The VLF method is suitable to address problems in which the target is a conductive sheet-like feature that is more or less vertically-oriented and found in a resistive host rock. A suitable application is mapping of faults and fracture zones in cristalline basement. Generally speaking, the ideal depth of investigation is 10-100 m, but this depends of course on the actual conductivity of target and host.
Reference: McNeill, J.D. and V. Labson, 1991, Geological mapping using VLF radio fields. Chapter 7 in Electromagnetic Methods in Applied Geophysics, Volume 2. Edited by M. N. Nabighian.