• 한국지구물리탐사학회:학술대회논문집
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• 2005.09a
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• pp.83-92
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• 2005

Fluid conductivity logging has been applied in the boreholes to identify the permeable fi:actures and estimate the origin of saline groundwater in coast area. Fluid replacement technique measures the fluid electrical conductivity with depth at different times in a well after the borehole is first washed out with different water by passing a tube to the borehole bottom. Then formation water flows into the borehole through aquifer such as permeable fractures or porous formation during ambient or pumping condition. Measured conductivity profiles with times therefore indicate the locations of permeable zone or fractures within the open hole or the fully slotted casing hole. As a result of fluid conductivity logging for three boreholes in the study area, it is interpreted that saline groundwater is caused by seawater intrusion through fractured rock, although the effect by land reclamation partially remains. We are planning the quantitative analysis to estimate the hydraulic characteristics using fluid replacement technique, and this approach might be usefully utilized for assessing the characteristics of seawater intrusion, the design of optimal pumping, and estimating the hydraulic properties in coastal aquifer.

• Geophysics and Geophysical Exploration
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• v.10 no.4
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• pp.299-307
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• 2007

Slowness time coherence (STC) technique has been applied to 3-receiver slim hole sonic log using 3 NX sized concrete model holes of different physical properties. We analyzed the effects of different source center frequencies on the wave forms, their amplitude spectra, and their STC results. We could determine the sonic velocity of each mode accurately by the application of STC method with the semblance projection and efficient selection of center frequency. Theoretical model and experimental model hole studies indicate that 4-receiver condition is the most ideal for STC in near surface slim hole sonic log. The result also indicates that favorable STC result can be obtained from three-receiver sonic log provided with the help of the first arrival picking method.

• Geophysics and Geophysical Exploration
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• v.7 no.3
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• pp.174-183
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• 2004

The knowledge of rock strength is important in assessing wellbore stability problems, effective sanding, and the estimation of in situ stress field. Numerous empirical equations that relate unconfined compressive strength of sedimentary rocks (sandstone, shale, and limestone, and dolomite) to physical properties (such as velocity, elastic modulus, and porosity) are collected and reviewed. These equations can be used to estimate rock strength from parameters measurable with geophysical well logs. Their ability to fit laboratory-measured strength and physical property data that were compiled from the literature is reviewed. While some equations work reasonably well (for example, some strength-porosity relationships for sandstone and shale), rock strength variations with individual physical property measurements scatter considerably, indicating that most of the empirical equations are not sufficiently generic to fit all the data published on rock strength and physical properties. This emphasizes the importance of local calibration before one utilizes any of the empirical relationships presented. Nonetheless, some reasonable correlations can be found between geophysical properties and rock strength that can be useful for applications related to wellhole stability where haying a lower bound estimate of in situ rock strength is especially useful.

• Journal of the Korean Geophysical Society
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• v.7 no.4
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• pp.237-245
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• 2004

Series of basic experiments for current density calibration by user process and for density calibration using geophysical model borehole were made. We tried to find the sonde response characteristics for current calibration using water and aluminium field jig, and using the equation of half life of 137Cs source. The result of calibration test made in a geophysical model borehole built first in Korea shows a perfect linear calibration equation. By adopting this calibration equation we could estimate the limitation as well as possibility of current density calibration by user process.

• The Journal of Engineering Geology
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• v.23 no.4
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• pp.457-465
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• 2013

We integrated and correlated datasets from surface and subsurface geophysics, drilling cores, and engineering geology to identify geological interfaces and characterize the joints and fracture zones within the rock mass. The regional geometry of a geologically weak zone was investigated via a fence projection of electrical resistivity data and a borehole image-processing system. Subsurface discontinuities and intensive fracture zones within the rock mass are delineated by cross-hole seismic tomography and analyses of dip directions in rose diagrams. The dynamic elastic modulus is studied in terms of the P-wave velocity and Poisson's ratio. Subsurface discontinuities, which are conventionally identified using the N value and from core samples, can now be identified from anomalous reflection coefficients (i.e., acoustic impedance contrast) calculated using a pair of well logs, comprising seismic velocity from suspension-PS logging and density from logging. Intensive fracture zones identified in the synthetic seismogram are matched to core loss zones in the drilling core data and to a high concentration of joints in the borehole imaging system. The upper boundaries of fracture zones are correlated to strongly negative amplitude in the synthetic trace, which is constructed by convolution of the optimal Ricker wavelet with a reflection coefficient. The standard deviations of dynamic elastic moduli are higher for fracture zones than for acompact rock mass, due to the wide range of velocities resulting from the large numbers of joints and fractures within the zone.

• Journal of the Korean earth science society
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• v.34 no.6
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• pp.588-601
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• 2013

In geophysics and geophysical exploration fields, we can use information about inclination and azimuth in various ways. These include borehole deviation logging for inversion process, real-time data acquisition system, geophysical monitoring system, and so on. This type of information is also necessarily used in the directional drilling of shale gas fields. We thus need to develop a subminiature, low-powered, multi-functional inclination and azimuth measurement system for geophysical exploration fields. In this paper, to develop real-time measurement system, we adopt the high performance low power Micro Control Unit (made with state-of-the-art Complementary Metal Oxide Semiconductor technology) and newly released Micro Electro Mechanical Systems Attitude Heading Reference System sensors. We present test results on the development of a multifunctional real-time inclination and azimuth measurement system. The developed system has an ultra-slim body so as to be installed in 42mm sonde. Also, this system allows us to acquire data in real-time and to easily expand its application by synchronizing with a depth encoder or Differential Global Positioning System.

• The Journal of Engineering Geology
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• v.23 no.4
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• pp.329-340
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• 2013

A geologic survey of the Bukpyeong and Pohang basins, as candidate basins for the geological storage of $CO_2$, was performed to evaluate storage capacity and security. To analyze the mechanical stability of the storage reservoir and cap rocks, we measured the porosity, seismic velocity, uniaxial strength, internal frictional angle, and Young's modulus of core samples recovered from the two basins. It is costly and sometimes impossible to conduct tests over the entire range of drill holes, and continuous logging data do not yield the mechanical parameters directly. In this study, to derive the mechanical properties of geologic formations from the geophysical logging data, we determined the empirical relations between the physical properties (seismic velocity, porosity, and dynamic modulus) and the mechanical properties (uniaxial strength, internal friction angle) of the core samples. From the comparison with our core test data, the best fits to the two basins were selected from the relations suggested in previous studies. The relations between uniaxial strength, Young's modulus, and porosity of samples from the Bukpyeong and Pohang basins are more consistent with certain rock types than with the locality of the basins. The relations between the physical and mechanical properties were used to estimate the mechanical rock properties of geologic formations from seismic logging data. We expect that the mechanical properties could also be used as input data for a modeling study to understand the mechanical instability of rock formations prior to $CO_2$ injection.

• Geophysics and Geophysical Exploration
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• v.25 no.4
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• pp.167-176
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• 2022

We designed a borehole deviation survey tool applicable for steel-cased holes, K-DEV, and developed a prototype for a depth of 500 m aiming to development of own equipment required to secure deep subsurface characterization technologies. K-DEV is equipped with sensors that provide digital output with verified high performance; moreover, it is also compatible with logging winch systems used in Korea. The K-DEV prototype has a nonmagnetic stainless steel housing with an outer diameter of 48.3 mm, which has been tested in the laboratory for water resistance up to 20 MPa and for durability by running into a 1-km deep borehole. We confirmed the operational stability and data repeatability of the prototype by constantly logging up and down to the depth of 600 m. A high-precision micro-electro-mechanical system (MEMS) gyroscope was used for the K-DEV prototype as the gyro sensor, which is crucial for azimuth determination in cased holes. Additionally, we devised an accurate trajectory survey algorithm by employing Unscented Kalman filtering and data fusion for optimization. The borehole test with K-DEV and a commercial logging tool produced sufficiently similar results. Furthermore, the issue of error accumulation due to drift over time of the MEMS gyro was successfully overcome by compensating with stationary measurements for the same attitude at the wellhead before and after logging, as demonstrated by the nearly identical result to the open hole. We believe that the methodology of K-DEV development and operational stability, as well as the data reliability of the prototype, were confirmed through these test applications.

• Economic and Environmental Geology
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• v.48 no.2
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• pp.115-130
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• 2015

Extended elastic impedance (EEI) is an extension of elastic impedance (EI) which is a generalization of acoustic impedance (AI) for nonzero angles of incidence and can be tuned to be proportional to reservoir properties. In this study, we evaluated EEI inversion by estimating the P-($V_p$) and S-wave velocities ($V_s$), P-wave to S-wave velocity ratio ($V_p/V_s$), and Poisson's ratio of the Second Wall Creek Sand of the Teapot Dome field, Wyoming, USA. We also applied the EEI inversion technique to estimate porosity, gamma-ray values, and density of the Second Wall Creek Sand. Data used in the study include 3-D pre-stack seismic data from the southern part of the field and four wells, selected from a large well database. The $V_s$ logs at the wells were constructed from the $V_p$ logs using the empirical relationships. The percent prediction errors for the four velocity properties are less than about 5% except for Poisson's ratio at one well, supporting that the EEI inversion can be used in the prediction of rock properties. However, the results from the EEI inversion analysis of porosity, gamma-ray values, and density at the wells were unsatisfactory and thus these properties, which are not directly computed from velocities, may not be suitable for EEI inversion.

• Geophysics and Geophysical Exploration
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• v.13 no.1
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• pp.61-68
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• 2010

A magnetotelluric (MT) survey has been performed to delineate deeply extended fracture systems at the geothermal field in Seokmo Island, Korea. To assist interpretation of the MT data, geological surveying and well logging of existing wells were also performed. The surface geology of the island shows Cretaceous and Jurassic granite in the north and Precambrian schist in the south. The geothermal regime has been found along the boundary between the schist and Cretaceous granite. Because of the deep circulation along the fracture system, geothermal gradient of the target area exceeds $45^{\circ}C/km$, which is much higher than the average geothermal gradient in Korea. 2D and 3D inversions of MT data clearly showed a very conductive anomaly, which is interpreted as a fracture system bearing saline water that extends at least down to 1.5 km depth and is inclined eastwards. After drilling down to the depth of 1280 m, more than 4000 tons/day of geothermal water overflowed with temperature higher than $70^{\circ}C$. This water showed very similar chemical composition and temperature to those from another existing well, so that they can be considered to have the same origin; i.e. from the same fracture system. A new geothermal project for combined heat and power generation was launched in 2009 in Seokmo Island, based on the survey. Additional geophysical investigations including MT surveys to cover a wider area, seismic reflection surveys, borehole surveys, and well logging of more than 20 existing boreholes will be conducted.