• Geophysics and Geophysical Exploration
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• v.13 no.3
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• pp.286-294
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• 2010

Korea Institute of Geoscience and Mineral Resources (KIGAM) has installed and operated seven seismoacoustic (infrasound) arrays as well as seismic stations in Korea. The seismo-acoustic array, which consists of co-located seismometers and micro-barometers, can observe both seismic and infrasonic signals from distant explosive phenomena. The infrasound is defined as low frequency (<20 Hz) acoustic waves in atmosphere. In particular, it can be detectable at long distance due to its low energy attenuation during propagation in atmosphere. KIGAM adopted the infrasound technology to discriminate surface explosions from earthquakes only because the surface explosion generally generates infrasound following seismic signal. In addition to surface explosions, these arrays have detected diverse geophysically natural and artificial phenomena, such as infrasound signal from the North Korean nuclear test. This review introduced the state-of-the-art studies and examples of infrasonic signals in and around the Korean Peninsula. In conclusion, infrasound technology would be clearly accepted itself as a new Earth monitoring technology by expanding its detectable regime to lithosphere-Earth surface-atmosphere. In future, an advanced technology, which allows to analyze seismic and infrasonic wave fields together, will enlarge the understanding of geophysical phenomena and be used as a robust analysis method for remote explosive phenomena in the broad infrasound regime.

• Geophysics and Geophysical Exploration
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• v.10 no.1
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• pp.29-36
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• 2007

Recent seismic surveys have shown that the presence of methane hydrate (MH) in sediments has significant influence on seismic attenuation. I have used vertical seismic profile (VSP) data from a Nankai Trough exploratory well, offshore Tokai in central Japan, to estimate compressional attenuation in MH-bearing sediments at seismic frequencies of 30-110 Hz. The use of two different measurement methods (spectral ratio and centroid frequency shift methods) provides an opportunity to validate the attenuation measurements. The sensitivity of attenuation analyses to different depth intervals, borehole irregularities, and different frequency ranges was also examined to validate the stability of attenuation estimation. I found no significant compressional attenuation in MH-bearing sediments at seismic frequencies. Macroscopically, the peaks of highest attenuation in the seismic frequency range correspond to low-saturation gas zones. In contrast, high compressional attenuation zones in the sonic frequency range (10-20 kHz) are associated with the presence of methane hydrates at the same well locations. Thus, this study demonstrated the frequency-dependence of attenuation in MH-bearing sediments; MH-bearing sediments cause attenuation in the sonic frequency range rather than the seismic frequency range As a possible reason why seismic frequencies in the 30-110 Hz range were not affected in MH-bearing sediments, I point out the effect of thin layering of MH-bearing zones.

• 한국지구물리탐사학회:학술대회논문집
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• 2007.06a
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• pp.323-328
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• 2007

We derived the dispersion curves and spectral density for several models constructed from physical property data of representative geology in Korea. Comparison between monopole and dipole sources, between physical properties, and between dispersion curves and spectral density maps have been made. The result was very helpful in sonic data acquisition as well as velocity determination.

• 한국해양학회지
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• v.28 no.4
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• pp.313-322
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• 1993

The Western south Sea of Korea can be divided into 4 acoustic facies (AF I-AF IV) according to the variations of acoustic characters. Typical acoustic characters revealed in high-resolution seismic profiles (3.5kHz) are prolonged, internal reflected, non-penetrated, and transparent types. These acoustic types probably controlled by bottom condition and sediment properties such as composition and compaction of sediments. Acoustic facies I is characterized by prolonged type which is produced by absorbing of acoustic signals on the coarse sediments including gravels and shell fragments and irregular bedforms. Acoustic facies II is characterized by internal reflected type which is probably produced by differential sediments compaction. Acoustic facies III is characterized by non-penetrated type caused by scattering of acoustic signals on the well sorted fine ad very fine sand sediments. Acoustic facies IV is characterized by transparent type with non-internal reflector in limited thickness. Acoustic types in high-resolution profiles provide important information not only about the stratigraphy of sub bottom but also abut the sedimentary processes in shallow sea.

• Proceedings of the Korean Society of Surveying, Geodesy, Photogrammetry, and Cartography Conference
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• 2006.11a
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• pp.33-45
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• 2006

The interest of sediment has been increased daily because most of domestic dam reservoir's operation time have been extended and wide basin area is the main characteristics for artificial reservoir which the speed of water flow in artificial reservoir is slower than that of natural reservoir. Therefore a lot of sediment has been significantly accumulated. In this study, the accurate topographic data were obtained using echo-sounding system. GPS survey, low-frequency sub-bottom profiler, and high-frequency echo-sounding system were used to compute the exact amount of sediment. Based on the results, DEM(Digital Elevation Model) and DSM(Digital Surface Model) were generated. The GIS system for the management of sediment was created based on topographic data on the riverbed and this system can be efficiently used for the management of sediment which caused the problems of reservoir capacity and water quality.

• Journal of the Korean Society of Surveying, Geodesy, Photogrammetry and Cartography
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• v.26 no.3
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• pp.285-292
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• 2008

The establishment of the scheme which could be managed the national reservoir efficiently and consideration of a variable management scheme by the application of a new technique for it is very important for the improvement of national service quality and application of related techniques. In this study, sediment information which was accumulated in the reservoir for a long time and occasioned the decrease of pondage and increase of water pollution was constructed rapidly and accurately using GPS, sub bottom profiler and echo sounder. And the calculation system of sediment was efficiently designed through the applications of 2D and 3D spatial information processing techniques. As a results, the calculation system of sediment which was accomplished a various functions was developed and a basic information of management for the reservoir which was constructed quite a while ago was presented.

• Geophysics and Geophysical Exploration
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• v.1 no.3
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• pp.183-187
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• 1998

Seismic waveform estimation is based on the assumption that the seismic trace tying a well is one dimensional convolution of the propagating seismic waveform and the reflectivity series derived from well logs (sonic and density). With this assumption, the waveform embedded in a seismic trace can be estimated using a Wiener match filter. In this paper, I experimented a preprocessing procedure that applies both on the seismic trace and on the reflectivity series. The procedure is based on the assumption that the travel time can be estimated better from the seismic trace and that the instantaneous reflectivity values can be measured better on the well log. Thus the procedure is, 1) start-time adjustment and dynamic differential stretches are applied on the sonic log, and 2) seismic amplitudes are balanced such that the low frequency part of the seismic are matched to that of the reflectivities derived from well logs.

• 한국지구물리탐사학회:학술대회논문집
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• 1999.08a
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• pp.202-216
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• 1999

The deep foundation is frequently used for the infrastructures. Since the quality control of the cast-in-place concrete foundations such as CIP piles and slurry walls is not so easy as that of the ready made PC(prestressed concrete) piles, it is necessary to get the information on the integrity of the concrete of the foundation. The depth estimation of foundations whose depths are unknown is also very important in repair and reinforcement works or in safety inspection and assessment to the big structures. The cross-hole sonic logging(CSL) system and the single channel reflection seismic measurement system were developed to test the integrity of pile. The former is well applied to CIP structures, while the later to all kinds of piles with less accurate result compared to that of CSL. To estimate the depth of the deep foundations, parallel seismics, borehole RADAR, and borehole magnetics can be used.

• Journal of the Korean Geophysical Society
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• v.4 no.2
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• pp.95-102
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• 2001

As a basic study of characterizing fracture condition in bore hole, conventional geophysical well log data consisting of sonic, gamma-gamma and resistivity logs were compared with geological core log and acousic televiewer log in limestone geology. Discontinuities shown on the acoustic televiewer log and the core log show fairly good correlation. The conventional Geophysical log is also shown to bi effective in locating discontinities in limestone geology; sonic log shows the best result and resistivity shows the worst. Particularly, the combination method of density and sonic logs is shown to be the most effective in delineating discontinuities.

• Proceedings of the Acoustical Society of Korea Conference
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• autumn
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• pp.197-200
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• 2001

입사된 음파에 대한 배진동 주파수의 발생과 입사 음파들간의 합$\cdot$차 주파수 발생은 물체내 결함이 존재할때에 나타나는 중요한 비선형 효과라는 것을 이용하여 단순화된 실험실 조건의 겹쳐진 두 장의 유리판에 적용하였다. 본 논문에서는 적층 접합 물체에 있어서의 비파괴 검사법을 위해 접합되지 않은 결함 부분은 두 장의 유리판 사이의 공기 층으로 단순화하였고, 접합되어진 부분은 물 층으로 단순화하여 실험을 진행하였다. 서로 다른 조건의 두 접합 부분으로부터 발생된 입사된 기본 주파수의 배진동 주파수 발생과 합 $\cdot$차 주파수 발생을 관측함으로써 구조물 내의 결함 유$\cdot$무를 판별하였다. 배진동 주파수의 발생과 합$\cdot$차 주파수의 발생은 결함이 존재할 경우에 두드러지게 나타났지만, 결함이 존재하지 않는 경우에서는 비선형 반응의 발생이 억제되었다. 이 결과로부터 비선형 현상의 발생은 이차원적인 적층 구조물에도 적용 가능하며, 비선형 비파괴 음향 탐사법에 의한 결함의 존재 유$\cdot$무 판별이 가능하였다.