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Sensitivity Analysis and Estimation of the Depth of Investigation in Small-Loop EM Surveys  

Song Yoonho (Geophysical Exploration and Mining Division, Korea Institute of Geoscience and Mineral Resource)
Chung Seung-Hwan (Geophysical Exploration and Mining Division, Korea Institute of Geoscience and Mineral Resource)
Publication Information
Geophysics and Geophysical Exploration / v.5, no.4, 2002 , pp. 299-308 More about this Journal
Abstract
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.
Keywords
small-loop EM survey; Sensitivity; Depth of investigation; In-phase component;
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