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REFERENCE LINKING PLATFORM OF KOREA S&T JOURNALS
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Geophysics and Geophysical Exploration
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Korean Society of Earth and Exploration Geophysicists
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Volume & Issues
Volume 3, Issue 4 - Nov 2000
Volume 3, Issue 3 - Aug 2000
Volume 3, Issue 2 - May 2000
Volume 3, Issue 1 - Feb 2000
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A Dispersion Analysis for Minimum Grids in the Frequency Domain Acoustic Wave Equation
Jang Seong-Hyung ; Shin Chang-Soo ; Yoon Kwang-Jin ; Suh Sang-Young ; Shin Sung-Ryul ;
Geophysics and Geophysical Exploration, volume 3, issue 2, 2000, Pages 39~47
A great deal of computing time and a large computer memory are needed to solve wave equation in a large complex subsurface layers using the finite difference method. The computing time and memory can be reduced by decreasing the number of grid points per minimum wave length. However, the decrease of grids may cause numerical dispersion and poor accuracy. In this study we performed the grid dispersion analysis for several rotated finite difference operators, which was commonly used to reduce grids per wavelength with accuracy in order to determine the solution for the acoustic wave equation in frequency domain. The rotated finite difference operators were to be extended to 81, 121 and 169 difference stars and studied whether the minimum grids could be reduced to 2 or not. To obtain accuracy (numerical errors less than
) the following was required: more than 13 grids for conventional 5 point difference stars, 9 grids for 9 difference stars, 3 grids for 25 difference stars, and 2.7 grids for 49 difference stars. After grid dispersion analysis for the new rotated finite difference operators, more than 2.5 grids for 81 difference stars, 2.3 grids for 121 difference stars and 2.1 grids for 169 difference stars were needed. However, in the 169 difference stars, there was no solution because of oscillation of the dispersion curves in the group velocity curves. This indicated that the grids couldn't be reduced to 2 in the frequency acoustic wave equation. According to grid dispersion analysis for the determination of grid points, the more rotated finite difference operators, the fewer grid points. However, the more rotated finite difference operators that are used, the more complex the difference equation terms.
EM Responses of Buried Conductive Pipes Calculated by 3-D Finite Element Method
Chung Ho-Joon ; Jung Hyun-Key ; Park Yeong-Sue ; Jo Chul-Hyun ;
Geophysics and Geophysical Exploration, volume 3, issue 2, 2000, Pages 48~52
We have calculated and analyzed the electromagnetic responses of buried conductive pipes due to a horizontal magnetic dipole source on the pound using a three-dimensional (3-D) finite element method to provide useful guidelines for designing electromagnetic pipe locator and for field operation of the system. For single buried pipe, the horizontal component and the horizontal difference of the vertical component of magnetic field show peaks above the pipe. When comparing the width of response curves of both cases around the peak, horizontal difference of vertical component of magnetic field shows much narrower peak, 2 times narrower at a half of maximum amplitude, than that of horizontal component of magnetic field. Accordingly, we can pinpoint the horizontal location of pipe on the ground more accurately by measuring the horizontal difference of vertical component of magnetic fold. Moreover, it will have a merit in determining the depth of pipe, because the equation for depth estimation is defined just above the pipe. When there are two buried pipes separated by two meters with each other, the response of horizontal difference of vertical component of magnetic field has two separate peaks each of which is located above the pipe whereas horizontal magnetic field response has only one peak above the pipe just below the transmitter. Thus, when there exist more than a buried pipe, measuring the horizontal difference of vertical magnetic field can effectively detect not only the pipe under transmitter but also adjacent ones. The width of response curves also indicates higher resolving ability of horizontal difference of vertical component of magnetic field.
Development of Network Based MT Data Processing System
Lee Heuisoon ; Kwon Byung-Doo ; Chung Hojoon ; Oh Seokhoon ;
Geophysics and Geophysical Exploration, volume 3, issue 2, 2000, Pages 53~60
The server/client systems using the web protocol and distribution computing environment by network was applied to the MT data processing based on the Java technology. Using this network based system, users can get consistent and stable results because the system has standard analysing methods and has been tested from many users through the internet. Users can check the MT data processing at any time and get results during exploration to reduce the exploration time and money. The pure/enterprised Java technology provides facilities to develop the network based MT data processing system. Web based socket communication and RMI technology are tested respectively to produce the effective and practical client application. Intrinsically, the interpretation of MT data performing the inversion and data process requires heavy computational ability. Therefore we adopt the MPI parallel processing technique to fit the desire of in situ users and expect the effectiveness for the control and upgrade of programing codes.
The Treatment of the Free-surface Boundary Conditions by Finite-Difference Midpoint-Averaging Scheme for Elastic Wave Equation Modeling
Park, Kwon-Gyu ; Suh, Jung-Hee ; Shin, Chang-Soo ;
Geophysics and Geophysical Exploration, volume 3, issue 2, 2000, Pages 61~69
The free-surface boundary conditions are persistent problem in elastic wave equation modeling by finite-difference method, which can be summarized with the degradation of the accuracy of the solution and limited stability range in Poisson's ratio. In this paper, we propose the mid-point averaging scheme as an alternative way of implementing the free-surface boundary conditions, and present the solution to Lamb's problem to verify our approach.
Prestack Reverse Time Depth Migration Using Monochromatic One-way Wave Equation
Yoon Kwang Jin ; Jang Mi Kyung ; Suh Jung Hee ; Shin Chang Soo ; Yang Sung Jin ; Ko Seung Won ; Yoo Hae Soo ; Jang Jae Kyung ;
Geophysics and Geophysical Exploration, volume 3, issue 2, 2000, Pages 70~75
In the seismic migration, Kirchhoff and reverse time migration are used in general. In the reverse time migration using wave equation, two-way and one-way wave equation are applied. The approach of one-way wave equation uses approximately computed downward continuation extrapolator, it need tess amounts of calculations and core memory in compared to that of two-way wave equation. In this paper, we applied one-way wave equation to pre-stack reverse time migration. In the frequency-space domain, forward propagation of source wavefield and back propagration of measured wavefield were executed by using monochromatic one-way wave equation, and zero-lag cross correlation of two wavefield resulted in the image of subsurface. We had implemented prestack migration on a massively parallel processors (MPP) CRAYT3E, and knew the algorithm studied here is efficiently applied to the prestck migration due to its suitability for parallelization.