It is now a well-established, unavoidable cliché, introducing many presentations over the last decade, that mineral deposit discovery rates are declining and geophysical methods are becoming ever more essential in finding deposits beneath cover. It is important therefore that those working in the minerals sector are able to make a basic evaluation of geophysical data to determine whether survey results are robust and interpretations are justified.
The following questions should be at the forefront of any geoscientists mind when presented with a survey proposal, results or an interpretation.
- Can geophysics identify the target?
All deposits are different. Host lithology, mineralisation, alteration, geometry, depth, overburden and the water table all have variable physical properties. Any given geophysical method can only detect anomalies where there is a strong contrast in those properties between target and background. Some geophysical methods are far better suited to certain deposits than others, so has there been careful consideration and selection of the technique?
- Has the data been collected appropriately?
Given a suitable geophysical method, survey parameters still need to be chosen that will give the spatial resolution, depth penetration and sensitivity to detect target anomalies. Sampling density, spatially or in the time and frequency domains, will all affect the ability to resolve target anomalies. Look for survey reports that detail field procedures and demonstrate how/why they have been used. What QAQC procedures were used during acquisition?
- How is the data presented?
Does the survey report detail how data has been processed? Filtering can smooth noisy data and highlight regional trends, or enhance small, localised anomalies. Filtering can also remove real geological anomalies if not carefully applied. Careless interpolation/gridding can mask areas of missing data and create false anomalies that are emphasised by poor choice of colour scale.
- Is the interpretation valid?
Three-dimensional models are great visually, but inversion of geophysical data is only useful if it produces geologically sound models that match the observed data to within a satisfactory error limit. Is the interpretation realistic, overcomplicated or maybe just too spurious to see in the maps/sections? The best interpretations will have been made by a collaboration of geologists and geophysicists using not just the geophysical survey data, but an integration of all the data available for the survey area.
- Beware voodoo geophysics!
Although rare, every now and then a new or supposedly ground-breaking technology comes along that could be described as “voodoo geophysics” (Hodges, 2005). Sadly, geophysical technology has advanced quite slowly over the decades, so great leaps forward, fantastic levels of instrument sensitivity, phenomenally accurate interpretations, or new secret proprietary technologies may not necessarily be legitimate. Enthusiastic overselling of conventional methods, misguided promotion by those who do not understand the physical principles or limitations of a method, or blatant scams – they are all unfortunately out there. Be inquisitive - if it sounds too good to be true, ask a geophysicist who can tell you if it is!
Geophysics is not “black box” technology to be wary of. With an informed approach to the use of geophysics, we have made some great mineral discoveries and will continue to do so as we are forced to explore deeper.
References:
Hodges, G., (2005), Voodoo Methods: Dealing with the Dark Side of Geophysics, Symposium on the Application of Geophysics to Engineering and Environmental Problems Proceedings : 315-327
