• Geophysics and Geophysical Exploration
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• v.3 no.1
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• pp.1-6
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• 2000

The 1 $km^2$ area studied is located in Sukchun-ri, Hwasung-koon, the southern part of Kyeonggi-do. Even though this site has been known as a contaminated area caused by seawater intrusions, geophysical and geochemical surveys have never been carried out at the site to determine the extent of the seawater contamination and to investigate whether the seawater intrusion is in progress. The purpose of this study is to determine the extent of seawater contamination and a preferred channel of the seawater intrusion using geophysical methods such as DC resistivity surveys with Schlumberger array and a dipole-dipole array. In order to determine whether the seawater intrusion is in progress in the area, DC resistivity monitoring with Schlumberger array was performed. According to the resistivity map obtained from the inversion of the resistivity data measured with Schlumberger array, the study area is divided into two districts as relatively lowly resistive (less than 30 ohm-m) and highly resistive (more than 30 ohm-m) areas. The distribution of the lowly resistive area is consistent with the distribution of the layer composed of clay minerals, and the resistivity of this layer decreases slowly as approaching to the old seashore. Hydrogeological analysis shows that the clay layer within a distance of about 200 m from the seashore has been already contaminated by sea-water and its electric conductivity is 8 times higher than that of the sand layer covered by the clay layer. According to the results of the 2-dimensional DC resistivity surveys with a dipole-dipole array, there are two preferred channels of the seawater intrusion in the site, and both the channels are in the NW-SE direction from the old seashore. The lowly resistive zone in the southern channel extends to a depth of 80 m. The DC resistivity monitoring with Schlumberger array was carried out along the preferred channel which has the low resistivity Bone (fracture zone) that extended to a depth of 80 m. The time series of apparent resistivity, measured at a distance of 260 m from the old coast line, fluctuates with a period of 12 hours. From these observations, it can be concluded that the seawater intrusion caused by tidal action is still in progress along the fractured zone interpreted by the DC resistivity surveys with a dipole-dipole array.

• Geophysics and Geophysical Exploration
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• v.1 no.2
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• pp.110-115
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• 1998

Self-potential, VLF-EM and dipole-dipole resistivity methods have been widely used for exploration of conductive sulfide ore deposit, because of the convenience and low cost of field work and the reliability of their results. The geophysical responses for vein-type sulfide outcrop of Changkoom mine located in Bukwi-Myon, Jinan-Gun, Chollabuk-Do were investigated and compared with its drilling results. The geology around the survey area is composed of acidic volcanics and sediments of Yuchon Group. And sulfides bearing pyrite, pyrrotite, galena etc. are deposited in disseminated or vein type within acidic volcanics. Typical geophysical responses were detected from the above vein type ore body, respectively. From the shape and extent of S.P. anomaly, ore body is dipping westward and extending about 50 m. It is detected that the VLF EM response matching the outline of ore zone is considered as indication of dyke dipping westward. And also resistivity response indicating conductive dipping dyke is detected. From drilling results for outcrop and geophysical anomalies, the shape of ore body is vein type dipping about $70^{\circ}W$ and extending about 50 m.

• Geophysics and Geophysical Exploration
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• v.4 no.4
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• pp.110-114
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• 2001

In order to obtain the geological information of the shallow subsurface and the characteristics of resistivity curves to the lateral extent of the gold bearing gravel layer underlaid by a thick layer, electrical resistivity surveys had been carried out in the northern area of Takaoi village, Kalimantan, Indonesia from September 30th to October 27th in 1999. The Dipole-Dipole (Dp-Dp) resistivity survey was carried out with a electrode spacing of A=3m, 5m and electrode separation index n=10, and the vertical electrical sounding (VES) of the Schlumberger type was performed at 6 sites. The interpretation of the Dp-Dp data and VES curves by computer modeling indicates 5 classified layers as the horizontal structure. And the characteristics of VES curves shows us KH type.

• Journal of the Korean Society of Groundwater Environment
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• v.4 no.4
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• pp.185-190
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• 1997

Electrical surveys were conducted at the Seokdae waste landfill in July,1996. Within the landfill, 4 lines of dipole-dipole surveys and 7 Schlumberger soundings were carried out and 2 soundings in front of the landfill. In the landfill, interpretations of the survey data show low resistivity zones below 10 Ωm to a depth of 50 m from the surface and such low resistivity zones of the D block are thicker than those of the other blocks by about 2~10 m. Considering the depth of the bedrock and the height of waste reclamation, no evidence of bedrock contamination by leachate is indicated. But it is inferred that the weathered zones are contaminated in the landfill. In the block A and B, minor fault having the strike of N$70^{\circ}$W have been confirmed by dipole-dipole surveys, so future contamination of the bedrock by leachate is possible The degree of ground contamination is the highest in the D block due to the leachate plume mainly heading for this block. On the other hand, electrical soundings do not indicate ground contamination by leachate in the front area of the landfill.

• Journal of the Korean Society of Groundwater Environment
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• v.3 no.2
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• pp.95-100
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• 1996

Schlumberger electrical soundings and Coincident loop time-domain electromagnetic soundings were made on the Nanji-do landfill to investigate the nature of fills and the subsurface structure. The measured data were transformed into apparent resistivity values and then inverted in terms of 1-D resistivity models. At 6 points, both measurements were carried out to check the validity of the interpreted subsurface electrical structures. Interpreted layered models from each method show a good agreement. Obtained models show that a conductive zone exist below the shallow resistive zone. Conductive zone, which is considered to be influenced by decomposition of organic waste materials and infiltration of precipitation, is terminated by resistive zone which is considered as basement. Considering the fact that conductive zone extends to the basement and there exist no barrier layers such as clay layers, contaminant plumes are likely to flow into the groundwater directly.

• Geophysics and Geophysical Exploration
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• v.5 no.4
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• pp.262-271
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• 2002

This study presents a practical procedure for the Bayesian inversion of geophysical data. We have applied geostatistical techniques for the acquisition of prior model information, then the Markov Chain Monte Carlo (MCMC) method was adopted to infer the characteristics of the marginal distributions of model parameters. For the Bayesian inversion of dipole-dipole array resistivity data, we have used the indicator kriging and simulation techniques to generate cumulative density functions from Schlumberger array resistivity data and well logging data, and obtained prior information by cokriging and simulations from covariogram models. The indicator approach makes it possible to incorporate non-parametric information into the probabilistic density function. We have also adopted the MCMC approach, based on Gibbs sampling, to examine the characteristics of a posteriori probability density function and the marginal distribution of each parameter.

• Economic and Environmental Geology
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• v.29 no.3
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• pp.345-355
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• 1996

The effectiveness of various electrode configurations in horizontal mappings and 1-D inversions of vertical sounding data for delineating 2-D structures was studied. Apparent resistivity values of three point, dipole-dipole, Wenner, and Schlumberger mappings were simulated for such structures as vertical dyke, tabular prism, buried vertical fault, ramp and complex structure by finite difference method (FDM) and they were compared with each other. Also 2-D cross sections for three structures obtained by interpolation of 1-D inverted sounding data in terms of three layers were compared for Schlumberger and Wenner arrays. On these cross sections, horizontal and vertical resistivity interfaces of the 2-D structures are revealed relatively clearly. Apparent resistivity curves of Schlumberger mapping show vertical resistivity discontinuities very well. On the whole, Schlumberger array is superior to the other arrays in electric sounding as well as mapping. This study clearly indicates that interpretations of 2-D structures based on 1-D inversion are possible.

• 한국지구물리탐사학회:학술대회논문집
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• 2008.10a
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• pp.47-50
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• 2008

Streamer resistivity surveys in shallow marine environments were carried out to analyze sediment characteristics at the bottom of reservoir. Because the resistivity values of reservoir water are very low and those of sediment are relatively high, apparent resistivity values increase with depth. And it is necessary to eliminate the apparent resistivity data decreased highly when the number of separation increases. According to the repeated data processing, we proposed the resistivity ratio of upper-to-lower layer is $0.6{\sim}0.8$ because the RMS error of inversion leads to the minimum in these range. As a result of the inversion for two- and three-layer model, the inversion including water depth is proved to be more effective than conventional method.

• Journal of the Korean Society of Groundwater Environment
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• v.5 no.1
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• pp.1-9
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• 1998

Schlumberger soundings and dipole-dipole electrical surveys were carried out in and around the Nanjido waste landfill in August and December, 1995. Survey points were set to be identical as those in August, 1994 and February, 1995 as possible. To elucidate the annual variation of resistivity structure in and around the landfill, 50 electrical soundings and 2 lines of dipole-dipole surveys were conducted. Interpretations of these data show that mean resistivity values become lower and thicknesses of contaminated layers by leachate thicker than those of the previous year in and around the landfill. Especially, mean thicknesses of saturated layers with leachate increased by about 3∼6 m and resistivities of bedrock decreased. Considering actual hydraulic conductivity, such increments of mean thicknesses are somewhat large. But resistivity variations in and around the Nanjido landfill clearly indicates contamination of layers by leachate is in progress even though some errors in measurements and interpretations are considered. On the other hand, sounding data in the back area of the landfill are almost identical to those of the previous year. From these results, it appears that contamination of weathered zone and bed rock is in progress mainly inside and in the front area of the landfill.

• Geophysics and Geophysical Exploration
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• v.10 no.2
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• pp.138-146
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• 2007

This paper describes 2.5D induced polarization (IP) modeling and inversion algorithms using complex resistivity. The complex resistivity method has merits for acquiring more valuable information about hydraulic parameters and pore fluid than the conventional IP methods. The IP modeling and inversion algorithms are developed by allowing complex arithmetic in existing DC modeling and inversion algorithms. The IP modeling and inversion algorithms use a 2.5D DC finite-element algorithm and a damped least-squares method with smoothness constraints, respectively. The accuracy of the IP modeling algorithm is verified by comparing its responses of two synthetic models with two different approaches: linear filtering for a three-layer model and an integral equation method for a 3D model. Results from these methods are well matched to each other. The inversion algorithm is validated by a synthetic example which has two anomalous bodies, one is more conductive but non-polarizable than the background, and the other is polarizable but has the same resistivity as the background. From the inverted section, we can cleary identify each anomalous body with different locations. Furthermore, in order to verify its efficiency to the real filed example, we apply the inversion algorithm to another three-layer model which includes phase anomaly in the second layer.