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REFERENCE LINKING PLATFORM OF KOREA S&T JOURNALS
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Geophysics and Geophysical Exploration
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Journal DOI :
Korean Society of Earth and Exploration Geophysicists
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Volume & Issues
Volume 10, Issue 4 - Nov 2007
Volume 10, Issue 3 - Aug 2007
Volume 10, Issue 2 - May 2007
Volume 10, Issue 1 - Feb 2007
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Derivation of Reverse-Time Migration Operator as Adjoint Operation
Ji, Jun ;
Geophysics and Geophysical Exploration, volume 10, issue 2, 2007, Pages 111~123
Unlike the conventional reverse time migration method which is implemented by simply extrapolating wavefield in reverse time, this paper presents a derivation of another reverse time migration operator as the exact adjoint of the presumed forward wavefield extrapolation operator. The adjoint operator is obtained by formulating the forward time extrapolation operator in an explicit matrix equation form and then taking the adjoint to this matrix equation followed by determining the corresponding operator. The reverse time migration operator as the exact adjoint to the implied forward operator can be used not only as a migration algorithm but also as an adjoint operator which is required in the imaging through an inversion such as least-squares migration.
An Efficient Implementation of Hybrid
Norm IRLS Method as a Robust Inversion
Ji, Jun ;
Geophysics and Geophysical Exploration, volume 10, issue 2, 2007, Pages 124~130
Least squares (
norm) solutions of seismic inversion tend to be very sensitive to data points with large errors. The
norm minimization gives more robust solutions, but usually with higher computational cost. Iteratively reweighted least squares (IRLS) method gives efficient approximate solutions of these
norm problems. I propose an efficient implementation of the IRLS method for a hybrid
minimization problem that behaves as
norm fit for small residual and
norm fit for large residuals. The proposed algorithm shows more robust characteristics to the decision of the threshold value than the l1 norm IRLS inversion does with respect to the threshold value to avoid singularity.
A Study in Seismic Signal Analysis for the First Arrival Picking
Lee, Doo-Sung ;
Geophysics and Geophysical Exploration, volume 10, issue 2, 2007, Pages 131~137
With consideration of the first arrival picking methodology and inherent errors in picking process, I propose, from the computerization point of view, a practical algorithm for picking and error computation. The proposed picking procedure consists of 2-step; 1) picking the first coherent peak or trough events, 2) derive a line which approximates the record in the interval prior to the pick, and set the intercept time of the line as the first break. The length of fitting interval used in experiment, is few samples less than 1/4 width of the arriving wavelet. A quantitative measure of the error involved in first arrival picking is defined as the time length that needed to determine if an event is the first arrival or not. The time length is expressed as a function of frequency bandwidth of the signal and the S/N ratio. For 3 sets of cross-well seismic data, first breaks are picked twice, by manually, and by the proposed method. And at the same time, the error bound for each trace is computed. Experiment results show that good performance of the proposed picking method, and the usefulness of the quantitative error measure in pick-quality evaluation.
IP Modeling and Inversion Using Complex Resistivity
Son, Jeong-Sul ; Kim, Junhg-Ho ; Yi, Myeong-Jong ;
Geophysics and Geophysical Exploration, volume 10, issue 2, 2007, Pages 138~146
This paper describes 2.5D induced polarization (IP) modeling and inversion algorithms using complex resistivity. The complex resistivity method has merits for acquiring more valuable information about hydraulic parameters and pore fluid than the conventional IP methods. The IP modeling and inversion algorithms are developed by allowing complex arithmetic in existing DC modeling and inversion algorithms. The IP modeling and inversion algorithms use a 2.5D DC finite-element algorithm and a damped least-squares method with smoothness constraints, respectively. The accuracy of the IP modeling algorithm is verified by comparing its responses of two synthetic models with two different approaches: linear filtering for a three-layer model and an integral equation method for a 3D model. Results from these methods are well matched to each other. The inversion algorithm is validated by a synthetic example which has two anomalous bodies, one is more conductive but non-polarizable than the background, and the other is polarizable but has the same resistivity as the background. From the inverted section, we can cleary identify each anomalous body with different locations. Furthermore, in order to verify its efficiency to the real filed example, we apply the inversion algorithm to another three-layer model which includes phase anomaly in the second layer.
Conjugate Gradient Least-Squares Algorithm for Three-Dimensional Magnetotelluric Inversion
Kim, Hee-Joon ; Han, Nu-Ree ; Choi, Ji-Hyang ; Nam, Myung-Jin ; Song, Yoon-Ho ; Suh, Jung-Hee ;
Geophysics and Geophysical Exploration, volume 10, issue 2, 2007, Pages 147~153
The conjugate gradient (CG) method is one of the most efficient algorithms for solving a linear system of equations. In addition to being used as a linear equation solver, it can be applied to a least-squares problem. When the CG method is applied to large-scale three-dimensional inversion of magnetotelluric data, two approaches have been pursued; one is the linear CG inversion in which each step of the Gauss-Newton iteration is incompletely solved using a truncated CG technique, and the other is referred to as the nonlinear CG inversion in which CG is directly applied to the minimization of objective functional for a nonlinear inverse problem. In each procedure we only need to compute the effect of the sensitivity matrix or its transpose multiplying an arbitrary vector, significantly reducing the computational requirements needed to do large-scale inversion.
A Comparative Study of 3D MT Modeling Methods
Han, Nu-Ree ; Nam, Myung-Jin ; Kim, Hee-Joon ; Song, Yoon-Ho ; Suh, Jung-Hee ;
Geophysics and Geophysical Exploration, volume 10, issue 2, 2007, Pages 154~160
This paper compares the characteristics of three different algorithms for three-dimensional (3D) magnetotelluric (MT) modeling. These methods are developed by Mackie et al. (1994), Sasaki (1999) and Nam et al. (2007). The first and second methods are based on the finite difference method (FDM), while the last one the finite-element method (FEM). MT responses, apparent resistivities and phases, for a COMMEMI 3D-2 model show a good agreement with integral equation solutions and only minor discrepancies are found over the anomalous bodies in the 3D model. The computation time of the two methods based on FDM is short and the static divergence correction contributes to speed up. The FEM modeling scheme is accurate but slow.