Geophysical follow-up
In mineral exploration the prime aim is to locate economic mineralisation, that is, orebodies. Geophysical components of green fields mineral exploration programmes typically comprise an initial literature research for and appraisal of pre-existing data, followed by commission and execution of appropriate broad-scale geophysical surveying, and analysis and interpretation of these results. Then, if things have gone to plan, there will be geophysical follow-up surveys designed to better discriminate geophysical anomalies of interest prior to drill-testing. It is aspects of this final follow-up stage that I’d like to address here. To maximise our chances of success we need to site drill-holes in the best possible position to test anomaly sources.
So, what sort of targets might we be investigating in the final follow-up stage? Ideally these would manifest as discrete geophysical anomalies reflecting direct detection of the targeted mineralisation. In some situations, e.g., electrical geophysical surveys targeting zinc sulphides, we might be relying on indirect detection via responses from other directly associated conductive metallic sulphides. In other scenarios, it could be environments favourable to mineralisation that are being sought, such as fold closures, structural intersections, facies changes within favourable horizons, etc. And in most cases the broad scale geophysical survey should also contribute to the understanding of the geology of the area under investigation.
Before going further, we might consider whether we even need to undertake geophysical follow-up. If the geophysical survey results confirm other indications, such as the presence of gossans or anomalous rock or soil geochemistry, we might have sufficient reason and information to drill-test without further geophysical work. In the blind mineralisation scenario relying solely on geophysics, if our initial survey technique was uniquely appropriate to the target style sought, and sufficiently detailed to accurately discriminate the resulting anomalies, we might reasonably dispense with geophysical follow-up surveys. However, while the accuracy and capabilities of modern geophysical survey techniques and processing regimes have dramatically improved, this ideal scenario is unlikely in most circumstances.
Typically then we face the prospect of designing and implementing follow-up geophysical surveys to further investigate our geophysical responses. But, to what end? Two inter-related aims are relevant: to deduce the nature of the source material and to sufficiently delineate its disposition. These two aims should be considered in tandem, but I will treat them sequentially below for ease of expression.
Anomaly source material
Before we embark on detailed anomaly delineation, we might consider whether the source material for the anomaly is what we’re after. Conventional geophysical follow-up surveys typically employ a ground-based version of the broad-scale survey technique – if we used airborne electromagnetics initially, then ground electromagnetics is the de facto choice for follow-up, detailed ground gravity would follow airborne or semi-regional ground gravity, etc. But this doesn’t necessarily advance the understanding of the nature of the anomaly source material – the follow-up is still targeting the same petrophysical property. Improved knowledge of source disposition from a detailed ground survey may provide clues on the nature of the anomaly source material, but where this could reasonably be one of several options, we are going to need more information to minimise drill-testing of unwanted target material.
One approach would be to undertake blanket follow-up ground geophysics using a range of different geophysical techniques over all anomalies of interest, but this could be an expensive and time-consuming process. A staged approach, starting with an appropriate technique (or techniques) specifically designed to highlight anomalies of interest and/or eliminate spurious anomalies before fine-tuning targets makes more sense. So, some thought on what petrophysical properties could be used to discriminate between our target style and that of other spurious anomalies is required. Using the electromagnetics example, if we are after conductive sulphides, magnetics might help identify those sulphides carrying pyrrhotite, gravity might help to discriminate graphite from more massive sulphides, IP might separate sulphides from non-sulphidic source material such as porous crush zones, etc. Using the gravity example, if we are after massive sulphides, passive seismics might help to identify those gravity anomalies due to basement highs, electromagnetics might highlight those anomalies more likely to be massive sulphides, magnetics might help to identify those anomalies due to denser basic rocks, etc.
Anomaly source disposition
Having hopefully eliminated the spurious anomalies, we need to give some consideration to the design and extent of detailed geophysical ground follow-up. The specific aim here is to delineate the disposition of the anomaly source material for optimal siting of drill-holes. In modern geophysical exploration, this delineation is typically achieved with 3D inversion, so the parameters and extent of the follow-up survey should be appropriate. For potential field methods, station density to provide sufficient detail and survey extent to establish ‘background’ would be considerations. For electrical and electromagnetic methods, survey parameters would be an additional consideration. 3D inversion of IP-resistivity surveys, for example, will greatly benefit from a tailored survey design – inverting the results from a series of parallel dipole-dipole lines can be fraught where the geology is more complex than simple 2D at right angles to the lines.
Finally, having possibly used more than one ground geophysical technique over our target, co-inversion of the different results may improve the final model.
Good hunting!