• Title/Summary/Keyword: Seismic velocity

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A proposal of seismic reference velocity ratio for the rock mass classification in tunnel area (터널구간 암반분류를 위한 탄성파 기준속도비의 제안)

  • Ko, Kwang-Beom;Ha, Hee-Sang;Lim, Hae-Ryong
    • 한국지구물리탐사학회:학술대회논문집
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    • 2005.09a
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    • pp.37-42
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    • 2005
  • Remote seismic tomography is regarded as one of the most valuable geophysical technique for the estimation of the rock mass classification in the tunnel area where hard data information such as drill logs are absent. But the results of rock mass classification based on the remote seismic tomography tend to be overestimated in practice. In this study, we propose the effective method to implement the seismic reference velocity ratio based on semblance for the improvement of rock mass classification. Also, to verify its feasibility, proposed technique was tested by using the real field data.

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Distinct Element Modelling of Stacked Stone Pagoda for Seismic Response Analysis (지진응답 해석을 위한 적층식 석탑의 개별요소 모델링)

  • Kim, Byeong Hwa;Lee, Do Hyung
    • Journal of the Earthquake Engineering Society of Korea
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    • v.22 no.6
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    • pp.345-352
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    • 2018
  • It is inevitable to use the distinct element method in the analysis of structural dynamics for stacked stone pagoda system. However, the experimental verification of analytical results produced by the discrete element method is not sufficient yet, and the theory of distinct element method is not universal in Korea. This study introduces how to model the stacked stone pagoda system using the distinct element method, and draws some considerations in the seismic analysis procedures. First, the rocking mode and sliding mode are locally mixed in the seismic responses. Second, the vertical stiffness and the horizontal stiffness on the friction surface have the greatest influence on the seismic behavior. Third, the complete seismic analysis of stacked stone pagoda system requires a set of the horizontal, vertical, and rotational velocity time histories of the ground. However, earthquake data monitored in Korea are limited to acceleration and velocity signals in some areas.

ARTICLES : MULTICHANNEL ANALYSIS OF SURFACE WAVES (MASW) - AN OVERVIEW

  • ChoonB.Park
    • Journal of the Korean Geophysical Society
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    • v.6 no.2
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    • pp.99-105
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    • 2003
  • Rayleigh waves which has more than 70% of the total seismic energy is the principal component of ground roll. Frequency component of a surface wave has a different propagation velocity, that is, phase velocity, which results in a different wavelength called dispersion. Rayleigh wave is one of the most common ways to use the dispersive properties of surface waves. MASW is a seismic method to evaluate shear-wave velocity information of the ground.

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P-wave Velocity Analysis Around the BSR Using Wide-angle Ocean-bottom Seismic Data (해저면 광각 탄성파 탐사자료를 이용한 BSR 부근의 P파 속도 분석)

  • Kim, Byoung-Yeop;Byun, Joong-Moo
    • Geophysics and Geophysical Exploration
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    • v.12 no.2
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    • pp.173-182
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    • 2009
  • In April 2008, KIGAM carried out an ocean-bottom seismometer (OBS) survey in the central Ulleung Basin where strong bottom simulating reflectors (BSRs) were revealed from previous surveys and some gas-hydrate samples were retrieved by direct sampling. The purpose of this survey is to estimate the velocity structure near the BSR in the gas hydrate prospect area using wide-angle seismic data recorded on the ocean-bottom seismometers. Along with the OBS survey, a 2-D seismic survey was performed whereby stratigraphic and preliminary velocity information was obtained. Two methods were applied to wide-angle data for estimating P wave velocity; one is velocity analysis in the $\tau$-p domain and the other is seismic traveltime inversion. A 1-D interval velocity profile was obtained by the first method, which was refined to layered velocity structure by the latter method. A layer stripping method was adopted for modeling and inversion. All velocity profiles at each OBS site clearly show velocity reversal at BSR depths due to the presence of gas hydrates. In addition, we could confirm high velocity in the column/chimney structure.

The effects of peak ground velocity of near-field ground motions on the seismic responses of base-isolated structures mounted on friction bearings

  • Tajammolian, H.;Khoshnoudian, F.;Talaei, S.;Loghman, V.
    • Earthquakes and Structures
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    • v.7 no.6
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    • pp.1259-1281
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    • 2014
  • This research has been conducted in order to investigate the effects of peak ground velocity (PGV) of near-field earthquakes on base-isolated structures mounted on Single Friction Pendulum (SFP), Double Concave Friction Pendulum (DCFP) and Triple Concave Friction Pendulum (TCFP) bearings. Seismic responses of base-isolated structures subjected to simplified near field pulses including the forward directivity and the fling step pulses are considered in this study. Behaviour of a two dimensional single story structure mounting on SFP, DCFP and TCFP isolators investigated employing a variety range of isolators and the velocity (PGV) of the forward directivity and the fling step pulses as the main variables of the near field earthquakes. The maximum isolator displacement and base shear are selected as main seismic responses. Peak seismic responses of different isolator types are compared to emphasize the efficiency of each one under near field earthquakes. It is demonstrated that rising the PGVs increases the isolator displacement and base shear of structure. The effects of the forward directivity are greater than the fling step pulses. Furthermore, TCFP isolator is more effective to control the near field effects than the other friction pendulum isolators are. This efficiency is more significant in pulses with longer period and greater PGVs.

Resolving a velocity inversion at the geotechnical scale using the microtremor (passive seismic) survey method

  • Roberts James C.;Asten Michael W.
    • Geophysics and Geophysical Exploration
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    • v.7 no.1
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    • pp.14-18
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    • 2004
  • High levels of ambient noise and safety factors often limit the use of 'active-source' seismic methods for geotechnical investigations in urban environments. As an alternative, shear-wave velocity-depth profiles can be obtained by treating the background microtremor wave field as a stochastic process, rather than adopting the traditional approach of calculating velocity based on ray path geometry from a known source. A recent field test in Melbourne demonstrates the ability of the microtremor method, using only Rayleigh waves, to resolve a velocity inversion resulting from the presence of a hard, 12 m thick basalt flow overlying 25 m of softer alluvial sediments and weathered mudstone. Normally the presence of the weaker underlying sediments would lead to an ambiguous or incorrect interpretation with conventional seismic refraction methods. However, this layer of sediments is resolved by the microtremor method, and its inclusion is required in one-dimensional layered-earth modelling in order to reproduce the Rayleigh-wave coherency spectra computed from observed seismic noise records. Nearby borehole data provided both a guide for interpretation and a confirmation of the usefulness of the passive Rayleigh-wave microtremor method. Sensitivity analyses of resolvable modelling parameters demonstrate that estimates of shear velocities and layer thicknesses are accurate to within approximately $10\%\;to\;20\%$ using the spatial autocorrelation (SPAC) technique. Improved accuracy can be obtained by constraining shear velocities and/or layer thicknesses using independent site knowledge. Although there exists potential for ambiguity due to velocity-thickness equivalence, the microtremor method has significant potential as a site investigation tool in situations where the use of traditional seismic methods is limited.

Monitoring and detecting $CO_2$ injected into water-saturated sandstone with joint seismic and resistivity measurements (탄성파 및 비저항 동시측정에 의한 수포화 암석시료에 주입된 $CO_2$ 모니터링 및 탐지)

  • Kim, Jong-Wook;Matsuoka, Toshifumi;Xue, Ziqiu
    • Geophysics and Geophysical Exploration
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    • v.14 no.1
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    • pp.58-68
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    • 2011
  • As part of basic studies of monitoring carbon dioxide ($CO_2$) storage using electrical and seismic surveys, laboratory experiments have been conducted to measure resistivity and P-wave velocity changes due to the injection of $CO_2$ into water-saturated sandstone. The rock sample used is a cylinder of Berea sandstone. $CO_2$ was injected under supercritical conditions (10 MPa, $40^{\circ}C$). The experimental results show that resistivity increases monotonously throughout the injection period, while P-wave velocity and amplitude decrease drastically due to the supercritical $CO_2$ injection. A reconstructed P-wave velocity tomogram clearly images $CO_2$ migration in the sandstone sample. Both resistivity and seismic velocity are useful for monitoring $CO_2$ behaviour. P-wave velocity, however, is less sensitive than resistivity when the $CO_2$ saturation is greater than ~20%. The result indicates that the saturation estimation from resistivity can effectively complement the difficulty of $CO_2$ saturation estimations from seismic velocity variations. By combining resistivity and seismic velocity we were able to estimate $CO_2$ saturation distribution and the injected $CO_2$ behaviour in our sample.

Utilization of Mean Shear Wave Velocity to a Depth Shallower than 30m for Efficient Seismic Site Classification in Korea (우리나라 지진공학적 지반 분류를 위한 30m 미만 심도 평균 전단파 속도의 활용)

  • Sun, Chang-Guk;Chung, Choong-Ki;Kim, Dong-Soo
    • Proceedings of the Korean Geotechical Society Conference
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    • 2006.03a
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    • pp.562-571
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    • 2006
  • Mean shear wave velocity of the upper 30m $(V_s30)$ used as the current site classification criterion for determining seismic design ground motions in Korea was established based on the typical depth of site investigations in western US, in which the depth to bedrock is much deeper than that in Korea. In this study, to establish appropriate site classification system for site conditions of Korea, site investigations including in-situ seismic tests to determine shear wave velocity $(V_s)$ were carried out at total 72 sites in Korean peninsula. The mean $V_s's$ to the depths of 5m, 10m, 15m, 20m and 25m together with the $V_s30$ at the testing sites were determined, and the correlation between the mean $V_s$ to a depth shallower than 30m and the $V_s30$ was drawn and suggested for the efficient seismic site classification in Korea. The proposed correlation could be utilized for the seismic design in case of the $V_s$ profiles shallower than 30 m in depth. The correlation in this study, nevertheless, requires further modification by means of the accumulation of various site data in Korea.

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Study of seismic wave propagation around tunnel (터널 주위의 탄성파 전파양상에 관한 연구)

  • Suh, Baek-Su;Oh, Seok-Hoon;Shon, Kwon-Ik;Lee, Sang-Chul
    • 한국지구물리탐사학회:학술대회논문집
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    • 2006.06a
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    • pp.291-296
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    • 2006
  • The aspect of wave propagation around cavity was investigated for the exact inversion of crosshole tomography data in order to understand the possibility of the existence of underground cavity. We found that the adequate frequency range for the tunnel investigation was about 2kHz to 5kHz, and the grid space was set up to 1/10 length of wavelength. The propagation of the seismic wave near the cavity may go through or detour the cavity according to the seismic velocity of inside of cavity. The detouring wave propagates with the seismic velocity of mother rock in spite of the velocity of inside of cavity. The smaller the velocity difference between the mother rock and cavity, the more frequent penetration of the seismic wave through the cavity was appeared.

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The Effect of the Shear Wave Velocity of a Seismic Control Point on Site Response Analysis (기반암 전단파속도의 부지응답특성 영향평가)

  • Lee, Jin-Sun
    • Journal of the Earthquake Engineering Society of Korea
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    • v.13 no.1
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    • pp.1-8
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    • 2009
  • In order to evaluate the effect of shear wave velocity of a seismic control point on site response analysis, one-dimensional equivalent linear site response analysis were performed on the model soil profile based on the results of a detailed site investigation of sedimentary layers at Incheon and Busan. The results of the analysis show that an increase of shear wave velocity on the seismic control point (base rock) results in an increase of acceleration in the soil layers. This was mainly due to an unclear definition of the seismic control point. For this reason, the Korean Seismic Design Standard requires a specific definition of the seismic control point, including spatial conditions and soil properties, similar to the MCE (Maximum Considered Earthquake) in FEMA 369.