• 한국지구물리탐사학회:학술대회논문집
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• 2006.06a
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• pp.3-6
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• 2006

There are three types of frequency-domain loop-loop EM induction method, depending on the loop separation and their location relative to the ground surface: horizontal-loop EM (HLEM), fixed small-loop EM, and helicopter-borne EM (HEM) methods. Multidimensional inversion provides tomographic images of the subsurface resistivity structure and thus enhances the interpretational accuracy of loop-loop EM data. HLEM method is shown to be effective for exploring groundwater resources in weathered and fractured crystalline basement terrains in semi-arid regions. Also, HEM method is useful for locating weak zones in landslide areas. The applicability of inversion to small-loop EM data depends solely on the S/N ratio. The quadrature response of small-loop EM data can only give the equivalent conductivity of a homogenous half-space model, and thus the in-phase component is essential in inverting EM data. However, the in-phase response is much lower and decreases more rapidly with decreasing frequency than the quadrature response. Further work is needed to obtain conductivity-depth images from small-loop EM data.

• 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.

• Geophysics and Geophysical Exploration
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• v.6 no.4
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• pp.207-214
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• 2003

To analyze soil properties with depth in rice field, we compared resistivity distributions obtained from soil analysis with one dimensional inversion of small loop electromagnetic (EM) data. Although it didn't show consistency exactly between the two resistivity distributions, low resistivity zones in soil analysis, appeared to agree with low resistivity zones in EM result. Therefore, small loop EM method can be applied to obtain rapidly the soil properties such as salt accumulation in a rice field. If research on soil property and EM responses of unsaturated zone would be conducted consistently, small loop EM method can be used effectively to detect salt accumulated zone in agricultural area.

• 한국지구물리탐사학회:학술대회논문집
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• 2007.06a
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• pp.245-250
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• 2007

The small loop EM method is a fast and convenient geophysical tool which can give shallow subsurface resistivity distribution. It can be a useful alternative of resistivity method in conductive environment. We applied the multi-frequency small loop EM method for the investigation of a soft ground landfill site which was constructed on a tideland since the resistivity of the survey area is extremely low. 3D resistivity distribution was obtained by merging 1D inversion results and shallow subsurface structure can be interpreted. By comparing the result with the drilling log and measured soil resistivity sampled at 16 drill holes, we can get lot of information such as groundwater level, thickness of landfill, salinity distribution, depth to the basement and etc.

• Geophysics and Geophysical Exploration
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• v.14 no.2
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• pp.152-157
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• 2011

The small-loop electromagnetic (EM) method is one of the rapid and non-destructive geophysical methods and has been used widely for many geophysical investigations, particularly for shallow engineering and environmental surveys. Especially in the shallow marine environment, the small-loop EM technique is very effective because of rapid and convenient data acquisition, large signal and low noise level. However, the method has been rarely applied in the very conductive marine environment since it's penetration or investigation depth might be considered too low. In this study, we demonstrated that the small-loop EM method can be effectively applied in the extremely conductive marine environment through the analysis of 1D small-loop EM data. Furthermore, we confirmed that the resistivity distribution under the sea bottom can be quantitatively predicted from the 1D inversion results of synthetic and field data.

• Journal of Korean Society of societal Security
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• v.2 no.3
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• pp.39-46
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• 2009

A small loop-loop multi-frequency electromagnetic(EM) induction method is a useful technique to map a resistivity distribution efficiently and non-destructively. However, for quantitative interpretation and depth sounding, the quality of measured data is crucial. In this paper, we propose a bias correction of measured data by using background noise measurements to obtain reliable data, and propose an evaluation technique of apparent that can provide a resistivity image easily. We have performed small loop-loop EM measurements to detect water saturation in a man-made test site. The application of our proposed techniques to the measured data was successful.

• 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.10 no.4
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• pp.308-313
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• 2007

The small-loop electromagnetic technique has been used successfully for many geophysical qualitative investigations, particularly for shallow engineering and environmental surveys. Recently, various geophysical imaging methods based on numerical modeling and inversion have been tried in order to get more quantitative subsurface structure. However, conventional 2.5D small loop EM modeling takes a lot of time because responses should be calculated for several wave numbers and transformed into space domain. In this study, we developed a 2.5D HCP small loop EM modeling algorithm using extended Born approximation, which does not require transformation. Also, we checked its validity by comparison with other numerical results.

• Geophysics and Geophysical Exploration
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• v.5 no.4
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• pp.299-308
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• 2002

We have derived an analytical expression for the sensitivity of the frequency domain small-loop electromagnetic (EM) surveys over a two-layer earth in order to estimate the depth of investigation with an instrument having the source-receiver separation of about 2 m. We analyzed the sensitivities to the lower layer normalized by those to the upper half-space and estimated the depth of investigation from the sensitivity analyses and the mutual impedance ratio. The computational results showed that the in-phase components of the sensitivity to the lower layer dominates those to the upper layer when the thickness of the upper layer is less than 20 m, while the quadrature components are not sensitive to the lower layer over the entire frequency range. Hence we confirmed that the accurate measurement of the in-phase component is essential to increase the depth of investigation in the multi-frequency small-loop EM survey. When conductive basement of 10 ohm-m underlies the upper layer of 100 ohm-m, an accurate measurement of the in-phase components ensures the depth of the investigation more than 10 m even accounting a noise effect, from which we conclude that the small-loop EM survey is quite effective in imaging the conductive plume down to a considerable depth. On the other hand, in the presence of the resistive basement of 1,000 ohm-m, the depth of investigation may not exceed 5 m considering the instrumental accuracy, which implies that the application of the small-loop EM survey is not recommended over the resistive environment other than detecting the buried conductor.

• Geophysics and Geophysical Exploration
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• v.6 no.3
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• pp.119-125
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• 2003

The small-loop electromagnetic (EM) technique has been used successfully for many geophysical investigations, particularly for shallow engineering and environmental surveys. In conventional small loop EM operating at small induction numbers, geometric sounding has been widely used because the depth of penetration of EM energy depends only on the source-receiver separation. Recently developed small loop EM system, however, measures the secondary magnetic field, $H^S$, at multiple frequencies with a fixed source-receiver separation and frequency sounding is tried actively. In this study, we analyzed the behavior of in-phase and quadrature components of ${H^S}_z$, for horizonal coplanar (HCP) configuration over two-layer models. Through this theoretical analysis, it was found that the in-phase component of ${H^S}_z$ is more suitable for frequency sounding than the quadrature component. But, the in-phase component of ${H^S}_z$ is too small to measure, especially in resistive and noisy environment like Korea. Using the fact that the quadrature component is much greater than the in-phase component and the difference of quadrature component of ${H^S}_z$ measured at two frequencies shows the same behavoir as the in-phase component, we suggested an alternative frequency sounding technique. Also, we defined an apparent conductivity, which reflects well the conductivity of subsurface layers.