Geophysical Methods in Mineral Exploration

Most of the Earth’s surface is covered by soil, vegetation, water or weathered rock, and most mineral deposits are hidden beneath that cover. Geophysics gives explorers a way to see through it. By measuring physical properties of the ground, such as magnetism, electrical behaviour and natural radioactivity, geophysicists can map rock types, structures, alteration and sometimes mineralisation itself, long before a drill is mobilised.

This article explains three of the most widely used methods, magnetics, induced polarisation and radiometrics. It covers what each measures, how surveys are designed and interpreted, what the limitations are, and how the methods work together in exploration for gold and other minerals.

How Geophysics Fits Into Exploration

Geophysical surveys do not find minerals directly. They detect contrasts in physical properties between rocks, and those contrasts must be interpreted in a geological context. Surveys are typically conducted in stages. Airborne surveys cover large areas quickly and cheaply to map regional geology and structure. Ground surveys then focus on priority areas with higher resolution. Results guide geochemical sampling, trenching and drilling, and are updated as new data arrive.

Because methods respond to different properties, using several together reduces ambiguity. A single anomaly can have many explanations, but an anomaly that appears in magnetics, induced polarisation and geochemistry at the same location is much more convincing.

Magnetic Surveys

What is measured. Magnetic surveys measure small variations in the Earth’s magnetic field caused by the magnetic minerals in rocks, principally magnetite and pyrrhotite. Readings are expressed in nanotesla. Rocks rich in magnetite, such as banded iron formation, mafic volcanics and some intrusions, produce high readings, while granites, sediments and strongly weathered rocks are often weakly magnetic.

What it reveals. Magnetics map lithology, structure and alteration. Faults and shear zones appear as linear breaks or offsets in magnetic patterns. Dykes appear as narrow linear highs. Folded iron formation shows the fold geometry. In many gold systems, hydrothermal alteration destroys magnetite, creating magnetic lows along mineralised shear zones, while in others, magnetite or pyrrhotite is added, creating highs.

Survey design. Airborne magnetic surveys fly at low altitude along parallel lines. For exploration, line spacings of about 50 to 200 metres and terrain clearances of around 30 to 80 metres are common, with tighter spacing giving better resolution of narrow features. Flight lines are oriented perpendicular to the expected geological strike. Ground magnetic surveys use hand-held or backpack sensors with typical line spacings of 10 to 50 metres and are useful for detailed work and for areas where airborne surveys are impractical. A base station records daily variations in the magnetic field, which are removed during processing.

Processing and interpretation. Raw data are corrected for diurnal variation and levelled, then gridded. Common enhancements include first vertical derivative, which sharpens edges, tilt derivative, which highlights structure regardless of amplitude, analytic signal, which locates sources, and upward continuation, which suppresses near-surface noise. Reduction to the pole is often applied to move anomalies over their sources, but near the magnetic equator, which includes much of Kenya, this operation becomes unstable. Specialist processing such as reduction to the equator or use of the analytic signal is preferred. Interpreters then trace lineaments, outline rock units, identify intersecting structures and construct a structural map.

Limitations. Magnetics respond only to magnetic minerals. A mineral deposit with no magnetic contrast can be invisible. Magnetic anomalies can also be caused by laterite, which contains iron oxides, and by cultural features such as buildings, fences, power lines and vehicles.

Induced Polarisation and Resistivity Surveys

What is measured. Induced polarisation, or IP, measures the ability of the ground to store electrical charge. An electrical current is injected through electrodes, then switched off, and the voltage decay is measured. The result is expressed as chargeability, commonly in millivolts per volt or milliseconds. At the same time, the apparent resistivity of the ground, in ohm-metres, is measured. Disseminated metallic minerals such as pyrite, pyrrhotite, chalcopyrite, graphite and some oxides produce strong chargeability responses. Clays and fine-grained minerals can also give a weaker response.

What it reveals. IP is a standard tool for detecting disseminated sulphides, which are commonly associated with gold, copper and nickel mineralisation. Resistivity maps rock type, weathering, water content and structures. Silicified zones appear as high-resistivity features, while clay-rich, weathered or water-saturated zones are conductive. Combined, IP and resistivity often outline a mineralised and altered shear zone more clearly than any other single geophysical method.

Survey design. Ground IP surveys use arrays of electrodes laid along lines. Common arrays include dipole-dipole, which gives good lateral resolution and is widely used for exploration, pole-dipole, which gives greater depth penetration, and gradient arrays, which cover areas quickly. Electrode spacing controls resolution and depth of investigation. Wider spacings reach deeper but blur detail. Line spacings of 100 to 200 metres are common for follow-up of geochemical or magnetic anomalies, with closer lines over drill targets. Surveys are slower and more expensive than magnetics, which is why they are used selectively.

Processing and interpretation. Data are plotted as pseudosections, which show apparent values at positions that approximate depth, and then converted to two-dimensional or three-dimensional models by inversion software. Inversion models are not unique, so interpretation depends on geological constraints. Interpreters look for zones of high chargeability, particularly where they coincide with moderate to high resistivity or with structural features.

Limitations. IP is sensitive to noise from power lines, fences and buried pipes. Conductive overburden, such as thick saprolite or clay-rich weathered profiles common in the tropics, reduces the depth of penetration and can mask deeper responses. Graphitic rocks and some barren sulphides, such as pyrite in unmineralised rocks, produce strong chargeability that does not indicate economic mineralisation. Access requirements for laying out lines, and cost, are practical constraints.

Radiometric Surveys

What is measured. Radiometric, or gamma-ray spectrometric, surveys measure the natural gamma radiation emitted by potassium, uranium and thorium in near-surface rocks and soils. Spectrometers record these three channels, usually reported as percent potassium and parts per million equivalent uranium and thorium. The gamma rays come from only the top few tens of centimetres of ground, so the method maps surface geology and soils.

What it reveals. Different rock types have characteristic radiometric signatures. Granites and felsic rocks are typically rich in potassium, thorium and uranium. Mafic rocks and banded iron formation have low counts. Hydrothermal alteration may add potassium through sericite or potassic alteration, so potassium highs, or ratios such as potassium over thorium, can highlight altered zones around gold or porphyry-style systems. Carbonatites, such as those at Mrima Hill, often have strong thorium and sometimes uranium responses, making radiometrics a useful tool for rare earth and niobium exploration. Radiometrics also help map regolith and soil types, which is valuable in planning geochemical surveys.

Survey design. Airborne radiometric surveys are usually flown together with magnetics, using large-volume crystal detectors and the same line spacing and terrain clearance. Ground surveys with hand-held spectrometers are used for detailed mapping and anomaly checking.

Processing and interpretation. Data are corrected for background, aircraft and cosmic radiation, altitude, and radon effects, then calibrated to standard units. Ternary images combining potassium, thorium and uranium in red, green and blue allow quick visual comparison of rock units.

Limitations. Because the signal comes from the very near surface, radiometrics reflect soil and regolith, which can differ from the underlying bedrock, especially where cover is transported. Moisture and dense vegetation attenuate gamma rays. Interpretation therefore requires care and is best used with geological mapping.

Complementary Methods

Other methods are often added. Electromagnetic surveys detect conductive bodies such as massive sulphides and graphite. Gravity surveys detect density contrasts, useful for mapping basins and intrusions. Vertical electrical sounding and electrical resistivity tomography map weathering depth and groundwater and are widely used in Kenya, including for gold exploration and for hydrogeological studies. Seismic methods are used mostly for deeper structure or in coal and petroleum exploration.

Using the Methods Together in a Greenstone Gold Terrain

In a typical greenstone belt like those of western Kenya, a practical sequence might look like this.

1. Airborne magnetics and radiometrics map greenstone lithologies, banded iron formation, granitoid margins and the major shear zones across the licence area.
2. Structural interpretation of the magnetic data identifies shear zone bends, intersections and folds as first-order targets.
3. Soil geochemistry is collected over the interpreted structures, guided by radiometric regolith maps.
4. Ground magnetics over anomalies provide higher-resolution structural detail.
5. Induced polarisation and resistivity are then run over geochemical anomalies to locate disseminated sulphide and silicified zones at depth.
6. Drill targets are defined where structural, geochemical and geophysical evidence coincide.

This workflow uses inexpensive wide-coverage methods first and expensive detailed methods later, in keeping with the principle of staged exploration.

Quality Control and Data Management

Good geophysics depends on good data. Contractors should provide calibration certificates, base station records, line coverage maps and raw data. Survey specifications such as line spacing, flight height, sampling interval and orientation should be agreed in writing before the survey. Results should be delivered in standard digital formats, with grids and processed images, so that they can be integrated into geographic information systems and three-dimensional models.

Cost Considerations

Costs depend on the method, area, location and contractor. Airborne magnetic and radiometric surveys are the cheapest per unit area and are typically quoted per line-kilometre. Ground magnetics cost more per line-kilometre, but are modest in absolute terms for small areas. IP surveys are the most expensive of the three because they require larger crews, more equipment and slower progress. Considering the value of the information, geophysics generally costs far less than drilling and can reduce the number of drill holes needed.

Conclusion

Magnetics, induced polarisation and radiometrics each measure a different property of the ground, and each provides a different piece of the geological puzzle. Magnetics map structure and lithology, IP detects disseminated sulphides and alteration, and radiometrics map surface geology, potassic alteration and radioactive carbonatites. Used together, interpreted with geological knowledge, and adapted to local conditions such as the low magnetic inclination and deep weathering of East Africa, they help explorers focus on the most promising ground and drill with greater confidence.