• Geophysics and Geophysical Exploration
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• v.9 no.4
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• pp.261-270
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• 2006

Two-dimensional (2-D) inversion of magnetotelluric (MT) data for two survey lines having south-north direction from Jeju Island has been carried out. Broad band MT sounding curves with good quality could be gathered by performing audio-frequency magnetotelluric (AMT) survey during the MT survey and by operating the remote reference in Kyushu Island, Japan. Comparison of the 2-D inversion model using MT band only and that using both AMT and MT bands for the field data as well as for the data from numerical 2-D modeling said that high frequency information from AMT survey can be useful for interpreting not only the shallow part but also the deep structures, especially when the formation is resistive. The 2-D inversion models of field data show a thick layer having around 10 ohm-m in the depth of a few hundred meters throughout the survey area, which can be considered as the unconsolidated sedimentary layer. And they also show a conductive anomaly at the central part of each survey lines. It can be either the effect of the surrounding sea water, or the structures due to ancient volcanic events. But unfortunately by now, we do not have any further information about the anomaly.

• Journal of the Korean Society for Nondestructive Testing
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• v.36 no.3
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• pp.195-201
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• 2016

Eddy current testing (ECT) is commonly applied for the inspection of automated production lines of metallic products, because it has a high inspection speed and a reasonable price. When ECT is applied for the inspection of a metallic object having an uneven target surface, such as the spline gear of a spline shaft, it is difficult to distinguish between the original signal obtained from the sensor and the signal generated by a defect because of the relatively large surface signals having similar frequency distributions. To facilitate the detection of defect signals from the spline gear, implementation of high-order filters is essential, so that the fault signals can be distinguished from the surrounding noise signals, and simultaneously, the pass-band of the filter can be adjusted according to the status of each production line and the object to be inspected. We will examine the infinite impulse filters (IIR filters) available for implementing an advanced filter for ECT, and attempt to detect the flaw signals through optimization of system design parameters for detecting the signals at the system level.

• Journal of the Korean Society of Hazard Mitigation
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• v.2 no.1 s.4
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• pp.135-141
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• 2002

Both non-linear damping values of the deep and shallow crustal materials and seismic source parameters are found from the observed near-field seismic ground motions at the South-eastern Korean Peninsula. The non-linear numerical algorithm applied in this study is Levenberg-Marquadet method. All the 25 sets of horizontal ground motions (east-west and north-south components at each seismic station) from 3 events (micro to macro scale) were used for the analysis of damping values and source parameters. The non-linear damping values of the deep and shallow crustal materials were found to be more similar to those of the region of the Western United States. The seismic source parameters found from this study also showed that the resultant stress drop values are relatively low compared to those of the Western United Sates. Consequently, comparisons of the various seismic parameters from this study and those of the United States Seismo-tectonic data suggest that the seismo-tectonic characteristics of the South eastern Korean Peninsula is more similar to those of the Western U.S.

• Journal of the Korean Institute of Illuminating and Electrical Installation Engineers
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• v.21 no.10
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• pp.59-65
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• 2007

Recently, ubiquitous sensor network(USN) techniques have been applied to electric power facility management. This paper dealt with the designed and fabricated ubiquitous sensors which monitor transformers and lightning arresters installed in overhead distribution systems. The sensors consist of a 8-[bit] microprocessor unit, a wireless communication nodule specified in IEEE 802.15.4, and associated electronics. A Rogowski coil was fabricated to measure load of transformer and surge current without saturation having good linearity up to 1000[A]. A zero-phase current transformer with a high relative permeability of $10^5$ at 180[Hz] was used to detect small leakage current of $50[{\mu}A]{\sim}1[mA]$ flowing lightning arrester, and the frequency bandwidth of the module is ranges from 12[Hz] to 1.24[kHz] at -3[dB].

• The Journal of the Acoustical Society of Korea
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• v.38 no.3
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• pp.290-301
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• 2019

The proposed algorithm deals with a detection problem of target echo for narrow-band continuous-wave active sonar in the underwater environment in this paper. In the active sonar systems, ping signal emitted for target detection produces a signal that consists of multiple reflections by many scatterers around, which is called reverberation. The proposed algorithm aims to detect the low-Doppler target echo in the reverberant environment. The proposed algorithm estimates the bearing, frequency, and temporal bases based on beamspace-domain multichannel nonnegative matrix factorization. In particular, the bases are divided into two basis groups - the reverberation group and the echo group, then the basis groups are estimated independently. In order to evaluate the proposed algorithm, a simulation with synthesized reverberation was performed. The results show that the proposed algorithm has enhanced performance than the conventional algorithms.

• Journal of Korean Tunnelling and Underground Space Association
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• v.8 no.1
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• pp.3-12
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• 2006

The purpose of this study is to describe the Non-Destructive Testing(NDT) of the rockbolt and investigate the applicability of the NDT methods to estimate the integrity of the rockbolt. To examine the rockbolt integrity including rockbolt itself and grouting material, two methods are adopted: numerical and experimental methods. In the numerical method, the numerical code DISPERSE is used to analyze the dispersion of the rockbolt. The dispersion curve shows the effects of the thickness and stiffness of grouted materials on the embedded rockbolt. Therefore, the optimal frequency for the integrity test of the rockbolt is obtained: 20~120kHz in L(1,0) mode. In the experimental methods, destructive and non-destructive tests are carried out in a laboratory. In the non-destructive test, the low frequency mode generated by an impact and t he high frequency mode generated by an ultrasonic transducer seem to characterize the rockbolt condition readily. The experimental results show that the guided waves attenuate more significantly when the stiffness of the grouted material increases and/or the zone of the defect increases. Meanwhile, the ultimate capacity of rockbolt was evaluated through the pull-out tests and is compared to the NDT results. This study demonstrates that the NDT is a valuable tool for the rockbolt integrity evaluation.

• Journal of Korean Society of Coastal and Ocean Engineers
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• v.13 no.3
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• pp.209-229
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• 2001

Simultaneous field measurements of short-period and long-period waves were made at five stations inside or outside Okkye Harbor, which is located in the east coast of Korea. Based on the measured data, spacial and temporal variations of the long-period wave energy were examined. Three smoothing methods were examined for the spectral estimates: fixed interval averaging method, incremental interval averaging method, and moving averaging method. It was shown that a proper smoothing method should be chosen depending on the period of first resonant mode and the length of data being used. By comparing the results obtained using the long-term data with those obtained using two-day data, we showed that it is necessary to analyze the data of calm seas and storm seas separately. The Helmholtz resonant period in Okkye Harbor was found to be about 9.6 minutes with its relative amplification ratio of 9 to 10, and local amplifications were apparent at the periods of 1.2 to 1.3 minutes and 0.7 minute. During calm seas, both at the harbor entrance and inside the harbor the energy of the waves of 9 minutes or longer period was larger than the infra-gravity wave energy by more than 100 times. However, during storm seas the energy level was very high all over the period band, and local amplification was larger than that during calm seas by more than 100 times, especially inside the harbor, Finally it was shown that the energies of the Helmholtz resonant mode and the infra-gravity waves of 1 to 2 minutes are proportional to the storm wave height.

• Journal of the Korean Institute of Intelligent Systems
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• v.20 no.1
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• pp.140-145
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• 2010

In this work, to theoretically analyze the nonlinear charging characteristic, a Radial Basis Function Neural Network (RBFNN) is adopted. Based on the RBFNN, an charging characteristic tendency of a Linear Type Magnetic Flux Pump (LTMFP) is analyzed. In the paper, we developed the LTMFP that generates stable and controllable charging current and also experimentally investigated its charging characteristic in the cryogenic system. From these experimental results, the charging current of the LTMFP was also found to be frequency dependent with nonlinear quality due to the nonlinear magnetic behaviour of superconducting Nb foil. On the whole, in the case of essentially cryogenic experiment, since cooling costs loomed large in the cryogenic environment, it is difficult to carry out various experiments. Consequentially, in this paper, we estimated the nonlinear characteristic of charging current as well as realized the intelligent model via the design of RBFNN based on the experimental data. In this paper, we view RBF neural networks as predominantly data driven constructs whose processing is based upon an effective usage of experimental data through a prudent process of Fuzzy C-Means clustering method. Also, the receptive fields of the proposed RBF neural network are formed by the FCM clustering.

• The Journal of the Petrological Society of Korea
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• v.21 no.1
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• pp.1-12
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• 2012

The Baegdusan volcano is one of the most active volcanoes in northeastern Asia, and the 10th century eruption was the most voluminous eruption in the world in recent 2,000 years. During the period from 2002 to 2005, volcanic earthquakes and abnormal surface distortions by suspected subsurface magma intrusion beneath the volcano were observed in the Baegdusan area. Seismic activity has gradually increased with earthquake swarms during 2002-2003 and hundreds of seismic event in a day, especially annual peak of 2,100 in 2003. Then the number of seismic activity has declined since 2006 to the background level in 1999-2001. According to the typical frequency of volcanic earthquakes in the Baegdusan volcano, the frequency distribution of typical volcanic earthquakes between 2002 and 2005 indicates that all the main frequency of the earthquakes basically falls down less than 5 Hz and 5-10 Hz. These events are all the VT-B and LP events caused by the shallow localized fracture and intrusion of magma. The horizontal displacement measurement by GPS during the period from 2000 to 2007 of the Baegdusan stratovolcano area indicates that an inflated process has been centered at the summit caldera since 2002. The displacement between 2002 and 2003 reached at a maximum value of 4 cm. After 2003, the deformation rate of the volcano continued to decrease with unusual variation during the period from 2006 to 2007. After 2003 the vertical displacement uplift rate falls down gradually but still keeps in an uplift trend in northern slope. It is generally believed that when $^3He/^4He(R)$ in a gas sample from a hot spring exceeds $^3He/^4He(R)$ in the atmosphere, it can be concluded that mantle-source. And temperatures of hot springs are rising steadily to $83^{\circ}C$. It is unrest signals at the Baegdusan, which is potentially active. The Baegdusan volcano is now in unrest status, there is eruption threat in the near future. Intensified monitoring and emergency response plan for volcanic risk mitigation are urgent for the volcano.