Constraints on interpreting magnetic spectral depths

Abstract

It is now possible to automate the extraction of magnetic depths over large areas as depth profiles. A depth profile is a graph of the probability of a layer at each depth. Presented in the form of a transect, depth profiles allow layers to be traced across significant distances. The appearance of discontinuous layers, and multiple layers, raises questions for interpretation, here addressed with modelling. Modelling of layers requires simulating the heterogeneity of the material. Accordingly, a method of modelling is demonstrated where flat prisms are populated with very large numbers of dipoles and their fields accumulated for spectral analysis. Thick layers give a depth signal in the transects about 20 m below their top surface. The distinction is minor given that the layer is assumed to extend across a 20 km square. In general, only one depth signal credibly represents the depth of its source. Multiple layers can be picked out on traverses when the deeper layer is sufficiently more magnetised than the layer above it. A weaker depth signal appears closer to a stronger signal. Signals within 100 m of each other tend to merge. The sensitivity of the method is significantly better when the survey has been flown north-south rather than east-west. Near-surface layers are not picked up by the method. The sampling frequency of the original survey dictates how close to the surface estimates can be provided. A rough rule of thumb is that no reliable depth estimates can be expected for sources shallower than half the flight line spacing.