Pseudo Resistivity Section By VLF-EM Method With a Single Frequency

Electromagnetic waves with single or limited frequencies from VLF transmitters generate secondary induced components of magnetic field, which are used to detect localized changes in electrical conductivity contrasts. This method, so-called VLF-EM, has been the powerful tool for mapping subsurface geological structures because of its low cost and short survey terms compared with electrical resistivity survey. 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. The VLF-EM data set was obtained numerically on a synthetic model. The cross section of NFG values derived from horizontal component of magnetic field clearly indicates high peaks at edges of a low resistivity anomaly buried below the surface. The peak of NFG values from the vertical component correspond with the centre of the anomaly. On the basis of the results, we estimated the pseudo-section of apparent resistivity from the VLF-EM data weighted with the NFG values at each depth. 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. We conclude that our simple technique give an approximate subsurface resistivity structures quickly, which is useful for geological interpretations.