Improvements in magnetic depth estimation: application of depth and width extent nomographs to standard depth estimation techniques

Abstract

We describe an example of a common geological scenario which results in magnetic bodies of limited depth extent. We then present examples of the errors in depth estimates, as high as 40% or more, incumbent in the use of many classic magnetic depth techniques when considering bodies of limited depth extent. Then using interpretation by characteristics applied to generalized dyke models we develop correction nomographs for a wide variety of classic magnetic depth techniques. We show the improvements in depth estimation that are obtained using our nomographs when applied to the ideal dyke model as well as a conceptualized passive margin cross-section model. We use the principle of isostatic equilibrium to develop one simple methodology for estimating magnetic crustal thickness in the absence of a priori information. We then demonstrate the effectiveness of these techniques by application to the 3D Bishop magnetic model. We show that by using reasonable geological assumptions one can develop acceptable first-pass estimates of the maximum thickness of magnetic bodies, which are key to accurate depth estimates. Using estimates of crustal thickness and applying appropriate thickness corrections based on the developed nomographs, errors of up to 40% in depth estimation can be reduced to a few per cent under ideal assumptions of isolated bodies and absence of noise.