• Geophysics and Geophysical Exploration
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• v.10 no.1
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• pp.29-36
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• 2007

Recent seismic surveys have shown that the presence of methane hydrate (MH) in sediments has significant influence on seismic attenuation. I have used vertical seismic profile (VSP) data from a Nankai Trough exploratory well, offshore Tokai in central Japan, to estimate compressional attenuation in MH-bearing sediments at seismic frequencies of 30-110 Hz. The use of two different measurement methods (spectral ratio and centroid frequency shift methods) provides an opportunity to validate the attenuation measurements. The sensitivity of attenuation analyses to different depth intervals, borehole irregularities, and different frequency ranges was also examined to validate the stability of attenuation estimation. I found no significant compressional attenuation in MH-bearing sediments at seismic frequencies. Macroscopically, the peaks of highest attenuation in the seismic frequency range correspond to low-saturation gas zones. In contrast, high compressional attenuation zones in the sonic frequency range (10-20 kHz) are associated with the presence of methane hydrates at the same well locations. Thus, this study demonstrated the frequency-dependence of attenuation in MH-bearing sediments; MH-bearing sediments cause attenuation in the sonic frequency range rather than the seismic frequency range As a possible reason why seismic frequencies in the 30-110 Hz range were not affected in MH-bearing sediments, I point out the effect of thin layering of MH-bearing zones.

• Journal of the Korean Geotechnical Society
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• v.31 no.5
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• pp.35-46
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• 2015

In this study, the behavior of self-supported earth retaining wall with stabilizing piles was investigated by using a numerical study and field tests in urban excavations. This earth retaining wall can provide stable support against lateral earth pressures through its use of stabilizing piles that provide passive resistance to lateral earth pressures arising due to ground excavations. Field tests at two sites were performed to verify the performance of instrumented retaining wall with stabilizing piles. Furthermore, detailed 3D numerical analyses were conducted to provide insight into the in situ wall behavior. The 3D numerical methodology in the present study represents the behavior of the self-supported earth retaining wall with stabilizing piles. A number of 3D numerical analyses were carried out on the self-supported earth retaining wall with stabilizing piles to assess the results stemming from wide variations of influencing parameters such as the soil condition, the pile spacing, the distance between the front pile and the rear pile, and the pile embedded depth. Based on the results of the parametric study, the maximum horizontal displacement and the maximum bending moment significantly decreased when the retaining wall with stabilizing piles is used. Moreover, the horizontal displacement reduction effect of influencing parameters such as the pile spacing and the distance between the front pile and the rear pile is more sensitive in sandy soil, with a higher friction angle compared to clayey soil. In engineering practice, reducing the pile spacing and increasing the distance between the front pile and the rear pile can effectively improve the stability of the self-supported earth retaining wall with stabilizing piles.

• Journal of the Korean Geotechnical Society
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• v.33 no.3
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• pp.49-62
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• 2017

In order to understand response of geo-structures to the freezing-thawing process in the ground, it is necessary to consider phase change of the pore water of the ground and also to understand soil interaction with structures. In this study, numerical analysis was carried out for freezing and thawing effect on the modular road system. Neumann's theoretical equation for freezing-thawing processes in porous media can be used to estimate frozen depth and heaving from basic soil properties and ground and surface temperature, but its application is limited to the case for the sediment with fully saturated condition and zero unfrozen water content. Numerical analysis of the modular road system was performed on various soil types and different ground water table as the varying freezing index. The amount of heaving in the silty soil was much larger than those in granite weathered soil or sandy soil, and lowering groundwater level reduced ground heaving induced by freezing. Numerical analysis for temperature history of the ground surface predicted residual heaving near the surface by the freeze-thaw process in silty soil. It ought to reduce stiffness and bearing capacity of the ground so that it will impair stability and serviceability of new road system. However, the amount of residual heaving was insignificant for the road system installed in weathered soil granite and sandy soil. Since modular road system is a pavement structure mounted on the supporting substructure unlike the prevalent road pavement system, strict criteria should be applied for uniform and differential settlement of the pavement system.

• Journal of Ecology and Environment
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• v.29 no.3
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• pp.287-293
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• 2006

We carried out pollen analysis and radiocarbon dating from sediments of Wangdeungjae moor (960 m a.s.l.) to clarify vegetation history of temperate broad-leaved deciduous forest zone of Mt. Jiri. Three local pollen zones and two sub-zones were recognized on the basis of variation in the palynofloral assemblage: I. Quercus zone (150-114 cm), II. Quercus-Pinus zone (114-43 cm): IIa. Quercus-Pinus subzone (114-83 cm), IIb. Quercus-Pinus-Herbs subzone (83-43 cm), III. Pinus-Quercus zone (43-27 cm). Radiocarbon date from sediment depth 111-116 cm was estimated $1,160{\pm}40yr$ B.P. From these results, overall vegetation around of Wangdeungjae moor and Mt. Jiri have changed from Q. mongolica dominant to steady decrease of Q. mongolica whereas steady increase of P. densiflora, finally abrupt increase of P. densiflora. 'Medieval Warm Period (MWP)' and 'Little Ice Age (LIA)' have not had an effect on change of vegetation around, at least, Mt. Jiri. But at maximum period of LIA, extension of grassland and steady increase of Pinus were recognized around the moor. After then, an abrupt increase of Pinus is supposed that vegetation change is reflected in human impact surging around foothills.

• Journal of Ecology and Environment
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• v.29 no.3
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• pp.213-217
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• 2006

The increase of human activities has been extensively changed the breeding habitats of birds directly or indirectly, where the reproductive success of the birds has been seriously reduced. Leisure activities on water, such as fishing, influenced the breeding biology of waterfowls. However, few researches have been done for this in Korea. We examined the effects of fishing activity on nest selection and density of four waterfowl species at Namyang Lake: Coot (Fulica atra), Moorhen (Gallinula chloropus), Little Grebe (Podiceps ruficollis), and Great Crested Grebe (Podiceps cristatus). We divided the Namyang Lake into three study areas. Except for the number of anglers, there were no significant differences among the three study areas in the kinds of avian predators, the water depth around study areas measured for possible territorial predatory mammals' passing over, and the values of Relative Importance Percentage of occupying plants. The results showed that waterfowls bred in Namyang Lake preferred the areas with few anglers for the nests. From this study, we recommend that leisure activities including fishing should be prohibited or restricted during the breeding periods of waterfowls for the stable breeding success and that further systematic researches and managements for the species are necessary.

• Journal of Environmental Science International
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• v.21 no.2
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• pp.181-188
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• 2012

Salicornia europaea (glasswort) is succulent, annual, halophytic plant mainly distributed throughout reclaimed land or salt marsh. It has strong tolerance to salt so that it plays the part of the pioneer species in the first succession. According to domestic and foreign studies, S. europaea contains plenty of minerals and antioxidant in the body. Since people take note of an availableness of this plant as health diet, the natural growth sites are threatened. In addition to development of salt marsh and sea shore, imprudent harvest has a bad effect to S. europaea population maintenance. To seek ways to preserve the population of this plant, we carried out the continuity of seed germination and restoration test. Seokmo Island, Daebu Island, Youngjong Island and Sudokwon landfill in Korea are selected sites for research. Result of germination continuity shows that most S. europaea seeds germinate on March but no more after July. However the germination was occurred after that time in the greenhouse. So we concluded that no germination after July is a matter of environmental condition not the number of remaining seeds. Also germination was seldom occurred in the spot where seeds production was not happened. In result of continuity test of seed germination by soil depth, germination was occurred vigorously only in top soil. From these results, we note that most S. europaea germinate in the early spring, and germination is finished by July. And this rapid germination speed makes the number of seeds in soil seed bank rare. If a large number of S. europaea in some area are harvested after July, the number of this species will dramatically decrease in that area the following year. In Seokmo Island, we carried out reintroduction experiment by sowing S. europaea seeds. On the first year, a small number of S. europaea settled and they produced seeds successfully. On April 2010, the second year, we observed many S. europaea in seed sowing sites. And we found out that plowing is more efficient than treatment sea water for settlement of S. europaea.

• Journal of Radiation Protection and Research
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• v.45 no.1
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• pp.45-52
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• 2020

Background: Recently, the International Commission on Radiological Protection (ICRP) lowered the dose limit for the eye lens from 150 mSv to 20 mSv, highlighting the importance of accurate lens dose estimation. The ICRP reference computational phantoms used for lens dose calculation are mostly based on the data of Caucasian population, and thus might be inappropriate for Korean population. Materials and Methods: In the present study, a detailed Korean eye model was constructed by determining nine ocular dimensions using the data of Korean subjects. The developed eye model was then incorporated into the adult male and female mesh-type reference Korean phantoms (MRKPs), which were then used to calculate lens doses for photons and electrons in idealized irradiation geometries. The calculated lens doses were finally compared with those calculated with the ICRP mesh-type reference computational phantoms (MRCPs) to observe the effect of ethnic difference on lens dose. Results and Discussion: The lens doses calculated with the MRKPs and the MRCPs were not much different for photons for the entire energy range considered in the present study. For electrons, the differences were generally small, but exceptionally large differences were found at a specific energy range (0.5-1 MeV), the maximum differences being about 10 times at 0.6 MeV in the anteroposterior geometry; the differences are mainly due to the difference in the depth of the lens between the MRCPs and the MRKPs. Conclusion: The MRCPs are generally considered acceptable for lens dose calculations for Korean population, except for the electrons at the energy range of 0.5-1 MeV for which it is suggested to use the MRKPs incorporating the Korean eye model developed in the present study.

• Journal of The Korean Association For Science Education
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• v.34 no.7
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• pp.635-645
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• 2014

The purpose of this study is to identify elementary school teachers' perceptions, practices, and background factors related to introducing students' everyday experience in science teaching process. The participants of this study were four elementary school teachers who have different features such as major, teaching period, gender, growth area, and age. The data was collected through semi-constructed and in-depth interviews. The results of the research are as follows: Teachers mostly used students' everyday experience during the introduction phase of science lessons for the purpose of motivation. They hold a positive view of using students' everyday experience during science lessons and thought that science teaching needs to actively use more of students' everyday experience, while in actual practice they disregarded or only passively introduced students' everyday experience. The various background factors found to affect teachers' practice are as follows: positive memory on their science class; educational experience of their own children; their own childhood environment; their learning style; their insufficient knowledge or enthusiasm; perceived educational value of everyday life in science education; teacher's duties; importance of students' achievement; difficulty in guiding experiment; reaction of students on introducing everyday experience; characteristics of science textbook and teacher's guidebook; lack of lesson time; realization of national common basic education; characteristics of their students; demands from parents or students; effect of introducing everyday experience. In addition, we found that the teachers behave not in accordance with what they thought due to external factors related to their profession and that, for a more active use of students' everyday experience in their teaching, teachers need support from textbooks and teachers' guidebooks.

• Korean Journal of Social Welfare
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• v.63 no.2
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• pp.155-177
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• 2011

This paper attempts to explore the universal foundation on which the discourses on the validity criteria in qualitative research should be based, and further discuss in depth the scope and direction of an adequate validity criterion in qualitative research by critical examination of previous discourses. To establish the foundation for qualitative research, first of all, the concept of quality and qualitative paradigm were examined in terms of ontology and epistemology, especially focused on the works carried out by Aristotle, Kenneth and Boyd. Secondly, the earlier studies on the validity of qualitative research were critically reviewed in terms of the theoretical findings on the concept of quality and qualitative paradigm. The major results include the following: First, the true content of quality is defined as the 'what-ness' or essence of things perceived by subjects, and qualitative paradigm as adequate framework for qualitative research is comprised of two intrinsic modes: one is the mode of awareness and the other is that of expression. Second, qualitative researchers are able to reach to the higher level of validity by considering not only the subjectivity and the originality but also the sociality in drawing a conclusion through qualitative perception. Third, the trend of previous validity discourses can be firmly summarized as the effort to overcome the effect of quantitative research and acquire the true identity for qualitative research, which is subjectivity, and as the quest to compromise the tension between subjectivity and sociality. Finally, based on the findings, several research topics are suggested. For example, the more attention should be paid to the area of the expression of qualitative awareness as a way of overcome the tension between subjectivity and publicness.

• Journal of the Korea Concrete Institute
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• v.18 no.5 s.95
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• pp.631-637
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• 2006

In this study, fundamental properties such as fresh and hardened performance of concrete mix(specification : 25-24-18) added fluorine-silicate hybrid based crack reducing agent(FS) were measured. Addition of FS ranged from 0.5% to 2.0% at intervals 0.5% based on cement weight. Adequate dosage(0.5%) of FS derived from basic properties measurements applied and compared resistance for shrinkage crack. The permeability of concrete in the absence(24-S-0.0) and presence(24-S-0.5) of evaluated at a mock-up sized concrete. Concrete added FS improved resistance for shrinkage crack and consequently crack number, length and area decreased to $50{\sim}74.4%$ compared non-added. As well, by the addition of FS, the resistance for permeability and penetration depth to concrete surface region increased 67% and 40%, respectively. Therefore it was confirmed that shrinkage crack resistance and permeability of concrete could be improved by the addition of FS.