Most junior explorers arrive in the field with a geology map, a compass, and a great deal of optimism. What they rarely have is a clear framework for interpreting the geophysical datasets that increasingly underpin every serious exploration programme. Airborne magnetic and radiometric surveys are now routinely flown over prospective ground across East Africa, yet the data are frequently misread, underused, or simply filed away. Understanding what these datasets actually measure — and what they cannot tell you — is not an academic exercise. It directly determines where you put your first trench.
What Magnetic Data Is Really Measuring
The total magnetic intensity (TMI) signal reflects variations in the concentration of magnetic minerals in the subsurface, predominantly magnetite. In greenstone belt terranes — the dominant host setting for orogenic gold across Tanzania, Uganda, and Kenya — magnetic contrasts map lithological boundaries with remarkable fidelity. Mafic and ultramafic units appear as magnetic highs; felsic intrusives and altered zones, where magnetite has been destroyed by hydrothermal fluids, appear as magnetic lows. That destruction of magnetite is geologically significant: it is a proxy for sulphidation, the same process that precipitates gold from hydrothermal fluids.
The most useful derivative products are not the raw TMI image but its transforms. The tilt derivative normalises signal amplitude across the survey area, making shallow and deep sources equally visible — critical in areas with strong remanent magnetism. The first vertical derivative (1VD) sharpens lateral boundaries, helping you trace lithological contacts and structural corridors. When you see a linear magnetic low cutting across stratigraphy, you are almost certainly looking at a fault or shear zone that has focused hydrothermal alteration. That is where your geological attention should go.
Decoding Radiometric Data: Potassium, Thorium, and Uranium
Airborne gamma-ray spectrometry measures the natural radioactive decay of three elements in the top 30–45 centimetres of the earth's surface: potassium (K), thorium (Th), and uranium (U). In exploration, the K channel is the workhorse. Potassium enrichment marks feldspathic lithologies, granite bodies, and — critically — zones of potassic and sericitic alteration associated with hydrothermal systems. A linear zone of elevated K cutting through otherwise low-K greenstones is not a geological curiosity; it is a structural corridor that has seen fluid flow.
The Th/K ratio is particularly diagnostic. Thorium is immobile in hydrothermal systems; potassium is highly mobile. Where the ratio drops anomalously low, potassium has been added by hydrothermal fluids relative to thorium — a classic signal of alteration. Uranium anomalies, while less directly linked to gold mineralisation, can indicate oxidising fluid pathways or late-stage remobilisation. Composite ternary images — K in red, Th in green, U in blue — are standard displays, but do not treat the colours as lithology maps without ground truth. Regolith cover, moisture, and vegetation all attenuate and distort the signal.
Integrating the Two Datasets: Where the Real Targeting Happens
Neither dataset alone gives you a drill target. The power comes from spatial correlation. A zone that shows a magnetic low (magnetite destruction, sulphidation) coinciding with a potassium high (hydrothermal fluid ingress) along a structural lineament is carrying multiple independent geophysical signatures of a hydrothermal system. In the Lake Victoria Goldfield, this tripartite signature — structure, demagnetisation, and K enrichment — has been a reliable first-pass guide to mineralised shear zones long before a single soil sample was collected.
Be precise about scale. Airborne surveys flown at 50-metre line spacing resolve features down to roughly 25–40 metres laterally; 200-metre line spacing surveys, common in regional programmes, will miss narrow shear zones entirely. Always check the survey parameters before drawing conclusions about the absence of anomalies. A clean dataset on coarse-spaced regional data tells you very little about what a detailed survey would reveal.
Practical Value for the Early-Stage Explorer
Airborne geophysics does not replace fieldwork — it directs it. Used correctly, magnetic and radiometric data compress the time between licence acquisition and prioritised target generation from months to weeks. The goal at this stage is not certainty; it is the ranked elimination of ground. Every square kilometre you can confidently de-prioritise based on the absence of structural, lithological, and alteration signatures saves money that belongs in the ground on your best targets. Build your interpretation systematically: structure first from magnetics, alteration second from radiometrics, then test the intersections on foot.
About Orex: Orex is a Tanzanian mineral exploration intelligence platform that delivers satellite-derived structural mapping, geochemical data integration, and targeting tools to geologists and investors operating across East Africa. Our tools are built for the realities of the field — practical, data-driven, and designed to reduce exploration risk at every stage of a programme.
Want to see fault structures and intersection targets on your area of interest — for free? Install GoldRadar Faults on your phone or desktop: it maps lineaments and automatically flags fault intersections derived from satellite elevation data, giving you a structural framework for preliminary exploration before you spend a dollar on the ground.
