• Economic and Environmental Geology
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• v.56 no.4
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• pp.365-384
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• 2023

We applied the partitioned waveform inversion to 2,026 event data recorded at 173 seismic stations from the Incorporated Research Institutions for Seismology Data Managing Center and the Ocean Hemisphere network Project to estimate S-wave velocity and radial anisotropy models beneath the Western Pacific. In the Philippine Sea plate, high-Vs anomalies reach deeper in the West Philippine basin than in the Parece-Vela basin. Low-Vs anomalies found at 80 km below the Parece-Vela basin extend deeper into the West Philippine Basin. This velocity contrast between the basins may be caused by differences in lithospheric age. Low-Vs anomalies are observed beneath the Caroline seamount chain and the Caroline plate. Overall positive radial anisotropy anomalies are observed in the Western Pacific, but negative radial anisotropy is found at > 220 km depth on the subducting plate along the Mariana trench and at ~50 km in the Parece-Vela basin. Positive radial anisotropy is found at > 200 km depth beneath the Caroline seamount chain, which may indicate the 'drag' between the plume and the moving Pacific plate. High-Vs anomalies are found at 40 ~ 180 km depth beneath the Ontong-Java plateau, which may indicate the presence of unusually thick lithosphere due to underplating of dehydrated plume material.

• Journal of the Korean earth science society
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• v.44 no.2
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• pp.119-134
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• 2023

The synoptic, thermodynamic, and dynamic characteristics of a snowfall event that occurred in the Yeongdong region on March 1-2, 2021, were investigated. Surface weather charts, ERA5 reanalysis data, rawinsonde data, GK-2A satellite data, and WISSDOM data were used for analysis. The snow depth, exceeding 10 cm, was observed at four weather stations during the analysis period. The maximum snow depth (37.4 cm) occurred at Bukgangneung. According to the analysis of the weather charts, old and dry air was trapped within relatively warm, humid air in the upper atmosphere over the East Sea and adjacent Yeongdong region. This caused unstable atmospheric conditions that led to developing convective clouds and snowfall over Bukgangneung. In particular, based on the thermodynamic and kinematic vertical analysis, we suggest that strong winds attributable to the vertical gradient of potential temperature in the low layer and the development of convective instability due to cold advection played a significant role in the occurrence of snowfall in the Yeongdong region. These results were confirmed from the vertical analysis of the rawinsonde data.

• Journal of Sensor Science and Technology
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• v.22 no.1
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• pp.28-37
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• 2013

This paper presents a fault-tolerant control technique for wind turbine systems with sensor and actuator faults. The control objective is to maximize power production and minimize turbine loads by calculating a desired pitch angle within their limits. Any fault with a sensor and actuator can cause significant error in the pitch position of the corresponding blade. This problem may result in insufficient torque such that the power reference cannot be achieved. In this paper, a fault-tolerant control technique using a robust dynamic inversion observer and control allocation is employed to achieve successful pitch control despite these faults in the sensor and actuator. The observer based detection method is used to detect and isolate sensor faults by checking whether errors are larger than threshold values. In addition, the control allocation technique is adopted to tolerate actuator fault. Control allocation is one of the most commonly used fault-tolerant control techniques, especially for over-actuated systems. Further, the control allocation method can be used to achieve the power reference even in the event of blade actuator fault by redistributing the lost torque due to erroneous pitch position into non-faulty blade actuators. The effectiveness of the proposed method is demonstrated through simulations with a benchmark model of the wind turbine.

• Proceedings of the IEEK Conference
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• 2001.06d
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• pp.151-154
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• 2001

In this Paper, a new harmonic imaging technique is proposed and evaluated experimentally. In the proposed method, a weighted chin signal with a hanning window is transmitted. The RF samples obtained on each array element are individually compressed by correlating with the reference signal defined as the 2nd harmonic (2f0) component of a transmitted chirp signal generated in a square-law system. The proposed method uses the compressed 2f0 component to form an image, for which the crosscorrelation term with f0 component should be suppressed below at least -60dB. After experiment, the 6dB pulse width and peak sidelobe level of the compressed 2f0 component were 0.7us and -60dB, respectively. This result shows that the proposed method can successfully eliminate the f0 component with a single transmit-receive event and therefore is more efficient than the conventional pulse inversion (PI) method in terms of frame rate. We also observed that the 2nd harmonic compont starts to decrease for source pressure higher than 210kPa in water, which implies that SNR of the 2nd harmonic imaging using short pulses cnanot be incresed beyond a certain limit.

• Geophysics and Geophysical Exploration
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• v.18 no.2
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• pp.64-73
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• 2015

Recently, hydraulic fracturing is used in various fields and microseismic monitoring is one of the best methods for judging where hydraulic fractures exist and how they are developing. When locating microseismic events using single vertical well data, distances from the vertical array and depths from the surface are generally decided using time differences between compressional (P) wave and shear (S) wave arrivals and azimuths are calculated using P wave hodogram analysis. However, in field data, it is sometimes hard to acquire P wave data which has smaller amplitude than S wave because microseismic data often have very low signal to noise (S/N) ratio. To overcome this problem, in this study, we developed a grid search algorithm which can find event location using all combinations of arrival times recorded at receivers. In addition, we introduced and analyzed the method which calculates azimuths using S wave. The tests of synthetic data show the inversion method using all combinations of arrival times and receivers can locate events without considering the origin time even using only single phase. In addition, the method can locate events with higher accuracy and has lower sensitivity on first arrival picking errors than conventional method. The method which calculates azimuths using S wave can provide reliable results when the dip between event and receiver is relatively small. However, this method shows the limitation when dip is greater than about $20^{\circ}$ in our model test.

• Economic and Environmental Geology
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• v.56 no.3
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• pp.277-291
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• 2023

To analyze the crustal velocity structures beneath 21 broadband seismic stations in Gangwon Province, South Korea, we first applied the H-κ stacking method to 139 teleseismic event data (Mw ≥ 5.8 and the epicentral distance of 30° - 90°) occurring between March 18, 2019 and December 31, 2022 to estimate the Moho depths and V_p/V_s ratios beneath each station. The Moho depths and V_p/V_s ratios from the H-κ stacking method range from 24.9 to 33.2 km depth and 1.695 - 1.760, respectively, and the estimated V_p/V_s ratios were applied to the joint inversion of receiver functions and surface wave dispersion to obtain 1-D crustal velocity models beneath each station. The resulting Moho depths range from 25.9 to 33.7 km depth, similar to the results from the H-κ stacking method. Moho depth results from the both methods are generally consistent with Airy's isostasy. The 1-D crustal velocity models confirm that the existence of 2 km thick low-velocity layers with P-wave velocities of 5 km/s or less at some stations in the Taebaeksan basin, and at the stations CHNB and GAPB in northern Gangwon Province, which are located above the Cenozoic sedimentary layer. The station SH2B, although not overlying a sedimentary layer, has a low P-wave velocity near the surface, which is probably due to various factors such as weathering of the bedrock. We also observe a velocity inversion with decreasing velocity with depth at all stations within 4 - 12 km depths, and mid-crustal discontinuities possibly due to density differences in the rocks at around 10 km depth below some stations.

• Economic and Environmental Geology
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• v.40 no.4
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• pp.473-490
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• 2007

Strata of the Kachi-1 well, Kunsan Basin, offshore western Korea, were analyzed by using integrated stratigraphy approach. As a result, five distinct unconformity-bounded units are recognized in the well: Triassic, Late Jurassic-Early Cretaceous, Early Cretaceous, Late Cretaceous, and Middle Miocene units. Each unit represents a tectono-stratigraphic unit that provides time-sliced information on basin-forming tectonics, sedimentation, and basin-modifying tectonics of the Kunsan Basin. In the late Late Jurassic, development of second- or third-order wrench faults along the Tan-Lu fault system probably initiated a series of small-scale strike-slip extensional basins. Continued sinistral movement of these wrench faults until the Late Cretaceous caused a mega-shear in the basin, forming a large-scale pull-apart basin. However, in the Early Tertiary, the Indian Plate began to collide with the Eurasian Plate, forming a mega-suture zone. This orogenic event, namely the Himalayan Orogeny, continued by late Eocene and was probably responsible for initiation of right-lateral motion of the Tan-Lu fault system. The right-lateral strike-slip movement of the Tan-Lu fault caused the tectonic inversion of the Kunsan Basin. Thus, the late Eocene to Oligocene was the main period of severe tectonic modification of the basin. After the Oligocene, the Kunsan Basin has maintained thermal subsidence up to the present with short periods of marine transgressions extending into the land part of the present basin.

• Geophysics and Geophysical Exploration
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• v.22 no.1
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• pp.21-28
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• 2019

We perform a 3D tomographic inversion using surface wave dispersion curves to obtain S-velocity model and radial anisotropy beneath Saudi Arabia. The Arabian Peninsula is geologically and topographically divided into a shield and a platform. We used event data with magnitudes larger than 5.5 and epicentral distances shorter than $40^{\circ}$ during 2008 ~ 2014 from the Saudi Geological Survey. We obtained dispersion curves by using the multiple filtering technique after preprocessing the event data. We constructed SH- and SV-velocity models and consequently radial anisotropy model at 10 ~ 60 km depths by inverting Love and Rayleigh group velocity dispersion curves with period ranges of 5 ~ 140 s, respectively. We observe high-velocity anomalies beneath the Arabian shield at 10 ~ 30 km depths and low-velocity anomalies beneath the Arabian platform at 10 km depth in the SV-velocity model. This discrepancy may be caused by the difference between the Arabian shield and the Arabian platform, that is, the Arabian shield was formed in Proterozoic thereby old and cold, while the Arabian platform is covered by predominant Paleozoic, Mesozoic, and Cenozoic sedimentary layers. Also we obtained radial anisotropy by estimating the differences between SH- and SV-velocity models. Positive anisotropy is observed, which may be generated by lateral tension due to the slab pull of subducting slabs along the Zagros belt.

• Atmosphere
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• v.24 no.1
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• pp.1-15
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• 2014

This study compared ensemble mean and probability forecasts of snow depth amount associated with winter storm over South Korea on 28 December 2012 at five operational forecast centers (CMA, ECMWF, NCEP, KMA, and UMKO). And cause of difference in predicted snow depth at each Ensemble Prediction System (EPS) was investigated by using THe Observing system Research and Predictability EXperiment (THORPEX) Interactive Grand Global Ensemble (TIGGE) data. This snowfall event occurred due to low pressure passing through South Sea of Korea. Amount of 6 hr accumulated snow depth was more than 10 cm over southern region of South Korea In this case study, ECMWF showed best prediction skill for the spatio-temporal distribution of snow depth. At first, ECMWF EPS has been consistently enhancing the indications present in ensemble mean snow depth forecasts from 7-day lead time. Secondly, its ensemble probabilities in excess of 2~5 cm/6 hour have been coincided with observation frequencies. And this snowfall case could be predicted from 5-day lead time by using 10-day lag ensemble mean 6 hr accumulated snow depth distribution. In addition, the cause of good performances at ECMWF EPS in predicted snow depth amounts was due to outstanding prediction ability of forming inversion layer with below $0^{\circ}C$ temperature in low level (below 850 hPa) according to $35^{\circ}N$ at 1-day lead time.

• Atmosphere
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• v.16 no.2
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• pp.67-83
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• 2006

The statistical analysis for the springtime windstorm in Korea shows that Yeongdong region has the highest occurrence frequency during recent 10 years. The objective of this study is to find possible mechanisms for the downslope windstorm formation in the Yeongdong region by using a mesoscale numerical model, WRF. Dynamical process, wave breaking (hereafter WB), is qualitatively investigated as the candidate mechanism for a windstorm event occurred in 5 April, 2005. WB is developed in upper troposphere downstream, since stable air is lifted by the Taebaek mountain. This process can cause and maintain the severe downslope windstorm by drawing the upper flow down to the surface. And the intensified downslope wind leads the hydraulic jump (hereafter HJ) in downstream region. Froude numbers at Chuncheon (upslope side), Seorak Mountain (crest), Yangyang (lee side), and the East Sea (distant downstream position) are estimated by about 0.4, 1.0, 1.6, and 0.6, respectively. This result implies that the accelerated and supercritical (Fr>1) flow adjusts to the ambient subcritical (Fr<1) conditions in the turbulent HJ. In addition, we find the formation of upstream inversion near top level of the mountain cause the intensification of HJ. Experiments to examine the orographic effect on the mechanisms suggest that the magnitudes of WB and HJ are larger in the experiment of higher topography, but there is no significant difference of windstorm magnitude among the experiments. Another important result from these sensitivity experiments is that the intensity of downslope windstorm strongly depends on the magnitude of upper (2~4 km) wind in upstream side.