Equivalent images derived from very-low-frequency (VLF) profile data

Abstract

We describe the implementation of a new fast imaging technique for filtering very-low-frequency (VLF) data measured on profiles into corresponding equivalent current systems in the earth. Single-frequency VLF data using magnetic measurements alone are often used to delineate lateral changes in electrical conductivity, e.g., fracture zones in crystalline terrains or changes in lithology in the sedimentary cover. Here, an attempt is made to add depth information to the conductivity distribution by realizing that the single-frequency VLF profile data contain information about (1) the background medium through their decay away from the conductors, (2) the position, and (3) the depth of the dominating conductors through the relative contribution of in-phase and quadrature components to the VLF anomaly in addition to the rate of change of the anomaly close to the conductors. Synthetic data from a model containing a shallow and a deeper conductor are filtered to show that the estimated current distributions coincide well with the horizontal position of the conductors, but even they provide some smeared images of the depth distribution of the conductors. A comparison with models obtained from regularized inversion of the same data shows good correspondence. The VLF field data from an area with clay lenses overlying wet sand and crystalline basement are filtered into current distributions that grossly mimic the electrical conductivity distribution of the clay lenses as obtained from radiomagnetotelluric measurements along the same profile.