• Journal of the Korean Society for Nondestructive Testing
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• v.20 no.5
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• pp.397-401
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• 2000

The corrosion-fatigue characteristics of the 12Cr steel, which is widely used in fossil power plants as a turbine blade material, are evaluated nondestructively by use of the Rayleigh surface wave. In this study, the frequency dependency of the Rayleigh surface wave is investigated indirectly by measuring the angular dependency of the backward radiation of the incident ultrasonic wave in the aged specimens, and then compared to the corrosion-fatigue characteristics. The width of the backward radiation profile decreases as the increase of the aging temperature, which seems to result from the increase of the effective degrading layer thickness. This parameter also shows an inversely proportionality to the exponent, m, in the Paris law which predicts the crack size increasement due to fatigue. The result observed in this study demonstrates high potential of the backward radiated ultrasound as a tool for the nondestructive evaluation of the corrosion-fatigue characteristics of the aged materials.

• Proceedings of the Korean Geotechical Society Conference
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• 2005.03a
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• pp.302-307
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• 2005

Love wave and Rayleigh wave are the major elastic waves belonging to the category of the surface wave. The fact that Love wave is not contaminated by P-wave which makes Love wave superior to Rayleigh wave and other body waves. Therefore, the information that Love wave carries is more distinct and clearer than the information of Rayleigh wave. Based on theoretical research, the joint inversion analysis which is used both Love wave dispersion information and Rayleigh wave dispersion information was proposed. Purpose of the joint inversion analysis is to improve accuracy and convergency of inversion results utilizing that frequency contribution of each wave is different. This analysis technique is consisted of the forward modeling using transfer matrix, the sensitivity matrix determined to the ground system and DLSS(Damped Least Square Solution) as a inversion technique. The application of this analysis was examined through the field test.

• Proceedings of the Korea Contents Association Conference
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• 2005.11a
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• pp.359-363
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• 2005

본 논문에서는 다양한 무선 통신 채널 환경에서 레일리 페이딩과 라이시안 페이딩 등을 모두 포함할 수 있고 이동 통신과 위성 통신에서도 적용할 수 있는 페이딩 모델로서 나카가미 페이딩 모델을 도입하고 전자기기에서 방출되는 임펄스 간섭파를 채널 환경으로 모델링하여 결합변조기법을 채용한 DS-CDMA 시스템의 성능을 분석하였다. 변조방식으로는 대역폭 효율 개선을 위해 MFSK와 MDPSK를 고려한 결합변조기법을 채용하였으며 실내 무선 페이딩 채널로써 직접파대 반사파 전력비를 나카가미 페이딩 심도지수 m으로 산출하여 임펄스 지수A와 가우스 잡음 전력 대 임펄스 잡음 전력비 $\Gamma'$를 파라미터로 하여 시스템 오율성능을 분석하였다.

• The Journal of Korean Institute of Communications and Information Sciences
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• v.27 no.1A
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• pp.1-12
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• 2002

Ultra Wideband (UWB) system uses wide band signal, which power spectral density likes as a noise floor, so UWB system can be used high speed communication without interfering with other communication system. As this system doesn't use radio frequency carrier. Its structure would be very simple and consume very low power. For the first time, we adopted Rayleigh monopulse multiple access system. And under the AWGN, we analysis and compare error probability, the number of user and data rate of the two methods that pulse position modulation and antipodal. In the result, We concluded that the antipodal method had much better probability of error, the number of user and data rate than PPM.

• Geophysics and Geophysical Exploration
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• v.9 no.1
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• pp.93-101
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• 2006

In this study, we propose a joint inversion method, using genetic algorithms, to determine the shear-wave velocity structure of deep sedimentary layers from receiver functions and surface-wave phase velocity. Numerical experiments with synthetic data indicate that the proposed method can avoid the trade-off between shear-wave velocity and thickness that arises when inverting the receiver function only, and the uncertainty in deep structure from surface-wave phase velocity inversion alone. We apply the method to receiver functions obtained from earthquake records with epicentral distances of about 100 km, and Rayleigh-wave phase velocities obtained from a microtremor array survey in the Kanto Plain, Japan. The estimated subsurface structure is in good agreement with the previous results of seismic refraction surveys and deep borehole data.

• Geophysics and Geophysical Exploration
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• v.17 no.1
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• pp.1-10
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• 2014

To reveal and classify site characteristics in densely populated areas in Chuncheon, Korea, Rayleigh-waves were recorded at 50 sites including four sites in the forest area using four 1-Hz velocity sensors and 24 4.5-Hz vertical geophones during the period of January 2011 to May 2013. Dispersion curves of the Rayleigh waves obtained by the extended spatial autocorrelation method were inverted to derive shear-wave velocity ($v_s$) models comprising 40 horizontal layers of 1-m thickness. Depths to weathered rocks ($D_b$), shear wave velocities of these basement rocks ($v_s^b$), average velocities of the overburden layer ($\bar{v}_s^s$), and the average velocity to a depth of 30 m ($v_s30$), were then derived from those models. The estimated values of $D_b$, $v_s^b$, $\bar{v}_s^s$, and $v_s30$ for 46 sites at lower altitudes were in the ranges of 5 to 29 m, 404 to 561 m/s, 208 to 375 ms/s, and 226 to 583 m/s, respectively. According to the Korean building code for seismic design, the estimated $v_s30$ indicates that the lower altitude areas in Chuncheon are classified as $S_C$ (very dense soil and soft rock) or $S_D$ (stiff soil). To determine adequate proxies for $v_s30$, we compared the computed values with land cover, lithology, topographic slope, and surface elevation at each of the measurement sites. Due to a weak correlation (r = 0.41) between $v_s30$ and elevation, the best proxy of them, applications of this proxy to Chuncheon of a relatively small area seem to be limited.

• Journal of the Korean Geotechnical Society
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• v.38 no.8
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• pp.17-27
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• 2022

To characterize the dynamic properties of Gimhae Plains sediments, we calculated natural frequencies using microtremor horizontal-to-vertical spectral ratios and derived shear wave velocity profiles by inversion of Rayleigh-wave dispersion curves obtained by the high frequency-wavenumber and modified spatial autocorrelation methods. Our results suggest that in this region, strong amplification of ground motion is expected in the vibration frequency (f ≥ 1 Hz). Additionally, obtained velocity profiles show that shear wave velocities are ~200 and 400 m/s for the shallow marine and old fluvial sediments, respectively. Bedrock is possibly encountered at depths of 60-100 m at most sites. We developed a simplified shear wave velocity model of shallow sediments based on the obtained profiles. Our results suggest that a large area in the Gimhae Plains could be categorized as an S6 site based on the Korean seismic design code (KDS 17 10 00).

• The Journal of the Acoustical Society of Korea
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• v.20 no.4
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• pp.94-100
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• 2001

In October 1997, low-frequency reverberation was measured at an experimental site off the west coast of the Jeju island using the explosive charge, Signals Underwater Sound (SUS). Received signals were separated into the noise, the reflection, and the scattering region, and then were analyzed for the spectral and statistical characteristics of each region. In the analysis of the spectrum we verified that each region had a unique frequency band and statistical characteristics as well. The results of this analysis showed that the real and imaginary portions were shown to be both normal distributions in each frequency bin. The reverberation envelope had a Rayleigh distribution and the phase had a uniform distribution.

• Journal of Advanced Navigation Technology
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• v.15 no.1
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• pp.61-68
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• 2011

DVB-T2 system developed by DVB project is the next generation digital terrestrial television broadcasting standard designed for offering HDTV service in a post-Analog Switch Off (AOS) environment. In this paper, the performance of DVB-T2 is evaluated with the help of computer simulation. The bits error rate(BER) performance is studied in both AWGN, Rayleigh, Ricean and 0db-echo channel. Firstly, we will give a brief introduction to DVB-T2 system and then compare its BER performance to that of Implementation Guideline.

• Geophysics and Geophysical Exploration
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• v.9 no.1
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• pp.73-85
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• 2006

Array observations of the vertical component of microtremors are frequently conducted to estimate a subsurface layered-earth structure on the assumption that microtremors consist predominantly of the fundamental mode Rayleigh waves. As a useful tool in the data collection, processing and analysis, the spatial autocorrelation (SPAC) method is widely used, which in practice requires a circle array consisting of M circumferential stations and one centre station (called "M-station circle array", where M is the number of stations). The present paper considers the minimum number of stations required for a circle array for efficient data collection in terms of analytical efficacy and field effort. This study first rearranges the theoretical background of the SPAC algorithm, in which the SPAC coefficient for a circle array with M infinite is solely expressed as the Bessel function, $J_0(rk)$ (r is the radius and k the wavenumber). Secondly, the SPAC coefficient including error terms independent of the microtremor energy field for an M-station circle array is analytically derived within a constraint for the wave direction across the array, and is numerically evaluated in respect of these error terms. The main results of the evaluation are: 1) that the 3-station circle array when compared with other 4-, 5-, and 9-station arrays is the most efficient and favourable for observation of microtremors if the SPAC coefficients are used up to a frequency at which the coefficient takes the first minimum value, and 2) that the Nyquist wavenumber is the most influential factor that determines the upper limit of the frequency range up to which the valid SPAC coefficient can be estimated.