• The Journal of Engineering Geology
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• v.14 no.2
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• pp.147-168
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• 2004

Purpose of this study is to quantitatively characterize the fracture roughness which was measured with a confocal laser scanning microscope. The roughness discrete data measured by confocal laser microscope were analyzed by spectral analysis and fast Fourier transform (FFT).The roughness data by used noise reduction filter were applied for fractal analysis to describe roughness features quantitatively. Artificial fractures created by Brazilian test on granites were used to measure fracture roughness under the confocal laser scanning microscope. Measurements were performed along three scan lines on each fracture surface. 36 scan lines were determined on 12 specimens in total. Features of roughness showed that coarse and medium grained granites tend to more rough features than those of fine grained granites. Continuous analog data of roughness is possible to described as discrete data of measure roughness with a fixed interval under the confocal laser microscope. Results of FFT with the measured data showed the highest values on the second harmonics. Distribution of average amplitude of second harmonics was observed 0.9853 in coarse grained granite, 1.0792 in medium grained granite and 0.6794 in fine grained granite. This indicates that the larger roughness has the higher energy of harmonics as the result of fractal analysis in low frequency zone.

• Journal of Korean Tunnelling and Underground Space Association
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• v.9 no.4
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• pp.403-413
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• 2007

As rock bolts become one of the main support systems in tunnels and underground structures, the integrity of the rock bolts affects the safety of these types of structures. The purpose of this study is the evaluation of rock bolt integrity using Fourier and wavelet transforms of the guided ultrasonic waves. After five rock bolt specimens with various defect ratios are embedded into a large scale concrete block, guided waves are generated by a PZT (lead zirconate titanate) element and measured by an acoustic emission (AE) sensor. The captured signals are analyzed in the frequency domain using the Fourier transform, and in the time-frequency domain using the wavelet transform based on a Gabor wavelet. The spectrum obtained from the Fourier transform shows that a portion of high frequency contents increases with increase in the defect ratio. Peak values in the time-frequency domain represent the interval of travel time of each echo. The energy velocities of the guided waves increase with the defect ratio. This study shows that the spectrum ratio and the energy velocity may be indicators fur the evaluation of rock bolt integrity.

• Journal of the Earthquake Engineering Society of Korea
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• v.13 no.6
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• pp.27-37
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• 2009

A method of instrumentally estimating seismic intensity (MMI) based on the Fourier Acceleration Spectrum of earthquake ground-motion, the so-called 'FAS MMI method' of Sokolov and Wald (2002), was evaluated for its applicability to Korea based on the empirical models of mean (m) and standard deviation (${\sigma}$) for Korea according to individual seismic intensity for MMI ${\leq}$ IV (Yun et al., 2009). This evaluation showed that the error in estimating the seismic intensity using the FAS MMI method is ${\sigma}$ = 0.74 MMI, and was further reduced to ${\sigma}$ = 0.61 MMI if the dependency of the error on earthquake magnitude and distance is additionally corrected. It is also shown that FAS MMI based on the FAS semi-empirically evaluated from small earthquakes for damaging earthquakes in Korea with maximum MMI ${\geq}$ VI could predict the observed MMI with the maximum error of 0.63 by using the combined FAS m-${\sigma}$ models of Korea for MMI ${\leq}$ IV and global region for MMI ${\geq}$ V.

• Proceedings of the KIEE Conference
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• 2008.07a
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• pp.219-220
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• 2008

이 논문은 이산푸리에변환을 이용하여 시계열데이터에 포함되어 있는 파라미터를 추정하는 새로운 방법과 그 응용 결과를 기술하고 있다. 제안한 방법은 푸리에스펙트럼으로부터 직접 파라미터를 추정하는 방법으로 FFT의 장점을 이용할 수 있으므로 신호에 포함되어 있는 중요 주파수를 비롯한 저주파 진동 파라미터를 빠르게 추정할 수 있다. 이 논문에서는 단순한 산술계산만 수행하여 제동계수를 추정할 수 있도록 알고리즘을 단순화하였다. 그리고 WSCC 계통에서 실측한 데이터에 제안한 알고리즘을 적용하여 효율성을 검증하였다.

• Proceedings of the Korean Institute of Information and Commucation Sciences Conference
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• 2010.05a
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• pp.254-257
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• 2010

본 논문에서는 신경회로망을 사용하여 이산푸리에변환에 의한 진폭성분과 위상성분을 복원하는 음성강조 시스템을 제안한다. 본 시스템은 신경회로망이 잡음이 부가된 음성신호의 이산푸리에변환의 진폭성분과 위상성분을 사용하여 학습된 후, 제안한 시스템은 배경잡음에 의하여 열화된 잡음이 부가된 음성신호를 강조한다. 배경잡음에 의하여 열화된 음성신호는 신경회로망을 사용하여 제안된 시스템에 의하여 강조되는 것을 실험결과로 증명하며, 제안한 시스템이 스펙트럼 왜곡율의 평가법을 사용하여 배경잡음에 의하여 열화된 음성신호에 대하여 효과적인 것을 실험으로 확인한다.

• The Journal of the Acoustical Society of Korea
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• v.42 no.3
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• pp.203-213
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• 2023

In medical ultrasound imaging systems, Side lobes may appear if signals outside the imaging point are not completely removed during receive focusing. If the time signal of the side lobe overlaps with the time signal (main lobe) from the image point, it is difficult to completely remove it using filter processing in the time domain. However, In the receive focusing process, when time-channel signals are Fourier-transformed, the main lobe and side lobe signals are spatially separated in the spectral domain. Therefore, the side lobes can be suppressed by multiplying the image with magnitude weights, which are determined by the magnitudes of the main and side lobes calculated in the spectral domain. In addition, when the main lobe and the side lobe spectrum are adjacent, the distance weight was applied based on the distance between them. In a 5 MHz ultrasound imaging system using a 64-channel linear transducer, point reflector and speckle images with cysts of various brightness were synthesized and weights were applied to the ultrasound image. Using computer simulations, we confirmed that the side lobes were greatly reduced without affecting the spatial resolution in the point reflector image, and the contrast was significantly improved in the cyst image with computer simulations.

• Journal of the Korea Institute of Information and Communication Engineering
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• v.14 no.5
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• pp.1078-1084
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• 2010

This paper presents a speech enhancement system which restores the amplitude components and phase components by discrete Fourier transform (DFT), using neural network training by back-propagation algorithm. First, a neural network is trained using DFT amplitude components and phase components of noisy speech signal, then the proposed system enhances speech signals that are degraded by white noise using a neural network. Experimental results demonstrate that speech signals degraded by white noise are enhanced by the proposed system using the neural network, whose inputs are DFT amplitude components and phase components. Based on measuring spectral distortion measurement, experiments confirm that the proposed system is effective for white noise.

• Journal of the Earthquake Engineering Society of Korea
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• v.13 no.6
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• pp.11-25
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• 2009

A method of instrumentally estimating the seismic intensity (MMI) based on Fourier Acceleration Spectrum, which is the so-called 'FAS MMI method' of Sokolov and Wald (2002), was considered for its applicability to Korea. In order to implement the FAS MMI method, the empirical models of mean (m) and standard deviation (${\sigma}$) for Korea were derived for MMI ${\leq}$ IV according to individual seismic intensity by using the site-consistent horizontal FAS of 580 records from 65 isoseismal maps prepared based on the reported MMI of Korea Meteorological Administration. The site-consistent FAS at a site were obtained by correcting the observed FAS for the difference of the site amplification function relative to that of the target site of Class D station (Yun and Suh, 2007) which was evaluated to be a representative site for the generic soil profile of Korea. The FAS m model for MMI ${\leq}$ IV follows the overall linear relation in log space according to seismic intensities, featuring the FAS mean model for MMI = IV similar to that of the global model of Sokolov and Wald (2002). The ${\sigma}$-values of the FAS model are found to be greater than those of the global model for MMI ${\geq}$ V, while significantly lower than those of the global model for MMI = IV.

• 유체기계공업학회:학술대회논문집
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• 2000.12a
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• pp.200-210
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• 2000

본 연구에서는 고속의 원심압축기에서 스톨 발단에 관한 연구를 수행하였다. 스톨을 일으키는 요인과, 스톨이 발생하기 전에 이를 미리 경고할 수 있는 방법을 주된 연구 주제로 삼았다. 원주방향으로 균일하게 분포된 8개의 고속응답 압력변환기를 사용하여 순간압력을 측정하였으며, 이 신호를 공간 푸리에 변환(space Fourier transform)을 사용하여 스톨의 발단을 알리는 신호를 측정하였고, 회전하는 파의 에너지(Traveling Wave Energy) 방법을 사용하여 스톨을 미리 경고하는 방법에 대하여 연구하였다. 회전하는 파의 에너지 방법은 스톨을 경고하는 데 좋은 성능을 보였으며, 저속에서는 약 임펠러 100회전, 중간속도에서는 약 200회전, 그리고 고속에서는 약 임펠러 1000회전의 경고시간을 보였다. 그리고 스톨 발단 근처에서 공간 푸리에 계수의 위상이 임펠러 주파수의 속도로 선형적인 증가를 보이는 구간이 나타났으며, 또한 임펠러 주파수의 스펙트럼이 스톨로 접근하면서 증가하는 것으로부터, 임펠러 주파수가 스톨을 일으키는 중요한 요인으로 작용함을 알 수 있었다. 또한 임펠러의 회전속도에 관계없이 스톨로 접근하면서 임펠러 주파수의 스펙트럼이 증가하므로, 이 값이 스톨을 경고하는 방법으로 사용될 수 있음을 보였으며, 약 임펠러 2n회전의 경고시간을 얻을 수 있었고, 임펠러의 속도가 빠를수록 긴 경고시간을 얻었다. 이 방법의 개발로 하나의 센서의 측정만으로도 효과적으로 스톨을 경고할 수 있는 기반을 마련하였다.

• Proceedings of the Earthquake Engineering Society of Korea Conference
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• 2002.09a
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• pp.43-50
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• 2002

부지고유의 강지진동모사를 위해 한반도 남부의 지각감쇠(Q)에 대한 두 가지 특성을 비선형 역산을 통해 규명하였다. 한 특성은 한반도 남부의 주요 지체구조구 및 특정 주향각에 대한 Q의 이방성이고 다른 특성은 Q의 수평방향으로의 공간적 변화 특성이다. 0의 이방성은 주향에 평행한 Q와 이에 수직한 Q의 비로 정의되었다. 사용된 지진자료는 190개 지진에 대한 3,400개 기록의 푸리에스펙트럼으로서 기록의 파선 분포는 한반도 남부 대부분을 조밀하게 덮었다. 역산 방법은 Levenberg-Marquardt 비선형역산 방법을 보다 안정화시킨 수정된 Levenberg-Marquardt 방법이 사용되었으며 기존에 도출된 광역적인 지진동파라미터를 초기해로 활용하였다. 역산 결과 국내의 중요 지체구조구 중 소백산육괴와 경상분지는 서로 다른 방향의 강한 Q 이방성을 나타내었으며 Q의 공간적 분포는 지진발생위치와 매우 밀접한 상관관계를 보여주었다