• Journal of the Korean Society of Manufacturing Technology Engineers
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• v.20 no.1
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• pp.47-52
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• 2011

It is well known that stiffness of composites depends on layup sequence of CFRP(carbon fiber reinforced plastics) laminates because the layup of composite laminates influences their properties. Ultrasonic NDE of composite laminates is often based on the backwall echoes of the sample. A pair of such transducers was mounted in a holder in a nose-to-nose fashion to be used as a scanning probe on composites. Miniature potted angle beam transducers were used (Rayleigh waves in steel) on solid laminates of composites. Experiments were performed to understand the behavior of the transducers and the nature of the waves generated in the composite (mode, wave speed, angle of refraction). C-scan images of flaws and impact damage were then produced by combining the pitch-catch probe with a portable manual scanner known as the Generic Scanner ("GenScan"). The pitch-catch signal was found to be more sensitive than normal incidence backwall echo of longitudinal wave to fiber orientation of the CFRP composites, including low level porosity, ply waviness, and cracks. Therefore, it is found that the experimentally Rayleigh wave variation of pitch-catch ultrasonic signal was consistent with numerical results and one-side ultrasonic measurement might be very useful to detect the defects.

• Geophysics and Geophysical Exploration
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• v.7 no.1
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• pp.14-18
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• 2004

High levels of ambient noise and safety factors often limit the use of 'active-source' seismic methods for geotechnical investigations in urban environments. As an alternative, shear-wave velocity-depth profiles can be obtained by treating the background microtremor wave field as a stochastic process, rather than adopting the traditional approach of calculating velocity based on ray path geometry from a known source. A recent field test in Melbourne demonstrates the ability of the microtremor method, using only Rayleigh waves, to resolve a velocity inversion resulting from the presence of a hard, 12 m thick basalt flow overlying 25 m of softer alluvial sediments and weathered mudstone. Normally the presence of the weaker underlying sediments would lead to an ambiguous or incorrect interpretation with conventional seismic refraction methods. However, this layer of sediments is resolved by the microtremor method, and its inclusion is required in one-dimensional layered-earth modelling in order to reproduce the Rayleigh-wave coherency spectra computed from observed seismic noise records. Nearby borehole data provided both a guide for interpretation and a confirmation of the usefulness of the passive Rayleigh-wave microtremor method. Sensitivity analyses of resolvable modelling parameters demonstrate that estimates of shear velocities and layer thicknesses are accurate to within approximately $10\%\;to\;20\%$ using the spatial autocorrelation (SPAC) technique. Improved accuracy can be obtained by constraining shear velocities and/or layer thicknesses using independent site knowledge. Although there exists potential for ambiguity due to velocity-thickness equivalence, the microtremor method has significant potential as a site investigation tool in situations where the use of traditional seismic methods is limited.

• Journal of Korean Society of Coastal and Ocean Engineers
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• v.2 no.1
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• pp.18-27
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• 1990

The calmness inside a harbor plays an important role in the appropriate disposition of har-bor structures. However, it is not easy to get the accurate computational results because they are affected by many factors concerning with the wave transformation. Successful solution also depends on determining the boundary values appropriately. This paper presents the numerical model which is able to calculate wave heights inside a harbor It is based upon the time-dependent mild-slope equation involving wave refraction, diffraction, shoaling effect and reflection. In particular, the arbitrary reflectivity is used at the boundary in order to simulate the real harbor reflection condition. This numerical model is applied for Hupo-Harbor and its validities are investgated by comparing with experimental values from the hydraulic model test as well as computational results from Taka-yama's numerical model (1981). It is shown that the model results are in good agreement with results from hydraulic model and Takayama＇s.

• Ocean and Polar Research
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• v.29 no.1
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• pp.9-31
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• 2007

Most seismic sea waves in the East Sea originate from earthquakes occurring near the Japanese west coast. While the waves propagate in the East Sea, they are deformed by refraction, diffraction and scattering. Though the Boussinesq equation is most applicable for such wave phenomena, it was not used in numerical modelling of seismic sea waves in the East Sea. To examine characteristics of seismic sea waves in the East Sea, numerical models based on the Boussinesq equation are established and used to simulate recent tsunamis. By considering Ursell parameter and Kajiura parameter, it is proved that Boussinesq equation is a proper equation for seismic sea waves in the East Sea. Two models based on the Boussinesq equation and linear wave equation are executed with the same initial conditions and grid size ($1min{\times}1min$), and the results are compared in various respects. The Boussinesq equation model produced better results than the linear model in respect to wave propagation and concentration of wave energy. It is also certified that the Boussinesq equation model can be used for operational purpose if it is optimized. Another Boussinesq equation model whose grid size is $40sec{\times}30sec$ is set up to simulate the 1983 and 1993 tsunamis. As the result of simulation, new propagation charts of 2 seismic sea waves focused on the Korean east coast are proposed. Even though the 1983 and 1993 tsunamis started at different areas, the propagation paths near the Korean east coast are similar and they can be distinguished into 4 paths. Among these, total energy and propagating time of the waves passing over North Korea Plateau(NKP) and South Korea Plateau(SKP) determine wave height at the Korean east coast. In case of the 1993 tsunami, the wave passing over NKP has more energy than the wave over SKP. In case of the 1983 tsunami, the huge energy of the wave passing over SKP brought about great maximum wave heights at Mukho and Imwon. The Boussinesq equation model established in this study is more useful for simulation of seismic sea waves near the Korean east coast than it is the Japanese coast. To improve understanding of seismic sea waves in shallow water, a coastal area model based on the Boussinesq equation is also required.

• Geophysics and Geophysical Exploration
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• v.10 no.1
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• pp.19-28
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• 2007

In this study, we propose a joint inversion method, using genetic algorithms, to estimate an S-wave velocity structure for deep sedimentary layers from receiver functions and surface-wave phase velocity observed at several sites. The method takes layer continuity over a target area into consideration by assuming that each layer has uniform physical properties, especially an S-wave velocity, at all the sites in a target area in order to invert datasets acquired at different sites simultaneously. Numerical experiments with synthetic data indicate that the proposed method is effective in reducing uncertainty in deep structure parameters when modelling only surface-wave dispersion data over a limited period range. We then apply the method to receiver functions derived from earthquake records at one site and two datasets of Rayleigh-wave phase velocity obtained from microtremor array surveys performed in central Tokyo, Japan. The estimated subsurface structure is in good agreement with the results of previous seismic refraction surveys and deep borehole data. We also conclude that the proposed method can provide a more accurate and reliable model than individual inversions of either receiver function data only or surface-wave dispersion data only.

• Journal of The Korean Astronomical Society
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• v.36 no.spc1
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• pp.101-107
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• 2003

The magnetosphere is often perturbed by impulsive input such as interplanetary shocks and solar wind discontinuities. We study how these initial perturbations are propagating within the magnetosphere over various latitude regions by adopting a three-dimensional numerical dipole model. We examine the wave propagation on a meridional plane in a time-dependent manner and compare the numerical results with multi-satellite and ground observations. The dipole model is used to represent the plasmasphere and magnetosphere with a realistic Alfven speed profile. It is found that the effects of refraction, which result from magnetic field curvature and inhomogeneous Alfven speed, are' found to become important near the plasmapause. Our results show that, when the disturbances are assumed at the subsolar point of the dayside magnetosphere, the travel time becomes smaller to the polar ionosphere compared to the equatorial ionosphere.

• The Proceeding of the Korean Institute of Electromagnetic Engineering and Science
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• v.2 no.4
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• pp.39-46
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• 1991

전자파 문제는 넓은 의미로 산란(scattering)문제와 역산란(inverse scattering)문제로 나눌 수 있다. 먼저 산란 문제는 에너지 또는 정보가 실린 전자파를 한 지점에서 다른 지점으로 보낼 때 통과하는 경로상의 매질 분포에 따라 왜곡 또는 변형되는 정도를 알아내는 것으로 반사(reflection), 굴절(refraction), 회절(diffraction)등 의 현상을 수반한다. 이 때 전자파를 왜곡시키는 물체를 산란체라고 부르며, 이러한 산란체로서는 전송선, 도파관, 광섬유 등과 같은 도파구조(guided wave structure)자체일 수 있으며 그들 내부에 고의로 부착된 첨가물일 수도 있다. 또한 공기나 지하와 같은 개방 구조 내의 물체나 비균일 매질 분포도 산란체가 될 수 있다. 이와는 반대로 역산란 문제는 알고 있는 전자파를 미지의 산란체에 가한 후, 여기서 산란된 전자파를 측정하여 얻은 자료로 부터 역으로 산란체의 위치, 크기, 모양, 매질 특성 등을 알아내는 것이다. 이러한 역산란 문제는지하 탐사(geophysical probing), 원격탐사(remote sensing), 레이다 영상(radar imaging), 의료진단(medical diagnosis), 비파괴 검사(nondestructive testing)등과 같은 많은 응용분야에 걸쳐 있다. 본 원고에서는 전자파 산란 및 역산란 문제에 대한 기존의 다양한 해석기법들을 체계적으로 분류하고, 이들의 적용범위와 한계에 대해 간략히 소개하기로 한다.

• Journal of the Korean Geophysical Society
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• v.9 no.4
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• pp.351-359
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• 2006

We investigated subsurface structures of the Bransfield Basin, the Antarctic with AUH (Autonomous Underwater Hydrophne) which was designed to record abyssal T-waves generated from submarine earthquakes. The data obtained from a multi-channel seismic survey and an AUH were used for this study. A seismic reflection method was applied to the multi-channel seismic survey data in order to identify bathymetry and sedimentary structures, and the signals recorded in the AUH were used to obtain deep structures as we applied a seismic refraction method. Even though we couldn’t investigate deeper and detailed structure in study area because of lack of Airgun’s capacity, the AUH showed possibilities for being used for a marine seismic survey. From this experiment, we decided the upper and lower sediment layer velocities, detected irregular basement topography probably caused by submarine volcanic/magmatic activities, and retrieved the velocity of the basement and the depth of the sediment layer/basement boundary.

• Journal of the Korean Society for Nondestructive Testing
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• v.26 no.1
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• pp.30-39
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• 2006

Feasibility is studied for an application of the mass-spring lattice model (MSLM), a numerical model previously developed for unidirectional composites, to the numerical simulation of ultrasonic inspection of austenitic welds modeled as transversely isotropic. Fundamental wave processes, such as propagation, reflection, refraction, and diffraction of ultrasonic waves in such an inspection are simulated using the MSLM. All numerical results show excellent agreement with the analytical results. Further, a simplified model of austenitic weld inspection has been successfully simulated using the MSLM. In conclusion, a great potential of the MSLM in numerically simulating ultrasonic inspections of austenitic welds has been manifested in this work, though significant further efforts will be required to develop a model with field practicality.

• KCID journal
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• v.19 no.1
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• pp.3-18
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• 2012

In recent years, as environmental problems have become great concerns among many people, th loss of beach sand has become one of the highly controversial issue. Major reasons for the beach erosion within the cases of West Coast can be classified as: 1) erosion at Unyeo, Baeksajang beaches are caused by the wave refraction according to the large-scale sand dredging, 2) erosion at Kkotji, Baeksajang, and Unyeo beaches are caused by large-scale embankment construction and the coastal road construction, and 3) erosion at Chollipo and Hakampo beaches are caused by construction of small ports. Erosion in the west coast of Korea coast beach erosion control measures, include groin, zeotube, terraced stone, jetty groin, and beach nourishment. Erosion control measures initially installed to prevent erosion showed a positive effect. However, if there is no continuous source of sand, the effect of measures is fewness.