Analysis and 3D visualisation of new compilations of potential field data to help constrain 3D models

The accuracy of broad-scale geological models and interpretations derived from gravity and airborne magnetic data is dependent upon long wavelength components of the data, which are in turn dependent upon the accuracy with which separate datasets are merged into single compilations. For magnetic data, a newly developed method (Minty, 2000) reduces this complex procedure to a single least-squares problem involving adjustments to all grids. The method has been tested using independent datasets for several regions across Australia (Milligan et al., 2001). For gravity data, Murray (1998) developed optimal gridding. Examples of derived potential field products and enhanced images are presented here for new compilations of airborne magnetic and gravity data for the north-eastern Gawler Craton (NEGC) in South Australia. In one example, directional filtering removes anomalies of the Gairdner Dyke Swarm, considerably enhancing features in the Gawler Range Volcanics. Application of automatic trend analysis and edge detection to merged compilations of potential field data are useful techniques for producing unbiased estimates of sharp lateral changes in physical properties of rocks (e.g. Blakely & Simpson, 1986; Phillips, 1997). A new method of edge detection (Archibald et al., 1999) for the analysis of magnetic and gravity data uses wavelet theory to identify points lying on the horizontal gradient maxima. Points calculated from these methods are aligned along the gradients, and have the appearance of continuous "worms". When the points generated for many levels of upward continuation of the original data are analysed in 3D, the worms provide information about the strength of the gradients, the localities of source-body edges, their depths, and their dip directions. For automatic analysis, it is more convenient if the worm points are converted into sets of vectors, or, further, into 3D surfaces (sheets) for each level of continuation. Examples are shown for the NEGC and for the Arunta area of the Northern Territory, where, in some cases, the orientation of discrete physical property contrasts are mapped. A useful analytical technique is to compute the best-fitting straight line for those worm vectors that have a reasonable linearity, and to plot rose diagrams of the orientation of the vectors. The results may be displayed either by their lengths and directions or as histograms of their directions. Discrete areas can be windowed separately to show how dominant trend directions change in different geological settings. Rose diagrams calculated for many levels of upward continuation may show how fracture sets vary vertically, both with changing dip and strike directions. Visualisation of this information provides useful constraints on 3D geological models. An example is the mapping of structural geology of basement for the NEGC.