• 한국지구물리탐사학회:학술대회논문집
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• 2005.09a
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• pp.73-82
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• 2005

We constructed a series of calibration models in the campus of Kangwon National University. A series of experiment for calibrating sonde response as well as casing effect in field measurement. Here we explained the process of construction of calibrating facility which consists of three model holes of different density and three model holes of different diameter. It was discussed the calibration equations obtained from its sonde response calibration test. A density correction chart for different separation of detector from hole wall was suggested, and verified the possibility of determining true density from measurements in cased hole.

• Geophysics and Geophysical Exploration
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• v.3 no.1
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• pp.7-12
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• 2000

The paper discusses a data processing methodology that derives a three dimensional porosity volume information from the 3-D seismic dataset. The methodology consists of preprocessing and inversion procedures. The purpose of the preprocessing is balancing the amplitudes of seismic traces by using reflectivity series derived from sonic and density logs. There are eight sonic logs are available in the study area; therefore, we can compute only 8 balance functions. The balance function for every seismic trace was derived from these 8 balance functions by kriging. In order to derive a wide-band acoustic impedance --similar to the one can be derived from a sonic log- from a band-limited reflection seismogram, we need to recover missing low- and high-frequency information of the seismic trace. For that Purpose we use the autoregressive method.

• Proceedings of the Korean Geotechical Society Conference
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• 2002.03a
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• pp.695-702
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• 2002

Groundwater flow is primarily influenced by the presence of fractures, functioning as conduits. To block the flow, grouting operation is commonly used. Thereby the fractures are then expected to be sealed, which will add to enhance the shear strength in rock. This far, regarding the assessment of grouting efficiency, however, there's been a considerable uncertainty That is, several geophysical methods of high resolution such as tomography, S-wave logging have produced a significant amount of measurable response caused by grouting, but they can inevitably be used only for the qualitative assessment. Thus, this paper deals with an accurate quantitative assessment about the grouting result. In this, a new strategy is introduced, based mainly on evaluating the opening of fractures. For fracture-opening investigation purposes, borehole Televiewer has already proven to be an excellent logging technique that produces both amplitude image and traveltime image. As well known, the traveltime image can be converted to a high precision 3D caliper log with max. 288 arms, which allows to observe the opening of fractures. To evaluate the fracture opening from the traveltime image, an algorithm of practical use was developed, in which image correction due to the borehole deviation, feature discrimination of wall roughness from fractures, automatic evaluation procedure etc. were considered. Field examples are shown to confirm the efficiency of the suggested method.

• Geophysics and Geophysical Exploration
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• v.15 no.2
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• pp.102-115
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• 2012

The purpose of this paper is to review several different methods calculating gas hydrate saturations. There are three methods using downhole log data, core data (including pressure core), and seismic velocity data. Archie's equation using electrical resistivity of downhole log data is widely used for saturation calculation. In this case, Archie's parameters should be defined accurately. And the occurrence types of gas hydrate significantly affect to saturation calculation. Thus saturation calculation should be carefully conducted. The methods using chlorinity and pressure core data are directly calculated from core sample. So far, the saturation calculated from pressure core gives accurate and quantitative values. But this method is needed much more time and cost. Thus acquisition of the continuous data with sediment depth is realistically hard. The recent several results show that the saturation calculated from resistivity data is the highest values, while the value calculated from pressure core is the lowest. But this trend is not always absolutely. Thus, to estimate accurate gas hydrate saturation, the values calculated from several methods should be compared.

• Geophysics and Geophysical Exploration
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• v.10 no.4
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• pp.369-376
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• 2007

Electrical resistivity tomography was carried out at seawater intrusion monitoring wells located at watershed in coastal areas. It is difficult to identify the characteristics of resistivity near monitoring well in case of using high signalto-noise ratio array due to the high conductivity condition in coastal aquifer although electrical resistivity survey is well adopted to delineate hydrogeological characteristics with the distribution of electrical resistivity. To improve the quality of electrical resistivity survey for two sites with seawater intrusion monitoring wells, inversion with the results of holeto-surface electrical resistivity tomography using single well was executed. The results of inversion for aquifer near wells were verified with the results of drilling log with the informations of fracture, electrical conductivity logging and normal resistivity logging. The inversion for aquifer near one of two wells was also performed at low and high tide with the same electrodes, respectively. From the inversion result, it is possible to obtain the resistivity images with high resolution and to identify the characteristics of aquifer related to seawater intrusion with tidal fluctuation. From this study, it was demonstrated that the hole-to-surface electrical resistivity tomography method accompanied with drilling log, electrical conductivity logging and normal resistivity logging would be useful to delineate the hydrogeological structures near monitoring wells in coastal areas.

• 한국지구물리탐사학회:학술대회논문집
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• 1999.08a
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• pp.137-155
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• 1999

Recently, Televiewer(Borehole Acoustic Scanner(Televiewer)) has come to be widely used specially for the general engineering construction design. The Televiewer tool using a focussed acoustic beam is to detect the amplitude and traveltime of each reflected acoustic signal at the wall, resulting in the amplitude- and traveltime image respectively. Fractures can be well detected, because they easily scatter the acoustic energy due to the highly narrow beam. In addition, the drilling work will rough the borehole wall so that the acoustic energy can be scattered simply due to the roughness of the wall. Thus, the amplitude level can be directed associated with the elastic properties(impedance) and the hardness of the rock as well. Meanwhile, the traveltime image provides an information about the borehole shape and can be converted to a high precision 3D caliper log(max. 288 arms). In this paper, based on the high resolution of Televiewer images, general evaluation methods are illustrated to derive very reliable rock parameters.

• Geophysics and Geophysical Exploration
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• v.17 no.1
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• pp.28-33
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• 2014

We investigate the spatial distribution of highly conductive layer using the one-dimensional inversions of the new magnetotelluric (MT) measurements obtained at the mid-mountain (400 ~ 900 m in elevation) western area of Jeju Island and the previous MT data over Jeju Island, Korea. The conductive layer indicates the sedimentary layer comprised of Seoguipo Fomation and U Formation. There is a definite positive correlation between the top of conductive layer and the earth surface in elevation. On the contrary, the bottom of conductive layer has a negative correlation with the surface elevation. In other words, the conductive layer has a shape of convex lens, which is thickest in the central part. The basement beneath the conductive layer could be concave in the central part of Jeju Island. A kriging considering the correlation between the layer boundary and the surface elevation provides a reliable geoelectric structure model of Jeju Island. However, further studies, i.e. three-dimensional modeling and interpretation integrated with other geophysical or logging data, are required to reveal the possible presence of three-dimensional conductive body near the subsurface vent of Mt. Halla and the causes of the bias in the depths of layer estimated from MT and core log data.

• Journal of the Korean Geophysical Society
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• v.2 no.3
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• pp.179-190
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• 1999

The overall structural framework was studied using the regional 2D seismic data, followed by the sequence stratigraphic study on the 3D seismic and well- log data in the margin of the South Con Son basin of the South China Sea. This research contributes to delineate depositional stratigraphy, depositional environment and geologic history in the 3D seismic area of highly complicated faulting. Eight Miocene sequences were indicated on the 3D seismic and well-log data, in which the structural maps of each sequence boundary and the isochron maps for the corresponding sequence were made. The seismic facies were analyzed for each sequence volume and sequence boundary surface. The 3D seismic area is characterized by coal beds deposited in the transgression environment (transgression systems tract) and channel distributions just above the sequence boundaries. During the Early Miocene, the coals and thick shales deposited in the mangrove swamp representing the lower coastal plain environment. During the Mid to Late Miocene, thick clastic sediments deposited in the coastal to shallow shelf by regional subsidence and marine transgression. The isochron maps and structural patterns indicate that the sediments were transported from west to east or from northwest to southeast.

• Geophysics and Geophysical Exploration
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• v.15 no.1
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• pp.1-7
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• 2012

To evaluate methods of determining near-surface shear-wave velocities (${\nu}_s$), we derived dispersion curves of Rayleigh waves generated by both passive and active sources in Chuncheon, Korea. Microtremors were recorded for 5 minutes in each of four triangular arrays with radii of 5 ~ 40 m. Those data were analyzed using the Spatial Autocorrelation method. Rayleigh waves were also generated by a hammer source and recorded in the same area for 2 s using 24 4.5-Hz geophones. Multichannel Analysis of Surface Waves was applied to those data. Velocity spectra were derived with relatively high signal-to-noise ratios in the frequency ranges of 7 ~ 19 and 11 ~ 50 Hz for the microtremors and synthetically generated Rayleigh waves, respectively. The resultant dispersion curves were combined as one and then input to inversion to derive shear wave velocities that were compared with a lithology log from a nearby well. Shearwave velocities in the top soil and soft-rock layers are almost constant with values of 221 and 846 m/s, respectively; while the inverse-modeled ${\nu}_s$ increases linearly in the gravelly sand, cobbles, and weathered-rock layers. If rock type is classified based on shear-wave velocity, the inversion-derived boundary between weathered-rock and soft rock may be about 5 m deeper than in the well log.

• Geophysics and Geophysical Exploration
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• v.8 no.1
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• pp.41-49
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• 2005

We have developed a random heterogeneous velocity model with bimodal distribution in methane hydrate-bearing Bones. The P-wave well-log data have a von Karman type autocorrelation function and non-Gaussian distribution. The velocity histogram has two peaks separated by several hundred metres per second. A random heterogeneous medium with bimodal distribution is generated by mapping of a medium with a Gaussian probability distribution, yielded by the normal spectral-based generation method. By using an ellipsoidal autocorrelation function, the random medium also incorporates anisotropy of autocorrelation lengths. A simulated P-wave velocity log reproduces well the features of the field data. This model is applied to two simulations of elastic wane propagation. Synthetic reflection sections with source signals in two different frequency bands imply that the velocity fluctuation of the random model with bimodal distribution causes the frequency dependence of the Bottom Simulating Reflector (BSR) by affecting wave field scattering. A synthetic cross-well section suggests that the strong attenuation observed in field data might be caused by the extrinsic attenuation in scattering. We conclude that random heterogeneity with bimodal distribution is a key issue in modelling hydrate-bearing Bones, and that it can explain the frequency dependence and scattering observed in seismic sections in such areas.