• Geophysics and Geophysical Exploration
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• v.13 no.4
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• pp.370-377
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• 2010

In this study, we investigated the current saturation which forces the apparent resistivity to converge when the conductivity contrast between the anomalous body and background medium is greater than a specific value. Analizing theoretical and numerical solutions for some simple models, we studied the behavior of the surface charge, and how the surface charge cause the current saturation and finally lead to the convergence of the apparent resistivity in the resistivity method. As a consequence of above analysis, we verified that the current saturation makes the apparent resistivity converge to a specific value and the magnitude of the apparent resistivity anomaly be less than that of the ideal conductor or insulator in the resistivity method. In general, current saturation is considered to occur when the conductivity contrast becomes larger than 100.

• Geophysics and Geophysical Exploration
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• v.5 no.3
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• pp.199-205
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• 2002

In the resistivity method, the potential difference between two grounded electrodes is measured and this can be positive or negative. The apparent resistivity and the potential difference have the same polarity. Since the electric field is the gradient of the potential, the polarity of the potential difference depends on the direction of the electric field. If the direction of the vector connecting two grounded electrodes is the same to that of the electric field, the measured potential difference and the apparent resistivity become positive. If the opposite is the case, they become negative. In general, the primary electric field and the vector connecting two potential electrodes have the same direction in a surface resistivity method. In this case, the measured potential difference is always positive because the primary electric field is greater than the secondary field. Therefore, the apparent resistivity is always positive if noise is free and topography is flat. The secondary field component, however, can be greater than the primary field component along the vector connecting two potential electrodes in the cross-hole resistivity method. Furthermore, if the secondary electric field and the vector connecting two potential electrodes have an opposite direction, the apparent resistivity become negative. Consequently, the apparent resistivity may be negative in the region where the primary electric field component along the vector connecting two potential electrodes is very small.

• Proceedings of the KSEEG Conference
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• 2002.04a
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• pp.110-112
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• 2002

• Journal of the Korea institute for structural maintenance and inspection
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• v.22 no.6
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• pp.164-169
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• 2018

In this study, the effect of the size of the specimen on the apparent resistivity was investigated at the laboratory level for electrical resistivity. The specimens were measured for apparent resistivity by fabricating specimens with different sides and heights. Experimental results show that the apparent resistivity increases as the side and height of the specimen become smaller. Also, it was confirmed that the influence of the size of the specimen on the electrical resistivity measurement was not linear.

• 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.

• Journal of the Korea institute for structural maintenance and inspection
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• v.23 no.6
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• pp.113-119
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• 2019

This study aims at the analysis using the Resistivity Estimation Model (REM) to examine the effect of specimen size on the measurement of electrical resistivity. In the experiment, specimens of concrete were fabricated and the apparent resistivity was measured for each electrode interval. The apparent resistivity measured was found to be distorted in the apparent resistivity as the specimen size became smaller and closer to the outside (edge). As a result of comparing the experimental and analysis values, it is expected that REM can be used to examine the effect of the size of the specimen.

• Journal of the Korea Institute of Building Construction
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• v.23 no.5
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• pp.519-526
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• 2023

This research endeavors to explore the nuances in apparent resistivity readings in concrete specimens due to the proximity of embedded reinforcement. To systematically gauge this, concrete samples incorporating singular and paired rebars were meticulously crafted. These rebars were strategically positioned at intervals of 0.03m, 0.04m, and 0.05m from each specimen's midpoint. Subsequent resistivity assessments were conducted at 0.01m increments up to the predetermined rebar location for each sample. A consistent observation was the nadir in apparent resistivity manifesting at the rebar's epicenter. Notably, dual-rebar configurations registered lower resistivity values at this central juncture compared to their single-rebar counterparts. This metric underscores the palpable impact of surrounding reinforcement on resistivity readings. Further, as the spatial separation between rebars increased, the distinctness in their locational identification via resistivity became increasingly pronounced.

• Geophysics and Geophysical Exploration
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• v.5 no.2
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• pp.108-117
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• 2002

We have introduced a new approach to obtain the conductivity information of subsurface using Cagniard impedance over two-dimensional (2-D) model in the presence of horizontal magnetic dipole source with the frequency range of $1\;kHz\~1\;MHz$. Firstly, we designed the method to calculate the apparent resistivity from the ratio between horizontal electric and magnetic fields, Cagniard impedance, considering the source effects when the plane wave assumption is failed in finite source EM problem, and applied it to several numerical models such as homogeneous half-space or layered-earth model. It successfully provided subsurface information even though it is still rough, while the one with plane wave assumption is hard to give useful information. Next, through analyzing Cagniard impedance and apparent resistivity considering source effect over 2-D models containing conductive- or resistive-block, we showed that the possibility of obtaining conductivities of background media and anomaly using this approach. In addition, the apparent resistivity considering source effect and phase pseudosections constructed from Cagniard impedance over the isolated conductive- and resistive block model well demonstrated outlines of anomalies and conductivity distribution even though there were some distortions came from sidelobes caused by 2-D body.

• Korean Journal of Materials Research
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• v.7 no.12
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• pp.1033-1040
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• 1997

Si가 첨가된 ZNR의 겉보기 유전상구, 겉보기 비유전손율, 유전비저항의 주파수 특성을 여러 측정온도에서 조사하였다. 모든 ZNR에 대하여 유전분산현상 및 유전흡수현상이 뚜렷이 나타났으며, 비슷한 경향의 유전성질을 나타냈다. SiO$_{2}$첨가량이 증가함에 따라 겉보기 유전상수, 피이크 겉보기 비유전손율은 감소하였으며, 유전비저항은 증가하였다. 온도상승시 겉보기 유전상수, 피이크 겉보기 비유전손율은 증가하였으며, 흡수 피이크 주파수는 고주파측으로 이동하였다. SiO$_{2}$첨가량이 증가함에 따라 ZNR은 Cole-Cole원호에서 0.68-0.72범위 내에서 증가하는 $\beta$값을 가지며, 완화시간의 분포가 좁아지는 유전특성을 나타냈다.

• Geophysics and Geophysical Exploration
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• v.15 no.2
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• pp.75-84
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• 2012

The sea layer in marine Controlled-Source Electromagnetic (mCSEM) survey changes the conventional definition of apparent resistivity which is used in the land CSEM survey. Thus, the development of a new algorithm, which computes apparent resistivity for mCSEM survey, can be an initiative of mCSEM data interpretation. First, we compared and analyzed electromagnetic responses of the 1D stratified gas hydrate model and the half-space model below the sea layer. Amplitude and phase components showed proper results for computing apparent resistivity than real and imaginary components. Next, the amplitude component is more sensitive to the subsurface resistivity than the phase component in far offset range and vice versa. We suggested the induction number as a selection criteria of amplitude or phase component to calculate apparent resistivity. Based on our study, we have developed a numerical algorithm, which computes appropriate apparent resistivity corresponding to measured mCSEM data using grid search method. In addition, we verified the validity of the developed algorithm by applying it to the stratified gas hydrate models with various model parameters. Finally, by constructing apparent resistivity pseudo-section from the mCSEM responses with 2D numerical models simulating gas hydrate deposits in the Ulleung Basin, we confirmed that the apparent resistivity can provide the information on the geometric distribution of the gas hydrate deposit.