• Geophysics and Geophysical Exploration
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• v.12 no.1
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• pp.33-40
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• 2009

In field surveys using the dipole-dipole electrical resistivity method, we often encounter negative apparent resistivity. The term 'negative apparent resistivity' refers to apparent resistivity values with the opposite sign to surrounding data in a pseudosection. Because these negative apparent resistivity values have been regarded as measurement errors, we have discarded the negative apparent resistivity data. Some people have even used negative apparent resistivity data in an inversion process, by taking absolute values of the data. Our field experiments lead us to believe that the main cause for negative apparent resistivity is neither measurement errors nor the influence of self potentials. Furthermore, we also believe that it is not caused by the effects of induced polarization. One possible cause for negative apparent resistivity is the subsurface geological structure. In this study, we provide some numerical examples showing that negative apparent resistivity can arise from geological structures. In numerical examples, we simulate field data using a 3D numerical modelling algorithm, and then extract 2D sections. Our numerical experiments demonstrate that the negative apparent resistivity can be caused by geological structures modelled by U-shaped and crescent-shaped conductive models. Negative apparent resistivity usually occurs when potentials increase with distance from the current electrodes. By plotting the voltage-electrode position curves, we could confirm that when the voltage curves intersect each other, negative apparent resistivity appears. These numerical examples suggest that when we observe negative apparent resistivity in field surveys, we should consider the possibility that the negative apparent resistivity has been caused by geological structure.

• The Journal of Engineering Geology
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• v.2 no.2
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• pp.113-130
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• 1992

Groundwater in crystalline basement is controlled primarily by tectonic fractures. It is evident that the delineation of the heavily faulted area and/or fractures deeply developped should be considerable value in deep-seated low enthalphy geothermal water. Electrical and electromagnetic methods have effectively been employed to map hydraulic faults and shear zones for groundwater exploration. In this study VLi; dipoledipole resistivity, controlled source audio~frequency magneto-telluric(CSAMT) and magnetic methods were applied in the Bomun resort area, adjacent to Kyongju city, southeastern part of Korea. The integrated geophysical tools employed in this experiment can be manifested themselves as: 1. Magnetic high for granite intrusions which is more favorable for geothermal gradient increase in depth. 2. VLF cross-over trends for mapping linear shallow conductive fractures and shear zones. 3. Dipole-dipole resistivity distributions for the deep-seated(less than 500m in depth) fractures and shear zones. The dipole-dipole resistivity field data were inverted to the true resistivity distribution with two-dimensional automatic inversion program based on the finite-difference method. 4. CSAMT provides an efficient way of delineating fractures and fault zones if the depth is greater than about 500m.

• Journal of the Korean Geophysical Society
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• v.1 no.1
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• pp.41-50
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• 1998

Schlumberger soundings, dipole-dipole survey and electrical conductivity mappings were carried out inside and in front of the entrance of the Okmyung waste landfill in August, 1997 and January, 1998. Inside and in front of the landfill, 11 and 4 electrical soundings and 1 dipole-dipole survey were carried out, respectively. Electrical conductivities were measured at 164 points along the 4 lines in front of the entrance of the landfill. Interpretations of survey data show that low resistivity zones of 0.3∼3 Ωm extend down to 65 m depth from the surface in the 6th landfill, which indicates subsurface contamination by leachate and leachate level at 3∼6 m depth from the surface. In the 9th landfill, low resistivity zones below 2 Ωm appear at 11∼15 m depth from the surface, which indicates a very slim chance of subsurface contamination. On the other hand, electrical surveys and electrical conductivity mappings reveal low resistivities at shallow depths in front of the entrance of the landfill, indicating a high possibility of contamination of weathered zone in this area. It appears that southern part of this area close to the 6th landfill is more contaminated by leachate.

• The Journal of Engineering Geology
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• v.29 no.3
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• pp.223-236
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• 2019

Resistivity surveys can identify the distribution of geological units and structures (including fragmented fault zones), the extent of weathered and modified geological strata, and the characteristics of groundwater. This study aims to analyze the underground sedimentary layers and geological structures near the Nari and Albong Basins of Ulleung-do, Korea, focusing on six survey lines to identify the spatial trends in subsurface resistivity. A modified dipole array method (D method) was employed, combining resistivity results obtained by existing dipole array methods (A and C methods). The modified method provides optimal analysis of the cross-section of underground resistivity, and shows a clear boundary between a low-resistivity zone (${\leq}500{\Omega}{\cdot}m$) of sedimentary layers and weak zones, and a high-resistivity zone (${\geq}5,000{\Omega}{\cdot}m$) of volcanic rock (trachyandesite). The estimated average thickness of the sedimentary layers is 50~100 m for the Albong Basin and 100~200 m for the Nari Basin. An anomaly zone, different from the weak zone in the bedrock, is identified as a caldera fault, and the low-resistivity zone extends from the surface down to the lowest survey depths.

• Journal of the Korean Geophysical Society
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• v.6 no.3
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• pp.155-164
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• 2003

The validity and efficiency of the roll-along technique widely used in 2-D electrical resistivity survey are analyzed in case of the dipole-dipole and the Wenner-Schlumberger arrays by numerical modelling. The shallow anomalous resistivity bodies are successfully inverted both in the dipole-dipole and in the Wenner-Schlumberger arrays because the shallow data of pseudosection are not omitted by the roll-along technique. However, the deep anomalous resistivity bodies can not be well resolved due to the skip of observed data which is more significant in the Wenner-Schlumberger array having relatively poor horizontal coverage of obtaining data. Carrying out electrical survey adopting the dipole-dipole array, the skip of data is insignificant because it is unfeasible to expand the electrodes to the maximum electrode separation coefficient($n_max$) owing to low S/N ratio. In case of the Wenner-Schlumberger array, however, because it is generally feasible to expand the electrodes $n_max$ to the owing to high S/N ratio, it is highly possible that skip of data from the roll-along technique causes significant distortion of inversion results. Therefore, adopting the Wenner-Schlumberger array having deeper median depth(Edwards, 1977) than do the dipole-dipole array on condition of the same unit electrode spacing( ($a$) ) and $n_max$, it is recommended to determine $a$ based on not $n_max$but $n_prob$free from the skip of observing data and forward electrodes with keeping overlap interval 3/4 of the survey line length in order to reduce the distortion of resistivity structure and perform resistivity survey efficiently. These results are confirmed by numerical modelling.

• Journal of the Korean Geophysical Society
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• v.2 no.3
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• pp.217-224
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• 1999

Although faults can be found by geological surveys, the surface traces of faults are not easily discovered by traditional geological surveys due to alluvia. In and around faults and fracture zones, the electrical resistivity appears to be lower than that of the surroundings due to the content of groundwater and clay minerals. Therefore, electrical resistivity surveys are effective to search buried faults and fracture zones. The dipole-dipole array electrical resistivity surveys, which could show the two dimensional subsurface electrical resistivity structure, were carried out in two areas, Yongdang-ri, Woongsang-eup, Yangsan-si, Kyungsangnam-do and Malbang-ri, Woedong-eup, Kyungju-si, Kyungsangpook-do. The one was next to the Dongrae Fault and the other near the Ulsan Fault was close to the region in which debatable quaternary fault traces had been found recently. From each measured data set, the electrical resistivity cross-section was obtained using the inversion program the reliability of which was analyzed using analytic solutions. A low resistivity zone was found in the inverted cross-section from the Yongdang-ri area survey data, and two low resistivity zones were found in that from the Malbang-ri area survey data. They were almost vertical and were 15∼20 m wide. Accounting the shape and the very low resistivity values of those zones (<100 Ωm)in the inverted section, they were interpreted as fracture zones although they should be proven by trenching. The reliability of the interpretation might be improved by adding some more parallel resistivity survey lines and interpreting the results in 3 and/or adding other geophysical survey.

• Journal of the Korean Geophysical Society
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• v.2 no.4
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• pp.259-268
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• 1999

Electrical resistivity surveys were conducted in the areas between Buji-ri and Seoak-dong, and between Nawon-ri and Yangdong-ri, Kyongju in order to investigate the geoelectric structure of the nothren part of the Yangsan Fault. In the area between Buji-ri and Seoak-dong south of Kyongju, the fracture zone east of the inferred fault develops more deeply, without significant north-south variation in depth, than west. In the area between Nawon-ri and Yangdong-ri north of Kyongju, the fault zone seems to be developed along the Hyungsan-river, and the resistivity structure west of the river is more affected by the fracture zone than east. Interpreted section of dipole-dipole survey conducted in Homyung-ri shows vertical contact of the Yangsan Fault. It appears that the boundary between the northern and central segment of the Yangsan Fault is located in the north of study areas since there is no significant variation in electrical resistivity structure near Kyongju.

• 한국지구물리탐사학회:학술대회논문집
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• 2006.06a
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• pp.73-78
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• 2006

To verify reinforcing effect of grouting materials on the water leakage region in dike, we performed various hydraulic test and we also applied electric resistivity survey including electrical resistivity tomography(ERT) to see resistivity variation before and after grouting. As the results of dipole-dipole array survey along dike, resistivity distribution after grouting was without noticeable spatial variation. Long term resistivity monitoring results at dike with Schlumberger array electric resistivity survey showed that the decreasing region of apparent resistivity and one dimensional inversion results were the occupied region by grout after grouting. From the ERT using check holes to inspect the effect of grouting, we could find that the ERT is quite effective to evaluate spatially grout region in dike.

• Geophysics and Geophysical Exploration
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• v.1 no.2
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• pp.92-100
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• 1998

Array CSAMT surveys were conducted in two areas where it was not easy to identify the presence of faults only with geological survey because of thick overburden. The purpose of these surveys were to locate the faults and to delineate the deep resistivity structures around the faults. The steep dip lineaments having high contrast in resistivity laterally and the low resistive zones having some width in the resistivity sections were interpreted as faults and fracture zones associated with faults, respectively, The good applicability of array CSAMT to the investigation of fault was recognized owing to the agreement between the interpretation results of array CSAMT and the conclusive evidences collected by the following geological survey. The evidences includes the recent exposure of fault and the trajectory of fault evidences of the survey line. A comparison of the applicabilities of array CSAMT method and the resistivity method using dipole-dipole array was presented with the results of both methods along a same traverse line.

• Geophysics and Geophysical Exploration
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• v.6 no.4
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• pp.187-194
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• 2003

A small-loop electromagnetic (EM) system using multiple frequencies has advantages in survey speed and cost despite of limitation on its depth of investigation. Therefore, small-loop EM surveys have been frequently used on various site investigations involving engineering and environmental problems. We have developed a subsurface imaging technique using small loop EM data. We used a one-dimensional (ID) inversion method to reconstruct a subsurface image from frequency EM sounding data. Tests using simulated data show that the method can reasonably recover the subsurface resistivity structure. Also, the method was tested on field data obtained with multiple frequency small loop EM system at a farm in Chunchon, Korea. The resistivity image obtained form field data compares favorably with the image from the dipole-dipole resistivity survey.