• Transactions of the Korean Society for Noise and Vibration Engineering
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• v.25 no.2
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• pp.124-132
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• 2015

The paper deals with the energy transmission of the elastic waves obliquely transmitting and refracting through a plate and a pipe. By calculating the transmitting ratio depending on the incidence angle, the angle corresponding to the maximum transmission was determined theoretically and considered in the design of ultrasonic transducers for flow velocimetry. Experiments were carried out by using prototypes of the transducers. Experimentally-obtained transmission ratios were compared with and confirmed the theoretically-calculated results.

• Transactions of the Korean Society for Noise and Vibration Engineering
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• v.20 no.3
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• pp.296-307
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• 2010

This paper deals with the energy transmission ratio of the elastic waves transmitting through a solid wall. Based on the displacement of the reflected and transmitted waves relative to the incident waves, the energy transmission ratio of the wave was obtained by multiplying the vibration velocity and stresses. Numerical calculation provided with the transmission ratio and refraction angle corresponding to the incidence angle, and it showed the mode conversion from the incident longitudinal wave to the transmitted transverse wave in particular incidence angle range. The paper established a procedure to find the incidence angle of the maximum energy transmission ratio and confirmed it by experiment.

• Proceedings of the Acoustical Society of Korea Conference
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• 1998.06e
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• pp.159-162
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• 1998

불발 음원에 따른 음원 배열의 빔 패턴 변화와 남극탐사자료에서 원거리장 파형 변화를 분석하였다. 원거리장 파형의 주신호 진폭은 전체 건 부피의 약 40%에 해당하는 음원들이 불발을 일으킬 경우, 탐사성능이 33% 정도 떨어지는 것으로 확인되었다. 같은 간격의 음원들이 불발 시에는 배열형태와 관계없이 길이 및 폭배열의 빔 폭이 동일하게 나타난다. 불발 음원의 부피가 증가됨에 따라 상대적으로 음파 에너지가 작아 지지만 빔 폭이 40$^{\circ}$ - 34$^{\circ}$로 좁아지는 경향을 나타낸다. 따라서 탄성파 탐사에서 필요로 하는 좁은 파형이 생성됨으로써 천부 지층탐사에 적합한 것으로 확인되었다.

• Journal of the Korean Geophysical Society
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• v.2 no.2
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• pp.100-110
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• 1999

In order to find a good seismic source for high-resolution reflection surveys on a tidal flat, characteristic features of several sources were examined through test recording by the walkaway configuration. The sources comprise portable vibrator, 1.2 kg monkey spanner, 4.7 kg sledge hammer, and weight drops with a 30 kg and a 100 kg iron ball, respectively. We analyzed raw and filtered seismic data for the resolution of individual events, then compared various seismic sources to define suitable one for the high-resolution survey in terms of energy level and frequency contents. The study reveals that the portable vibrator is the most suitable source for the detection of and imaging geologic structures less than 20-30 m deep in a tidal flat. We ascribe this to the wide frequency band and high-frequency contents of the portable vibrator. In contrast, the hammer may be an alternative where the target depth increases up to 100 m.

• Geophysics and Geophysical Exploration
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• v.25 no.4
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• pp.214-216
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• 2022

Distributed acoustic sensing (DAS), an increasingly growing acquisition technique in the oil and gas exploration and seismology fields, has been used to record seismic signals using optical cables as receivers. With the development of imaging methods for DAS data, full waveform inversion (FWI) is been applied to DAS data to obtain high-resolution property models such as P- and S-velocity. However, because the DAS systems measure strain from the phase distortion between two points along optical cables, DAS data must be transformed from strain to particle velocity for FWI algorithms. In this study, a plane-wave FWI algorithm based on the relationship between strain and horizontal particle velocity in the plane-wave assumption is proposed to apply FWI to DAS data. Under the plane-wave assumption, strain equals the horizontal particle velocity, which is scaled by the velocity at the receiver position. This relationship was confirmed using a numerical experiment. Furthermore, 4-layer and modified Marmousi-2 velocity models were used to verify the applicability of the proposed FWI algorithm in various survey environments. The proposed FWI was implemented in land and marine survey environments and provided high-resolution P- and S-velocity models.

• Geophysics and Geophysical Exploration
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• v.24 no.3
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• pp.78-88
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• 2021

Owing to the abnormal weather conditions due to global warming, carbon capture and storage (CCS) technology has attracted global attention as a countermeasure to reduce CO₂ emissions. In the Pohang CCS demonstration project in South Korea, 100 tons of CO₂ were successfully injected into the subsurface CO₂ storage in early 2017. However, after the 2017 Pohang earthquake, the Pohang CCS demonstration project was suspended due to an increase in social concerns about the safety of the CCS project. In this study, to reconfirm the structural suitability of the CO₂ storage site in the Pohang Basin, we employed seismic imaging based on reverse-time migration (RTM) to analyze small-scale ocean-bottom seismic data, which have not been utilized in previous studies. Compared with seismic images using marine streamer data, the continuity of subsurface layers in the RTM image using the ocean-bottom seismic data is improved. Based on the obtained subsurface image, we discuss the structural suitability of the Pohang CO₂ storage site.

• Journal of the Korean Institute of Telematics and Electronics
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• v.11 no.6
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• pp.1-15
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• 1974

In this paper, the SH mode of elastic surface waves which are used for delay lines of elastic surface wave is the cretically analysed. It is shown that the SH mode has very large electromechanical coupling factor and propagates on the surface with very small decaying coefficient into the medium. In the case of P2T-4, the depth cf piezoelectric medium that contains 80% of energy is 190 wavelengths. An elastic surface wave delay line is discussed from the view point of 2-port network. Center frequency is shifted by the ratio of transducer electrode width to gap between transducer ellcerodes when electromechanical coupling factor is large. Tempera _ore coefficients for bulk waves of LiNbO3 and LiTaO3 are also calculated and the minimum temperature coefficient value of delay time is 5.4X 10-6/$^{\circ}C$ ia the case of transverse wave propagating along Z axis on LiTaO3. Experimental data are in good agreement with theoretical values of the temperature coefficients of delay time for elastic surface waves propagating along X axis of 130$^{\circ}$ and 64$^{\circ}$ rotated Y cut planes of the LiNbO3.

• 한국신재생에너지학회:학술대회논문집
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• 2009.06a
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• pp.573-573
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• 2009

지열에너지는 지구가 생성될 당시부터 지구 내부에 존재하는 무한한 열에너지로 온실가스 배출이 적으며 태양광이나 풍력 등 다른 신재생 에너지와는 달리 일정한 에너지를 공급할 수 있는 항상성 에너지로 기저부하를 담당할 수 있다. 지열을 이용한 전력 생산은 1904년에 이탈리아 라데렐로에서 처음으로 시작되었으며, 현재까지 화산지대를 중심으로 활발히 이루어지고 있다. 2001년에서 2005년 사이에 전세계 지열발전용량은 약 13% 증가하였으며, 2005년을 기준으로 약 8,933MWe의 지열발전설비가 가동 중이다. 최근 들어 지하 심부까지 시추하여 지열저장소(geothermal reservoir)를 형성하고 이를 통해 지열에너지를 생산하는 새로운 시스템인 EGS(Enhanced Geothermal Systems)가 개발됨에 따라 비화산지대에서도 지열발전소를 건설하려는 움직임이 가속화되고 있다. EGS는 지하 심부의 불투수성 결정질 암반에 존재하는 지열에너지의 경제적인 생산뿐만 아니라 물을 주입하여 생산시키는 순환 방식을 이용하여 지열에너지 획득의 매개 역할을 하는 지열수의 고갈 문제를 해결하였다. 결정질 암반에서의 지열저장소의 형성은 암반 내에 분포하는 불연속면에서 주로 발생하며, 이를 위한 압력 조건은 현지 암반의 응력 분포 특성과 암반 및 불연속면의 물성에 좌우된다. 시추공을 통해 지하 심부의 암반에 수압이 가해지면 물의 주입으로 불연속면의 마찰력이 감소하며, 이로 인해 불연속면에 전단변형이 발생하게 된다. 전단변형은 불연속면을 열린 상태로 유지시켜 지열저장소를 형성하게 된다. 불연속면의 전단 변형시 발생하는 미소 탄성파는 시추공 주변에 설치한 모니터링 장비에서 측정되며, 모니터링 장비에 의해 측정된 미소 탄성파 발생 지점의 클러스터는 지열저장소의 공간적 분포 및 규모를 추정할 수 있는 자료가 된다. 현재 EGS를 이용한 지열발전 프로젝트는 프랑스 슐츠, 스위스 바젤, 호주 하바네로에서 대표적으로 진행 중이다. 슐츠는 현재 1.5MWe의 파일럿 플랜트를 가동 중이며, 하바네로는 파일럿 플랜트 건설 단계를 진행중이다. 스위스 바젤은 지열저장소를 형성시킬 목적으로 수행된 주입시험에서 발생된 문제에 대한 기술의 신뢰성을 확보할 목적으로 잠시 중단된 상태다. 제주도는 신생대에 분출하여 형성된 대표적인 한국의 화산지형으로 지열부존 가능성이 높을 것으로 예상되는 지역이다. 따라서 폐사는 지열에너지 부존 특성을 파악하기 위한 심부 물리 탐사 및 탐사정 시추가 실시될 예정이며 궁극적으로 국내 최초의 상용화된 지열발전소 건설을 목표로 하고 있다.

• Geophysics and Geophysical Exploration
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• v.14 no.4
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• pp.282-288
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• 2011

One of the most challenging issues facing shallow seismic survey is how to generate large amplitude of high frequency signal with small seismic sources. We tested the performance of the most commonly used shallow seismic source, hammer, with four plates: PE, nylon, aluminum, and steel plates. We compared their signal powers in terms of impulsive forces, accelerations, and ground vibration velocities caused by hammer impacts. According to a previous work, hammer blowing to an aluminum plate would generate the largest amplitude among four combinations. However, it was found in this experimental research that aluminum plate delivers seismic wave energy to the ground less than that generated with steel or PE plate. Even though the amplitude is relatively small, plastic plates could provide seismic pulses of 180 ~ 200 Hz in the bandwidth, and it seems to be very hard to generate seismic energy over the frequency of 250 Hz.

• Geophysics and Geophysical Exploration
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• v.25 no.3
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• pp.109-119
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• 2022

The computational efficiency of reverse time migration (RTM) based on numerical modeling is not secured due to the high-frequency band of several hundred Hz or higher for data acquired through a three-dimensional (3D) ultra-high-resolution (UHR) seismic survey. Therefore, this study develops an RTM program to derive high-quality 3D geological structures using UHR seismic data. In the traditional 3D RTM program, an excitation amplitude technique that stores only the maximum amplitude of the source wavefield and a domain-limiting technique that minimizes the modeling area where the source and receivers are located were used to significantly reduce memory usage and calculation time. The program developed through this study successfully derived a 3D migration image with a horizontal grid size of 1 m for the 3D UHR seismic survey data obtained from the Korea Institute of Geoscience and Mineral Resources in 2019, and geological analysis was conducted.