• Journal of the Korean earth science society
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• v.28 no.7
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• pp.903-913
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• 2007

In this study, the researchers compared the S-wave velocity structures obtained by two kinds of dispersion curves: phase and group dispersions from a tidal flat located in the SW coast of the Korean peninsula. The ${\tau}-p$ stacking method was used for the phase velocity and two different methods (multiple filtering technique: MFT and continuous wavelet transform: CWT) for the phase velocity. It was difficult to separate higher modes from the fundamental mode phase velocities using the ${\tau}-p$ method, whereas the separation of different modes of group velocity were easily achieved by both MFT and CWT. Of the two methods, CWT was found to be more efficient than MFT. The spatial resolutions for the inversion results of the fundamental mode for both phase and group velocities were good for only a very shallow depth of ${\sim}1.5m$. On the other hand, the spatial resolutions were good up to ${\sim}4m$ when both the fundamental and the 1st higher mode poop velocities obtained by CWT were used for S-wave inversion. This implies that the 1st higher mode Rayleigh waves contain more information on the S-wave velocity in deeper subsurface. The researchers applied the CWT method to obtain the fundamental and the 1st higher mode poop velocities of the S-wave velocity structure of a tidal flat located in SW coast of the Korean peninsula. Thea the S-wave velocity structures were compared with the borehole description of the study area.

• Journal of the Korea Academia-Industrial cooperation Society
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• v.17 no.12
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• pp.17-25
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• 2016

Revetments help protect levee slopes from erosion. If the design of the revetment is not appropriate, the levee may collapse as a result of scouring due to the strong flow velocity and tractive force. Therefore, when designing a revetment, it is very important to calculate the representative velocity. However, the average velocity and depth calculated by 1-D varied flow analysis are generally applied to the design, which do not reflect the increase in velocity caused by the free and force vortex. Therefore, it is necessary to correct the representative velocity in order to ensure the stability of the revetment in a meandering channel. In this study, the applicability of the method of calculating the representative velocity considering the curve and scour was studied (by comparing it with) the average and maximum velocities determined by numerical simulation. The representative velocity corrected for the effect of the curve and scour and the maximum velocity calculated by the numerical simulation were found to match quite well. In addition, the riprap size of the gabion in the meandering and straight channels were compared by applying them to the conventional design formulas. In the future, it is necessary to perform additional numerical simulations for various rivers with different characteristics, in order to propose a method of designing a suitable revetment for Korean characteristics. At this time, the results of this study are expected to be able to be used as basic data.

• Geophysics and Geophysical Exploration
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• v.14 no.4
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• pp.305-315
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• 2011

A total of 864 measurements for P- and S- wave velocity of acrylic and stainless steel core samples have been performed with respect to their lengths and axial load applied. S-wave velocity measurement was much harder than P-wave velocity, so that it showed higher deviation in measured S-wave velocity with respect to repeated measurement, length of the cores, and the axial load applied. Velocity measurements for acrylic cores showed more stable and less than half of the variation between the measurements than the stainless steel cores. This seems to be come from better coupling between the transducers and acrylic cores than stainless cores, and from larger value of the first arrival time in a similar system noise environments. From the analysis of the 864 measurements, it is recommended that the length of the core be 60 ~ 90 mm, axial load between 20 kg (27.7 $N/cm^2$) and 30 kg (41.6 $N/cm^2$) for measurement of wave velocity of the acrylic and stainless steel cores. Especially for measuring S-wave velocity of stainless steel core, core length should be less than 50 mm, otherwise it will be affected by mode conversion or others. These results can be used in measurement and correction for system delay in wave velocity measurement for rock cores.

• Geophysics and Geophysical Exploration
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• v.14 no.4
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• pp.324-334
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• 2011

Seismic wave velocity change has been monitored due to the accumulation of micro-cracks by uniaxial loads on the rock samples from Seokmo Island with stepwise increase in 5 stages. After the load was applied up to 95% of UCS, P- and S-wave velocities varied in ranges of 0.9 ~ 18.3% and 2.8 ~ 14.8% of fresh rock sample velocities, respectively. Unlike seismic velocity of the dry rock samples that showed overall decreases after the loading, velocity changes of saturated rock samples were much more complicated. These seemed to be due to the mixture of two contradictory mechanisms; i.e. accumulation of micro-crack causes an increase in porosity and a decrease in wave velocity, while saturation causes an increase in wave velocity. Most of tested rocks showed a trend of velocity increase with low axial load and then velocity decrease at later stages. Starting stage of velocity decrease differs from samples to samples. After the failure of rock occurred, noticeable increases of porosity and decreases of wave velocity have been observed. It showed overall trend that the more the quartz contents and the lower the silicate, the higher the Young's modulus.

• 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.

• Journal of Korean Tunnelling and Underground Space Association
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• v.9 no.2
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• pp.121-131
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• 2007

Recently, the probability of rock joints being exposed to free faces is getting higher for the scale of rock mass structures gets larger. Also, the frequency of occurring dynamic events such as earthquakes and blasting has been increasing. Thus, the shear behavior of rock joints under different conditions needs to be investigated. In this study, a series of direct shear tests were carried out under various conditions to examine the velocity-dependent shear behavior of saw-cut rock joints. Two types of direct shear test were carried out. The first was to examine the velocity-dependent shear behavior of saw-cut rock joints at seven different shear velocities, each with three different normal stresses. The second was to examine the shear behavior of saw-cut rock joints when three different instantaneous shear velocities changed. As a result, the coefficient of friction was affected by normal stress. The breakpoint velocity, the point when the change of shear velocity starts to affect the frictional behavior, became lower as normal stress increased. Also, as the shear velocity became lower, the degree of stress-drop on stick-slip behavior became larger. As a result of examining the changes of friction coefficient, velocity weakening (decrease of friction coefficient) was observed. The decrement of friction coefficient due to the changes of shear velocity under slow shear velocity was larger than that under fast shear velocity.

• Journal of the Korean Society for Aeronautical & Space Sciences
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• v.47 no.2
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• pp.107-113
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• 2019

In this paper, a robust filter based wind velocity estimation algorithm without an air velocity sensor in an air vehicle is presented. The wind velocity is useful information for the air vehicle to perform precise guidance and control. In general, the wind velocity can be obtained by subtracting an air velocity which is obtained by an air velocity sensor such as a pitot-tube, and a ground velocity which is obtained by a navigation equipment. However, in order to simplify the configuration of the air vehicle, the wind estimation algorithm is necessary because the wind velocity can not be directly obtained if the air velocity measurement sensor is not used. At this time, the aerodynamic coefficient of the air vehicle changes due to the turbulence, which causes the uncertainty of the system model of the filter, and the wind estimation performance deteriorates. Therefore, in this study, we propose a wind estimation method using $H{\infty}$ filter to ensure robustness against aerodynamic coefficient uncertainty, and we confirmed through simulation that the proposed method improves the performance in the uncertainty of aerodynamic coefficient.

• Journal of Navigation and Port Research
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• v.43 no.3
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• pp.186-196
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• 2019

Berthing energy is majorly influenced by the berthing velocity. It is necessary to design an appropriate berthing velocity for each pier, since excessive berthing velocity can cause berthing accident causing damage to the ship and pier. In this study, as a statistical approach for berthing velocity, the probability distributions suitable for the berthing velocities were confirmed using the K-S test, the A-D test and the Q-Q plot. As a result, the frequency distribution of the berthing velocity was found to be suitable using the Weibull distribution as well as the lognormal distribution. Additionally, the predicted values obtained through estimation of the berthing velocity using the concept of probability of exceedance in this study is proposed as a reference of design berthing velocity. It can be observed that the design berthing velocity is set to be somewhat low so that it does not practically match with the reality. This study and its results can be expected to contribute to the development of a proper design velocity calculation method.

• Journal of Korean Society of Coastal and Ocean Engineers
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• v.31 no.1
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• pp.9-16
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• 2019

Flow velocity changes in the shroud system for tidal current power generation due to experimental flow velocities and blade geometry changes were analyzed by hydraulic experiment and numerical simulation. Through the hydraulic experiment, flow velocities at inlet of shroud system and RPM according to blade geometry were measured, and numerical simulation was used to analyze flow velocity changes in shroud. When the experimental flow velocity was increased by about 28% and the shape of the airfoil was applied, the measured flow velocity at the shroud inlet tended to increase by up to about 56%. On the other hand, when airfoil-shaped blades were installed, the flow velocity at the inlet tended to increase by up to 14% compared to conventional blades, and RPM was also the highest at the same conditions. The hydraulic experiment and numerical simulation results showed an error of about 13%, and the trends of the flow velocity changes in each result are similar. Numerical simulation of the flow velocity changes in the shroud showed that the flow velocity tended to increase 1.7 times at the front of the blade compared to the inlet. The results of the flow velocity change analysis in the shroud system obtained from this study will provide the basic data necessary for the development of efficient shroud system for tidal current power generation.

• Ecology and Resilient Infrastructure
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• v.8 no.1
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• pp.22-31
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• 2021

Surface image velocimetry (SIV) is a noncontact velocimetry technique based on images. Recently, studies have been conducted on surface velocity measurements using drones to measure a wide range of velocities and discharges. However, when measuring the surface velocity using a drone, reference points must be included in the image for image correction and the calculation of the ground sample distance, which limits the flight altitude and shooting area of the drone. A technique for calculating the surface velocity that does not require reference points must be developed to maximize spatial freedom, which is the advantage of velocity measurements using drone images. In this study, a technique for calculating the surface velocity that uses only the drone position and the specifications of the drone-mounted camera, without reference points, was developed. To verify the developed surface velocity calculation technique, surface velocities were calculated at the Andong River Experiment Center and then measured with a FlowTracker. The surface velocities measured by conventional SIV using reference points and those calculated by the developed SIV method without reference points were compared. The results confirmed an average difference of approximately 4.70% from the velocity obtained by the conventional SIV and approximately 4.60% from the velocity measured by FlowTracker. The proposed technique can accurately measure the surface velocity using a drone regardless of the flight altitude, shooting area, and analysis area.