• Geophysics and Geophysical Exploration
• /
• v.19 no.2
• /
• pp.76-83
• /
• 2016

Migration image can be distorted due to reflected waves in the source and receiver wavefields when discontinuities of input velocity model exist in seismic imaging. To remove reflected waves coming from layer interfaces, it is a common practice to smooth the velocity model for migration. If the velocity model is smoothed, however, the subsurface image can be distorted because the velocity changes around interfaces. In this paper, we attempt to minimize the distortion by reducing reflection energy in the source and receiver wavefields through acoustic impedance homogenization. To make acoustic impedance constant, we define fake density model and use it for migration. When the acoustic impedance is constant over all layers, the reflection coefficient at normal incidence becomes zero and the minimized reflection energy results in the improvement of migration result. To verify our algorithm, we implement the reverse-time migration using cell-based finite-difference method. Through numerical examples, we can note that the migration image is improved at the layer interfaces with high velocity contrast, and it shows the marked improvement particularly in the shallow part.

• Journal of the Computational Structural Engineering Institute of Korea
• /
• v.23 no.1
• /
• pp.1-8
• /
• 2010

The Fourier series uses a vibrating wave that possesses an amplitude that is like the one of the sine curve. Therefore, the functions used in the Fourier series do not change due to the value of the frequency and that set a limit to express irregular signals with rapid oscillations or with discontinuities in localized regions. However, the wavelet series analysis(WSA) method supplements these limits of the Fourier series by a linear combination of a suitable number of wavelets. By using the wavelet that is focused on time, it is able to give changes to the range in the cycle. Also, this enables to express a signal more efficiently that has singular configuration and that is flowing. The main objective of this study is to propose a scheme called wavelet series analysis for the application of wavelet theory to one-dimensional problems represented by the second-order elliptic equation and to evaluate theperformance of proposed scheme comparing with the finite element analysis. After a through evaluation of different types of wavelets, the HAT wavelet system is chosen as a wavelet function as well as a scaling function. It can be stated that the WSA method is as efficient as the FEA method in the case of axial bars with distributed loads, but the WSA method is more accurate than the FEA method at the singular points and its computation time is less.

• Journal of the Korean Geotechnical Society
• /
• v.19 no.2
• /
• pp.227-236
• /
• 2003

The evaluation of the reliable strength and deformation characteristics of weathered granite masses is very important for the design of geotechnical structure under working stress conditions. Various types of laboratory test such as triaxial compression test can be performed to determine the strength parameters. However, it is very difficult to obtain the representative undisturbed samples on the site and also the rock specimen cannot represent rock mass including discontinuities, fracture zone, etc. This study aims to investigate the strength and deformation characteristics of granite masses corresponding to its weathering and develop a practical strength parameter evaluation method using the results of PMT. To predict weathering intensity and strength parameters of the weathered granite masess in the field, various laboratory tests and in-situ tests including field triaxial test and PMT are carried out. Based on the results of weathering index tests, the classification method is proposed to identify the weathering degree in three groups for the weathered granite masses. Using the analytical method based on the Mohr-Coulomb failure criteria and the cavity expansion theory, the strength parameters of rock masses were evaluated from the results of PMT. It shows that weathering intensity increases with decreasing the strength parameters exponentially. The strength parameters evaluated with the results of PM almost coincide with the results of field triaxial test.

• The Journal of Engineering Geology
• /
• v.12 no.4
• /
• pp.405-419
• /
• 2002

Fracture roughness of rock specimens is observed by a new confocal laser scanning microscope (CLSM; Olympus OLS1100). The wave length of laser is 488 nm, and the laser scanning is managed by a light polarization method using two galvano-meter scanner mirrors. The function of laser reflection auto-focusing enables us to measure line data fast and precisely. The system improves resolution in the light axis (namely z) direction because of the confocal optics. Using the CLSM, it is Possible to measure a specimen of the size up to $10{\;}{\times}{\;}10{\;}cm$ which is fixed on a specially designed stage. A sampling is managed in a spacing $2.5{\;}\mu\textrm{m}$ along x and y directions. The highest measurement resolution of z direction is $10{\;}\mu\textrm{m}$, which is more accurate than other methods. Core specimens of coarse and fine grained granite are provided. Fractures are artificially maneuvered by a Brazilian test method. Measurements are performed along three scan lines on each fracture surface. The measured data are represented as 2-D and 3-D digital images showing detailed features of roughness. Line profiles of the coarse granites represent more frequent change of undulation than those of the fine granite. Spectral analyses by the fast Fourier transform (FFT) are performed to characterize the roughness data quantitatively and to identify influential frequency of roughness. The FFT results suggest that a specimen loaded by large and low frequency energy tends to have high values of undulation change and large wave length of fracture roughness.

• The Journal of Engineering Geology
• /
• v.24 no.1
• /
• pp.39-52
• /
• 2014

Limestone caves that consist of main passages and branches are formed by a variety of processes, and have the characteristic of developing with a preferred orientation controlled by discontinuities such as bedding, joints, and faults around the cave. However, it is difficult to analyze a representative orientation from various orientations. To interpret the overall development orientation of limestone caves, this study proposes new development orientation analysis methods, termed the Average Span Ratio Method (ASRM) and the Individual Development Ratio Method (IDRM), using the weighting of persistence. Nine limestone caves in Korea were randomly selected for testing the new methods. The methods show a stronger development orientation for limestone caves than that obtained by traditional methods, which consider only the distribution of development orientations. Based on an analysis of the relationship between the average span and the dip angle of bedding, it is confirmed that shallowly dipping bedding is a major contributor to the expansion of span in limestone caves. In addition, using scan-line survey data acquired in the field, we perform an RMR analysis of stability of the ground around limestone caves.

• The Journal of Engineering Geology
• /
• v.27 no.2
• /
• pp.119-124
• /
• 2017

The waviness of Large Discontinuity rock is the one of important elements that judges the stability of rock slope. When the waviness of large discontinuity is measured in the field, there are many limitations Therefore this research was carried out to measure waviness of large rock discontinuities using 3D laser scanner to supplement this problem. This research established one 3D model that actual X, Y and Z coordinates through the integrated data gained from one that calculates waviness of base lock using CAD program was compared and analyzed to that of disc-clinometer. As its results, the high reliability of results could be recognized as it belongs to mechanical tolerance $1{\sim}2^{\circ}$ and the results belong to the measured values of Mean DIP and Mean are all within $1^{\circ}$. So, the investigation method of waviness of large discontinuity rock face using 3D laser scanner was verified as more prompt, effective and reliable method than conventional direct site measuring method.

• Journal of the Society of Disaster Information
• /
• v.14 no.3
• /
• pp.367-378
• /
• 2018

Purpose: The objective of this study is to present a reasonable safety management of the anchored in-situ wall systems constructed in the ground conditions consisting of multi-layered soils underlain by bedrocks in the urban area of Korea. Method: Field measurements collected from collapse case histories with deep excavations were analyzed for the safety management of the wall systems supported by the earth anchors in terms of lateral displacement rates. Results: The average maximum lateral displacement rate in a collapsed zone of the in-situ wall significantly increased upon the completion of the excavation. Particularly, the collapse of the in-situ wall system due to the sliding occurring along the discontinuities of the rock produced a considerably large lateral displacement rate over a relatively short period. Conclusion: For predicting and preventing the collapse of the wall system during or after the excavation work, the utilization of the safety management criteria of the in-situ wall system by the lateral displacement rate was found to be much more reasonable in judging the safety of earthworks than the application of the quantitative management criteria which have been commonly used in the excavation sites.

• Tunnel and Underground Space
• /
• v.15 no.2 s.55
• /
• pp.119-128
• /
• 2005

Although the evaluation of the mechanical properties and behavior of jointed rock masses is very important for the design of tunnel and underground openings, it has always been considered the most difficult problem. One of the difficulties in describing the rock mass behavior is the selection of the appropriate constitutive model. This limitation may be overcome with the progress in discrete element software such as PFC, which does not need the user to prescribe a constitutive model for rock mass. In this paper, a 30\;m\;\times\;30\;m\;\times\;30\;m m jointed rock mass of road tunnel site was analyzed. h discrete fracture network was developed from the joint geometry obtained from core logging and surface survey. Using the discontinuities geometry from the DFN model, PFC simulations were carried out, starting with the intact rock and systematically adding the joints and the stress-strain response was recorded for each case. With the stress-strain response curves, the mechanical properties of jointed rock masses were determined. As expected, the presence of joints had a pronounced effect on mechanical properties of the rock mass. More importantly, getting the mechanical response of the PFC model doesn't require a user specified constitutive model.

• Journal of Korea Water Resources Association
• /
• v.40 no.2 s.175
• /
• pp.135-146
• /
• 2007

FLDWAV model was modified such that it can adequately simulate the effect of Jamsil and Singok submerged weirs in the main reach of the Han River. The enhanced model combines weir-type discharge equations for overflow at fixed weir and Manning equation for fluvial-type flow at the movable weir. Equations for weir overflow include those for submerged weir flow and free overflow. Gates of the movable weir may be open or closed for the simulation. In order to test the simulation capabilities, the enhanced model was applied for various flow conditions at submerged weirs. Backwater effect due to Jamsil and Singok submerged weirs were well simulated. Simulations were carried out for spring and neap tides extracted from artificial tide generated by combining $M_2\;and\;S_2$ tidal constituents. Simulation results cleared indicated that tidal effect extends further upstream as the flood discharge decreases. Low flow simulation capabilities of the enhanced model was tested. Discontinuities of water surface elevation due to the submerged weirs were successively simulated.

• Tunnel and Underground Space
• /
• v.20 no.2
• /
• pp.82-91
• /
• 2010

Grouting technology is one of the ground improvement methods used in water controlling and reinforcement of rock mass in underground structure construction. It is necessarily required to find out the characteristics of grout flow through discontinuities in a rock mass for an adequate grout design and performance assessment. Laminar flow is not always applicable in simulating a grout flow in a rock mass, since the rock joints usually have apertures at a micro-scale and the flow through these joints is affected by the joint roughness and the velocity profile of the flow changes partially near the roughness. Thus, the influence of joint roughness and aperture on the grout flow in rough rock joint was numerically investigated in this study. The commercial computational fluid dynamics code, FLUENT, was applied for this purpose. The computed results by embedded Herschel-Bulkley model and VOF (volume of fluid) model, which are applicable to simulate grout flow in a narrow rock joint that is filled with air and water, were well compared with that of analytical results and previously published laboratory test for the verification. The injection pressure required to keep constant injection rate of grout was calculated in a variety of Joint Roughness Coefficient (JRC) and aperture conditions, and the effect of joint roughness and aperture on grout flow were quantified.