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REFERENCE LINKING PLATFORM OF KOREA S&T JOURNALS
> Journal Vol & Issue
Geophysics and Geophysical Exploration
Journal Basic Information
Journal DOI :
Korean Society of Earth and Exploration Geophysicists
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Volume & Issues
Volume 6, Issue 4 - Nov 2003
Volume 6, Issue 3 - Aug 2003
Volume 6, Issue 2 - May 2003
Volume 6, Issue 1 - Feb 2003
Selecting the target year
A Study on the Detection of Small Cavity Located in the Hard Rock by Crosswell Seismic Survey
Ko, Kwang-Beom ; Lee, Doo-Sung ;
Geophysics and Geophysical Exploration, volume 6, issue 2, 2003, Pages 57~63
For the dectection of small cavity in the hard rock, we investigated the feasibility of crosswell travel-time tomography and Kirchhoff migration technique. In travel-time tomography, first arrival anomaly caused by small cavity was investigated by numerical modeling based on the knowledge of actual field information. First arrival delay was very small (<0.125 msec) and detectable receiver offset range was limited to 4m with respect to
normalized first arrival anomaly. As a consequence, it was turned out that carefully designed survey array with both sufficient narrow spatial spacing and temporal (<0.03125 msec) sampling were required for small cavity detection. Also, crosswell Kirchhoff migration technique was investigated with both numerical and real data. Stack section obtained by numerical data shows the good cavity image. In crosswell seismic data, various unwanted seismic events such as direct wave and various mode converted waves were alto recorded. To remove these noises und to enhance the diffraction signal, combination of median and bandpass filtering was applied and prestack and stacked migration images were created. From this, we viewed the crosswell migration technique as one of the adoptable method for small cavity detection.
Estimation of Two-dimensional Distribution of Coefficient of Permeability from Electrical Logging and AMT Data in Yangsan Area
Lee, Tae-Jong ; Park, Nam-Yoon ; Choo, Seok-Yeon ; Lee, Jong-Ho ; Koh, Sung-Yil ;
Geophysics and Geophysical Exploration, volume 6, issue 2, 2003, Pages 64~70
A new approach of estimating the coefficient of permeability (COP) from resistivity has been developed, which can provide another good application tool of geophysical methods to geo-technical field. Borehole electrical logging and Lugeon test results in Yangsan area showed that resistivity is inversely proportional to the COP. For granite and andesite in Yansan area, the relation between the resistivity (
) and the COP (k) revealed that,
. Derived relation is applied to AMT data acquired from a survey line along the tunnel. Two-dimensional resistivity distribution from AMT data was converted to two-dimensional COP section. The final COP section can be used as good input data for groundwater modeling.
A Study to Estimate the Onset Time of an Impulsive Borehole Source
Lee, Doo-Sung ;
Geophysics and Geophysical Exploration, volume 6, issue 2, 2003, Pages 71~76
Accurate estimation of the first arrival travel time is an essential task to obtain a high resolution velocity tomogram. Accuracy of the travel time estimation may be influenced by two factors; geological and mechanical. A serious mechanical factor is the source firing control problems. We found the control problems in the records generated by tome impulsive borehole sources. The problems are; irregular firing control and uncertainty in estimation of the absolute firing-times shown in records. Definitely, the time difference will introduce an error to the first arrival times, and accordingly; it will cause some distortion in the resulting velocity tomogram. A method to determine the firing time is suggested here. The method determines the optimum onset time by comparing the horizontal and the NMO velocity with various amount of delay time adjustment.
Free-surface Boundary Condition in Time-domain Elastic Wave Modeling Using Displacement-based Finite-difference Method
Min Dong-Joo ; Yoo Hai Soo ;
Geophysics and Geophysical Exploration, volume 6, issue 2, 2003, Pages 77~86
We designed a new time-domain, finite-difference, elastic wave modeling technique, based on a displacement formulation. which yields nearly correct solutions to Lamb's problem. Unlike the conventional, displacement-based, finite-difference method using a node-based grid set (where both displacements and material properties such as density and Lame constants are assigned to nodal points), in our new finite-difference method, we use a cell-based grid set (where displacements are still defined at nodal points but material properties within cells). In the case of using the cell-based grid set, stress-free conditions at the free surface are naturally described by the changes in the material properties without any additional free-surface boundary condition. Through numerical tests, we confirmed that the new second-order finite differences formulated in the cell-based grid let generate numerical solutions compatible with analytic solutions unlike the old second-order finite-differences formulated in the node-based grid set.
UHF Electromagnetic Perturbation due to the fluctuation of Conductivity in a Fault Zone
Lee Choon-Ki ; Lee Heuisoon ; Kwon Byung-Doo ; Oh SeokHoon ; Lee Duk Kee ;
Geophysics and Geophysical Exploration, volume 6, issue 2, 2003, Pages 87~94
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.
Seismic Reflection Tomography by Cell Parameterization
Seo, Young-Tak ; Shin, Chang-Soo ; Ko, Seung-Won ;
Geophysics and Geophysical Exploration, volume 6, issue 2, 2003, Pages 95~100
In this study, we developed reflection tomography inversion algorithm using Straight Ray Technique (SRT) which can calculate travel time easily and fast for complex geological structure. The inversion process begins by setting the initial velocity model as a constant velocity model that hat only impedance boundaries. The inversion process searches a layer-interface structure model that is able to explain the given data satisfactorily by inverting to minimize data misfit. For getting optimal solution, we used Gauss-Newton method that needed constructing the approximate Hessian matrix. We also applied the Marquart-Levenberg regularization method to this inversion process to prevent solution diverging. The ability of the method to resolve typical target structures was tested in a synthetic salt dome inversion. Using the inverted velocity model, we obtained the migration image close to that of the true velocity model.
3D Seismic Travel-time Tomography using Fresnel Volume
Ko, Kwang-Beom ; Lee, Doo-Sung ;
Geophysics and Geophysical Exploration, volume 6, issue 2, 2003, Pages 101~107
3D seismic travel-time tomography algorithm baled on Fresnel volume was developed and its feasibility was investigated by the numerical experiments. To testify the field applicability of the developed algorithm, frequency characteristics and way coverage of the crossholel seismic raw data were investigated and 3D velocity tomogram cube with about 8m spatial resolution was obtained. When compared this 3D velocity cube with the conventional 2D ray tomogram, two results were matched well. We concluded that 3D seismic tomography algorithm developed in this study has enough potential to the field application.