• Journal of the Korean earth science society
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• v.43 no.1
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• pp.188-198
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• 2022

Some deep faults do not reach the ground surface and are seldom recognized. Gokgang Fault area in the east of the Heunghae area of the Pohang basin has been selected to confirm the feasibility of the Horizontal-to-Vertical Spectral Ratio (HVSR) approach to identify blind faults. Densely spaced microtremor data have been acquired along two lines in the study area and processed to obtain resonance frequencies. An empirical relationship between the resonance frequency and the bedrock depth was proposed using borehole data available in the study area. Resonance frequencies along two lines were then converted to bedrock depths. The resulting depth profiles show significant lateral variations in the bedrock depth. As expected, considerable variation in the resonance frequency is observed near the Gokgang fault. The depth profiles also present additional significant variations in the resonance frequencies and the bedrock depths. The feature is presumably related to a blind fault that is previously unknown. Therefore, this case study confirms the feasibility of the HVSR technique to identify faults otherwise not recognized on the surface.

• Journal of Soil and Groundwater Environment
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• v.27 no.spc
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• pp.34-50
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• 2022

Principal component analysis (PCA) was conducted using hydrochemical data in four testbeds (A to D) built for the development of site characterization technologies to assess the hydrochemical processes controlling the hydrochemistry in each site. The PCA results indicated the nitrogen loading to deep bedrock aquifers through permeable fractures in Testbed A, the chemical weathering enhanced with the biodegradation of petroleum hydrocarbons in Testbed B, the reductive dechlorination in Testbed C, and the different hydrochemistry depending on the depth to bedrock in Testbed D, consistent with the characteristics of each site. In Testbeds B and D, outliers seemed to affect the PCA result probably due to the small number of samples, whereas the PCA result was still consistent with site characteristics. This study result indicates that the PCA is widely applicable to hydrochemical data for the assessment of major hydrochemical processes in contamination sites, which is useful for site characterization when combined with other site characterization technologies, e.g., geological survey, geophysical investigation, borehole logging. It is suggested that PCA is applied in contaminated sites to interpret hydrochemical data not only for the distribution of contamination levels but also for the assessment of major hydrochemical processes and contamination sources.

• Journal of Soil and Groundwater Environment
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• v.12 no.4
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• pp.35-53
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• 2007

Geochemical and isotope studies on the groundwater system of the Youngcheon area were carried out. Most groundwaters belong to Ca-$HCO_3$ and Ca-$SO_4$ types and some groundwaters belong to Na-$HCO_3$ type. Geochemical characteristics of these groundwaters were mainly affected by their basement rocks around the boreholes. High $SO_4$ content of groundwater is the result of reaction with sulfate or sulfide minerals in the host rock. Ca was originated from the carbonate minerals in the sedimentary rock. After the groundwater was saturated with calcite, the Na-$HCO_3$ type groundwaters were evolved by the reaction with plagioclase for a relatively long residence time. This explanation was supported by low tritium contents of Na-$HCO_3$ type groundwaters. ${\delt}a^{18}O$ and ${\delta}D$ data indicate that the groundwaters are of meteoric water origin and there was no difference between the various types of waters. Grondwaters from the boreholes BH-1, BH-9 and BH-12 showed the geochemical and isotopic characteristics of deep groundwater. Most borehole groundwaters except them did not show the systematic geochemical variations with sampling depth indicating that the shallow and deep groundwaters were mixed with each other throughout the study area. The results of water quality analysis indicate that the study area is highly contaminated by the introduction of agricultural sewage.

• Economic and Environmental Geology
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• v.39 no.5 s.180
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• pp.607-613
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• 2006

For the reasonable use of low grade-shallow geothermal energy by Standing Column Well(SCW) system, the basic requirements are depth-wise increase of earth temperature like $2^{\circ}C$ per every 100m depth, sufficient amount of groundwater production being about 10 to 30% of the design flow rate of GSHP with good water quality and moderate temperature, and non-collapsing of borehole wall during reinjection of circulating water into the SCW. A closed loop type-vertical ground heat exchanger(GHEX) with $100{\sim}150m$ deep can supply geothermal energy of 2 to 3 RT but a SCW with $400{\sim}500m$ deep can provide $30{\sim}40RT$ being equivalent to 10 to 15 numbers of GHEX as well requires smaller space. Being considered as an alternative of vertical GHEX, many numbers of SCW have been widely constructed in whole country without any account for site specific hydrogeologic and geothermal characteristics. When those are designed and constructed under the base of insufficient knowledges of hydrgeothermal properties of the relevant specific site as our current situations, a bad reputation will be created and it will hamper a rational utilization of geothermal energy using SCW in the near future. This paper is prepared for providing a guideline of SCW design comportable to our hydrogeothermal system.

• 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.

• The Journal of Engineering Geology
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• v.14 no.1
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• pp.1-20
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• 2004

This study focuses on the hydrochemical characteristics in goundwater affected by reclamation at 2000 Sydney Olympic Games site, Sydney, Australia. The Olympic Games site can be divided into three areas, i.e. reclaimed areas; landfill areas and non-infilled areas. In the current work, 'reclaimed areas' were previously estuarine, and were filled with waste materials and are now above present high tide level, whereas 'landfill areas' are areas where deposition of waste materials occurred above sea level. No deposition of waste took place in 'non-infilled areas'. This study was also evaluated by three different types such as deep boreholes, shallow boreholes and standpipes. The hydrochemishy of groundwaters in reclaimed and non-in-filled areas is characterized by Mg- and Ca-enrichment, whereas groundwaters in landfill areas are elevated in K and NO₃. Na, K and Mg are the dominant cations in groundwater from reclaimed areas and Na and K are the dominant cations in groundwater in landfill areas. Na and Mg are the dominant cations in groundwater in deep boreholes, whereas Na and K are the dominant cations in groundwater in shallow boreholes and standpipes. There is no distinct trend in heavy metals with electrical conductivity in the groundwater between the re-claimed, landfill and non-infilled areas. Fe and Mn in landfill areas with respect to reclaimed areas and non-infilled areas show a distinct increase in concentration with declining pH. Mean electrical conductivity values in the deep and shallow boreholes are higher than that of standpipes, but the minimum and maximum value of electrical conductivity in groundwater in standpipes shows remarkably different value, probably due to perched pond. There is no correlation between Cu, Pb, Zn, Cr concentrations in groundwater with pH, from deep boreholes, shallow boreholes and standpipes, except for Fe and Mn, which demonstrate increasing concentrations with declining pH. The results revealed a close association between elevated concentrations in groundwater and the presence of fill materials at the site. Trace metals teachability from re-claimed soils adjacent to estuary plays a significant role in determining their potential environmental risk to surrounding environment.

• Geophysics and Geophysical Exploration
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• v.11 no.1
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• pp.34-40
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• 2008

Underwater seismic refraction with advanced interpretation approaches makes important contributions to shallow marine exploration and geotechnical investigations in Australia's coastal areas. A series of case studies are presented to demonstrate the recent applications of continuous and static USR methods to river crossing and port infrastructure projects at various sites around Australia. In Sydney, static underwater seismic refraction (USR) with bottom-placed receivers and borehole seismic imaging assisted the development of improved geotechnical models that reduced construction risk for a tunnel crossing of the Lane Cove River. In Melbourne, combining conventional boomer reflection and continuous USR with near-bottom sources and receivers improved the definition of a buried, variably weathered basalt flow and assisted dredging assessment for navigation channel upgrades at Geelong Ports. Sand quality assessment with continuous USR and widely spaced borehole information assisted commercial decisions on available sand resources for the reclamation phase of development at the Port of Brisbane. Buried reefs and indurated layers occur in Australian coastal sediments with the characteristics of laterally limited, high velocity, cap layers within lower velocity materials. If these features are not recognised then significant error in depth determination to deeper refractors can occur. Application of advanced refraction inversion using wavefront eikonal tomography to continuous USR data obtained along the route of a proposed offshore pipeline near Fremantle allowed these layers and the underlying bedrock refractor to be accurately imaged. Static USR and the same interpretation approach was used to image the drowned granitic regolith beneath sediments and indurated layers in the northern area of Western Australia at a proposed new berthing site where deep piling was required. This allowed preferred piling sites to be identified, reducing overall pile lengths. USR can be expected to find increased application to shallow marine exploration and geotechnical investigations in Australia's coastal areas as economic growth continues and improved interpretation methods are developed.

• Geophysics and Geophysical Exploration
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• v.13 no.1
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• pp.61-68
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• 2010

A magnetotelluric (MT) survey has been performed to delineate deeply extended fracture systems at the geothermal field in Seokmo Island, Korea. To assist interpretation of the MT data, geological surveying and well logging of existing wells were also performed. The surface geology of the island shows Cretaceous and Jurassic granite in the north and Precambrian schist in the south. The geothermal regime has been found along the boundary between the schist and Cretaceous granite. Because of the deep circulation along the fracture system, geothermal gradient of the target area exceeds $45^{\circ}C/km$, which is much higher than the average geothermal gradient in Korea. 2D and 3D inversions of MT data clearly showed a very conductive anomaly, which is interpreted as a fracture system bearing saline water that extends at least down to 1.5 km depth and is inclined eastwards. After drilling down to the depth of 1280 m, more than 4000 tons/day of geothermal water overflowed with temperature higher than $70^{\circ}C$. This water showed very similar chemical composition and temperature to those from another existing well, so that they can be considered to have the same origin; i.e. from the same fracture system. A new geothermal project for combined heat and power generation was launched in 2009 in Seokmo Island, based on the survey. Additional geophysical investigations including MT surveys to cover a wider area, seismic reflection surveys, borehole surveys, and well logging of more than 20 existing boreholes will be conducted.

• Journal of the Korean earth science society
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• v.34 no.6
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• pp.575-587
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• 2013

DC electrical and electromagnetic survey was applied to evaluate the reserve of an iron mine site. We analyzed the borehole cores and the cores sampled from outcrops in order to decide which geophysical method was efficient for the evaluation of iron mine site and to understand the geological setting around the target area. Based on the core tests for specific weight, density, porosity, resistivity and P-wave velocity, showing that the magnetite could be distinguishable by the electrical property, we decided to conduct the electrical survey to investigate the irone mine site. According to previous studies, the DC electrical survey was known to have various arrays with high resolutions effective to the survey of the iron mine site. However it was also known that the skin depth is too shallow to grasp the deep structure of iron mine. To compensate the weakness of the DC electrical method, we applied the MagnetoTelluric (MT) survey. In addition, a Controlled Source MT (CSMT) method was also applied to make up the shortcoming of MT method which is weak for shallow targets. From the DC electrical and MT survey, we found a new low resistivity zone, which is believed to be a magnetite reserve beneath the old abandoned mine. Therefore, this study was confirmed for additional utility value.

• Journal of Korean Tunnelling and Underground Space Association
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• v.4 no.4
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• pp.277-286
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• 2002

Most of the problems in dealing with the tunnel construction are the uncertainties and complexities of the stress conditions and rock strengths in ahead of the tunnel excavation. The limitations on the investigation technology, inaccessibility of borehole test in mountain area and public hatred also restrict our knowledge on the geologic conditions on the mountainous tunneling area. Nevertheless an extensive and superior geophysical exploration data is possibly acquired deep within the mountain area, with up to the tunnel locations in the case of alternative design or turn-key base projects. An appealing claim in the use of artificial neural networks (ANN) is that they give a more trustworthy results on our data based on identifying relevant input variables such as a little geotechnical information and biological learning principles. In this study, error back-propagation algorithm that is one of the teaching techniques of ANN is applied to presupposition on Rock Mass Ratings (RMR) for unknown tunnel area. In order to verify the applicability of this model, a 4km railway tunnel's field data are verified and used as input parameters for the prediction of RMR, with the learned pattern by error back propagation logics. ANN is one of basic methods in solving the geotechnical uncertainties and helpful in solving the problems with data consistency, but needs some modification on the technical problems and we hope our study to be developed in the future design work.