A case study on geophysical gridding techniques: INTREPID perspective

Abstract

A wide variety of numerical procedures in potential field geophysics require data modelled on a regular grid. However, airborne data tend to be highly sampled along the flight line and sparsely sampled in the perpendicular direction. A gridding method commonly called 'bi-cubic spline' is widely used in potential field geophysics. Standard bi-cubic spline methods used on aeromagnetic data produce artefacts when a geological feature's 'line of strike' is not perpendicular to the direction of the acquisition line. This method has a tendency to break up thin elongated magnetic anomalies, at an oblique angle, into a series of bulls eye artefacts. A method of finding local anomalies and their strike along lines based upon minimum variance principles reduces these effects. This technique has significant impact on the quality of output grids. In association with the Magnetic Image Project (MAGMAGE) developed by Gunn and collaborators, that involved work on complex attributes of aeromagnetic anomalies, the gridding of phase posed some unique problems. Raw phase is a spiralling function which is periodic and cyclic. Unwrapping of the phase, therefore, is necessary to give a spatially coherent grid for interpretation. By focussing on two developments in gridding ? trending in a bi-cubic spline method and unwrapping of cyclic data ? these methods are shown to increase the accuracy of representation of actual data being interpolated. Case studies of these solutions are presented using the INTREPID geophysical processing and visualisation system.