• Journal of the Korean earth science society
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• v.28 no.3
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• pp.367-373
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• 2007

A geoacoustic modeling has been developed to predict sound transmission through the submarine layers of sediment and rock. It demands a geoacoustic model with the measured, extrapolated, and predicted values of geoacoustic parameters controlling acoustic propagation. In the coastal areas of Okgye and Bukpyeong, the East Sea, the marine succession consists of Quaternary/Tertiary deposits and acoustic basement. The basement of Okgye coastal area is indicative of siliciclastics of the Pyeongan Group in Paleozoic, and the average velocities of P-wave and S-wave are 4276 m/s and 2400 m/s, respectively. The basement of Bukpyeong coastal area is indicative of limestone of the Joseon Supergroup in early Paleozoic, and the average velocities of P-wave and S-wave are 5542 m/s and 2742 m/s, respectively.

• Geophysics and Geophysical Exploration
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• v.15 no.4
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• pp.155-162
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• 2012

The Thomsen-Haskell method was used to determine sensitivities of the Rayleigh-wave phase velocities and spectral amplitude of vertical ground motion to insertion of a single velocity-anomaly layer into overburden underlain by a basement. The reference model comprised a 9-m thick overburden with shear-wave velocity (${\nu}_s$ of 300 m/s above a half-space with ${\nu}_s$ = 1000 m/s. The inserted layer, with a velocity of 150, 225, 375, or 450 m/s and a thickness of 1, 2, or 3 m, was placed at depths increasing from the surface in increments of 1 m. Phase velocities were computed for frequencies of 4 to 30 Hz. For inserted layer models, we placed an anomalous layer with thickness of 1 ~ 3 m, shear-wave velocity of 150 ~ 450 m/s, and at depths of 0 ~ 8 m in the overburden. The frequency range of 8 ~ 20 Hz were the most sensitive to the difference of $C_R$ between the inserted and reference models (${\Delta}C_R$) for h = 1 m and the frequency range got wide as h increased. For all of the models, the spectral amplitudes of the fundamental mode exceeded those of the $1^{st}$-higher mode except at frequencies just above the low-frequency cutoff of the $1^{st}$-higher mode.

• Geophysics and Geophysical Exploration
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• v.12 no.2
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• pp.199-207
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• 2009

From 329 earthquake data in the Korean Peninsula, we separated the intrinsic and scattering quality factor ($Q_i^{-1}$ and $Q_s^{-1}$) using the Multiple Lapse Time Window Analysis (MLTW) method. For the homogeneous velocity structure, $Q_i^{-1}$ reduces the amplitudes of both direct and coda waves; $Q_s^{-1}$ diminishes the direct wave amplitude but enlarges the coda wave amplitude. Based on this phenomenon, MLTW method analytically derives theoretical curves and obtains $Q_i^{-1}$ and $Q_s^{-1}$ by least square fit with observation curves. This study is the first approach for the seismic stable region by MLTW method, and show that $Q_i^{-1}$ and $Q_s^{-1}$ in the Korean Peninsula are very low at lower than frequencies of 5 Hz. This low value seems to be related to the inactive tectonism of the Korean Peninsula.

• Journal of the Korean earth science society
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• v.30 no.4
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• pp.401-408
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• 2009

We carried out the Suspension PS log and gamma-gamma log in andesite and sedimentary rocks distributed near the area of Myodo-dong, Yeosu. The main purpose of this study was to find out the distribution of seismic velocities and densities and to analyze the correlation of the two rocks by comparing their geophysical well logs with rock cores. The distributions of P and S-wave velocities for two rocks represented in the study area showed a difference of about 2,000 m/sec and 1,500 m/sec with depth, respectively, while the distributions of densities had a discrepancy of about $0.35g/cm^3$ with depth. However, the results of geophysical well logs analysis were similar to the two rocks. Consequently, it is clear that these results are interpreted as a difference due to weathering rather than rock type. In particular, the RQD values showed a good correlation to geophysical well log data.

• 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.

• Journal of the Korean Geophysical Society
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• v.4 no.2
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• pp.113-120
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• 2001

The discrete wavelet transform(DWT) has potential as a tool for supplying discriminatory attributes with which to distinguish seismic events. The wavelet transform has the great advantage over the Fourier transform in being able to localize changes. In this study, a discrete wavelet transform is applied to seismic traces for identifying seismic events and picking of arrival times for first breaks and S-wave arrivals. The precise determination of arrival times can greatly improve the quality of a number of geophysical studies, such as velocity analysis, refraction seismic survey, seismic tomography, down-hole and cross-hole survey, and sonic logging, etc. provide precise determination of seismic velocities. Tests for picking of P- and S- wave arrival times with the wavelet transform method is conducted with synthetic seismic traces which have or do not have noises. The results show that this picking algorithm can be successfully applied to noisy traces. The first arrival can be precisely determined with the field data, too.

• KSCE Journal of Civil and Environmental Engineering Research
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• v.33 no.4
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• pp.1499-1508
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• 2013

Freeze-thaw tests were performed on gneiss samples collected from Terra Nova Bay, Antarctica in order to examine the engineering properties of rocks with slightly weathered (SW) and moderately weathered (MW). The tests were conducted under temperature ranging from $20{\pm}2^{\circ}C$ to $-20{\pm}2^{\circ}C$. A cycle of test consisted of 5 hours of freezing followed by another 5 hours of thawing under full saturation. In this paper, total 200 cycles of freeze-thaw test were performed with measurements of porosity, absorption, ultrasonic velocity, and shore hardness per each 20 cycle and that of uniaxial compressive strength (UCS) per each 50 cycle. The UCS of the SW rocks approximately decreased 0.07 MPa per a single cycle, while that of MW rocks decreased around 0.2 MPa per a single cycle. During the 200 cycles of SW rocks, the absorption increased from 0.23% to 0.39%, the P-wave velocity decreased from 4,054 m/s to 3,227 m/s and S-wave velocity decreased from 2,519 m/s to 2,079 m/s. Similarly, those of MW rocks changed from 0.65% to 1.6%, 3,207 m/s to 2,133 m/s and 2,028 m/s to 1,357 m/s. In conclusion, it was inferred that the properties of SW rocks experienced approximately 200-300 cycles of freeze-thaw process become close to those of MW rocks.

• Geophysics and Geophysical Exploration
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• v.13 no.1
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• pp.102-108
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• 2010

The internal structure of a river embankment must be delineated as part of investigations to evaluate its safety. Geophysical methods can be most effective means for that purpose, if they are used together with geotechnical methods such as the cone penetration test (CPT) and drilling. Since the dyke body and subsoil in general consist of material with a wide range of grain size, the properties and stratification of the soil must be accurately estimated to predict the mechanical stability and water infiltration in the river embankment. The strength and water content of the levee soil are also parameters required for such prediction. These parameters are usually estimated from CPT data, drilled core samples and laboratory tests. In this study we attempt to utilise geophysical data to estimate these parameters more effectively for very long river embankments. S-wave velocity and resistivity of the levee soils obtained with geophysical surveys are used to classify the soils. The classification is based on a physical soil model, called the unconsolidated sand model. Using this model, a soil profile along the river embankment is constructed from S-wave velocity and resistivity profiles. The soil profile thus obtained has been verified by geotechnical logs, which proves its usefulness for investigation of a river embankment.

• Geophysics and Geophysical Exploration
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• v.16 no.3
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• pp.109-118
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• 2013

On and near the 23-m high earthen Cheongcheon dam in Boryeong City, Korea, short seismic refraction and surface-wave profiles were conducted using a 5-kg sledgehammer. From vertical and horizontal components of the seismic waves, near-surface P-wave velocities (${\nu}_p$) and S-wave velocities (${\nu}_s$) were derived by inverting first-arrival refraction times and dispersion curves of Rayleigh waves. Average ${\nu}_p$ and ${\nu}_s$ for the Jurassic sedimentary basement were determined to be 1650 and 950 m/s at a depth of 30 m directly beneath the dam and 1650 m/s and 940 m/s at a depth of 10 m at the toe of the dam, respectively. The dynamic Poisson's ratio for these strata were therefore in the range of 0.24 to 0.25, which is consistent with ratios for consolidated sedimentary strata. Near a 45-m borehole 152 m downstream from the dam crest, an SH tomogram indicates a refraction boundary with an average ${\nu}_s$ of 870 m/s at depths of 10 ~ 12 m. At this site, the overburden comprises the upper layer with relatively constant ${\nu}_p$ and ${\nu}_s$ around 500 and 200 m/s, respectively, and the lower layer in which both ${\nu}_p$ and ${\nu}_s$ increase with depth almost linearly. The dynamic Poisson's ratios for the overburden were in the range of 0.30 to 0.43.

• 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.