• 한국지구물리탐사학회:학술대회논문집
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• 2008.10a
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• pp.21-26
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• 2008

A multi-geophysical surveys were carried out at Hwasan caldera which is located in Euisung Sub-basin. In order to overcome the limitation of the previous studies, dense gravity data and magnetotelluric (MT) data were obtained and integrated. In this study, the independent inversion models from gravity and MT method were integrated using a correlation and classification approaches to map geologic structure. The results of integration analysis indicated followings; 1) pyroclastic rocks around the central area of Hwasan caldera have lower density and resistivity when compared with those of neighborhood regions and are extended to around 1 km in depth, 2) the high resistivity and density intrusive igneous rocks are imaged around the ring fault boundary, and 3) the basement structure, which has low resistivity and high density, 5 km deep inferred by integration analysis. Also, for integration analysis, we suggested Structure Index method. This method is analyzed using Type Angle and Type Intensity, which are calculated by the spatial correlation of the physical properties. In this study, we can perform the integration analysis effectively using Structure Index method.

• Geophysics and Geophysical Exploration
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• v.20 no.2
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• pp.61-71
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• 2017

Recently, ground-penetrating radar (GPR) surveys have been actively carried out for precise subsurface void investigation because of the rapid increase of subsidence in urban areas. However, since the interpretation of GPR data was conducted based on the interpreter's subjective decision after applying only the basic data processing, it can result in reliability problems. In this research, to solve these problems, we analyzed the difference between the events generated from subsurface voids and those of strong diffraction sources such as the buried pipeline by applying the edge detection technique, which is one of image processing technologies. For the analysis, we applied the image processing technology to the GRP field data containing events generated from the cavity or buried pipeline. As a result, the main events by the subsurface void or diffraction source were effectively separated using the edge detection technique. In addition, since subsurface voids associated with the subsidence has a relatively wide scale, it is recorded as a gentle slope event unlike the event caused by the strong diffraction source recorded with a sharp slope. Therefore, the directional analysis of amplitude variation in the image enabled us to effectively separate the events by the subsurface void from those by the diffraction source. Interpretation based on these kinds of objective analysis can improve the reliability. Moreover, if suggested techniques are verified to various GPR field data sets, these approaches can contribute to semiautomatic interpretation of large amount of GPR data.

• Journal of the Korean earth science society
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• v.23 no.4
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• pp.380-392
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• 2002

The purpose of the present study is to evaluate the usage of microtremor in estimation of subsurface structure and ground response to strong ground motion. To accomplish the purpose, the current status of microtremor study are reviewed and microtremors recorded at several stations are analysed. First of all, the stability of microtremor is examined through the analysis of microtremors recorded for 80 seconds per hour during the time from 10 p.m. to 6 a.m. for eight hours at night time. It is found that the shape of microtremor spectra of low frequency below 10Hz is approximately invariable with time and the spectra contain informations about subsurface structure. The subsurface structures estimated from the predominant frequency determined from the recorded microtremors are compared with the known ones from geophysical surveys at several stations in Kyungju. The comparison of structures shows rough agreements at most stations. Horizontal to vertical spectral ratio(HVSR) technique for microtremor has been proposed as an indirect method to determine ground response to strong ground motion. The HVSR for microtremors recorded in Kyungju is calculated and compared with theoretical transfer function calculated from the known structures. The comparison shows rough coincidence of the peak frequency of spectra between them.

• Geophysics and Geophysical Exploration
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• v.11 no.2
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• pp.85-92
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• 2008

Magnetotelluric (MT) methods are widely applied as an effective exploration technique to geothermal surveys. Two-dimensional (2-D) analysis is frequently used to investigate a complicated subsurface structure in a geothermal region. A 2-D finite-element method (FEM) is usually applied to the MT analysis, but we must pay attention to the accuracy of so-called auxiliary fields. Rodi (1976) proposed an algorithm of improving the accuracy of auxiliary fields, and named it as the MOM method. Because it introduces zeros into the diagonal elements of coefficient matrix of the FEM total equation, a pivoting procedure applied to the symmetrical band matrix makes the numerical solution far less efficient. The MOM method was devised mainly for the inversion analysis, in which partial derivatives of both electric and magnetic fields with respect to model parameters are required. In the case of forward modeling, however, we do not have to resort to the MOM method; there is no need of modifying the coefficient matrix, and the auxiliary fields can be elicited from the regular FEM solution. The computational efficiency of the MOM method, however, can be greatly improved through a sophisticated rearrangement of the total equation.

• Journal of the Korean Geophysical Society
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• v.2 no.1
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• pp.45-56
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• 1999

Seismic refracrion and reflection surveys were conducted along an E-W trending track of 482 m long in Ilwall-dong, Pohang. End-on spread was employed as source-receiver configuration with 2 m for both geophone interval and offset. Seismic data were acquired using 24 channels at every shot fired every 2 m along the track. Refraction data were interpreted using equations for multi-horizontal layers. Reflection data were processed in the sequence of trace edit, gain control, CMP sorting, NMO correction, mute, common offset gathering, and filtering to produce a single fold seismic section. There are two layers in shallow subsurface of the study area. Upper layer has the P-wave velocities ranging from 267 to 566 m/s and is interpreted as a layer of unconsolidated sediments. Lower layer has P-wave velocities of 1096-3108 m/s and is interpreted as weathered rock to hard rock. Most of the lower layer classified as soft rock. Upper layer has lateral variations in both P-wave velocity and thickness. The upper layer in the eastern part of the seismic line is 3-5 m thick and has P-wave velocity of 400 m/s in average. The upper layer in the western part is 8-10 m thick and has P-wave velocity of 340 m/s in average. The eastern part is interpreted as unconsolidated beach sand, while the western part is interpreted as infilled soil to develop a construction site. Three fault systems of high angle are imaged in seismic reflection section. It is interpreted that the area between these fault systems are relatively safe. Large buildings should be located in the safe ground condition of no fault and footings should be designed to be in the basement rock of 3-10 m deep below the surface.

• Geophysics and Geophysical Exploration
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• v.6 no.4
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• pp.165-170
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• 2003

Recently, resistivity surveys have been frequently carried out over the irregular terrain such as mountainous area. Such an irregular terrain itself can produce significant anomalies which may lead to misinterpretations. In this study, topographic effects in resistivity survey were studied using the physical scale modeling as well as the numerical one adopting finite element method. The scale modeling was conducted at a pond, so that we could avoid the edge effect, the inherent problem of the scale modeling conducted in a water tank in laboratory. The modeling experiments for two topographic features, a ridge and a valley with various slope angles, confirmed that the results by the two different modeling techniques coincide with each other fairly well for all the terrain models. These experiments adopting dipole-dipole array showed the distinctive terrain effects, such that a ridge produces a high apparent resistivity anomaly at the ridge center flanked by zones of lower apparent resistivity. On the other hand, a valley produces the opposite anomaly pattern, a central low flanked by highs. As the slope of a terrain model becomes steeper, the terrain-induced anomalies become stronger, and moreover, apparent resistivity can become even negative for the model with extremely high slope angle. All the modeling results led us to the conclusion that terrain effects should be included in the numerical modeling and/or the inversion process to interpret data acquired at the rugged terrain area.

• Geophysics and Geophysical Exploration
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• v.23 no.1
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• pp.13-21
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• 2020

The acquisition and processing of long-offset data are essential for imaging deep geological structures in marine seismic surveys. It is challenging to derive an accurate subsurface image by employing conventional data processing to long-offset data owing to the normal moveout (NMO) stretch and non-hyperbolic moveout phenomena induced by seismic anisotropy. In 2017, the Korea Institute of Geoscience and Mineral Resources conducted a simultaneous two-dimensional multichannel streamer and ocean-bottom seismic survey using a 5.7-km streamer and an ocean-bottom seismometer to identify the deep geological structure of the Ulleung Basin. Herein, the actual geological subsurface structure was obtained via the sequential iterative updating of the velocity and anisotropic parameters of the long-offset data obtained using a multichannel streamer, and anisotropic prestack Kirchhoff migration was performed using the updated velocity and anisotropic parameters as input parameters. As a result, the reflection energy in the long-offset traces, which showed non-hyperbolic moveout owing to seismic anisotropy, was well aligned horizontally and NMO stretches were also reduced. Thus, a more precise and accurate migrated image was obtained, minimizing the distortion of reflectors and mispositioned reflection energy.

• Geophysics and Geophysical Exploration
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• v.8 no.1
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• pp.86-91
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• 2005

We present the results of electrical resistivity surveys carried out to estimate the seasonal variation of the water table level in a large-scale landslide area of Tertiary geology in Japan. One long profile, trending NE-SW, was established perpendicular to the main regional geology of the region. Three boreholes are located very close to the profile. The profile was surveyed twice, once before snowfall and once after snow had melted. The relationship between resistivity and water saturation of pyroclastic materials was clarified through laboratory tests. We did this in order to estimate the water content of the pyroclastic layer from the observed resistivity distribution in the landslide area. The resistivity of the saturated pyroclastic deposit calculated using an empirical formula was found to be $570{\Omega}.m$. Based on this computed resistivity, the groundwater level was deduced by assuming that the pyroclastic deposits were fully saturated beneath the water table. We show that the estimated water table before snowfall is lower than that inferred after snow has melted, by about 1.1 to 4.7 m. This suggests that the water table in the upper part of the pyroclastic layer in the landslide area fluctuates greatly, compared to the lower part. This seasonal groundwater fluctuation is possibly caused by the infiltration of water into the subsurface after snowmelt.

• Geophysics and Geophysical Exploration
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• v.8 no.1
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• pp.59-66
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• 2005

The main purpose of this paper is to describe a highly efficient common mid-point (CMP) data acquisition method for ground-penetrating radar (GPR) surveying, which is intended to widen the application of GPR. The most important innovation to increase the efficiency of CMP data acquisition is continuous monitoring of the GPR antenna positions, using a real-time kinematic Global Positioning System (RTK-GPS). Survey time efficiency is improved because the automatic antenna locating system that we propose frees us from the most time-consuming process-deployment of the antenna at specified positions. Numerical experiments predicted that the data density and the CMP fold would be increased by the increased efficiency of data acquisition, which results in improved signal-to-noise ratios in the resulting data. A field experiment confirmed this hypothesis. The proposed method makes GPR surveys using CMP method more practical and popular. Furthermore, the method has the potential to supply detailed groundwater information. This is because we can convert the spatially dense dielectric constant distribution, obtained by using the CMP method we describe, into a dense physical value distribution that is closely related to such groundwater properties as water saturation.

• Geophysics and Geophysical Exploration
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• v.22 no.3
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• pp.99-106
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• 2019

Resistivity method has been used for the dam safety inspection and, for the convenience of fieldwork, two-dimensional (2D) resistivity data has been usually measured along the dam crest. However, since the dam has three-dimensional (3D) structure, 2D resistivity survey along the dam crest violates 2D assumption and 3D effects caused by 3D topography and material properties in the dam distort the inversion result of 2D resistivity data acquired along the dam crest. Furthermore, it is really hard to evaluate the 3D structure of the dam and 3D leakage pathway using 2D resistivity survey because 2D resistivity survey can provide only 2D resistivity section beneath the survey line. In this study, 3D resistivity survey was conducted at a dam in Korea. By comparing the results from 3D and 2D resistivity surveys, merit and demerits of 3D survey were investigated. Finally, it was confirmed that 3D survey can provide more accurate information about the dam status and 3D leakage pathway compared to the 2D survey. Therefore the 3D resistivity survey should be actively expanded for more accurate dam safety inspection even though more time and expense are required.