Method for depth estimation on aeromagnetic vertical gradient anomalies

Abstract

The straight-slope technique introduced some years ago by Vacquier et al. (1951) is employed to develop simple empirical procedures that can be used to determine depth to the top/center of anomalous sources on measured aeromagnetic vertical gradient profiles. Five geologic bodies/structures in the form of their magnetic/geometric model equivalents, namely, point pole, point dipole, finite dipole, dipping dike, and dipping contact are considered. From analysis of the normalized theoretical curves due to those models it is observed that the horizontal projection of the straight part of the steepest sections of each curve is insensitive to changes in the inclination of the Earth's magnetic field and also to the dip angle of dipping models. Further analysis of the curves using this observation leads to the conclusion that, when dealing with the interpretation of observed vertical gradient profiles, the length of the horizontal projection on a given profile must be doubled to obtain depth to the point-pole, point-dipole, or finite-dipole source. For a geologic contact and a wide but shallow (i.e., the width more than twice the depth) dike, the length of the projection gives the depth for either source. However, a thin but deeply buried (i.e., the width less than twice the depth) dike, requires use of characteristic curves such as those developed in this study. Application of the procedures to observed vertical gradient results from the White Lake region of Ontario, Canada has proven quite successful.