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Geophysical methods in mineral exploration measure physical properties of the ground, such as magnetism, natural radioactivity and electrical response, to see through soil and cover. Magnetics, induced polarization (IP) and radiometrics are among the most widely used tools for gold and base metal exploration. This guide explains what each one measures, how surveys are designed and processed, and how to choose between them, with context for Kenya.
Start with Physical Properties
Geophysics works only when the target differs from its surroundings in some measurable property. Magnetic minerals such as magnetite and pyrrhotite change the local magnetic field. Potassium, thorium and uranium in rocks and soil emit gamma rays. Sulphide minerals hold electrical charge, which IP detects. Gravity senses density, and electromagnetic (EM) methods sense conductivity.
No method finds gold directly in most orogenic systems. They find the rocks, structures, alteration and sulphides that accompany gold.
Choose Airborne or Ground Surveys
Airborne surveys cover huge areas quickly and cheaply per kilometre, which suits regional work. Ground surveys are slower and costlier, but give finer detail and are the only practical option for methods like IP. A common sequence is airborne first to map structure and rock units, then ground work over chosen targets.
Magnetics: Mapping Rocks and Structures
A magnetic survey measures small variations in the Earth’s magnetic field caused by magnetic minerals. It is fast and low-cost. In greenstone belts, magnetics helps to map rock units, dykes, iron-rich layers, faults and folds. Where rocks are non-magnetic, faults and alteration may show as magnetic lows because magnetite is destroyed by fluids.
Common processing steps make the data easier to read:
- Reduction to the pole (RTP) shifts anomalies to sit directly above their sources, as if the survey were at the magnetic pole. It matters most at low latitudes, such as near the equator in Kenya, where it needs care because of stability problems.
- First vertical derivative sharpens edges and highlights shallow, narrow features.
- Analytic signal produces peaks over magnetic bodies and edges, and depends less on the direction of magnetisation.
Interpreters combine these images to trace lineaments and map contacts.
Radiometrics: Potassium, Thorium and Uranium
Radiometric surveys measure gamma rays from the top 30 to 40 cm or so of ground. They report potassium (K), equivalent thorium (eTh) and equivalent uranium (eU), the word equivalent meaning that thorium and uranium are inferred from their decay products. Maps of the three channels, often shown as a ternary image, help to separate rock types and regolith.
Potassium alteration, such as sericite or K-feldspar around veins, can raise potassium counts, which makes radiometrics a way to map alteration. Thick transported cover and wet soil dampen signals, so results need to be read together with regolith maps.
Induced Polarization: Detecting Disseminated Sulphides
IP injects current into the ground and measures how the ground holds and releases charge after the current stops. This property is called chargeability. Resistivity is measured at the same time. Disseminated sulphides, such as pyrite and arsenopyrite that often accompany gold, give high chargeability, so IP can pick out sulphide-bearing zones even when little gold is present.
Course notes from the University of British Columbia stress an important limitation. Clay and graphite can also be strongly chargeable, so chargeability alone should not be used to identify the mineral type. Field values and units also vary with the method, which makes it hard to compare surveys.
Array Types and 3D IP
The array is the layout of current and voltage electrodes. In dipole-dipole, current and voltage pairs are spaced apart along a line, which gives good lateral resolution and is popular for exploration. Pole-dipole places one current electrode far away, giving stronger signals and deeper reach in some settings, but an asymmetric response. Increasing electrode separation probes deeper, and results are plotted as pseudosections. Three-dimensional (3D) IP uses grids of lines to image sulphide bodies more accurately, at a higher cost. It is usually reserved for advanced targets.
Gravity and EM in Brief
Gravity surveys detect density contrasts and are useful for dense massive sulphide bodies, intrusions or deep basement structure. Electromagnetic (EM) surveys detect conductive bodies, and are the standard first choice for massive sulphide targets, such as nickel and copper. They are less relevant for disseminated gold, though they can help to map conductive weathered zones and faults.
Plan Survey Design: Line Spacing and Sensor Height
Line spacing and sensor height set the detail you can see. Closer lines and lower flying height resolve smaller features, but cost more. A rule of thumb is that line spacing should be smaller than the width of the target you want to detect, and lines should run across the geological strike. For ground IP, station spacing and line separation are chosen from the target size and depth. Always record survey parameters, since they limit what can be interpreted later.
Process Data and Use Inversion
Raw data is corrected for things like base station drift, flight height and instrument noise, then levelled and gridded. Inversion is a computer process that turns surface measurements into a model of the properties underground, such as magnetic susceptibility or chargeability at depth. Inversions are non-unique, meaning many different models can fit the same data. That is the central ambiguity of geophysics.
For this reason, constrain the models with geology: outcrop, drill core, density and susceptibility measurements. Geophysics supports geological thinking and does not replace it.
Kenya Context: The National Airborne Geophysical Survey
Kenya has completed a nationwide airborne geophysical programme, run through the State Department for Mining. Public reporting describes it as a survey of the whole country that collected magnetic and radiometric data, with more than three million line-kilometres of data reported, and produced updated geological and mineral prospectivity maps. Mayer Brown’s overview reports that the preliminary results identified about 970 mineral occurrences. Details such as line spacing and flying height should be confirmed with official data releases before being used for design.
For explorers, this dataset is a cost-saving starting point: regional magnetics to trace shear zones in the Migori and Kakamega-Vihiga greenstone belts, radiometrics to map regolith and alteration, and an early ranking of ground before applying for licences through the online Mining Cadastre portal. Prospecting licences under the Mining Act 2016 are limited to three years, so efficient early targeting matters.
Cost and When to Use Which
| Method | Typical role | Relative cost | Best for |
|---|---|---|---|
| Airborne magnetics | Regional structure and geology | Low per km | First-pass mapping |
| Radiometrics | Regolith and alteration | Low (often with magnetics) | Mapping cover and alteration |
| Ground magnetics | Detailed structure | Low to moderate | Following up airborne data |
| IP and resistivity | Sulphide zones | Moderate to high | Drill targeting |
| 3D IP | Detailed sulphide imaging | High | Advanced targets |
| Gravity and EM | Dense or conductive bodies | Varies | Specific deposit types |
Costs vary by country, contractor, and terrain, so get quotes. As a general rule, start with the cheapest method that answers the question, and reserve expensive methods for targets that have already passed cheaper tests.
Integrate with Geology
The best results come when geophysics, geochemistry, and structural geology agree. A magnetic lineament that crosses a bend, carries a soil gold and arsenic anomaly, and has an IP chargeability high beneath it, is a strong drill target. Any single dataset on its own is much weaker.
Conclusion
Magnetics map structure and rock units, radiometrics map alteration and regolith, and IP finds sulphides. Each has limits and ambiguities, and each works best when combined with geological knowledge. In Kenya, the national airborne dataset offers a valuable base layer for planning efficient exploration.
