An airborne geophysical survey flies sensors along closely spaced parallel lines to measure the physical properties of the rocks below: magnetism, natural radioactivity, electrical conductivity and density. It is the fastest way to see through soil, sand and forest over a large area, and it is usually the second layer of data an exploration team buys, after geological maps and satellite imagery.

Try it: Airborne survey line-km, flying-hours and calendar planner →

The sensors are only half the story. The same magnetometer gives a sharp, useful map at 50 m height and 100 m line spacing, and a blurred regional one at 300 m height and 800 m spacing. So most of this guide is about specification and planning, which is where a client’s decisions actually change the result.

“Gamma-rays can travel about 35 centimetres through rock and several hundred metres through air.” (Geoscience Australia)

What an airborne geophysical survey measures

Four sensor families do most of the work, and many aircraft carry two or three at once.

• Magnetics: the magnetometer records variations in the Earth’s field caused by magnetic minerals, mainly magnetite. Geoscience Australia notes modern surveys are flown less than 100 m above ground. Our geomagnetic survey guide covers line spacing and height in detail.
• Radiometrics (gamma-ray spectrometry): measures potassium, uranium and thorium at the surface. Geoscience Australia’s radiometrics page says gamma-rays travel about 35 cm through rock, so the method maps the ground’s skin, including potassium alteration linked to some hydrothermal deposits.
• Electromagnetics (AEM): a transmitter induces currents in conductive ground and receiver coils detect the response, to several hundred metres according to GA’s AEM page. See our guide to TEM, VTEM and CSAMT systems for depth and system choice.
• Gravity and gravity gradiometry: measure density variations; GA’s national gravity grids include 451,000 line-km of airborne gravity and gradiometry.

In the USA, the USGS Earth Mapping Resources Initiative flies helicopter magnetic and radiometric surveys to support critical-mineral assessments, and the USGS says the data are publicly downloadable. That pairing, magnetics plus radiometrics from one aircraft, is a standard aerial geophysical survey package in mineral exploration: both sensors are passive and fly together on the same lines.

Radiometrics and gravity in more detail

Radiometric coverage can be as complete as magnetics. Geoscience Australia says the country has been surveyed systematically for about 40 years, and its latest national radiometric grids compile more than 600 surveys at 100 m resolution. It lists the uses as geological mapping, finding mineral deposits, identifying potassium alteration around hydrothermal ore deposits, uranium and thorium exploration, and environmental mapping. Because gamma-rays come from the top few tens of centimetres, radiometric maps are strongly shaped by soils and transported cover, which is useful for regolith mapping and a trap for anyone expecting to see bedrock.

Airborne gravity is the specialist end of the market. Geoscience Australia’s gravity page explains that gravity gradiometers measure the difference in gravity between two points very close together, in several directions at once, which is what lets them work from a moving aircraft. Its national grid, at 400 m cells, combines those airborne lines with 1,430,447 ground gravity observations.

Helicopter vs fixed-wing vs drone: choosing an airborne geophysical platform

The platform decides how low and how slowly you can fly, and height is the single biggest control on resolution. Magnetic, EM and gamma-ray signals all weaken with distance, so the closer the sensor to the ground, the sharper the anomaly.

Fixed-wing aircraft

The USGS fact sheet on aeromagnetics describes surveys flown by airplane or helicopter, either at a constant barometric elevation or draped at a constant height above terrain. Fixed-wing aircraft are the common choice for large, gently rolling blocks, and the CSEG Recorder review of AEM systems cited below lists fixed-wing time-domain EM systems such as MEGATEM and GEOTEM at up to 2 million NIA of transmitter moment. In steep country, ask how closely the proposed aircraft can hold the drape, because height above ground is what you are paying for.

Helicopter geophysical survey

A helicopter geophysical survey trades cost per line-km for terrain-following and low, slow flying. The USGS Earth MRI programme uses helicopters, flying a nominal 100 m in its west-central Utah survey and rising to 330 m over populated areas for safety. For EM, the CSEG Recorder review of AEM systems credits helicopters’ lower receiver height with larger primary fields and more penetration, which explains the popularity of helicopter TEM in exploration.

Drones

Drones now carry magnetometers, and increasingly other sensors, lower still. A review of 70 UAV magnetic studies in Drones found multi-rotor drones had become the most widely used type, and notes that traditional aeromagnetic surveys usually fly 100–500 m while drones fly from a few metres to tens of metres above ground. One Ontario comparison it cites flew a drone at 35, 45 and 70 m against a regional helicopter survey at about 85 m.

Drones have their own limits. The review explains that the standard magnetic compensation used on manned aircraft cannot be applied directly to drones, partly because the field close to the ground is not uniform and partly because the required compensation manoeuvres are hard to fly with multi-rotors. The simplest fix, it says, is to move the sensor away from the drone on a rope, rod or towed bird, but that brings instability, swing and positioning errors of its own. Short endurance and small payloads also limit the area a drone can cover per flight.

Airborne geophysical survey design and how long the job will take

Four numbers define an airborne geophysical survey: line spacing, tie-line spacing, line direction and height. Line spacing sets the smallest feature you can map between lines. Government programmes show the range. The Geological Survey of Canada’s federal compilation says older surveys generally used 800 m lines, with most surveys since 2000 at 400 m or less; Nigeria’s national survey used mostly 500 m; recent USGS Earth MRI helicopter blocks use 200–250 m. Geoscience Australia’s AusAEM, flown for national mapping, uses 20 km lines, while targeted AEM surveys have historically been about 200 m.

Tie lines run perpendicular to the main lines, typically about ten times further apart, and exist to level the data. Line direction should cross the geological strike. Height, as above, is the resolution lever with the steepest safety cost.

Matching line spacing to the target

Start from the smallest feature that matters. If the target is a 1 km porphyry alteration footprint, lines a few hundred metres apart will cross it several times. If it is a 50 m wide shear hosting gold, the lines need to be much closer, or the survey will only hint at it between lines. Many explorers fly an airborne geophysical survey at moderate spacing over the whole licence first, then infill the best blocks, which keeps the expensive tight lines where the geology justifies them.

From specification to line-kilometres

Line-kilometres are how most contractors price and schedule work. For a rectangular block, traverse line-km is roughly the area divided by the line spacing; tie lines add a fraction on top. Flying time follows from the survey speed, plus turns and ferry to and from the airstrip. The IAGSA annex then caps what a crew can fly: a single flight of five hours for a one-pilot crew or eight hours for a two-pilot crew (excluding transit), and 40 flight hours in any 7 consecutive days per pilot.

Processing: what happens between the aircraft and the map

Raw airborne data need several corrections before they are usable. For magnetics, the USGS describes removing the International Geomagnetic Reference Field for the survey date and location and subtracting the daily variations recorded by a base-station magnetometer. The tie lines are then used to level the traverse lines against each other so that the grid has no stripes. At national scale the same idea applies to whole surveys: the Geological Survey of Canada says its compilation was levelled survey to survey to correct for arbitrary datums, slow changes in the Earth’s field and differing specifications. Ask any contractor to document each of these steps in the final report.

Flight-hour limits shape the schedule

The IAGSA annex sets stepped caps on pilot flight hours: 40 in any 7 consecutive days, 70 in any 14 and 120 in any 30, plus 1,200 in a calendar year, with maximum duty of 14 hours a day. The longer the job, the lower the average daily flying allowed, which matters for big blocks flown by a single crew.

Public airborne geophysical data you may already have

Before commissioning anything, check what governments have already flown. The scale of some national archives is striking, and even older data can rule ground in or out.

Geoscience Australia’s national magnetic grids draw on 33.5 million line-km from more than 1,200 surveys, and its gravity grids add 451,000 line-km of airborne gravity and gradiometry. In Canada, the federal aeromagnetic compilation covers more than 700 surveys since 1947. In Nigeria, the Nigeria Geological Survey Agency reports about 2 million line-km from its national magnetic, radiometric, gravity and EM programme, including 24,000 line-km of Tempest EM at 200 m spacing. In the USA, Earth MRI data are free to download.

For how each method compares on the ground as well as in the air, see our overview of geophysics for mineral exploration.

Airborne geophysics companies: how to choose a contractor

Many people searching for airborne geophysics companies want a shortlist. A good neutral starting point is the membership of the International Airborne Geophysics Safety Association (IAGSA), whose member companies conduct low-level survey flights and commit to its safety practices. Its published active members include, among others, Geotech, Sander Geophysics, SkyTEM, Spectrem Air, New-Sense Geophysics, EON Geosciences, Precision Geosurveys, Thomson Airborne and Xcalibur Smart Mapping. We list them for orientation only; we have no relationship with any of them and this is not a recommendation.

What to ask airborne geophysical survey companies

1. Systems and track record. Which sensors, what specifications, and similar surveys in comparable terrain.
2. Safety. Will they work to the IAGSA Survey Contract Annex, and will they supply a Job Safety Analysis before award, as the annex requires?
3. Height policy. What clearance they propose over your terrain, and how deviations are reported.
4. Pay structure. The annex says flight crews should not be paid by hours or kilometres flown, removing an incentive to push on in bad conditions.
5. Deliverables. Raw and processed line data, grids, flight-path and altitude records, calibration records, and a logistics and processing report.
6. Permits. Who obtains aviation and landholder approvals, especially for low flying near settlements.

Safety practices that belong in the contract

The IAGSA annex recommends that anyone hiring an airborne geophysical survey company include the annex, or an equivalent, at tender stage so every bidder prices the same safety standard. Among its provisions: low-level turns limited to 30 degrees of bank at constant altitude, satellite flight-following that reports aircraft position at least every 120 seconds, survival items carried by each crew member, a minimum operating temperature of minus 35°C, and the rule that the pilot in command has the final word on safety.

Surveys over water, at night or in the cold

The annex adds provisions for harder conditions. If a survey crosses water, every crew member should have completed underwater escape training within the past three years. Night surveys need an IFR-certified aircraft with two independent and dissimilar navigation systems, flown at least 1,000 feet above all obstacles in the operating area plus a 10-nautical-mile buffer. Flights above 10,000 feet, or wherever the aviation authority requires it, need continuous oxygen for all crew in an unpressurised aircraft. These are exactly the costs a cheaper bidder may leave out, so check them line by line.

“Flight crews shall not be paid on the basis of hours or kilometres flown.” (IAGSA Survey Contract Annex)

Where satellite targeting fits before an airborne geophysical survey

An aero geophysical survey is fast, but it is still priced by the line-kilometre. Satellite remote sensing costs far less per square kilometre and answers a complementary question: where does the surface show alteration minerals, iron oxides and structures that fit the deposit model? Using that to decide where to infill tightly is the simplest way to cut airborne costs. Our remote sensing guide explains how the spectral side works.

Our satellite-based mineral detection analyses multispectral and hyperspectral imagery of your boundary and flags likely mineralised target zones, alteration halos, faults and fractures, with no ground disturbance. Send coordinates, a KML/KMZ file or a polygon with the country and target mineral, and we deliver in 5–20 business days depending on area and mineral complexity.

• 📊 Timelines: early-stage screening in days rather than months.
• 📊 Cost: up to 80–85% lower early-exploration cost.
• Deliverables: prospectivity heatmaps, estimated location and depth ranges, geological interpretation, and GIS files to plan infill blocks and flight lines.
• Premium+: TargetMax™ Drilling Intelligence with drilling-angle recommendations and 3D subsurface models.
• Experience: 100,000+ hectares scanned for 20+ mineral types across 25+ countries.
• Limits: we are not an airborne survey contractor, and satellite targets are exploration targets, not resources.

Draw your licence on mining.farmonaut.com: Map Your Mining Site, see a sample of satellite-driven 3D mineral prospectivity mapping, or ask for pricing through the mining query form. Our mineral exploration guide sets out the whole sequence.

A staged airborne geophysical programme

1. Download public data and note its line spacing, height and age.
2. Screen the licence from orbit for alteration and structure.
3. Fly a first-pass airborne geophysical survey (usually magnetics and radiometrics) at moderate spacing over the whole licence.
4. Infill and add EM over the zones where satellite and first-pass data agree.
5. Follow up on the ground with mapping, sampling, ground geophysics (for example resistivity and IP surveys) and then drilling.

Frequently asked questions

Reviewed September 2026 against Geoscience Australia’s magnetics, radiometrics, gravity and airborne electromagnetics pages, the Geological Survey of Canada aeromagnetic compilation, the Nigeria Geological Survey Agency’s airborne survey overview, USGS Earth MRI survey releases, the CSEG Recorder review of AEM systems, the 2021 Drones review of UAV magnetic surveys, and IAGSA’s member list and Survey Contract Annex (Revision 3).

Company names are listed from IAGSA’s published membership for orientation only and are not an endorsement. Safety rules and survey specifications change; confirm current versions with IAGSA, your aviation regulator and the agency concerned. Satellite targets are exploration targets, not mineral resources.