• Geophysics and Geophysical Exploration
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• v.14 no.3
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• pp.214-219
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• 2011

We perform the frequency-domain waveform inversion based on the residual-selection strategy. In the residual-selection strategy, we classify time-domain residual wavefields into several groups according to the order of absolute amplitudes. Because the residual wavefields are normalized after regularization of the gradient directions within each group, the residual-selection strategy plays a role in enhancing the small-amplitude wavefields, which contributes to improving the deep parts of inverted subsurface images. After classifying residuals in the time domain, they are transformed to the frequency domain. Waveform inversion is performed in the frequency domain using the back-propagation technique which has been popularly used in reverse-time migration. The residual-selection strategy is applied to the SEG/EAGE salt and IFP Marmousi models. Numerical results show that the residual-selection strategy yields better results than the conventional frequency-domain waveform inversion.

• Geophysics and Geophysical Exploration
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• v.26 no.2
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• pp.84-93
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• 2023

The supervised learning-based deep-learning seismic inversion techniques have demonstrated successful performance in synthetic data examples targeting small-scale areas. The supervised learning-based deep-learning seismic inversion uses time-domain wavefields as input and subsurface velocity models as output. Because the time-domain wavefields contain various types of wave information, the data size is considerably large. Therefore, research applying supervised learning-based deep-learning seismic inversion trained with a significant amount of field-scale data has not yet been conducted. In this study, we predict subsurface velocity models using Laplace-domain wavefields as input instead of time-domain wavefields to apply a supervised learning-based deep-learning seismic inversion technique to field-scale data. Using Laplace-domain wavefields instead of time-domain wavefields significantly reduces the size of the input data, thereby accelerating the neural network training, although the resolution of the results is reduced. Additionally, a large grid interval can be used to efficiently predict the velocity model of the field data size, and the results obtained can be used as the initial model for subsequent inversions. The neural network is trained using only synthetic data by generating a massive synthetic velocity model and Laplace-domain wavefields of the same size as the field-scale data. In addition, we adopt a towed-streamer acquisition geometry to simulate a marine seismic survey. Testing the trained network on numerical examples using the test data and a benchmark model yielded appropriate background velocity models.

• Proceedings of the Korea Water Resources Association Conference
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• 2019.05a
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• pp.88-88
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• 2019

컴퓨터 성능향상과 수치해석기법의 발달로 인해 Navier-Stokes 방정식에 기초한 수치모델을 활용한 3차원 유동/파동장 해석이 증가하고 있는 추세이다. 그러나 아직까지 Navier-Stokes 방정식 모델의 계산부하를 PC에서 소화하기에는 무리가 따른다. 게다가 실험실 스케일을 벗어나, 실제 현장을 계산영역으로 설정할 경우에는 계산량이 엄청나게 증가하게 된다. 이것을 극복하기 위해서는 반듯이 병렬계산을 수행하여야 한다. 본 연구에서는 계산부하가 큰 Navier-Stokes 방정식 기반의 3차원 수치모델 LES-WASS-3D를 활용한 대용량 병렬계산체계를 구축한다. 나아가 3차원 정밀 또는 광역의 유동/파동장 해석에 있어서 병렬계산체계의 성능과 적용성을 검토한다. 현재 보급되고 있는 PC들은 모두 멀티프로세서가 장착됨으로 손쉽게 병렬계산을 수행할 수 있다. 그러나 정밀 또는 광역해석을 위해서는 대용량 병렬계산 컴퓨터가 요구된다. 따라서 본 연구에서는 보조프로세서를 장착한 공유메모리 환경의 고성능 병렬계산체계를 구축한다. 나아가 포트란 기반의 순차코드로 구축된 기존 3차원 Navier-Stokes 방정식 모델 LES-WASS- 3D를 병렬코드로 변환한다. 병렬계산 성능 및 적용성을 검토하기 위한 수치해석을 수행한다. 이상의 과정을 통해 본 연구에서 구축한 병렬계산체계의 성능 및 적용성을 확인할 수 있었다. 그리고 3차원 유동/파동장 해석에 있어서 정확도 향상뿐 아니라, 계산영역을 확장할 수 있는 계기가 마련되었다. 또한 유동/파동 해석보다 많은 계산시간이 필요한 지형변동 해석에도 충분히 적용될 수 있다고 판단된다.

• Geophysics and Geophysical Exploration
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• v.3 no.2
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• pp.70-75
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• 2000

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.

• Geophysics and Geophysical Exploration
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• v.1 no.2
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• pp.116-126
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• 1998

I present a datuming scheme for poststack data that uses wavefield depth extrapolation. The method I have developed allows the use of any depth extrapolation technique, such as phase-shift, split-step, and finite-difference extrapolation. I derive the datuming algorithms by transposing and taking the complex conjugate (i.e. taking adjoint) of the corresponding forward modeling operator, which does upward extrapolation from a flat surface to an irregular surface. The exact adjoint relation between the forward modeling operator and the datuming operator is demonstrated algebraically. Testing the poststack datuming algorithms with synthetic data, using several depth extrapolation algorithms, has shown that the method works well.

• Journal of Korean Society of Coastal and Ocean Engineers
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• v.14 no.2
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• pp.128-135
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• 2002

An existing numerical model fo r determining the wave field and pore pressures inside rubble mound breakwater was reformulated here especially to enhance the predictability of interior pore pressures. The pore pressures strongly depend on the nonlinear wave field occurring along frontal slope which is very difficult to be numerically reproduced. In the present study, hence, the amplitude and phase informations of wave pressures along the frontal slope are obtained directly through a hydraulic model test and are incorporated into the numerical model. The interior wave field is analyzed by a boundary element method, and thereby the pore pressures are determined. It was found that the calculated pore pressures agreed quite well with experimental values.

• Journal of Korean Society of Coastal and Ocean Engineers
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• v.14 no.1
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• pp.19-33
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• 2002

A numerical method to efficiently secure necessary design informations of the hydraulic characteristics of rubble mound breakwater was attempted here. The method combines the exterior wave field with the interior wave field which is formulated incorporating porous media flow inside the breakwaters. An approximate method based on the long wave assumption was used for the exterior wave field while a boundary element method was used for the interior wave field. A hydraulic experiment was also performed to verify the validity of the numerical analysis. The numerical results were compared with experimental data and results from existing formulae. They generally agreed in both reflection and transmission coefficients. The calculated pore pressures also showed a similar pattern with experimental data, even if they gave some significant differences in their values fur some cases. The main cause of such differences can be attributed to the strongly nonlinear wave field occurring on the frontal slope of the breakwater. The direct input of dynamic pressures(measured from hydraulic experiment) into the numerical method was suggested as a promising method to enhance the predictability of pore pressures.

• Proceedings of the Acoustical Society of Korea Conference
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• spring
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• pp.267-270
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• 2000

강제가진을 받는 진동장은 직접장과 반사장으로 이루어진다. 직접장은 무한구조요소가 점입력을 받을 때의 해와 같으며, 반사장은 직접장에 의해 발생되는 1차 반사파 및 추가로 발생하는 무수한 반사파들의 합으로 나타낼 수 있다. 본 논문에서는 점가진을 받는 유한한 평판의 단일 주파수 해석을 수행하기 위한 레이추적기법을 연구하였다. 이를 위해, 직접장은 고주파수 가정을 이용하여 원형전달파로 근사화하고. 이 원헝전달파를 다수의 파동관 (wave tube)으로 이산화하였다. 균일한 경계조건과 무시할 만큼의 미약한 굴절효과를 가정하고 경계에서의 정반사 (specular reflection)만을 고려하여, 경계에서의 입사파동관. 전달 및 반사파동관의 기하학적 관계를 제안하였다. 이들 파동관이 평판 내부를 진행하면서 관측점에 미치는 영향들을 합성하여 비교적 정확한 강제진동응답을 얻을 수 있음을 단일 평판의 예제를 통하여 확인할 수 있었다 그러나, 연성된 평판의 경우에는 다소 부정확한 결과를 얻었다.

• Geophysics and Geophysical Exploration
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• v.2 no.1
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• pp.54-62
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• 1999

This paper describes a datuming scheme for a prestack dataset which uses wavefield depth extrapolation. The formulation of the prestack datuming algorithm is performed by finding the adjoint operator to the corresponding forward prestack wavefield extrapolation from a flat surface to an irregular surface. Here I used double-square-root (DSR) equation to extrapolate wavefield in prestack sense. This correspond to the forward model of the well known `survey sinking` prestack imaging algorithm.

• The Optical Journal
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• s.156
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• pp.61-77
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• 2015

광학세계에서는 한동대학교 홍경희 교수의 저서 "기초광공학"을 연재하고 있다. 이 책에는 산업에 많이 활용되는 빛의 성질 및 그에 대한 이용 방법이 명확하고 알기 쉽게 설명돼 있다. 제1장에는 광학의 기본 개념과 역사, 빛을 파동으로 생각할 때의 파동운동과 운동방정식 등이 소개돼 있다. 제2장부터 제5장까지는 빛의 입자적 성질로 나타나는 기하광학 원리와 응용법이, 6장부터 제9장까지는 빛의 파동 성질로 나타나는 갑섭, 회절, 편광현상에 대한 원리와 응용법이 상세히 실려 있다. 이어 제10장에는 광섬유 광학이, 제11장에는 광원이, 제12장은 빛의 검출과 인식 및 측정이, 제13장에는 색깔의 원리와 응용법이 구체적으로 나열돼 있다. 마지막으로 제14장과 제15장에는 다양한 학문 분야나 첨단장비에 활용되며 그 발전에 영향을 주고 있는 레이저의 원리와 그 응용법이 소개돼 있다. "기초광공학"이 광학 업계에 종사하시는 분들은 물론 광학에 관심을 갖고 있는 분들에게도 유용한 자료가 되었으면 하는 바람에 광학세계에서도 2014년 1월호부터 연재를 하게 되었다.