Pseudo resistivity cross section imaging using VLF-EM data

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 working hours. However, it has not been tested to estimate a pseudo-resistivity section, both the apparent resistivity and the depth of the 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 field data, is applied to the magnetic components at the surface in order to estimate the information of subsurface pseudo resistivity below the survey area. A VLF-EM data set is first created numerically for a synthetic model including a region of low resistivity anomaly buried below the surface. The cross section of NFG values derived from the horizontal components of magnetic field clearly indicates high peaks at the edges of the resistivity anomaly. The peak of NFG values from the vertical component correspond with the center of the anomaly. We then estimated the pseudo-section of apparent resistivity from the VLF-EM data using the NFG values of the horizontal and the vertical magnetic fields. It was finally confirmed that the weighted apparent resistivity values probably become lower in the vicinity of low resistivity anomaly than the surrounding area, although the estimated value is a little higher than the true resistivity value. This method is also applied to the noisy synthetic VLF-EM data with additive Gaussian noise. We conclude that our simple technique gives approximate subsurface resistivity structures in a quick manner, and is useful for geological interpretations.