3D Inversion of Magnetics at the Raglan Deposit
Interpretations

The variable for the inversion is susceptibility contrast and the recovered model is shown on this page in various ways. Inversion constructs 3D models of susceptibility contrast, so interpretation requires an appropriate display for studying the resulting structures. A 3D cube of susceptibilities can be displayed in cross-sectional cuts or as a 3D-isometric image after volume rendering. Cross-section or plan-section slices have the advantage that values of the physical property in each cell are displayed. These are the numbers which reproduce the field data. To the left is an example of a plan-section cut at a depth of 400 metres. High susceptibillity values are blue and low values are red. Click the image for an animated view of a series of such slices.

Recall that our objective function constructs smooth models, so discrete geological boundaries may appear as gradational changes, unless a particular value is used in the image, as in the iso-surface renderings.

One disadvantage of the cross-sections is that it is sometimes difficult to get a sense of 3D structure. This is where 3D volume rendered images have an advantage. But care must be taken since a different threshold for volume rendering will generate a different image. The next figure illustrates a volume rendering for which a susceptibility value of 0.04 was chosen to render the iso-surface.

The inversion basically recovered a single elongated magnetic body whose ends coincided with the known outcropping. It was an isosurface image like this one that persuaded the project geologist to spot a deep 1100 metre hole, which confirmed the inversion results that the apparently isolated outcrops on the so-called "5-8 ultramafic flow" and the "Katinniq flow" to the west were in fact connected at depth. The targeted magnetic source was intersected at 650 metres. As a bonus they intersected a 5 metre thick mineralized section (sub-ore grade, approximately 1% nickel) within the 350 metre thick intersection of magnetic ultramafics. The image supplied by Falconbridge is provided by this link (reduced to 800 x 429 pixels, or 46 kbytes).

© UBC-GIF  January 9, 2007

preproc.