By Ignatius Kamwanje
Geophysics is a high tech, highly mathematical and specialized way of sampling rock formation within the earth’s crust. Interestingly it can also sample invisible things like resistivities, seismicity, electrical currents and magnetic response. This is because minerals have a lot of different properties including their ability to conduct electricity. These characteristics can actually be measured and sampled. The science of geophysics applies the principles of physics to the study of the earth. Geophysical investigations/surveys of the interior of the earth involve taking measurements at or near the earth’s surface that are inﬂuenced by the internal distribution of physical properties. Analysis of these measurements can reveal how the physical properties of the earth’s interior vary vertically and laterally. The geophysical tools used in mineral finding is much the same and to a layman, the resulting geophysical map looks like an illusion, weird, mysterious, nothing to interpret.
Geophysical data collection/Investigation/Survey
The collection of geophysical data is left to experts who can afford expensive geophysical equipment while shooting electrical currents into the ground so that other probes can read the current. There is a likely danger in the use of geophysical data however, because no one really knows what it is saying, since there is also an inherent danger in being attracted to the red hotspots(pseudo-anomalies). Inevitably, when looking at geophysical maps, the question becomes; where should drilling be carried out? And equally, the answer that comes back is the hotspot. Not because that is the best place to drill, but because it just looks like to. Geophysical survey is a very difficult temptation to fight. For example, radiometric surveys (one method of geophysics) were ﬂown in the Solomon Islands primarily for detecting phosphates in the 1960’s. A ground follow-up team sent to the Island expected to ﬁnd phosphatic rock. A careful analysis showed that there was very little phosphate, and instead the team found out that the source of the anomaly was uraniferous bauxite, in mineable quantities. However, geophysical surveying, although sometimes prone to major ambiguities or uncertainties of interpretation, it provides a relatively rapid and cost-effective means of deriving distributed information on subsurface geology. In exploring for subsurface resources, the methods are capable of detecting and delineating local features of potential interest that could not be discovered by any realistic drilling programme. The most commonly used geophysical survey methods include gravity, electromagnetic, magnetic, radiometric, electrical resistivity, Self-Potential, Induced Polarization, Seismic just to mention but a few. Airborne geophysical surveys, on the other hand, provides the quickest, and often the most cost-effective, ways of obtaining geological information about large areas. In some cases, airborne indications have been so clear and deﬁnitive that ground follow-up work was limited to deﬁning drill sites, but this is unusual. More often, extensive ground geological, geochemical, and geophysical surveys are required to prioritize airborne anomalies.
When is geophysical survey carried out?
An exploration company has a geological map showing all the rock types and the structures otherwise known as a basemap. Sometimes based on geochemical sampling the company can find some higher than expected values called an anomaly of a mineral. So a geophysical survey program is undertaken to develop a map of where the high conductivity rock occurs. All this data is put together and analyzed with greater expectation that there might be something there.
In Malawi, the recent geophysical mapping exercise was done to find the hotspots. The reader may be cautioned that the results of geophysical survey do not necessarily tell and guarantee the company that the mineral has been found. The geophysical map produced looks like an illusion of undecipherable patterns of pretty colours where one, including a geologist may not necessarily even interpret anything let alone a geophysicist. Geophysics uses properties of rocks to determine the likelihood of a mineral based on those properties and the subsurface geology. Most people are cheated that the geophysical survey results can yield instant mining of minerals and they do not need to proceed to another exploration exercise. This is totally unacceptable because the company is not so yet there as to mine. It is the ground follow up exercise through geological mapping that will give an indication of certain traces of a mineral prior to drilling. In Malawi for example, the recent geophysical survey that was done through countrywide mapping is being followed up by geological mapping exercise called the GEMMAP (Geological Mapping and Mineral Assessment Project). This is being done to provide ground proofing data so that the geological map produced shall be overlain by the geophysical map and also the geochemical map (if available) to correlate if there is some sort of similarity and this will guide the way forward to find places where anomalies of certain minerals are likely to be located ready for drilling exercise at a later stage.
Geophysical Survey methods based on reasons/aims.
In prospective areas,geophysical methods allow for rapid regional appraisal of areas where ground access may be difﬁcult – for example a forest, difficult to reach terrain. Geophysics typically measure objective characteristics that are possessed by all rocks to some degree and result in the collection of large amounts of geo- referenced digital data. Geophysicists undertake two different kinds of surveys: those that are aimed at deﬁning regional geology and those that aim to directly locate ore though in some cases there is an overlap between these two types.
The ﬁrst type of geophysical survey is a mapping of the areal distribution of a particular rock or soil characteristic – it could be, for example, patterns of electromagnetic reﬂectance, magnetic susceptibility, rock conductivity or element concentrations/ratios in rocks soils or drainage sediments. These measurements need not have any immediate or direct relevance to the ore body sought. The data is used in conjunction with bedrock or regolith maps from direct surface observations inorder to produce an interpretation of 3D geology. Geological models are then used to predict where the ore might be found and so guide.
The second type of geophysical survey is aimed at measuring unusual or a typical feature of rocks that directly reﬂect, and have close spatial relationships to, economic mineralization. Since ore bodies are in most cases small relative to the earth’s crust, such surveys have to be based on detailed, close-spaced measurements and are generally expensive sample/measuring point, whereas a large number may have come from a relatively small source close to the sample point. For example, the effects of gold both dense and electrically conductive, is negligible in deposits suitable for large-scale mining because of the very low concentrations. Diamonds are also present in deposits in very low concentrations and have, moreover, no outstanding physical properties. In these and similar cases, geophysicists must rely on detecting associated minerals or, as in the use of seismic reﬂection to locate offshore subsequent search. placers, and magnetics and electromagnetics to locate kimberlites, on deﬁning favourable environments The role of computer processing of geophysical data is to present it in such a way as to facilitate the human judgement process and sometimes involve running simulations. This problem of deﬁning anomalous values can often be partly overcome by looking for natural groupings and patterns within the dataset and making reasonable assumption that such groupings reﬂect the operation of fundamental geological factors, including mineralization processes.
This is the main reason why no exploration technique should be conducted in isolation. The most powerful exploration programme is normally the one that combines data gathered from several different appropriate geological, geophysical and geochemical surveys. Ultimately, once all processing and presentation steps have been performed, the key to interpreting the results of geophysical and geochemical surveys is an understanding of the geology and ore-forming processes of the area.