• Journal of the Korean earth science society
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• v.32 no.6
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• pp.654-664
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• 2011

A field campaign of magnetotelluric (MT) and audio-frequency MT (AMT) survey was done at 36 measurement points as a complementary for the previous 44 MT measurements completed during the period of 2005-2006. The purpose of additional MT survey is to investigate the possible fracture system in Seokmo Island, which is conceived to be crucial in accumulation and migration of geothermal hot spring in this area. We have done 2D and 3D inversions of overall MT and AMT data distributed on a grid to interpret subsurface of extended area. The inversion results reveal that at least two major faults are imaged in the inversion results, one of which is in NNE-SWW with steep dip, and another is in E-W direction.

• Geophysics and Geophysical Exploration
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• v.5 no.2
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• pp.78-83
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• 2002

The finite element method combined with the sensitivity method is adopted for 2-dimensionl Fourier transform inversion. To improve the efficiency of inversion calculation, multi-resolution wavelet method is proposed., Theoretical data which is obtained from above method is shown to examine the proposed method. Theoretical model assumes that underground cavity is located in limestone area. In theoretical model, 16 current and potential electrodes are located to get theoretical data. It is shown that the about inversion method is very exact and useful calculation method, in case the larger model is very small such as under ground cavity.

• Journal of the Korean Geophysical Society
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• v.2 no.2
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• pp.123-134
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• 1999

Magnetotelluric responses may be affected by strong anisotropy of the high-conductivity layers (HCL) in the upper mantle or lower crust. We have studied two-dimensional anisotropy MT modelling to examine the effect of high anisotropic media. Electrical properties of a homogeneous anisotropic body are defined by a symmetric conductivity tensor and the problem is described by coupled diffusion equation in the frequency domain. In two-dimensional anisotropic environments, diagonal elements of the impedance tensor have higher values than those in isotropic environments. In some cases, TM mode phases reach more than 90°and apparent resistivities decrease for some frequency range because of telluric distortion. GB decomposition may be used to recover regional responses, but can be affected by the regional anisotropic effect. Considering these results, BC87 dataset was interpreted with a modified anisotropic model.

• The Journal of Engineering Geology
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• v.33 no.3
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• pp.371-388
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• 2023

A stratum with a complex composition and a distributed low-permeability soil layer is difficult to remediate quickly because the soil remediation does not proceed easily. For efficient purification, the permeability should be improved and the soil remediation agent (H₂O₂) should be injected into the contaminated section to make sufficient contact with the TPH (Total petroleum hydrocarbons). This study analyzed a method for crack formation and effective delivery of the soil remediation agent based on pneumatic fracturing, plasma blasting, and vacuum suction (the PPV method) and compared its improvement effect relative to chemical oxidation. A demonstration test confirmed the effective delivery of the soil remediation agent to a site contaminated with TPH. The injection amount and injection time were monitored to calculate the delivery characteristics and the range of influence, and electrical resistivity surveying qualitatively confirmed changes in the underground environment. Permeability tests also evaluated and compared the permeability changes for each method. The amount of soil remediation agent injected was increased by about 4.74 to 7.48 times in the experimental group (PPV method) compared with the control group (chemical oxidation); the PPV method allowed injection rates per unit time (L/min) about 5.00 to 7.54 times quicker than the control method. Electrical resistivity measurements assessed that in the PPV method, the diffusion of H₂O₂2 and other fluids to the surface soil layer reduced the low resistivity change ratio: the horizontal change ratio between the injection well and the extraction well decreased the resistivity by about 1.12 to 2.38 times. Quantitative evaluation of hydraulic conductivity at the end of the test found that the control group had 21.1% of the original hydraulic conductivity and the experimental group retained 81.3% of the initial value, close to the initial permeability coefficient. Calculated radii of influence based on the survey results showed that the results of the PPV method were improved by 220% on average compared with those of the control group.

• 한국지구물리탐사학회:학술대회논문집
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• 2007.12a
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• pp.31-42
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• 2007

Dc resistivity monitoring has been increasingly used in order to understand the changes of subsurface conditions in terms of conductivity. The commonly adopted interpretation approach which separately inverts time-lapse data may generate inversion artifacts due to measurement error. Eventually the contaminated error amplifies the artifacts when reconstructing the difference images to quantitatively estimate the change of ground condition. In order to alleviate the problems, we defined the subsurface structure as four dimensional (4-D) space-time model and developed 4-D inversion algorithm which can calculate the reasonable subsurface structure continuously changing in time even when the material properties change during data measurements. In this paper, we discussed two case histories of resistivity monitoring to study the ground condition change when the properties of the subsurface material were artificially altered by injecting conductive materials into the ground: (1) dye tracer experiment to study the applicability of electrical resistivity tomography to monitoring of water movement in soil profile and (2) the evaluation of cement grouting performed to reinforce the ground. Through these two case histories, we demonstrated that the 4-D resistivity imaging technique is very powerful to precisely delineate the change of ground condition. Particularly owing to the 4-D inversion algorithm, we were able to reconstruct the history of the change of subsurface material property.

• Geophysics and Geophysical Exploration
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• v.7 no.3
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• pp.164-173
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• 2004

To delineate geothermal water movement at the Pohang geothermal development site, Self-Potential (SP) survey and monitoring were carried out during pumping tests. Before drilling, background SP data have been gathered to figure out overall potential distribution of the site. The pumping test was performed in two separate periods: 24 hours in December 2003 and 72 hours in March 2004. SP monitoring started several days before the pumping tests with a 128-channel automatic recording system. The background SP survey showed a clear positive anomaly at the northern part of the boreholes, which may be interpreted as an up-flow Bone of the deep geothermal water due to electrokinetic potential generated by hydrothermal circulation. The first and second SP monitoring during the pumping tests performed to figure out the fluid flow in the geothermal reservoir but it was not easy to see clear variations of SP due to pumping and pumping stop. Since the area is covered by some 360 m-thick tertiary sediments with very low electrical resistivity (less than 10 ohm-m), the electrokinetic potential due to deep groundwater flow resulted in being seriously attenuated on the surface. However, when we compared the variation of SP with that of groundwater level and temperature of pumping water, we could identify some areas responsible to the pumping. Dominant SP changes are observed in the south-west part of the boreholes during both the preliminary and long-term pumping periods, where 3-D magnetotelluric survey showed low-resistivity anomaly at the depth of $600m\~1,000m$. Overall analysis suggests that there exist hydraulic connection through the southwestern part to the pumping well.

• Geophysics and Geophysical Exploration
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• v.7 no.2
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• pp.148-154
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• 2004

This article reviews the development of three-dimensional (3-D) magnetotelluric (MT) modeling. The 3-D modeling of electromagnetic fields is essential in understanding the physics of MT soundings, and in implementing an inversion method to reconstruct a 3-D resistivity image. Although various numerical schemes have been developed over the last two decades, practical methods have been quite limited. However, the recent rapid improvement in computer speed and memory, as well as the advance in iterative solution algorithms for a large system of equations, makes it possible to model the MT responses of complex 3-D structures, which have been very difficult to simulate before. The use of staggered grids in finite difference method has become popular, conserving a magnetic flux and an electric current and allowing for realistic discontinuous fields. The convergence of numerical solutions has been greatly accelerated by adopting Krylov subspace methods, proper preconditioning techniques, and static divergence corrections. The vector finite-element method using edge elements is also free from the discontinuity problem, and seems a natural choice for modeling complex structures including irregular topography because its flexibility allows one to capture full geometric complexity.

• Journal of Korean Tunnelling and Underground Space Association
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• v.4 no.2
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• pp.135-141
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• 2002

This research has been conducted to investigate the influence of mined cavities on a tunnel to be constructed around a coal mine. The location and dimension of cavities were supposed by analysing synthetically geological structures and condition of coal beds as well as gangway map since there does not exist any map describing mined cavities. Detailed geological and geophygical survey have also been carried out for the purpose of understanding the geological structure and rock mass conditions. The two dimensional numerical analysis with FLAC has been performed on the geological sections reconstituted from the obtained information and the affects of mined cavities on the tunnel have been assessed.

• Geophysics and Geophysical Exploration
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• v.6 no.2
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• pp.87-94
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• 2003

ULF geomagnetic field anomalies related to earthquakes have been reported and a mechnism that magnetic field variations could be generated by the induced telluric current due to the high frequency fluctuation of conductivity in a fault Bone have been proposed. In this study, we calculated electromagnetic anomalies using a simple fault model and investigated the possibility of significant perturbation. Since low frequency electromagnetic fields are modulated by the high frequency oscillation of conductivity and the modulated fields are concentrated in a narrow ULF band, the electromagnetic fields in ULF band could be perturbed significantly. The amplitude of electromagnetic field anomaly depends on various factors: the geometry and conductivity of fault zone, the magnitude and frequency of conductivity fluctuation, the resistivity structure of crust or mantle, the frequency bandwidth of observational data and so on. Therefore, it is strongly required to reveal the deep resistivity structure of crust a.: well ah the structure of fault zone and to ,select the optimal observation frequency band for the observation of electromagnetic activities related with earthquakes.

• Journal of the Korean earth science society
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• v.32 no.6
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• pp.602-612
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• 2011

Jeju Island, a volcanic island located in South Korea, has been one of the main targets of geophysical and/or geological studies because of its tectonic importance related to the volcanism and tectonic link to the southern part of the Korean Peninsula. Recently, as a number of broad-band magnetotelluric (MT) measurements were made, we have examined the deep part of the island. In such an insular setting, it is not easy to properly recover the deep structures such as the lower crust and the upper crust using MT data, because their low-frequency components are strongly affected by the surrounding sea of the island. In this study, we apply the sea-effect correction to the existing MT data collected at a total of 102 sites in Jeju Island. The sea-effect correction makes remarkable changes in the observed MT data at frequencies below 1 Hz, clearly indicating the existence of a conductive lower crust. The 2-D inversion results for both Jeju Southern Line (JSL) and Jeju Northern Line (JNL) show that the transition zone separating the resistive upper crust and conductive lower crust exists at a depth of 20 km on average.