A method for calculating equivalent layers corresponding to large aeromagnetic and radiometric grids

Abstract

It is commonly known that, for every magnetic field, whether observed on a plane surface or not, a distribution of magnetisation can be calculated on a surface below the surface of observation such that the magnetic field produced by the distribution duplicates the observed magnetic field. The distribution of magnetisation, which does not necessarily have any physical reality, is referred to as the "equivalent layer". The value of the equivalent layers is such that, once one has been calculated for an observed magnetic field, it can be used to recompute fields corresponding to the equivalent layer at other magnetic inclinations, on other surfaces of observation or on arbitrary spatial grid intervals. Equivalent layers can thus be used as a basis for reduction to the pole, elevation corrections and gridding algorithms. Equivalent layers can also be calculated for airborne radiometric data. The same properties apply to radiometric equivalent layers as apply to magnetic equivalent layers, with the added significance that a radiometric equivalent layer corresponding to the ground surface gives a mapping of the actual radioelement concentrations corresponding to the airborne observations. Accurate, stable, iteration-based algorithms can be used to calculate magnetic and radiometric equivalent layers corresponding to large grids of aeromagnetic and radiometric data.