• Journal of the Korean Geophysical Society
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• v.8 no.2
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• pp.53-61
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• 2005

2-D and 3-D resistivity surveys were carried out at the Deok-In2 levee during the period of arid and rainy seasons to assess the waterproof effectiveness of sheet pile and routing sections and detect the location of pipings. Inverted resistivity sections clearly indicated the boundaries of sheet pile and grouting sections and the locations of pipings observed at the ground surface. It is necessary that proper survey parameters are determined considering inverted depth and resolution and contacting resistance is decreased to obtain favorable result.

• Geophysics and Geophysical Exploration
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• v.10 no.1
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• pp.89-97
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• 2007

A three-dimensional (3D) magnetotelluric (MT) survey has been carried out to delineate subsurface structures and possible fractures, for development of low-temperature geothermal resources in Pohang, Korea. Quite good quality MT data could be obtained throughout the survey region by locating the remote reference in Kyushu, Japan, which is ${\sim}480\;km$ from the centre of the field site. 3D modelling and inversion are performed taking into account the sea effect in MT measurements near the seashore. The nearby sea in the Pohang area affects MT data at frequencies below $1\;Hz{\sim}0.2\;Hz$, depending on the distance from the seashore. The most severe sea effects were observed in the south-east parts of the survey area, closer to Youngil Bay. 3D inversion with and without the seawater constraint showed very similar results at shallow depths, roughly down to 2 km. At greater depths, however, a strong sea effect seems to form a fictitious conductive structure in ordinary 3D inversion, especially in the south-eastern part of the survey region. Comparison between drilling results and the resistivity profiles from inversions showed that five layered structures can be distinguished the subsurface beneath the target area. They are: (a) semi-consolidated mudstones with resistivity less than $10\;{\Omega}m$, which are ${\sim}300\;m$ thick in the northern part and ${\sim}600\;m$ thick in the southern part of the survey area; (b) occasional occurrence of trachybasalt and lapilli tuff within the mudstone layer has resistivity of a few tens of${\Omega}m$, (c) intrusive rhyolite ${\sim}400\;m$ thick has resistivity of several hundreds of ${\Omega}m$, (d) alternating sandstone and mudstone down to 1.5 km depth shows resistivity of ${\sim}100\;{\Omega}m$, (e) a conductive structure was found at a depth of ${\sim}3\;km$, but more geological and geophysical study should be carried out to identify this structure.

• The Journal of Engineering Geology
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• v.28 no.2
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• pp.297-312
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• 2018

In order to assess the safety diagnosis and grouting reinforcement effect of old reservoir facility, local governments and public offices mainly use electrical resistivity survey. However, electrical resistivity survey is a qualitative evaluation that varies the resistivity value by various exploration conditions. It is also difficult to grasp the stiffness change directly related to the stability of reservoir, thus an electrical resistivity survey is not applicable to continuous stability monitoring after grouting. The purpose of this study is to investigate and validate the quantitative evaluation of reinforcement effect of reservoir with cement grouting through shear velocity (Vs), which is closely related to the stiffness (${\mu}$) of the ground. This study was carried out on two reservoir facilities. The reinforcement effect was evaluated by comparing the permeability test, standard penetration test, down-hole test and MASW(Multi-channel Analysis of Surface wave) survey before and after cement grouting. Shear wave velocity changes before and after grouting were analyzed by phase velocity difference and inversion analysis, respectively, and the reliability of the analytical results was evaluated by comparing with field test results. Shear wave velocity increases to 5~10% in case of the D levee, and 10~20% in the levee of H reservoir. These results are showed similar pattern to the field test results.

• 한국지구물리탐사학회:학술대회논문집
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• 2003.11a
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• pp.390-397
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• 2003

A three-dimensional (3D) inversion technique has been developed for interpretation of magnetotelluric (MT) data. The inversion method is based on the linearized least-squares (Gauss-Newton) method with smoothness regularization. In addition to the underground 3D resistivity distribution, static shifts are also treated as unknown parameters in the inversion. The forward modeling is by the staggered-grid finite difference method. A Bayesian criterion ABle is applied to search the optimum trade-off among the minimization of the data misfit, model roughness and static shifts. The method has been applied to several MT datasets obtained at geothermal fields in Japan and other Asian countries. In this paper, two examples will be discussed: one is the data at the Ogiri geothermal area, southwestern Japan, and the other is at the Pohang low-enthalpy geothermal field, southeastern Korea. The inversion of the Ogiri data has been performed stably, resulting in a good fitting between the observed and computed apparent resistivities and phases. The recovered 3D resistivity structure is generally similar to the two-dimensional (2D) inversion models, although the deeper portion of the 3D model seems to be more realistic than that of the 2D model. The 3D model is also in a good agreement with the geological model of the geothermal reservoirs. 3D interpretation of the Pohang MT data is still preliminary. Although the fitting to the observed data is very good, the preliminary 3D model is not reliable enough because the station coverage is not sufficient for a 3D inversion.

• Geophysics and Geophysical Exploration
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• v.6 no.4
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• pp.165-170
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• 2003

Recently, resistivity surveys have been frequently carried out over the irregular terrain such as mountainous area. Such an irregular terrain itself can produce significant anomalies which may lead to misinterpretations. In this study, topographic effects in resistivity survey were studied using the physical scale modeling as well as the numerical one adopting finite element method. The scale modeling was conducted at a pond, so that we could avoid the edge effect, the inherent problem of the scale modeling conducted in a water tank in laboratory. The modeling experiments for two topographic features, a ridge and a valley with various slope angles, confirmed that the results by the two different modeling techniques coincide with each other fairly well for all the terrain models. These experiments adopting dipole-dipole array showed the distinctive terrain effects, such that a ridge produces a high apparent resistivity anomaly at the ridge center flanked by zones of lower apparent resistivity. On the other hand, a valley produces the opposite anomaly pattern, a central low flanked by highs. As the slope of a terrain model becomes steeper, the terrain-induced anomalies become stronger, and moreover, apparent resistivity can become even negative for the model with extremely high slope angle. All the modeling results led us to the conclusion that terrain effects should be included in the numerical modeling and/or the inversion process to interpret data acquired at the rugged terrain area.

• Journal of the Korean earth science society
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• v.21 no.4
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• pp.408-422
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• 2000

Multiple geophysical methods were applied over the Manjang cave area in Cheju Island to compare and contrast the effectiveness of each method for exploration of underground cavities. The used methods are gravity, magnetic, electrical resistivity and GPR(Ground Pentrating Radar) survey, of which instruments are portable and operations are relatively economical. We have chosen seven survey lines and applied appropriate multiple surveys depending on the field conditions. In the case of magnetic method. two-dimensional grid-type surveys were carried out to cover the survey area. The geophysical survey results reveal the characteristic responses of each method relatively well. Among the applied methods, the electric resistivity methods appeared to be the most effective ones in detecting the Manjang Cave and surrounding miscellaneous cavities. Especially, on the inverted resistivity section obtained from the dipole-dipole array data, the two-dimensional distribution of high resistivity cavities are revealed well. The gravity and magnetic data are contaminated easily by various noises and do not show the definitive responses enough to locate and delineate the Manjang cave. But they provide useful information in verifying the dipole-dipole resistivity survey results. The grid-type 2-D magnetic survey data show the trend of cave development well, and it may be used as a reconnaissance regional survey for determining survey lines for further detailed explorations. The GPR data show very sensitive response to the various shallow volcanic structures such as thin spaces between lava flows and small cavities, so we cannot identify the response of the main cave. Although each geophysical method provides its own useful information, the integrated interpretation of multiple survey data is most effective for investigation of the underground caves.

• Journal of Korean Society of Environmental Engineers
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• v.28 no.5
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• pp.463-471
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• 2006

Electrical resistivity surveys were conducted at areas of abandoned landfills in Cheonan and Wonju. Geology and extent of leachate migration around the landfills were evaluated with collected resistivity data by 2-D and 3-D resistivity inverse modeling. The Cheonan landfill is located above the paddy fields and the resistivity survey lines were crossed to examine possible pollution at the paddy fields by leakage of the landfill leachate. In Wonju, the landfill and the downgradient paddy fields are divided by a concrete barrier wall. At the bottom of the landfill, there is a leachate settlement system, which has not been in operation. To evaluate leachate leakage into the paddy fields, a total of 4 survey lines were used. According to the resistivity survey results, the landfill leachate in Cheonan appeared to be restricted only within the interior of the landfill, not to migrate into the subsurface of the paddy fields. These results are well consistent with electrical conductivity values of groundwaters obtained from a periodic analysis of water qualities. In Wonju, however, it was inferred that the leachate emanating from the landfill migrated beneath the abandoned leachate settlement system and the leachate would reach the downgradient paddy fields. Low resistivity area was observed in the old reservoir area and it appeared to be derived from convergence of groundwater flows from the surrounding valley and the moist wet land. In addition, groundwater flow into the paddy fields occurs beneath the old reservoir embankment at depths of $7{\sim}8m$. This paper reports details of the resistivity surveys for the uncontrolled landfills.

• Geophysics and Geophysical Exploration
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• v.13 no.2
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• pp.175-180
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• 2010

The small loop electromagnetic (EM) method is a fast and convenient geophysical tool which can provide resistivity distribution of shallow subsurface. Especially, it can be a useful alternative of resistivity method in a very conductive environment such as a reclaimed saline land. We applied the multi-frequency small loop EM method for the site investigation of reclaimed saline land. We inverted the measured EM data using one dimensional (1D) inversion program and merged to obtain three dimensional (3D) resistivity distribution over the survey area. Finally, comparing he EM results with the drill log and measured soil resistivity sampled at 16 drill holes, we can define the site character such as thickness of landfill, salinity distribution, and etc.

• Geophysics and Geophysical Exploration
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• v.13 no.1
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• pp.69-79
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• 2010

In order to reduce leakage from a reservoir, a large amount of cement milk (grout) was injected from boreholes drilled around the shores of the reservoir, and monitored to establish the infiltration of cement milk into the bedrock under the reservoir. From laboratory tests using rock core samples, it was revealed that the resistivity of cement milk is much lower than that of the groundwater at this location. Therefore, it was expected that the resistivity of the zones filled with cement milk would be significantly reduced. Geophysical surveys are expected to be suitable methods to check the effectiveness of grouting in improving the water-retaining performance of a reservoir. DC electrical surveys (seven in total) and two Controlled Source Audio-frequency Magneto-Telluric (CSAMT) surveys were conducted along survey lines in the reservoir to monitor the infiltration of cement milk during the grouting. Extremely low resistivity zones ($10\;{\Omega}m$ or less) were observed in resistivity sections obtained by 2D inversion. The zones are inferred to be fractured zones filled with cement milk. In sections showing the rate of change of resistivity, three zones that showed significant change showed gradual expansion to deeper parts as the grouting progressed. These zones correspond to highly permeable zones detected by Lugeon tests at grout boreholes. We have confirmed that it is possible to measure the resistivity change by DC electrical and CSAMT surveys from the surface of the reservoir. It seems that such monitoring results could be reflected in future grouting plans.

• Journal of the Korean Geophysical Society
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• v.6 no.2
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• pp.89-97
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• 2003

For safety evaluation of a rockfill dam, it is often necessary to investigate spatial distribution and dynamic characterization of weak zones such as fractures. For this purpose, both seismic and electric methods are adopted together in this research. The former employs the multichannel analysis of surface waves (MASW) method, and aims at the mapping of 2-D shear-wave velocity (Vs) profile along the dam axis that can be associated with dynamic properties of filled materials. The latter is carried out by DC- resistivity survey with a main purpose of mapping of spatial variations of physical properties of dam materials. Results from both methods are compared in their signature of anomalous zones. In addition, downhole seismic survey was carried out at three points within the seismic survey lines and results by downhole seismic survey are compared with the MASW results. We conclude that the MASW is an efficient method for dynamic characterization of dam-filling materials, and also that joint analyses of these two seemingly unrelated methods can lead to an effective safety evaluation of rock-fill dam.