가 쉽게 인지되지 않으나, 부정류 상태에서는 각 시추공간의 수리적 연결성을 보여준다. 각 시추공별 검층에서 나타나는 유동성 단열구조는 모두 일치하지 않으며, 특히 부정류 상태에서의 유속 검층에서는 타 검층에서 인지된 유동성 단열구조중 일부만이 시추공간에 서로 수리적으로 연결되어 있음을 보여준다.

• Journal of the Korean Society of Groundwater Environment
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• v.5 no.2
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• pp.80-87
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• 1998

Since groundwater flow paths have one of the major roles to transport the radioactive nuclides from the radioactive waste repository to the biosphere, the discrete fracture network model is used for the rock block scale flow instead of the porous continuum model. This study aims to construct a three dimensional discrete fracture network to interpret the groundwater flow system in the study site. The modeling work includes the determination of the probabilistic distribution function from the fracture geometric and hydraulic parameters, three dimensional fracture modeling and model calibration. The results of the constant pressure tests performed in a fixed interval length at boreholes indicate that the flow dimension around boreholes shows mainly radial to spherical flow pattern. The fracture transmissivity value calculated by Cubic law is 6.12${\times}$10$\^$-7/ ㎡/sec with lognormal distribution. The conductive fracture intensity estimated by FracMan code is 1.73. Based on this intensity, the total number of conductive fractures are obtained as 3,080 in the rock block of 100 m${\times}$100 m${\times}$100 m.

• Journal of the Korean Society of Groundwater Environment
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• v.4 no.2
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• pp.61-72
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• 1997

With increasing activities for groundwater withdrawal and for the construction of underground cavern in the coastal areas, the seaward flow of groundwater has been disturbed or even reversed, resulting in seawater intrusion in aquifers. This phenomenon would be attributed to the freshwater contamination and the corrosion of steel materials. The hydrogeological and geochemical investigations have performed to characterize the seawater intrusion into the underground caverns located in the coastal area. Assumimg the inland aquifer as unconfined one, we have found out that the theoretical interface of freshwater-seawater is far different from the pathways identified. In the study site, the main pathways of seawater intrusion into the underground cavern are characterized as the sub-horizontal fractures (zones). The seawater intrusion in granitic terrane would depend mainly on the characteristics of conductive fracture system developed along the coastal area.

• Geophysics and Geophysical Exploration
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• v.12 no.4
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• pp.299-308
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• 2009

A magentotelluric (MT) survey at the Uiseong area has been performed for the site investigation of pilot scale $CO_2$ sequestration. The purpose of the MT survey is to delineate deeply extended fracture systems that can act as a leakage path of injected $CO_2$ Plume. Since the target area is extremely noisy in electromagentic sense, low frequency data below 1 Hz cannot be used for inversion. Two- and three-dimensional interpretation of the MT data showed a very clear conductive anomaly, which has the direction of $N55\sim65^{\circ}W$ and is extended roughly down to 1.6 km. It have the same direction with the strike-slip faults, the Gaeum and Geumcheon Faults. On the contrary, the eastern part of the survey area shows relatively homogeneous to the depth of 2 km though some small fractures at shallow depths can be found. Test drilling and high-definition borehole surveys should be followed at the eastern part of the survey area and hydraulic fracturing is required for injection of $CO_2$, because mean porosity of the sedimetary rock in the area is only 1.47%.

• The Journal of Engineering Geology
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• v.14 no.1
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• pp.93-104
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• 2004

This paper intended to assess the hydro-structure characteristics of volcanic rocks based on the hydrogeological data obtained from the underground storage cavern during construction. The variation of groundwater levels was periodically measured from the 28 surface monitoring holes(NX size) and the hydraulic pressures and injection rates were daily monitored from the water curtain holes(95 horizontal holes and 63 vertical holes). The hydraulic interference tests were performed in whole water curtain holes. The distribution patterns of hydraulic pressure are closely related to the dip angles of fracture intersected to the water curtain holes. Three domains can be grouped by the distribution of hydraulic pressures in the horizontal water curtain holes. The initial hydraulic pressures measured immediately after drilling of water crutain holes are high in ascending order of the cavern C-2, C-1, and C-3. The priliminary hydrochemical data also indicate that the portions of the deep groundwater composition is relatively great in the cavern C-3 area. Some of the horizontal water curtain holes in the cavern C-3 show a steady higher groundwater pressure with the composition of shallow groundwater indicating the outer boundary as constant hydraulic boundary. The water curtain holes in the cavern C-2 is characterized as low initial hydraulic pressure and less injection rates, suggesting poor hydraulic connectivity to a shallow groundwater system. The results of the study can help to understand a hydraulic compartment concept in a fracture hydro-geology and be utilized during the surface investigation for a groundwater system.

• Geophysics and Geophysical Exploration
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• v.10 no.1
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• pp.89-97
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• 2007

A three-dimensional (3D) magnetotelluric (MT) survey has been carried out to delineate subsurface structures and possible fractures, for development of low-temperature geothermal resources in Pohang, Korea. Quite good quality MT data could be obtained throughout the survey region by locating the remote reference in Kyushu, Japan, which is ${\sim}480\;km$ from the centre of the field site. 3D modelling and inversion are performed taking into account the sea effect in MT measurements near the seashore. The nearby sea in the Pohang area affects MT data at frequencies below $1\;Hz{\sim}0.2\;Hz$, depending on the distance from the seashore. The most severe sea effects were observed in the south-east parts of the survey area, closer to Youngil Bay. 3D inversion with and without the seawater constraint showed very similar results at shallow depths, roughly down to 2 km. At greater depths, however, a strong sea effect seems to form a fictitious conductive structure in ordinary 3D inversion, especially in the south-eastern part of the survey region. Comparison between drilling results and the resistivity profiles from inversions showed that five layered structures can be distinguished the subsurface beneath the target area. They are: (a) semi-consolidated mudstones with resistivity less than $10\;{\Omega}m$, which are ${\sim}300\;m$ thick in the northern part and ${\sim}600\;m$ thick in the southern part of the survey area; (b) occasional occurrence of trachybasalt and lapilli tuff within the mudstone layer has resistivity of a few tens of${\Omega}m$, (c) intrusive rhyolite ${\sim}400\;m$ thick has resistivity of several hundreds of ${\Omega}m$, (d) alternating sandstone and mudstone down to 1.5 km depth shows resistivity of ${\sim}100\;{\Omega}m$, (e) a conductive structure was found at a depth of ${\sim}3\;km$, but more geological and geophysical study should be carried out to identify this structure.