• Journal of the Korean Association of Geographic Information Studies
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• v.19 no.4
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• pp.17-35
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• 2016

Earthquake-induced disasters are often more severe in locations with soft soils than firm soils or rocks due to differences in ground motion amplification. On a regional scale, such differences can be estimated by spatially predicting subsurface soil thickness over the entire target area. In general, soil deposits are generally deeper in coastal or riverside areas than in inland regions. In this study, a coastal metropolitan area, Incheon, was selected to assess site effects and provide information on seismic hazards. Spatial prediction of geotechnical layers was performed for the entire study area within the GIS framework. Approximately 7,000 existing borehole drilling data in the Incheon area were gathered and archived into the GIS Database (DB). In addition, surface geotechnical data were acquired from a walkover survey. Based on the built geotechnical DB, spatial zoning maps of site-specific seismic response parameters were created and presented for use in a regional seismic strategy. Site response parameters were performed to determine site coefficients for seismic design over the entire target area and compared with each other. Site classifications and subsequent seismic zoning were assigned based on site coefficients. From this seismic zonation case study in Incheon, we verified that geotechnical GIS-DB can create spatial zoning maps of site-specific seismic response parameters that are useful for seismic hazard mitigation particularly in coastal metropolitan areas.

• The Journal of Engineering Geology
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• v.20 no.4
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• pp.381-390
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• 2010

This study assessed the hydrogeological characteristics of a seepage area of white leachate. The geological characteristics of the leachate were determined by a surface survey, and an electrical resistivity survey and borehole image processing system (BIPS) were applied to estimate the distribution of discontinuities, to assess the geological structure of the seepage areas. Fluctuations in groundwater level within boreholes were measured during periods of precipitation in the dry and wet seasons. The results show that electrical resistivity is lower in the seepage section than in non-seepage sections. The distribution of fracture zones and limestone cavities was inferred from the logging data and BIPS data. Variations in groundwater level and groundwater recharge, related to rainfall events, show the direct effect of rainfall events during the rainy season. We obtained a strong relationship between seepage amount and rainfall (correlation coefficients of 0.83-0.97).

• 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 Korean Tunnelling and Underground Space Association
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• v.14 no.5
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• pp.503-516
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• 2012

Rock mass rating (RMR) is the key factor when designing the appropriate support pattern of tunnel projects. Borehole drilling is usually performed along the tunnel route in order to determine the rock mass rating to be used for tunnel design. The rock mass rating at the non-boring region between boreholes is usually assessed through geophysical surveys such as electrical prospecting, seismic prospecting, etc. Many studies were carried out to find out the correlation between electrical resistivity and rock mass rating. However, most researches were aimed at obtaining the relationship between the two parameters utilizing experimental results obtained from laboratory tests or electrical prospectings. In this paper, efforts were made to analyze and obtain relationships between the electrical resistivity obtained from in-situ electrical resistivity logging data and the rock mass rating. Correlation studies using field data showed that the electrical resistivity is highly correlated with the rock mass rating with the determination coefficient more than 90%. The correlation analysis was also carried out between RMR classification parameters and the electrical resistivity. It was shown that the correlation between the condition of discontinuities and the electrical resistivity was very high with the determination coefficient more than 80%; that between the groundwater condition and the electrical resistivity was very low with the determination coefficient less than 57%.

• Geophysics and Geophysical Exploration
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• v.20 no.4
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• pp.216-225
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• 2017

Recently, steel casing became an interesting issue when applying controlled-source electromagnetic (EM) method to various fields because effects of steel casing on EM responses are not negligible. This study employed an approach that approximates the steel casing as a series of electric dipole sources in order to develop the numerical algorithm for the efficient simulation of EM responses in the presence of steel casing. After verifying the validity of the developed algorithm, we analyze effects of steel casing on EM responses with the synthetic model simulating geothermal reservoir environment. The analysis showed that the effects of steel casing on EM responses are localized near the casing and increase as the transmitter becomes close to the casing. In addition, through the analysis on the EM responses by the injection of clean water, we confirm that the effects of casing are negligible when interpreting the after-injection data acquired using the transmitter located far enough from the casing. Considering the difference in EM responses between before and after injection in inversion, the effects of the casing can be neglected although after-injection data shows considerable difference due to the close distance between the transmitter and casing. To investigate this kind steel casing effect, the precise analysis on EM responses should be preceded. The algorithm introduced in this study will contribute to the reliable calculations of EM responses distorted by the conductive steel casing.

• Geophysics and Geophysical Exploration
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• v.12 no.4
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• pp.299-308
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• 2009

A magentotelluric (MT) survey at the Uiseong area has been performed for the site investigation of pilot scale $CO_2$ sequestration. The purpose of the MT survey is to delineate deeply extended fracture systems that can act as a leakage path of injected $CO_2$ Plume. Since the target area is extremely noisy in electromagentic sense, low frequency data below 1 Hz cannot be used for inversion. Two- and three-dimensional interpretation of the MT data showed a very clear conductive anomaly, which has the direction of $N55\sim65^{\circ}W$ and is extended roughly down to 1.6 km. It have the same direction with the strike-slip faults, the Gaeum and Geumcheon Faults. On the contrary, the eastern part of the survey area shows relatively homogeneous to the depth of 2 km though some small fractures at shallow depths can be found. Test drilling and high-definition borehole surveys should be followed at the eastern part of the survey area and hydraulic fracturing is required for injection of $CO_2$, because mean porosity of the sedimetary rock in the area is only 1.47%.

• Journal of the Korean Association of Geographic Information Studies
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• v.12 no.4
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• pp.136-145
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• 2009

Site characteristic is an important input parameter in the geologic hazard assessments including, but not limited to, earthquakes, liquefaction and landslides. Although it is a routine to use data collected by boreholes or seismic prospecting for site classifications, we used indirect methods using the geologic and the topographic maps. A site classification map in the Gyeongsang Province has been produced by GIS tools based on geologic age, rock types, and elevations from the geologic map and the topographic map of Korea. Site B (rock site) is dominant in the study area, although softer soils are observed along rivers and in reclaimed lands. We have found that 73% of the site classification results in the study are in concordance with those obtained from borehole data. Observed discrepancies are attributed to errors in the geologic and the topographic maps. For some sites, the origin of the differences is not clear, which requires a further field study or a drilling. Site classification from this study provides essential information for reliable hazard assessments of earthquakes, floods, landslides and liquefaction. Results obtained in the study also play a crucial role in land use planning for developing areas.

• Geophysics and Geophysical Exploration
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• v.8 no.1
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• pp.26-33
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• 2005

Inversion of multi-mode surface-wave phase velocity for shallow engineering site investigation has received much attention in recent years. A sensitivity analysis and inversion of both synthetic and field data demonstrates the greater effectiveness of this method over employing the fundamental mode alone. Perturbation of thickness and shear-wave velocity parameters in multi-modal Rayleigh wave phase velocities revealed that the sensitivities of higher modes: (a) concentrate in different frequency bands, and (b) are greater than the fundamental mode for deeper parameters. These observations suggest that multi-mode phase velocity inversion can provide better parameter discrimination and imaging of deep structure, especially with a velocity reversal, than can inversion of fundamental mode data alone. An inversion of the theoretical phase velocities in a model with a low velocity layer at 20 m depth can only image the soft layer when the first higher mode is incorporated. This is especially important when the lowest measurable frequency is only 6 Hz. Field tests were conducted at sites surveyed by borehole and PS logging. At the first site, an array microtremor survey, often used for deep geological surveying in Japan, was used to survey the soil down to 35 m depth. At the second site, linear multichannel spreads with a sledgehammer source were recorded, for an investigation down to 12 m depth. The f-k power spectrum method was applied for dispersion analysis, and velocities up to the second higher mode were observed in each test. The multi-mode inversion results agree well with PS logs, but models estimated from the fundamental mode alone show f large underestimation of the depth to shallow soft layers below artificial fill.

• Journal of the Korean GEO-environmental Society
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• v.20 no.5
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• pp.5-12
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• 2019

On November 15, 2017, a unpredictable liquefaction damage was occurred at the $M_L=5.4$ Pohang earthquake and after, many researches have been conducted in Korea. In Korea, where there were no cases of earthquake damage, it has been extremely neglectable in preparing earthquake risk maps and building earthquake systems that corresponded to prevention and preparation. Since it is almost impossible to observe signs and symptoms of drought, floods, and typhoons in advance, it is very effective to predict the impacts and magnitudes of seismic events. In this study, 14,040 borehole data were collected in the metropolitan area and liquefaction evaluation was performed using the amplification factor. Based on this data, liquefaction hazard maps were prepared for ground accelerations of 0.06 g, 0.14 g, 0.22 g, and 0.30 g, including 200years return period to 4,800years return period. Also, the correlation analysis between the earthquake acceleration and LPI was carried out to draw a real-time predictable liquefaction hazard map. As a result, 707 correlation equations in every cells in GIS map were proposed. Finally, the simulation for liquefaction risk mapping against artificial earthquake was performed in the metropolitan area using the proposed correlation equations.

• Journal of Korean Society of Disaster and Security
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• v.12 no.1
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• pp.45-56
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• 2019

In this study, the seismic risk has been evaluated by setting the bedrock acceleration to 0.154g which, was taking into consideration that the earthquake return period for the buried electric power tunnels in the metropolitan area to be 1,000 years. In this case, the risk assessment during the earthquake was carried out in three stages. In the first stage, the site classification was performed based on the site investigation data of the target area. Then, the LPI(Liquefaction Potential Index) was applied using the site amplification factor. After, candidates were selected using a hazard map. In the second stage, risk assessment analysis of seismic response are evaluated thoroughly after the recalculation of the LPI based on the site characteristics from the boring logs around the electric power area that are highly probable to be liquefied in the first stage. The third Stage visited the electric power tunnels that are highly probable of liquefaction in the second stage to compensate for the limitations based on the borehole data. At this time, the risk of liquefaction was finally evaluated based off of the reinforcement method used at the time of construction, the application of seismic design, and the condition of the site.