A geomagnetic survey measures small changes in the strength of the Earth’s magnetic field and turns them into a map of the rocks below. Magnetite and a few other minerals distort the field; most sedimentary rocks barely do. So a magnetic map shows you intrusions, volcanic belts, faults and the places where hydrothermal fluids destroyed or created magnetite, even where soil, sand or forest hides the outcrop.

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Magnetics is usually the first geophysical layer an exploration team buys, and often the one it already owns, because many governments have flown national coverage and give the data away. Knowing how a geomagnetic survey is designed tells you whether that free data is good enough for your target or whether you need a tighter survey of your own.

“The Earth’s field ranges from about 25,000 nT at the magnetic equator to 70,000 nT at the poles.” (USGS)

What a geomagnetic survey actually measures

Every magnetometer reading is the sum of three things: the main field from the Earth’s core, short-term variations from the Sun, and the local field from magnetised rocks. According to the USGS introduction to potential-field magnetics, instruments measure field intensity to about 0.1 nanotesla (nT), while the core field itself ranges from 25,000 nT at the magnetic equator to 70,000 nT at the poles. The geology you care about is a tiny ripple on top of that.

Processing removes the big parts. The International Geomagnetic Reference Field (IGRF), a global model updated every five years, is subtracted for the survey date and location. A base-station magnetometer parked at a fixed point records the daily (diurnal) swings and magnetic storms, and those are removed from the moving readings. What remains, the USGS explains, is mostly the field associated with magnetic minerals in the crust. That residual is the magnetic anomaly.

Susceptibility: why some rocks light up

Magnetic susceptibility describes how strongly a rock becomes magnetised in the Earth’s field. It is dimensionless in SI units, and it is dominated by a handful of minerals. Magnetite is the big one. Pyrrhotite matters in nickel and some base-metal systems. Hematite and pyrite are hundreds to thousands of times weaker, which is why a hematite-rich iron formation and a magnetite-rich one can look completely different on the same map.

Induced and remanent magnetisation

Rocks carry two kinds of magnetisation. Induced magnetisation exists only while the Earth’s field is present. Remanent magnetisation is frozen in when the rock forms and can point in a different direction, even reversed. Both vanish above the Curie temperature, about 580°C for magnetite. Remanence is why a single body can produce a positive anomaly, a negative one or a paired high and low, and why you should never read a magnetic low as “no rock”.

Ground magnetic survey: running a land magnetic survey on foot

A ground magnetic survey is the simplest geophysics you can run. One person walks straight lines across the ground carrying a magnetometer with a GPS, while a second magnetometer sits still at a base station. The US EPA’s magnetic method page describes exactly this: field staff walking a grid with a handheld instrument, and a base station to monitor the daily changes and correct for drift.

Three instrument types are common. The EPA lists cesium-vapour, proton-precession and fluxgate magnetometers, each with its own strengths. Cesium-vapour units sample fast and suit continuous “walking mag” work. Proton-precession instruments read slower and are often used at stations. Fluxgates measure field components and are light, which is why they appear on many drones.

Designing the grid

• Line direction: run lines across the expected strike of the geology, so every dyke, shear or contact is crossed rather than followed.
• Line spacing: tight enough that your smallest target is crossed by at least two lines. A 20 m wide shear needs closer lines than a kilometre-wide intrusion.
• Reading interval: much closer along the line than across it, so the shape of each anomaly is well sampled.
• Base station: in a quiet spot away from fences, vehicles and power lines, read throughout the day.
• ⚠ Cultural noise: steel fences, pipes, drill casing and even the operator’s boots and phone produce anomalies. Log them in the field notes.

How much ground can one crew cover? That depends on your walking speed, terrain and how many productive hours you get once travel and base-station setup are subtracted. The planner below turns a grid design into line-kilometres, readings and field days, using your own speed and hours rather than any generic rate.

When a ground survey beats an airborne one

A ground-based geomagnetic survey is slower per square kilometre, but the sensor sits a metre or two from the source, so narrow and shallow bodies show sharply. It makes most sense over small licences, for follow-up on airborne anomalies, and on ground where low flying is unsafe or not allowed. For larger areas, a magnetometer geophysical survey from the air covers in hours what a crew walks in weeks.

Aeromagnetic survey design: airborne magnetic survey specifications

An airborne geomagnetic survey, usually called an aeromagnetic survey, puts the magnetometer on an aircraft (our guide to airborne geophysical survey platforms compares helicopter, fixed-wing and drone options). The USGS describes the setup plainly: an airplane or helicopter carries a magnetometer that records total field intensity along continuous flight lines a fixed distance apart. The aircraft either holds a constant barometric elevation or follows the terrain at a constant height, called a draped survey. Perpendicular tie lines, flown more widely spaced, let processors level the data between lines.

People search for this as an aerial magnetic survey, an aero magnetic survey or an airborne magnetic survey. They are the same thing. What changes from one survey to the next is the specification: line spacing, tie-line spacing, height above ground and sensor type. Those four numbers decide what the map can resolve.

Line spacing and tie lines

Line spacing sets the smallest feature you can map between lines. Tie lines are typically five to ten times further apart. Recent USGS Earth MRI helicopter surveys show the pattern: 200 m lines with 1,000 m ties over the Colorado Mineral Belt, 200 m lines with 2,000 m ties in west-central Utah, and 250 m lines with 2,500 m ties over parts of Florida and Georgia. Nigeria’s national programme used 500 m lines, with ties reported at 5,000 m in a published interpretation study.

Height above ground

The closer the sensor, the sharper the anomaly. The USGS notes that the shallower a magnetic object, the sharper and narrower its anomaly, and the deeper the source, the broader and gentler the gradients. Flying height works the same way. Geoscience Australia says modern Australian surveys are flown less than 100 m above ground. Nigeria’s surveys averaged 80 m terrain clearance. The USGS Utah survey flew a nominal 100 m, rising to 330 m over populated areas for safety.

How government aeromagnetic data improved, and where to get it

Much of the world’s regional geomagnetic survey coverage was flown by government geological surveys, and the specifications have tightened a great deal. This matters when you inherit old data on a licence: its age tells you a lot about what it can and cannot show.

In Australia, Geoscience Australia reports that surveys before 1990 mostly had flight-line spacing of 1,500 m or more, while since 1990 they have usually been 400 m or less. Its national grids draw on 33.5 million line-kilometres from more than 1,200 surveys, and governments there have run airborne magnetic programmes since 1951. In Canada, the Geological Survey of Canada has acquired aeromagnetic data since 1947. The federal compilation on the Open Government Portal says more than 700 surveys were flown, generally at 800 m line spacing and 305 m above the ground, with most surveys since 2000 at 400 m or less.

Nigeria, the USA and other public datasets

Nigeria is one of the best-covered countries in Africa. The Nigeria Geological Survey Agency describes a national airborne programme flown by Fugro: a 2003 pilot over Ogun State, Phase I (2005–07) covering 44% of the country, and Phase II (2007–10) covering the remaining 55%, funded through the World Bank’s Sustainable Management of Mineral Resources Project. Surveys were mostly at 500 m spacing and 80 m mean terrain clearance, about 2 million line-kilometres in total. The Niger Delta block was flown at 1 km spacing.

In the USA, the USGS Earth Mapping Resources Initiative (Earth MRI) runs helicopter magnetic and radiometric surveys to support critical-mineral assessments, and the USGS says the data are publicly downloadable through its map viewer. Check with your own country’s geological survey too: many sell or share data by map sheet.

Commissioning a geomagnetic survey: questions for the contractor

Most juniors buy magnetics from a specialist contractor rather than owning the kit. A short, specific brief saves arguments later. These are the questions we would want answered in writing before any geomagnetic survey starts, whether it is walked, flown by helicopter or flown by drone.

1. Line and tie spacing, and line direction. Are lines across strike, and are ties roughly ten times the line spacing?
2. Nominal and actual height. What clearance will be held over ridges and valleys, and how will deviations be reported?
3. Sensor and sample rate. Cesium-vapour, proton-precession or fluxgate, and how many readings per second or per metre?
4. Base station. Where it sits, what it records and what diurnal limit stops flying or walking for the day.
5. Deliverables. Raw and processed line data, the IGRF model used, levelled grids, RTP and derivative grids, and a logistics report.
6. Safety and permits. Low flying and drone flights need aviation approval; ask who holds it.

Four mistakes we see in geomagnetic survey reports

The first is gridding below the line spacing: a 400 m survey gridded at 20 m looks crisp but invents detail between lines. The second is ignoring remanence, which shifts or flips anomalies. The third is reading a single total-field image without an RTP or derivative version. The fourth is treating an anomaly as an orebody. A geomagnetic survey ranks ground; only sampling and drilling prove metal.

For how magnetics sits alongside gravity, radiometrics, EM and seismic, see our overview of geophysics for mineral exploration.

Aeromagnetic interpretation: turning a magnetic map into targets

A raw geomagnetic survey map is hard to read because of the dipole effect: outside the magnetic poles, one body gives a paired high and low offset from its true position. Interpreters therefore work from a set of derived images. The USGS describes the main ones. A reduced-to-the-pole (RTP) map recalculates the field as if the survey sat at the magnetic pole, which simplifies anomaly shapes and centres each anomaly over its source. Derivative and gradient maps sharpen edges, which often mark faults and contacts.

Reading depth and shape

Anomaly width is the first depth clue. Sharp, narrow anomalies come from shallow sources; broad, smooth ones come from deep sources. Filtering by wavelength separates the two, so a regional high from a deep intrusion does not hide a narrow near-surface dyke. Quantitative tools (Euler deconvolution, profile modelling, 3D inversion) go further, but every one of them gives a family of possible answers rather than a single one. Magnetic data alone never fix depth uniquely.

Alteration shows up as magnetite created or destroyed

Hydrothermal fluids change magnetite content, and that is the link between a magnetic geophysical survey and ore. Clark’s review of magnetic effects of hydrothermal alteration in Tectonophysics notes that the giant porphyry copper deposits of the Chilean Andes show extensive magnetite-destructive alteration in their mineralised zones. The USGS study of porphyry systems in the Silverton caldera, Colorado used geophysics to image such systems in three dimensions. In contrast, some skarns and iron-oxide copper-gold systems add magnetite and appear as highs.

• Linear breaks and offsets: faults and shear zones, the plumbing for many gold and base-metal systems.
• Round or elliptical highs: intrusions, which may drive porphyry or skarn systems.
• Lows inside or beside intrusions: possible magnetite-destructive alteration worth checking.
• Strong, narrow highs: banded iron formation, magnetite skarn or mafic dykes.
• ⚠ Not an assay: none of these patterns proves mineralisation. They rank ground for sampling.

“Before 1990, most Australian aeromagnetic surveys had lines 1,500 m or more apart.” (Geoscience Australia)

Gravity and magnetic surveys together

Gravity and magnetics are called potential-field methods, and they are often flown or interpreted together because they respond to different rock properties. The USGS explains that magnetics senses magnetisation while gravity senses density, and that both are relatively inexpensive methods that cover large areas quickly. A gravity magnetic survey pairing is powerful because many ore-related bodies differ in both: a dense but weakly magnetic sulphide lens, a magnetic but ordinary-density dyke, or a light, non-magnetic granite.

Geoscience Australia’s national gravity grids combine 1,430,447 ground gravity observations with 451,000 line-kilometres of airborne gravity and gradiometry. That kind of coverage lets explorers see basin shapes and dense intrusive bodies that magnetics alone would miss. For a single licence, a gravity and magnetic survey package usually means ground gravity stations over the best targets from the geomagnetic survey rather than a full airborne gravity job.

Drone magnetometry fills the gap

Drones now carry magnetometers between walking height and helicopter height. A 2021 review in Drones of 70 studies found multi-rotor drones had become the most widely used type for magnetic surveys, because they are easy to operate, low cost and can fly very low. A drone geomagnetic survey sits in the middle on both resolution and speed. In one Ontario test the review cites, a six-rotor drone flew about 48 line-km at 35 to 70 m above ground in 7 hours, while the follow-up ground survey took about two weeks in the same rough, forested terrain.

Where satellite targeting fits before a magnetic survey mineral exploration budget

Magnetic survey mineral exploration spending, and the cost of any geomagnetic survey you commission, is most efficient when you already know which part of a licence deserves tight lines. Satellite remote sensing answers a different question from magnetics: it maps surface alteration minerals, iron oxides and structures from reflected light. Put together, the two narrow the ground faster than either does alone. Our guide to remote sensing in mineral exploration explains the spectral side.

Our satellite-based mineral detection analyses multispectral and hyperspectral imagery of your boundary and flags likely mineralised target zones, alteration halos, faults and fractures. You send coordinates, a KML/KMZ file or a polygon plus the country and target mineral, and we deliver in 5–20 business days depending on area and mineral complexity. The report includes prospectivity heatmaps, estimated location and depth ranges and georeferenced GIS files your geophysicist can load next to the magnetic grids.

• 📊 Timelines: early screening in days rather than months.
• 📊 Cost: up to 80–85% lower early-exploration cost.
• 📊 Track record: 100,000+ hectares scanned for 20+ mineral types in 25+ countries.
• Premium+: TargetMax™ Drilling Intelligence with drilling-angle recommendations and 3D subsurface models.
• What it is not: a substitute for a geomagnetic survey or for drilling. Targets are exploration targets, not resources.

Start by drawing your licence on mining.farmonaut.com: Map Your Mining Site, look at a sample of satellite-driven 3D mineral prospectivity mapping, or ask for pricing through the mining query form. For the full exploration sequence from desktop study to drilling, read our mineral exploration guide.

A sensible sequence

1. Download public magnetics and check its line spacing and height against your target size.
2. Screen the licence from orbit for alteration and structure.
3. Fly or walk a detailed magnetic survey over the best zones, with lines across strike.
4. Interpret RTP and derivative maps alongside geology and satellite targets.
5. Test anomalies with mapping, soils or rock chips, then IP, EM or gravity where the target needs it (see our guides to resistivity and IP surveys and electromagnetic survey methods).
6. Drill the targets that survive.

Frequently asked questions

Reviewed September 2026 against the USGS fact sheet on potential-field magnetics, the US EPA’s magnetic method guidance, Geoscience Australia’s magnetics and gravity pages, the Geological Survey of Canada aeromagnetic compilation, the Nigeria Geological Survey Agency’s airborne survey overview, USGS Earth MRI survey releases, UBC’s Geophysics for Practicing Geoscientists susceptibility tables, Clark’s review in Tectonophysics and the 2021 Drones review of UAV magnetic surveys.

Survey specifications and data availability change; check the agency’s own page before relying on a figure. Satellite targets are exploration targets, not mineral resources, and must be confirmed by sampling and drilling.