• Economic and Environmental Geology
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• v.35 no.5
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• pp.455-463
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• 2002

There have been few geophysical studies on the crustal structure of the continent-ocean zone around the middle eastern part of Korean peninsula, because of the lack of database in both land and ocean. The area for the study on the internal crustal structure using gravity data is bounded by the latitude of 37$^{\circ}$-38"N and longitude of 128$^{\circ}$-132$^{\circ}$E. WCA correction is applied to shipborne gravity data to integrate with gravity anomalies obtained on land. The high frequency components of the shipborne gravity data which are considered as the noise on survey track are effectively removed by means of correlating with satellite gravity data. The corrected shipborne free-air gravity anomaly is integrated with the Bouguer gravity anomaly on land under the same condition. The integrated gravity anomaly is divided into four areas for power spectrum analysis. The depths of Moho discontinuity increases gradually from inland to Ulleung basin. As the result of modeling based on power spectrum analysis, Moho discontinuity depth is about 33-35 km in the continental zone of Korea and 18-28 km at the continental margin. Such structural character is well elucidated in changing gravity data around Ulleung basin. The depths of Moho discontinuity in the southern ocean of Ulleung-island is 16--17 km, which is much lower than in the land. The result of crustal structure modeling in this study is similar to that computed by prior seismic exploration around this area.

• Geotechnical Engineering
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• v.14 no.3
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• pp.95-108
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• 1998

Downhole testing and seismic CPT (SCPT) have been widely used to evaluate stiffness profiles of the subgrade. Advantages of downhole testing and SCPT such as low cost, easy operation and a simple seismic source have got these testings more frequently adopted in site investigation. For the automated analysis of downhole testing and SCPT, the concept of interval measurements has been practiced. In this paper. a new inversion procedure to deal tilth the interval measurements for the automated downhole testing and SCPT (including a newlydeveloped continuous SCPT) is proposed. The forward modeling in the new inversion procedure incorporates ray path theory based on Snell's law. The formulation for the inversion analysis is derived from the maximum likelihood approach, which estimates the maximum likelihood of obtaining a particular travel time from a source to a receiver. Verification of the new inversion procedure was performed with numerical simulations of SCPT using synthesized profiles. The results of the inversion analyses performed for the synthetic data show that the new inversion analysis is a valid procedure which enhances Va profiles determined by downhole testing and SCPT.

• The Journal of the Acoustical Society of Korea
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• v.18 no.1
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• pp.10-15
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• 1999

We have studied a method to design and fabricate the Intermediate Frequency(IF) band pass filter with low shape factor which is used for CDMA base station on the 35°Y-cut X-propagation Quartz substrate. In order to fabricate a device of the low shape factor for the IF SAW filter on this substrate, we employed apodization weighted type interdigital transducer(IDT) as an input and withdrawal weighted type IDT as an output by using impulse modelling method. Also, using the Kaiser-Bessel window function, we have adopted 2200pairs and 1000pairs of input and oueut IDT respectively to minimize the effect of ripple. Furthermore, the width and the space of IDT finger are 3.6 ㎛ and 3.5 ㎛ respectively. Thus, we can have optimal results when the IDT thickness is 6000Å in consideration of the ratio of SAW's wavelength while it's aperture is 2mm for impedance matching. The fabricated SAW filter for CDMA had the property of almost 115.2MHz of a center frequency, less then 1.27MHz of bandwidth, less than 1.3 of shape factor, - l5dB of out band attenuation insertion loss and -45dB of rejection band.

• Tunnel and Underground Space
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• v.18 no.5
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• pp.328-337
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• 2008

Quantifying the extent and characteristics of the excavation damage zone(EDZ) is important for the nuclear waste industry which relies on the sealing of underground openings to minimize the risk for radionuclide transport. At AECL's Underground Research Laboratory(URL) the Tunnel Sealing Experiment(TSX) was conducted and the tunnel geometry and orientation relative to the stress field had been selected to minimize the potential for the development of an EDZ. The extent and characteristics of the EDZ was measured using velocity profiling and permeability measurements in radial boreholes. The results from this EDZ characterization are used in this paper to evaluate a modeling fir estimating the extent of the EDZ. The methodology used a damage model formulated in the Universal Distinct Element Code and calibrated to laboratory properties. This model was then used to predict the extent of crack initiation and growth around the TSX tunnel and the results compared to the measured damage. The development of the damage zone in the numerical model was found to be in good agreement with the field measurements.

• Geophysics and Geophysical Exploration
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• v.4 no.3
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• pp.84-88
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• 2001

For the elastic migation, the velocity errors between the initial velocity model and true velocity model seriously affect the migrated images. The assumption of an initial velocity model, thus, is one of the critical factor for the successful migration. In case of applying the layered earth model as an initial velocity model, the layer boundary having large velocity contrast can not be defined well with conventional traveltime calculation algolithms and we have the difficulties for expressing the characteristics of the real subsurface. Smooth Background Model (SBM) we have applied as an initial velocity model in our study is characterized to be linearly varying the velocity with the depth, which can express the velocity variation in the subsurface properly. Thus it can properly be applied to traveltime calculation algolithms such as Vidale's method. In this study, Kirchhoff operator for prestack migration was used and the absolute amplitude obtained by modeling was applied as a weighted value to consider the true amplitude for initial model. Initial velocity model for migration was determined by using stacking velocity and we applied this model to real data.

• Geotechnical Engineering
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• v.13 no.1
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• pp.5-18
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• 1997

In order to make reliable ground shock predictions in saturated geological media, it is necessary to use multi -phase material models and numerical codes. This paper presents the results of theoretical study of the fundamental behavior of multi-phase porous media subjected to high dynanlic loadings, and deals with the development of numerical code MPDAP with JWL(Jones-Wilkins-Lee) model, which is capable of considering the kinds and characters of explosives. To check the global equilhorium equations of the numerical code, we carried out some verifications. In the cases of the elastic spherical wave propagation in a single phase medium, one-dimensional linear ronsolidation, and one timensional wave propagation in saturated linear elastic soils and rocks, the results calculated by MPDAP show close agreement with closed-form solutions or numerical solutions generated with two phase code.

• Journal of the Computational Structural Engineering Institute of Korea
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• v.32 no.4
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• pp.241-247
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• 2019

A band gap refers to a certain frequency range where the propagation of mechanical waves is prohibited. This work focuses on engineering three-dimensional Kelvin lattices having external band gaps at low audible frequency ranges using a gradient-based design optimization method. Elastic wave propagation in an infinite periodic lattice is investigated by employing the Bloch theorem. We model the ligaments using a shear-deformable beam model obtained by consistent linearization in a geometrically exact beam theory. For a given lattice topology, we enlarge band gap sizes by controlling the configuration of the beam neutral axis and cross-section thickness that are smoothly parameterized by B-spline basis functions within the isogeometric analysis framework.

• Geophysics and Geophysical Exploration
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• v.22 no.1
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• pp.12-20
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• 2019

The first-arrival traveltime calculation method based on the damped wave equation overcomes the shortcomings of ray-tracing methods. Since this algorithm needs to solve the damped wave equation, numerical modeling is essential. However, it is not desirable to use the finite-difference method (FDM), which has good computational efficiency, for simulating the land seismic data because of irregular topography. Thus, the finite-element method (FEM) which requires higher computational cost than FDM has been used to correctly describe the irregular topography. In this study, we computed first-arrival traveltimes in an irregular topographic model using FDM incorporating embedded boundary method (EBM) to overcome this problem. To verify the accuracy and efficiency of the proposed algorithm, we compared our results with those of FEM. As a result, the proposed method using EBM not only provided the same accuracy as the FEM but also showed the improved computational efficiency.

• Tunnel and Underground Space
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• v.22 no.4
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• pp.266-275
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• 2012

In this study, a method is devised to implement a supercritical $CO_2$ ($scCO_2$) injection environment on a laboratory scale and to investigate the effects of $scCO_2$ on the properties of rock specimens. Specimens of shale and sandstone normally constituting the cap rock and reservoir rock, respectively, were kept in a laboratory reactor chamber with $scCO_2$ for two weeks. From this stage, a chemical reaction between rock surface and the $scCO_2$ was induced. The effect of saline water was also investigated by comparing three conditions ($scCO_2$-rock, $scCO_2-H_2O$-rock and $scCO_2$-brine(1M)-rock). Finally, we checked the changes in the properties before and after the reaction by destructive and nondestructive testing procedures. The swelling of shale was a main concern in this case. The experimental results suggested that $scCO_2$ has a greater effect on the swelling of the shale than pure water and brine. It was also observed that the largest swelling displacement of shale occurred after a reaction with the $H_2O-scCO_2$ solution. The results of a series of the destructive and nondestructive tests indicate that although each of the property changes of the rock differed depending on the reaction conditions, the $H_2O-scCO_2$ solution had the greatest effect. In this study, shale was highly sensitive to the reaction conditions. These results provide fundamental information pertaining to the stability of $CO_2$ storage sites due to physical and chemical reactions between the rocks in these sites and $scCO_2$.

• The Journal of Engineering Geology
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• v.7 no.2
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• pp.139-149
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• 1997

Electrical resistivity monitoring methods were adopted to detect groundwater table change in alluvium. Numerical modelling test using finite element method(FEM) and field resisfivity monitoring were conducted in the study. The field monitoring data were acquired in the alluvium deposit site in Jeong-Dong Ri, Geum River where pumping test had been conducted continuously for 20 days to make artificial changes of groundwater table. The unit distance of the electrode array was 4m and 21 fixed electrodes were applied in numerical calculation and field data acquisition. "Modified Wenner" and dipole-dipole array configurations were used in the study. The models used in two-dimensional numerical test were designed on the basis of the simplifving geological model of the alluvium in Jeong Dong Ri, Geum River. Numerical test results show that the apparent resistivity pseudosections were changed in the vicinity of the pootion where groundwater table was changed. Furthermore, there are some apparent resistivity changes in the boundary between aquifer and crystalline basement rock which overlays the aquifer. The field monitoring data also give similar results which were observed in numerical tests. From the numerical test using FEM and field resistivity monitoring observations in alluvium site of Geum River, the electrical monitoring method is proved to be a useful tool for detecting groundwater behavior including groundwater table change. There are some limitations, however, in the application of the resistivity method only because the change of groundwater table does not give enough variations in the apparent resistivity pseudosections to estimate the amount of groundwater table change. For the improved detection of groundwater table changes, it is desirable to combine the resistivity method with other geophysical methods that reveal the underground image such as high-resolution seismic and/or ground penetrating radar surveys.