• Proceedings of the Korean Geotechical Society Conference
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• 2001.03a
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• pp.167-174
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• 2001

Reflection method using ultrasonic source has been attempted to obtain the information about tunnel lining structures composed of lining, shotcrete, water barrier and voids at the back of lining. In this work, two different types of sources, i.e. single-pulse source and sweep source, can be used. Single-pulse source with short time duration has the frequency content whose amplitudes tend to be concentrated around the dominant frequency, whereas sweep source with long time duration denotes a flat distribution of relatively larger amplitude over a broad frequency band, although the peak to peak amplitude of single-pulse source wavelet is equivalent to that of sweep source one. In traditional seismic application, a single-pulse source(weight drop, dynamite) is typically used. However, to investigate the fine structure, as it is the case in the tunnel lining structure, the sweep wavelet can be also a desirable source waveform primarily due to the higher energy over a broad frequency band. For the investigation purposes of sweep source, a physical modeling is a useful tool, especially to study problems of wave propagation in the fine layered media. The main purpose of this work was using a physical modeling technique to explore the applicability of sweep source to the delineation of inner layer boundaries. To this end, a two-dimensional physical model analogous to the lining structure was built and a special ultrasonic sweep source was devised. The measurements were carried out in the sweep frequency range 10 ∼ 60 KHz, as peformed in the regular reflection survey(e.g. roll-along technique). The measured data were further rearranged with a proper software (cross-correlation). The resulting seismograms(raw data) showed quitely similar features to those from a single-pulse source, in which high frequency content of reflection events could be considerably emphasized, as expected. The data were further processed by using a regular data processing system "FOCUS" and the results(stack section) were well associated with the known model structure. In this context, it is worthy to note that in view of measuring condition the sweep source would be applied to benefit the penetration of high frequency energy into the media and to enhance the resolution of reflection events.

• Journal of the Korean Geophysical Society
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• v.8 no.1
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• pp.45-48
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• 2005

In order to investigate the velocity structure of the southern part of the Korean peninsula, exploded seismic signals were recorded for 120 s along a 294-km WNW-ESE line and 150 s along a 335-km NNW-SSE line in 2002 and 2004, respectively. Velocity tomograms were derived from inverting first arrival times. One-dimensional velocity models derived by joint analyses of teleseismic receiver functions and surface wave dispersion at several stations near the profiles were uesd to build initial models. The raypaths indicate several midcrust interfaces including ones at approximate depths of 2.0 and 14.9 km with refraction velocities of approximately 6.0 and 7.1 km/s, respectively. The deepest significant interface varies in depth from 30.8 km to 36.1 km. The critically refracting velocity varies from 7.8 to 8.1 km/s along this interface which may correspond to the Moho discontinuity. The velocity tomograms show (1) existence of a low-velocity zone centered at 6-7 km depth under the Okchon fold belt, (2) extension of the Yeongdon fault down to greater than 10 km, and (3) existence of high-velocity materials under the Gyeongsan basin whose thickness is less than 4.2 km.

• Geophysics and Geophysical Exploration
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• v.18 no.1
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• pp.1-8
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• 2015

To estimate the shear-wave velocity (${\nu}_s$ beneath the OTVZ seismic station on Ngauruhoe volcano in New Zealand, we calculated receiver functions (RFs) using 127 teleseismic data ($Mw{\geq}5.5$) with high signal-to-noise ratios recorded during November 11, 2011 to September 11, 2013. The genetic inversion algorithms was applied to 21 RFs calculated by the iterative time-domain deconvolution method. In the 1-D ${\nu}_s$ model derived by the inversion, the Moho is observed at a 14 km depth, marked by a ${\nu}_s$ transition from 3.7 km/s to 4.7 km/s. The average ${\nu}_s$ of the overlying crust is 3.4 km/s, and the average ${\nu}_s$ of a greater than 9-km thick low-velocity layer (LVL) in the lower crust is 3.1 km/s. The LVL becomes thinner with increasing distance from the station. Another LVL thicker than 10 km with ${\nu}_s$ less than 4.3 km/s is found in the upper mantle. Those LVLs in the lower crust and the upper mantle and the relatively thin crust might be related to the magma activity caused by the subducting Pacific plate.

• Proceedings of the International Union of Geodesy And Geophysics Korea Journal of Geophysical Research Conference
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• 2003.05a
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• pp.15-15
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• 2003

The first crustal refraction survey in the Korean Peninsula was carried out over the survey line connecting Seosan-Yeongdong-Kyeongiu on Dec. 15, 2002. The total length of the survey line was about 300 Km and 198 portable seismometers were deployed with approximately 1.5-km interval. The survey line itself was geologically important since it was almost normal to the so-called Sino-Korean structural trend. Two shots, one at Seosan (west end point) and the other at Yeongdong (mid-point), were exploded. They were 100-m deep drill well explosions. The Seosan shot consisted of a ton emulsion type explosive, while Youngdong consisted of 500 kg one. Both shots generated signals with good S/N ratios to the farthest receivers. Seismic signals were recorded by 195 receivers out of 198 ones. Although the originally planned Kyeongju shot (east end point) could not be exploded due to public discontent, the experiment was evaluated very successful. First breaks in all recorded traces were picked up and two preliminary analyses were carried out. The one is conventional flat layer analysis and the other was refraction tomographic analysis. The one resulted in average 32-km thick two-layer crust and the underlying mantle with 8.05-km/s P-velocity. The top crust layer with 3.86 kw/s P-velocity was 2.5-km thick and the lower crust layer with 6.0l km/s P-velocity was 29.5-km thick. The other resulted in a velocity cross-section. The confidence level of the velocity cross-section could not be evaluated at this time because only two shot were exploded. Detailed analyses such as surface wave dispersion are on going. Continuing crustal scale refraction surveys are planned in Korea.

• Tunnel and Underground Space
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• v.23 no.3
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• pp.180-191
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• 2013

The purpose of this research was to find an optimal quarrying for marble by analyzing the applicability and the work efficiency of a chain saw machine newly introduced in the underground Baekwoon mine. From the test results of the physical properties of Baekwoon marble, which affects the efficiency of rock cutting, it was found to have similar physical characteristics as the ones which are now being produced in the other areas in Korea. And especially it shows isotropic property, which can be thought to be advantageous as a dimensional stone. To check the long-term quality of the marble as a stone material, several tests such as corrosion resistance test and abrasion test were carried out. It was found to be vulnerable to acid rain with decrease of weight and seismic wave velocity after applying artificial rain at pH 5.6 for 50 times. The percentage of wear from abrasion test was 22.67%. The working time and cutting speed of the chain saw machine were recorded and analyzed during the test-run at the quarry. The overall work cycle was assorted into 9 unit operations and the operating time per each unit was drawn. The operating times for the two cutting patterns, which could be possibly applicable to the work site, were compared. The results indicated that the pattern B, that the cutting sequence was set to minimize the movement of the machine, showed 6% less working hours than the pattern A, which first cuts the outer boundary. With cutting pattern analysis, the ore body in the Baekwoon mine was 3 dimensionally modeled and a quarrying plan considering the existing conditions of the marble was suggested.

• Journal of the Korean GEO-environmental Society
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• v.15 no.2
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• pp.37-43
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• 2014

Natural disasters such as earthquakes and tsunamis occur suddenly, so that they cause massive loss of lives and property. Especially earthquakes represent a particularly severe threat because of the extensive damage accompanied by them. In Korea, an earthquake-resistant design has been rarely applied to a design or construction of slope. However, in resent years, the researches for earthquake-resistance have been performed because the importance on the earthquake-resistance is perceived and highlighted. Soil nail method, one of the slope stability methods, is excellent for its constructability and cost effectiveness, as compared with other stability methods. Also, this method has been widely used for reinforced construction for slope stability. The studies of soil nail method have been performed on the interaction behavior between nails and slopes as well as the varied load condition such as static load, dynamic load and so on. Nevertheless, there has been minimal research regarding the constraint condition of nail head. In this study, the numerical analysis was performed for identifying effect on slope stability for the constrain condition of the soil nail. The result shows that the resistance of constrained the nail head on reinforced slope is larger compared to the one of unconstrained nail head.

• Economic and Environmental Geology
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• v.51 no.6
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• pp.589-593
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• 2018

Democratic Peoples' Republic of Korea (DPRK) conducted the $6^{th}$ underground nuclear test at the Punggye-ri underground nuclear test site on September 27, 2017 12 hours 30 minutes of Korean local time. Comprehensive Nuclear-Test Ban Treaty Organization (CTBTO) under U.N. announced the body wave magnitude of the event was mb 6.1 while U.S. Geological Survey (USGS)'s calculation was mb 6.3. In this study, the differences of the magnitude estimates were investigated and verified. For this purpose, a source scaling between the $5^{th}$ and $6^{th}$ event, which's epicenters are 200 meters apart, was performed using seismic data sets from 30 broadband stations. The relative amplitude variations of the $6^{th}$ event compared to the $5^{th}$ event in the frequency domain was analyzed through the scaling. The increased amount of the bodywave magnitude $m_b$ for the $6^{th}$ event was calculated at 1 Hz, which was compared to those from USGS and CTBTO's calculations.

• The Journal of Engineering Geology
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• v.31 no.3
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• pp.381-393
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• 2021

Physical, mechanical, hydraulic, and geophysical tests were applied to validate methods of inspecting the effectiveness of grouting on an agricultural reservoir dam. Data obtained from series of in situ and laboratory tests considered four stages: before grouting; during grouting; immediately after grouting; and after aging the grouting for 28 days. The results of SPT and triaxial tests, including the unit weight, compressive strength, friction angle, cohesion, and N-value, indicated the extent of ground improvement with respect to grout injection. However, they sometimes contained errors caused by ground heterogeneity. Hydraulic conductivity obtained from in situ variable head permeability testing is most suitable for identifying the effectiveness of grouting because the impermeability of the ground increased immediately after grouting. Electric resistivity surveying is useful for finding a saturated zone and a seepage pathway, and multichannel analysis of surface waves (MASW) is suitable for analyzing the effectiveness of grouting, as elastic velocity increases distinctly after grouting injection. MASW also allows calculation from the P- and S- wave velocities of dynamic properties (e.g., dynamic elastic modulus and dynamic Poisson's ratio), which can be used in the seismic design of dam structures.

• Journal of the Korean Geotechnical Society
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• v.37 no.11
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• pp.71-81
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• 2021

The purpose of this study is liquefaction phenomenon was simulated using the 1g shaking table test. Analysis of liquefaction and Re-liquefaction behavior according to the ground conditions was analyzed when an embankment exists above the ground. The soil used in the experiment was silica sand and the ground composition was a liquefied layer of 50cm (Case 1), a non-liquefied layer of 17.5cm and a liquefied layer of 32.5cm (Case 2). The embankment was formed by fixing the height of 10cm and the slope of the slope at a ratio of 1:1.8. For seismic waves, excitation of a 5Hz sine wave was performed for 8 seconds, and a total of 5 case excitations were performed. In Case 1, it was confirmed that liquefaction occurred at all depths during the first vibration excitation at the free-field and that liquefaction did not occur at all depths except 5cm at the third vibration excitation. At the center of the embankment, liquefaction occurred up to a depth of 20cm during the first vibration excitation, and it was confirmed that liquefaction did not occur at all depths except for a depth of 5cm during the second vibration excitation.

• 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.