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| | Abstract | | VLF-EM method has been the powerful tool for mapping subsurface geological structures because of its low cost and short survey terms. However, it has not been tested to estimate a pseudo-resistivity section, both the apparent resistivity and the depth of conductive anomaly by using the measured magnetic components with a single frequency. In this study, the Normalized Full Gradient (NFG) method, generally used for the downward continuation of the potential filed data, was applied to the magnetic components at the surface. We confirmed that the weighted apparent resistivity values are lower in the vicinity of low resistivity anomaly than the surrounding area, although the estimated value is a little higher than the original value. Also, the phase-shift method, which is used in seismic migration, is applied to the horizontal magnetic components with multiple frequencies. The synthetic example shows that the migration method is effective for imaging the anomaly. We conclude that our simple technique give an approximate subsurface resistivity structures quickly, which is useful for geological interpretations and also for an initial model of the fullwave form inversion. |
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