• 한국지구물리탐사학회:학술대회논문집
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• 2006.06a
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• pp.117-122
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• 2006

In the geophysical monitoring to understand the change of subsurface material properties with time, the time-invariant static subsurface model is commonly adopted to reconstruct a time-lapse image. This assumption of static model, however, can be invalid particularly when fluid migrates very quickly in highly permeable medium in the brine injection experiment. In such case, the resultant subsurface images may be severely distorted. In order to alleviate this problem, we develop a new least-squares inversion algorithm under the assumption that the subsurface model will change continuously in time. Instead of sampling a time-space model into numerous space models with a regular time interval, a few reference models in space domain at different times pre-selected are used to describe the subsurface structure continuously changing in time; the material property at a certain space coordinate are assumed to change linearly in time. Consequently, finding a space-time model can be simplified into obtaining several reference space models. In order to stabilize iterative inversion and to calculate meaningful subsurface images varying with time, the regularization along time axis is introduced assuming that the subsurface model will not change significantly during the data acquisition. The performance of the proposed algorithm is demonstrated by the numerical experiments using the synthetic data of crosshole dc resistivity tomography.

• Geophysics and Geophysical Exploration
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• v.4 no.3
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• pp.89-95
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• 2001

The final goal of the seismic method is to verify the spatial characteristic of subsurface media. For this purpose, professional interpreters interprete the discontinuities of seismic events from seismic image. This paper introduces a method for automatic discontinuity extraction from 3D seismic image. The method consists of three steps. The first step is for estimating coherency of seismic events from seismic data. The second step is to express the potential region where discontinuities may exist in the form of binary image. The third step is to locate the discontinuities by thining the region found in the second step.

• Journal of the Korean Society for Nondestructive Testing
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• v.22 no.3
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• pp.275-283
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• 2002

To detect the position and depth of buried underground utilities, method of Ground Penetrating Radar(GPR) survey is the most commonly used. However, the skin-depth of GPR is very shallow, and in the places where subsurface materials are not homogeneous and are compose of clays and/or salts and gravels, GPR method has limitations in application and interpretation. The aim of this study is to overcome these limitations of GPR survey. For this purpose the site where the GPR survey is unsuccessful to detect the underground big pipes is selected, and soil tests were conducted to confirm the reason why GPR method was not applicable. Non-destructive high-frequency electromagnetic (HFEM) survey was newly developed and was applied in the study area to prove the effectiveness of this new technique. The frequency ranges $2kHz{\sim}4MHz$ and the skin depth is about 30m. The HFEM measures the electric field and magnetic field perpendicular to each other to get the impedance from which vertical electric resistivity distribution at the measured point can be deduced. By adopting the capacitive coupled electrodes, it can make the measuring time shorter, and can be applied to the places covered by asphalt an and/or concrete. In addition to the above mentioned advantages, noise due to high-voltage power line is much reduced by stacking the signals. As a result, the HFEM was successful in detecting the buried underground objects. Therefore this method is a promising new technique that can be applied in the lots of fields, such as geotechnical and archaeological surveys.

• Proceedings of the Korean Radioactive Waste Society Conference
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• 2005.11a
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• pp.134-142
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• 2005

The experimental setups for flow visualization and processes identification in laboratory scale (so cal led Flow Lab.) has developed to get ideas and answer fundamental questions of flow and migration in geologic media. The setup was made of a granite block of $50{\times}50cm$ scale and a transparent acrylate plate. The tracers used in this experiments were tritiated water, anions, and sorbing cations as well as an organic dye, eosine, to visualize migration paths. The migration plumes were taken with a digital camera as a function of time and stored as digital images. A migration model was also developed to describe and identify the transport processes. Computer simulation was carried out not only for the hydraulic behavior such as distributions of pressure and flow vectors in the fracture but also for the migration plume and the elution curves.

• Geophysics and Geophysical Exploration
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• v.21 no.4
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• pp.231-243
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• 2018

Streaming potential (SP) arises from fluid flow through effectively connected pores. From this potential, formation water information as well as fluid flow properties can be estimated. As micro particles being located in boundary between subsurface porous media and fluid are charged to form electrical double layer, fluid flow caused by several reasons generates SP, one of electrokinetic phenomena. Occurrence mechanism of SP is complex and signal strength is relatively weak compared to noise. However, application of self potential survey using SP to monitoring of formation fluid is expanding because of its' convenience of exploration without artificial source and repetitiveness of signal. This paper accounts for the occurrence mechanism of SP studied before, including governing equations and analyzes previous various case studies of SP according to the change of physical properties of materials. It helps to increase understanding about SP and also lays the foundations of the application of SP to fields.

• Geophysics and Geophysical Exploration
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• v.24 no.1
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• pp.6-19
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• 2021

Subsurface physical properties can be obtained and imaged by seismic exploration, and various algorithms have been developed for this purpose. In this regard, root mean square error (RMSE) has been widely used to quantitatively evaluate the accuracy of the developed algorithms. Although RMSE has the advantage of being numerically simple, it has limitations in assessing structural similarity. To supplement this, full-reference image quality assessment (FR-IQA) techniques, which reflect the human visual system, are being investigated. Therefore, we selected six FR-IQA techniques that could evaluate the obtained physical properties. In this paper, we used the full-waveform inversion, because the algorithm can provide the physical properties. The inversion results were applied to the six selected FR-IQA techniques using three benchmark models. Using salt models, it was confirmed that the inversion results were not satisfactory in some aspects, but the value of RMSE decreased. On the other hand, some FR-IQA techniques could definitely improve the evaluation.

• The Journal of Engineering Geology
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• v.7 no.3
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• pp.161-171
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• 1997

The exact information on geological structures and characteristics of the subsurface must be acquired to secure quality and safety of constructions. GPR technique, one of the most updated geophysical methods, is known for its applicability to shallow-depth underground surveys. The purpose of this study is to examine the usefulness of GPR method in constructions for detailed subsurface investigations, especially detecting the boundary between basement rock and its overburden. To find appropriate depths of the geological boundaries, it is necessary to obtain velocity of electromagnetic wave propagating into the ground. Wave velocity 0.096 m/ns estimated from velocity analysis using CMP gathers is used for depth conversion from time section. The depths of geological boundaries from GPR profiles are very well correlated with boring data. In addition, GPR survey has found some undulations of the geological boundaries due to weathering, which cannot be provided by conventional coring approaches.

• Tunnel and Underground Space
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• v.13 no.6
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• pp.465-475
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• 2003

Groundwater inflow into the caverns constructed in fractured rock mass was simulated by numerical modeling, NAPSAC (DFN, discrete fracture network model) and NAMMU (CPM, continuous porous media model), a finite-element software package for groundwater flow in 3D fractured media developed by AEA Technology, UK. The input parameters for modeling were determined on surface fracture survey, core logging and single hole hydraulic test data. In order to predict the groundwater inflow more accurately, the anisotropic hydraulic conductivity was considered. The anisotropic hydraulic conductivities were calculated from the fracture network properties. With a minor adjustment during model calibration, the numerical modeling is able to reproduce reasonably groundwater inflows into cavern and the travel length and times to the ground surface along the flow paths in the normal, dry and rainy seasons.

• Proceedings of the Korea Water Resources Association Conference
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• 2017.05a
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• pp.386-386
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• 2017

지하수 해석을 위해 3차원 수치해석 모형이 적용되고 있으나, 다양한 매개변수를 명확히 적용하기에는 한계가 있다. 특히, 수리전도도는 지층의 투수성정도를 나타내는 계수로 지하수 분석에 매우 중요한 매개변수이다. 신뢰성의 확보를 위해 양수시험을 통해 산출된 결과를 이용하고 있으나 소수관정의 시험결과를 유역의 대푯값으로 적용하기에 불확실도가 매우 높고, 수위변화가 독특한 지역의 정확한 수리특성을 적용하기에 한계가 있다. 이 연구에서는 수리전도도 특성을 해석하기 위하여 3차원 수치해석모형을 적용하였으며, 모델보정 방법 중 Regularization(정규화)라고 불리는 Pilot point 기법을 사용하였다. 제주도와 같이 수리전도도 값이 지역별로 차이가 크고 동일 유역 내에도 다른 지질구조를 보이는 등 동일 매질에서 동일 투수성을 보이지 않는 다양함으로 실측값들을 적용하기에 어려운 곳에서 정규화라는 보정방법은 최적화된 방법이다. 지하수위는 중제주유역 내 위치한 12개소 수위관측정의 2016년 연평균수위를 적용하였다. 미계측지역은 제주도 등수위선자료를 이용하여 DEM을 구축하였으며, 임의지점 17개소를 선정하여 대표수위로 적용하였다. 중제주유역의 평균 수리전도도는 82.90 m/day로 분석되었으며, 유역의 동측 하류부는 최대 1,745 m/day로 비교적 큰 결과가 산출되었다. 유역의 중앙에 위치하는 OR관측정을 기준으로 동 서지역의 지하수위를 검토한 결과 서측은 지형구배에 따라 지하수위가 형성하고 있으나, 동측의 경우 상 하류의 표고차가 30m이상 발생되지만 지하수위는 유사한 형태를 보이고 있다. 지하수 흐름에 해석되는 Darcy의 방정식은 수리전도도와 동수경사는 반비례관계를 나타내며, 이 이론에 의해 상 하류 지하수위가 유사하게 형성되는 동측지역은 국부적으로 수리전도도가 높게 형성되는 것으로 확인되었다. 실무에서는 유역경계에 따라 평균화된 매개변수가 적용되므로 명확한 지하수 해석이 어렵고, 수리전도도와 같이 지역적 편차가 심한 매개변수는 다양한 연구를 통해 적용성검토가 수행되어야 한다.

• KSCE Journal of Civil and Environmental Engineering Research
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• v.13 no.1
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• pp.97-103
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• 1993

This paper presents a quasi-analytical method using integral equations to obtain head distributions in unsaturated porous media with different hydrogeologic properties. One-dimensional soultion algorithms were developed for two cases of boundary conditions at the top: 1) constant head and 2) constant flux. Water table elevation at the bottom was assumed known for both cases. The methodology was applied to a fly ash disposal facility in an alluvium area. The results showed that the pressure head distributions had high nonlinearity with large gradients slightly above the interface of two media which made preliminary numerical solutions implausible. The developed method helped to structure finite element grids for improving convergence and accuracy.