• Economic and Environmental Geology
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• v.33 no.6
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• pp.525-536
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• 2000

Electromagnetic and electric surveys were performed to reveal the deep structure of the Yangsan fault in the Bong-gye and Unyang areas, Kyeongnam Province, Korea. Especially, high-frequency magnetotelluric (HFMT) method of EM survey was mainly employed to study the deep subsurface configuration of Yangsan fault. HFMT survey was performed at 25 points of spacing 50 m, making 1.3 km survey line in Unyang area and 13 points of spacing 50 m, making 0.6 km survey line in Bong-gye area. Two 2-D cross-sections (Unyang and Bong-gye areas) were achieved as results. Electric survey by dipole-dipole array was performed to study the structure of shallow subsurface and compare the results with HFMT surveys. The results of HFMT and electric surveys show that Yangsan fault is a geologic boundary. It is very narrow and steep (about $80^{\circ}C$), and extends to 1~1.5 km depth.

• Proceedings of the Korea Information Processing Society Conference
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• 2023.11a
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• pp.902-903
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• 2023

장애물이나 경사지가 많은 협소 지역에서 탐사 활동을 수행하는 로봇은 험지에서도 이동할 수 있는 자율주행 방법을 필수적으로 제공해야 한다. 본 논문은 협소 지역에서 탐사와 객체 탐지를 위해 주행 상황에 따라 바퀴 주행과 관절 주행을 동적으로 변경하면서 이동하는 다관절 로봇 시스템을 제안한다. 다관절 로봇은 마찰력과 수직항력, 토크 값 등을 고려해 설계한 운동 모델을 기반으로 바퀴와 관절 이동을 변경하면서 자율적으로 주행한다. 관리자는 관제 서버를 통해 로봇이 수집한 탐사 정보를 실시간으로 확인하고 필요시 로봇의 원격제어를 수행할 수 있다. 본 연구를 통해 사람이 접근하기 어려운 협소 지역 탐사나 재난지역 인명구조 활동에 활용할 수 있기를 기대한다.

• Geophysics and Geophysical Exploration
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• v.1 no.2
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• pp.127-135
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• 1998

The Born approximation is widely used for solving the complex scattering problems in electromagnetics. Approximating total internal electric field by the background field is reasonable for small material contrasts as long as scatterer is not too large and the frequency is not too high. However in many geophysical applications, moderate and high conductivity contrasts cause both real and imaginary part of internal electric field to differ greatly from background. In the extended Born approximation, which can improve the accuracy of Born approximation dramatically, the total electric field in the integral over the scattering volume is approximated by the background electric field projected to a depolarization tensor. The finite difference and elements methods are usually used in EM scattering problems with a 2D model and a 3D source, due to their capability for simulating complex subsurface conductivity distributions. The price paid for a 3D source is that many wavenumber domain solutions and their inverse Fourier transform must be computed. In these differential equation methods, all the area including homogeneous region should be discretized, which increases the number of nodes and matrix size. Therefore, the differential equation methods need a lot of computing time and large memory. In this study, EM modeling program for a 2D model and a 3D source is developed, which is based on the extended Born approximation. The solution is very fast and stable. Using the program, crosshole EM responses with a vertical magnetic dipole source are obtained and the results are compared with those of 3D integral equation solutions. The agreement between the integral equation solution and extended Born approximation is remarkable within the entire frequency range, but degrades with the increase of conductivity contrast between anomalous body and background medium. The extended Born approximation is accurate in the case conductivity contrast is lower than 1:10. Therefore, the location and conductivity of the anomalous body can be estimated effectively by the extended Born approximation although the quantitative estimate of conductivity is difficult for the case conductivity contrast is too high.

• Geophysics and Geophysical Exploration
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• v.2 no.2
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• pp.96-103
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• 1999

Most of saline water inousions have been diagnosed by geophysical or geochemical approach independently. The objective of this study is to provide the effective method to detect the saline water intrusion on the ground water in the vicinity of seashore using these two methods. Schulumberger sounding, frequency domain electromagnetic sounding and geochemical analysis of ground water were carried out to explore saline water intrusion. Schulumberger sounding was implemented in dry surface condition before irrigation water was introduced into the field, while electromagnetic sounding was carried out in wet ground condition after the irrigation. The purpose of duplicated measurements on the equivalent spot at different times was to investigate the variation of anomaly zone depending on the amount of ground water. It was possible to discriminate the anomalous zone due to high water saturation from the low electric resistivity zone by high salt concentrations through this way. For the verification of the geophysical result, the ground water samples in the study area were collected and analysed at the 23 points near the measuring spots. The groundwater at the spot nearest to the sea water intrusion identified by geophysical method indicates higher salinity than the standard limit concentration for agricultural irrigation water (250 mg/1). Isotope analysis of $D({^2}H)$ vs. is ${^18}O$ and PCA analysis were used to discriminate the anthropogenic pollution from those of high salinity from sea water intrusion.

• 한국지구물리탐사학회:학술대회논문집
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• 1999.08a
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• pp.92-116
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• 1999

Tomographic approaches to image underground structure using electrical properties, can be divided into DC resistivity, electromagnetic, and radar tomography, based on the operating frequency. DC resistivity and radar tomography methods have been recently applied to site investigation for engineering purpose in Korea. This paper review these two tomography methods, through the case histories acquired in Korea. As another method of borehole radar survey, borehole radar reflection method is included, and its inherent problem and solution are discussed, how to find the azimuth angle of reflector using direction-finding-antenna. Since the velocity anisotropy of radar wave has been commonly encountered in field data, anisotropic radar tomography is discussed in this paper. In DC resistivity tomography, two subjects are focussed, electrode arrays, and borehole effect owing to the conductive fluid in borehole. Using the numerical modeling data, various kinds of electrode ways are compared, and borehole effect is illustrated. Most of the case histories presented in this paper are compared with known geology, core logging data, and/or Televiewer images.

• 한국지구물리탐사학회:학술대회논문집
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• 2008.10a
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• pp.37-40
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• 2008

We have measured resistivities for undisturbed soil samples collected from dredged bank with sea sand, and analyzed with one-dimensional inversion results from small-loop electromagnetic survey data. From the relationship between the two resistivities, it appeared that calculated resistivities were remarkably consistent with measured resistivities. Correlation relationships between resistivity values and cone resistance were analyzed after comparing inversion results with cone resistance. It turns out that the region with below 1 ohm-m is correspondent to that of with less than $50\;kgf/cm^2$ in dredged bank with sea sand. From the study result, resistivity monitoring of small-loop EM periodically is proved to be more effective to maintain the stability of embankment dike.

• Geophysics and Geophysical Exploration
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• v.12 no.1
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• pp.8-16
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• 2009

The holistic inversion approach for frequency domain airborne electromagnetic data has previously been employed to simultaneously calibrate, process and invert raw frequency-domain data where prior information was available. Analternative formulation has been developed, which is suitable in the case where explicit prior information is not available. It incorporates: a multi-layer vertically-smooth conductivity model; a simplified bias parameterisation; horizontal smoothing with respect to elevation; and cluster computer parallelisation. Without using any prior data, an inversion of 8.0 million data for 3.4 million parameters yields results that are consistent with independently derived calibration parameters, downhole logs and groundwater elevation data. We conclude that the success of the holistic inversion method is not dependent on a sophisticated conceptual model or the direct inclusion of survey-area specific prior information. In addition, acquisition costs could potentially be reduced by employing the holistic approach which largely eliminates the need for high altitude zero-level measurements.

• Proceedings of the KSEEG Conference
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• 2000.04b
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• pp.171-174
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• 2000

• The Magazine of the IEIE
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• v.21 no.9
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• pp.58-68
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• 1994

• Geophysics and Geophysical Exploration
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• v.9 no.4
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• pp.299-303
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• 2006

Geologic noise, especially located at shallow depth, can be a great obstacle in the interpretation of geophysical data. Thus, it is important to suppress geologic noise in order to accurately detect major anomalous bodies in the survey area. In the inversion of geophysical data, model parameters at shallow depth, which have small size and low contrast of physical property, can be regarded as one of geologic noise. The least-squares method with smoothness constraint has been widely used in the inversion of geophysical data. The method imposes a big penalty on the large model parameter, while a small penalty on the small model parameter. Therefore, it is not easy to suppress small anomalous boies. In this study, we developed a new inversion scheme which can effectively suppress geologic noise by imposing a big penalty on the slowly varying model parameter and a small penalty on the largely varying model parameter. We call the method MTE (main-target emphasizing) inversion. Applying the method to the inversion of 2.5D small loop EM data, we can ensure that it is effective in suppressing small anomalous boies and emphasizing major anomalous bodies in the survey area.