Magnetometry and gamma-ray spectrometry: what each method reveals

When we talk about airborne geophysics for mineral exploration, two methods dominate the scene: magnetometry and gamma-ray spectrometry (radiometrics). They measure distinct physical properties and, for that reason, tell different parts of the same geological story.

Magnetometry: the structural skeleton

Magnetometry records small variations in the Earth's magnetic field caused by the magnetic minerals present in rocks — above all magnetite. Because the content of these minerals changes from one lithology to another and along faults and intrusions, the magnetic map reveals the structural framework of the area.

With it you can outline:

• Contacts between geological units;
• Structural lineaments, faults and fractures;
• Intrusive bodies and dikes;
• Zones where hydrothermal alteration destroyed or concentrated magnetite.

Magnetometry "sees" at depth and does not depend on outcrops, which makes it powerful in covered terrains.

Gamma-ray spectrometry: the surface signature

Gamma-ray spectrometry measures the natural gamma radiation associated with three elements: potassium (K), thorium (eTh) and uranium (eU). An important technical note: potassium is measured directly (from the potassium-40 isotope), while thorium and uranium are estimated from daughter isotopes of their decay — which is why they appear as equivalents (the "e" in eTh and eU). Unlike magnetometry, this signal comes only from the top tens of centimeters of the ground (about 30–45 cm) — it is a reading of the surface.

Each lithology has a characteristic radiometric signature. Granites are usually rich in potassium; lateritic covers concentrate thorium; and certain hydrothermal alteration zones change the proportions among the three elements. That is why radiometrics is excellent for:

• Discriminating lithologies and mapping surface units;
• Identifying potassic alteration, a classic vector for hydrothermal deposits;
• Composing the RGB ternary image — by convention, K in red, eTh in green and eU in blue —, where each color represents one element and patterns jump out.

Why use them together

The strength is in the combination. Magnetometry shows the structure at depth; radiometrics characterizes the surface and alteration. When a magnetic lineament coincides with a potassium anomaly, for example, you have a far more robust indication than either method alone.

Integrating this data with surface geology and known mineral occurrences is what turns "colored blotches" into well-founded exploration targets.

From theory to practice

Processing and cross-referencing these two methods manually requires time and specialized tools. The promise of automation is to standardize this integration: from the mineral tenement number, the platform generates the already-integrated magnetic and radiometric products, ready for interpretation.

How Lithogeo helps

This is exactly the kind of work the Lithogeo platform automates. From an ANM process number or a polygon of your area, it processes and integrates the public data — magnetometry, gamma-ray spectrometry, geology, terrain and mineral occurrences — and delivers the products described here ready for analysis, in minutes and right in the browser.

Educational content. Geophysical signatures vary with the regional geological context and must be interpreted by a qualified professional.