• 한국지구물리탐사학회:학술대회논문집
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• 2008.10a
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• pp.119-124
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• 2008

Due to the recent development of instruments which enabled the measurements of subtle IP effect in the ground and analysis algorithms, complex resistivity (CR) method was expanding its application to various field. In this study, we applied the CR method to the test site where the ground reinforcement had been done by injecting the cement mortar for investigating the effect of ground reinforcement. For this site, resistivity monitoring and tomography survey was carried out while the ground reinforcement had been made by the grouting. From the result, the anomalous region that was shown on the result of resistivity 4D monitoring was coincident with those of phase section in the CR method, because the cement grouting material had the strong IP effects. It might be expected that the CR method should be very powerful surveying tool for the similar purpose.

• 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.

• Journal of the Korean Geotechnical Society
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• v.23 no.7
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• pp.5-16
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• 2007

The monitoring and investigation of underwater landslide help to understand its mechanism, increase the usefuless of design and construction and reduce the losses. This paper presents three high resolution geophysical techniques electrical resisitance, ultrasonic wave reflection imaging, and shear wave tomography conducted to determine the lab-scaled submerged landslide. Electrical resistance profiles of a soil mass obtained by an electrical resistance probe provide detailed information to assess the spatial distribution of the soil mass with milimetric resolution. An ultrasonic wave image obtained by recording the reflections from interfaces of different impedance materials permits detecting layers and landslide with submilimetric resolution. The pixel based image of immersed landslides is created by the inversion of the boundary information achieved from the traveling time of shear waves. The experimental results show that the ultrasonic wave imaging and the electrical resistance can provide complementary information; and their association with S-wave tomography image can produce a 3-D view of the underwater landslide. This study suggests that geophysical techniques may be effective tools for the detection of the underwater landslides and spatial distribution offshore.

• Geophysics and Geophysical Exploration
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• v.5 no.3
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• pp.178-184
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• 2002

For economy and convenience, seismic refraction survey is widely used in surveying for large civil engineering work. The purpose of this study is to obtain the numerical responses of various models using Kirchhoff migration, and to analyze its application to the real data processing. Synthetic traveltime curve was calculated by vidale's algorithm, and various models such as 2 or 3 layer model and irregular topography model are tested to simulate the response of real structure. In order to compare the effect of initial velocity model, true velocity models, inversion results by tomography, smooth velocity models are used as an initial guess. The responses of model data show that the algorithm of this study is more sensitive to initial velocity model than the reflection survey, so choosing a suitable initial velocity model will be the most important thing in real data processing.

• Economic and Environmental Geology
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• v.39 no.4 s.179
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• pp.507-524
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• 2006

3-D seismic tomographic inversion is applied to investigation on velocity structure in and near Korean Peninsula. Firstly, it is applied to the region in southeastern Korean Peninsula. According to the results low-velocity zone seems to be clearly appeared in the so called Gyeongsang sedimentary basin and high-velocity zone is shown at the section of 7.5 km depth it implies the inclusion of plutonic rocks at the sedimentary basin. At the depth about $20{\sim}30$ km existence of low-velocity zone seems to be related with the development of Yangsan fault system. Secondly it is applied to the region not only in Korean Peninsula but also East Sea using data from both Korean Peninsula and Japan Islands. Accorging to the results, subduction zone starting from eastern part of Japan seems to be extended to the region beneath the East Sea.

• Geophysics and Geophysical Exploration
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• v.21 no.1
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• pp.33-40
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• 2018

We investigate the crustal and uppermost mantle SV- and SH-wave velocity structure and radial anisotropy beneath East Asia including Korea, China and Japan. Rayleigh waves and Love waves were extracted from the seismic data recorded at broadband seismic stations in East Asia. Using the MFT (Multiple Filter Technique), we obtained group velocity dispersion curves of Rayleigh and Love waves with a period range of 3 to 200 s. We obtained 62466 Rayleigh-waves dispersion-curve measurements in vertical components and 54141 Love-waves dispersion-curve measurements in transverse components, respectively. The inverted models using these data sets provide SV- and SH-wave velocity structure of crust and uppermost mantle down to 100 km depth. In both cases of the S-wave velocity structures, strong high-velocity anomalies are observed down to 30 km depth beneath the East Sea, and deeper than 30 km depth, strong low-velocity anomalies are found beneath the Tibetan plateau. In the case of the SH-wave velocity structure, strong low-velocity anomalies are observed beneath the East Sea deeper than 30 km depth, leading to negative anisotropy. On the other hand, positive anisotropy is usually observed beneath the Tibetan plateau.

• Geophysics and Geophysical Exploration
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• v.11 no.1
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• pp.1-14
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• 2008

We present a method for interpreting seismic records with arrivals and waveforms having characteristics which could be generated by extremely inhomogeneous velocity structures, such as non-typical oceanic crust, decollement at subduction zones, and seamounts in oceanic regions, by comparing them with synthetic waveforms. Recent extensive refraction and wide-angle reflection surveys in oceanic regions have provided us with a huge number of high-resolution and high-quality seismic records containing characteristic arrivals and waveforms, besides first arrivals and major reflected phases such as PmP. Some characteristic waveforms, with significant later reflected phases or anomalous amplitude decay with offset distance, are difficult to interpret using only a conventional interpretation method such as the traveltime tomographic inversion method. We find the best process for investigating such characteristic phases is to use an interactive interpretation method to compare observed data with synthetic waveforms, and calculate raypaths and traveltimes. This approach enables us to construct a reasonable structural model that includes all of the major characteristics of the observed waveforms. We present results here with some actual observed examples that might be of great help in the interpretation of such problematic phases. Our approach to the analysis of waveform characteristics is endorsed as an innovative method for constructing high-resolution and high-quality crustal structure models, not only in oceanic regions, but also in the continental regions.