Minerals geophysics Shallow or deep? Gravity and magnetic potential field anomalies have a maximum theoretical source body depth. However, a plethora of other source body configurations is possible at shallower depths. That is, a sharply defined anomaly must have its source body near surface, but a broader anomaly, although conventionally attributed to a deeper source body, may actually have its source at shallower depths. In mineral geophysics, such source body properties and configurations will be constrained by mineralogically appropriate density contrasts and magnetic susceptibility values, and geologically realistic geometric distributions. To see how equivalent source body configuration might change with depth, I first calculated the gravity response at surface for a prismatic source body topped at 50 m depth, then inverted the resulting gravity anomaly but constrained the modelled source body to be within the first 20 m below surface. The two bodies are shown in Figure 1. Not surprisingly, the domed shape of the shallow modelled body somewhat mimicked that of the gravity anomaly itself. Is this shape geologically reasonable? Absolutely. We’re dealing with density contrasts not absolute densities here, so this shape could easily represent a localised palaeotopographic basement high beneath younger less dense transported cover, or the localised shallowing of a base-of-weathering profile (weathered material is less dense). So, if you’re drilling a gravity target and encounter bedrock/basement much shallower than expected, you might have already explained your anomaly. However, unless you have a good understanding on the surrounding depths to bedrock/ basement and a localised shallowing is indicated, I wouldn’t abandon the drilling. In contrast to positive gravity anomalism, consider the response of a localised deepening of the weathering profile, or of a depression in the palaeotopographic profile, i.e. negative gravity anomalism. In strongly weathered environments, primary sulphide mineralisation is often characterised by associated enhanced weathering and by a localised deepening of the weathering profile, both of which would create localised negative gravity anomalism. Superimposing this weathering-related gravity low over the expected positive gravity anomaly from deeper massive primary sulphides will seriously downgrade, distort or even obliterate the positive gravity response (see Figure 2). Perhaps this explains why some primary sulphide mineralisation Terry Harvey Associate Editor for Minerals geophysics terry.v.harvey@glencore.com.au Figure 1.  Equivalent gravity anomaly source bodies. Figure 2.  Weathered mineralisation gravity anomalies.  Minerals geophysics 38 PREVIEW JUNE 2019