Electromagnetic Survey Methods: TEM, VTEM, CSAMT and Depth

An electromagnetic survey sends a changing magnetic field into the ground and listens for the echo from anything that conducts electricity. Massive sulphides, graphite and salty groundwater carry current well; most dry rock does not. That contrast is why EM, flown from helicopters and aircraft or laid out as loops on the ground, is the workhorse for base-metal exploration in Canada and Australia.

The physics sounds exotic, but the practical questions are simple. What does the target conduct compared with its host rock? How deep is it? What conductive junk sits above it? Answer those three and you can choose between a helicopter TEM survey, a fixed-wing system, a ground loop or a controlled-source sounding with some confidence.

“The conductivity of rocks and minerals can vary by 20 orders of magnitude.” (USGS, citing Grant and West)

How an electromagnetic survey works

Every EM system has a transmitter and a receiver. The transmitter drives a changing current through a loop of wire, which creates a primary magnetic field. When that field passes through a conductive body it induces eddy currents, and those currents create a secondary magnetic field of their own. The receiver coil picks up the secondary field. The US EPA’s frequency-domain EM page sets out the same chain: primary field, induced electromotive force, eddy currents, secondary field.

Because nothing has to touch the ground, EM can be flown. Geoscience Australia’s AEM page describes a transmitter on a plane or helicopter inducing eddy currents that are detected by receiver coils towed below and behind the aircraft, and says AEM can detect conductivity changes to a depth of several hundred metres. That is the basic idea of electromagnetic induction geophysics, whether the loop hangs from a helicopter or lies on the ground.

What conducts, and what does not

Conductivity is measured in siemens per metre (S/m) or millisiemens per metre (mS/m); resistivity in ohm-metres is its inverse. Our electrical resistivity geophysics guide covers the electrode-based methods. The USGS chapter on the geophysics of volcanogenic massive sulphide deposits gives the contrast that makes EM work: VMS deposits exceed 500 mS/m, similar to graphite and salt water, while igneous and metamorphic rocks are typically below 1 mS/m and sedimentary rocks range from 1 to 500 mS/m.

Frequency domain versus time domain: choosing an electromagnetic geophysical survey

There are two ways to separate the weak secondary field from the strong primary one. Frequency-domain EM (FDEM) transmits continuously at one or more frequencies and measures how much the returning signal is shifted in amplitude and phase. Time-domain or transient EM (TEM, also TDEM) switches the transmitter off and listens to the decay in the quiet gap. The EPA’s TEM page explains that currents decay more slowly in conductive materials, so the shape of the decay curve tells you about conductivity with depth.

A review of the state of the art in the CSEG Recorder sums up the trade-off for airborne systems. Time-domain systems suit mineral exploration better: lower frequency content, greater depth penetration, and transmitter dipole moments typically 0.1 to 2 million NIA, which it puts at 100 to 1,000 times more powerful than frequency-domain systems (typically under 300 NIA). FDEM wins on near-surface resolution, under about 50 m, and on poorly conductive targets in very resistive ground.

Passive and semi-airborne options

Some systems transmit nothing. Passive AFMAG-type systems such as ZTEM measure natural electromagnetic fields instead and, according to the CSEG review, reach the largest depths, of the order of kilometres; it reports ZTEM has flown over 350,000 line-km. VLF systems work in the 3 to 30 kHz band, and the review notes one has been in continuous production since the 1970s. Semi-airborne methods put a grounded transmitter on the surface and fly the receiver, and the review cites one operating at 4 to 20 Hz for conductive bodies below 1.5 km.

VTEM geophysics: the specifications in one real survey report

Searches for VTEM geophysics usually want to know what the system actually records. A survey report filed with the Quebec mines ministry shows it. Geotech flew a VTEM Plus helicopter survey over the Adam Project near Matagami, Quebec, for Exploration Midland between December 2016 and February 2017. The block covered 32 km2 with traverse lines 150 m apart and tie lines every 1,500 m, for 224 planned line-kilometres (one line-kilometre is one kilometre flown along a survey line).

The transmitter loop was 26 m across, with a 30 Hz base frequency and a peak dipole moment of 367,403 nIA. The loop and receiver flew on average 37 m above the ground. Two caesium magnetometers flew with the system, so the job returned a magnetic gradient grid as well as the EM data. The report also notes that actual line-kilometres normally exceed the planned figure; here 234 were flown against 224 planned, about 4% more, which is worth allowing for in a budget.

Skin depth: how deep an electromagnetic survey can see

EM energy weakens as it travels into conductive ground. The standard yardstick is skin depth, the distance over which the field falls to about 37% of its surface strength. UBC’s EM GeoSci reference gives the working formula: skin depth in metres is about 503 times the square root of resistivity (ohm-m) divided by frequency (Hz). Low frequency and resistive ground mean deep penetration; high frequency and conductive ground mean shallow.

Time-domain surveys have an equivalent. The same reference gives a diffusion distance of about 1,260 times the square root of time divided by conductivity, so the later the time channel, the deeper the currents have travelled. At 1 kHz its table lists skin depths of about 1.6 m in sulphide skarn, 8.76 m in seawater, 160 m in wet sediments and 500 m in dry sediments.

Skin depth is a yardstick, not a guarantee. The depth at which a real system can detect a target also depends on its size, its conductivity, the transmitter power, the receiver noise and the host rock. Use the calculator for a first sense of scale, then ask your contractor for a forward model of your actual target.

Airborne electromagnetic survey: helicopter, fixed-wing and national programmes

An airborne electromagnetic survey, sometimes searched as an aerial electromagnetic survey or AEM, covers ground fast and can fly over swamp, forest and rough terrain. There are two platform families. The CSEG review notes that helicopter systems fly their receivers lower, which gives a larger primary field at the ground and more penetration, and that this explains the popularity of helicopter TEM in exploration.

Helicopter electromagnetic survey systems

A helicopter electromagnetic survey typically slings a large transmitter loop with the receiver at its centre beneath the aircraft. VTEM is the best-known example in searches for VTEM geophysics. Geotech, its developer, says VTEM has surveyed more than two million line-kilometres. The CSEG review lists VTEM variants, HELITEM (which it calls the highest-power helicopter TDEM system in operation, at 2 million NIA) and SkyTEM models among the main helicopter time-domain systems.

Fixed-wing AEM

Fixed-wing time-domain systems such as MEGATEM and GEOTEM tow the receiver behind the aircraft. They are usually the economic choice for large, flat, regional blocks, while helicopters win in rugged ground and on tight, detailed grids. Fixed-wing frequency-domain systems are rarer; the CSEG review says frequency-domain development has largely stalled, with only four or five systems still operating out of 22 described in 1997.

National AEM programmes

Governments increasingly fly AEM for mapping and groundwater as well as minerals. Geoscience Australia runs AusAEM at 20 km line spacing, which it describes as the most extensive AEM acquisition programme ever conducted, and says more than 14 AEM systems operate in Australia. Historical targeted surveys were flown at about 200 m spacing. In Nigeria, the Nigeria Geological Survey Agency reports Tempest time-domain EM flown at 200 m spacing over three blocks in 2008–09, totalling 24,000 line-km.

Designing an airborne EM survey

Three settings do most of the work. Line spacing should cross a target at least twice; Geoscience Australia notes that targeted AEM has historically been flown at around 200 m, while its 20 km AusAEM lines are strictly for regional mapping. Line direction should cut across the expected strike of the conductors, because a flat plate crossed end-on gives a weak, confusing response. Height should be as low as safety and terrain allow, since the CSEG review attributes much of the helicopter systems’ advantage to lower receiver height. For platforms and contractors, see our airborne geophysical survey guide.

Beyond those, ask about the waveform, base frequency and timing of the receiver channels, and have your geophysicist check them against the forward model for your target. Ask too whether magnetics is recorded at the same time. Geotech’s published description of VTEM, for example, includes a caesium magnetometer mounted above the loop, so an airborne electromagnetic survey of that kind should come with a magnetic grid at no extra flying cost. Our guide to geomagnetic survey specifications sets out what that grid should meet.

Airborne electromagnetic survey cost: line-km, spacing and mobilisation

An airborne electromagnetic survey is usually quoted per line-kilometre, plus a mobilisation charge to bring the crew and aircraft to site. Geoscience Australia says AEM is more expensive to acquire than airborne magnetics and radiometrics, which is why AEM has mostly been flown in small, isolated blocks at about 200 m line spacing. One published cost benchmark comes from groundwater work: the University of Nebraska-Lincoln Conservation and Survey Division describes about US$500 per line-km as a maximum cost target for surveys flown from 2014 to 2020, covering planning, acquisition, processing, inversion and interpretation. It is a cap for water projects, not a mineral-exploration quote, so use it only as a sanity check against bids. The same guideline says mobilisation is a substantial share of the cost and that larger surveys cost less per kilometre.

The line-kilometres you pay for follow from the area and the spacing: divide the area in kilometres squared by the spacing in kilometres. A 100 km2 block flown at 200 m spacing needs about 500 line-km; at 150 m it needs about 667 before tie lines. Halving the spacing doubles the bill, so the spacing should come from the size of the target, not from habit. The table shows how far apart the lines were on four published surveys.

Ground methods: TEM geophysical survey, CSAMT and EM induction

Airborne conductors are usually followed up on the ground, where loops can be larger, transmit times longer and positions exact. A TEM geophysical survey on the ground, also called a transient electromagnetic survey, lays out a square wire loop. The EPA says typical land surveys use transmitter loops with 20, 40 and 100 m sides, and that bigger loops and longer transmit times give deeper investigation, from tens to hundreds of metres depending on the ground.

CSAMT geophysics

Controlled-source audio-frequency magnetotellurics (CSAMT) is a frequency-domain sounding. A grounded wire one to two kilometres long acts as the source, and the electric and magnetic fields are measured at receiver stations some distance away, ideally at least four skin depths from the transmitter. Summaries collected on Science.gov’s CSAMT topic page describe data sampled from 0.1 Hz to 10 kHz and resistivity mapping in the top two to three kilometres. CSAMT geophysics is popular for deep structure beneath porphyry and epithermal systems and in geothermal work.

Electromagnetic induction geophysics for shallow work

Small frequency-domain instruments carried by one person map conductivity in the top few metres to tens of metres. The EPA notes that new FDEM systems allow rapid data collection by a single operator. In exploration they help map clay-filled faults, regolith and shallow conductors, and they are routine for mine-site environmental work.

Interpreting EM conductors without fooling yourself

A good EM survey produces a list of conductors, each with a position, depth estimate, strike, dip and conductance. The hard part is ranking them. The USGS VMS chapter lists the traps plainly: graphite- or sulphide-bearing anoxic sediments are highly conductive and hard to tell apart from massive sulphides, and barren pyrite- or pyrrhotite-rich bodies look the same as economic ones. It also notes that EM detects massive sulphide only where the grains are electrically connected; disseminated stockwork below a lens needs induced polarisation instead.

• Check the magnetics. A conductor with a coincident magnetic high may be pyrrhotite-rich; one along a magnetic unit boundary may be a stratigraphic graphitic horizon.
• Check the geology. A conductor that follows a known black shale for kilometres is less interesting than a short one cutting across stratigraphy.
• Check the cover. Broad, flat-lying responses often come from conductive overburden or saline groundwater.
• Check culture. Power lines, fences, rail and pipelines produce sharp anomalies; the logistics report should flag them.
• ⚠ Drill the best, not the most. A handful of well-ranked conductors beats a long list drilled in order of strength.

What the contractor should hand over

An EM job is only as useful as its deliverables. Ask in the contract for raw and processed data for every time channel or frequency, flight-path and altitude records, the magnetic data flown alongside, conductivity-depth sections or inversions with their settings, a conductor pick list with confidence ratings, and a logistics report that flags power lines and other cultural noise. Without the raw channels, a second opinion or a later re-inversion is impossible.

It also pays to keep the forward models you commissioned before the survey. When the results come in, compare the modelled response of your target with what was actually recorded. A conductor that matches the model in size and decay deserves more attention than one that does not, and a target that should have been visible but was not tells you something too.

“Massive sulphide can be detected by EM only if its grains are electrically connected.” (USGS)

For how EM fits beside magnetics, gravity, radiometrics and IP, see our overview of non-intrusive geophysics for mineral exploration.

Airborne magnetic vs electromagnetic surveys: which is better for exploration?

Neither is better in general, because they measure different things. A magnetic survey maps changes in the Earth’s field caused by magnetic minerals, so it outlines rock units and large geological structures (Geoscience Australia; US EPA). An EM survey maps conductivity, so it responds to sulphides, graphite, clay and salty water (Geoscience Australia). Magnetics costs less, so it is often flown first over the whole licence, and EM is added over the ground that survives. Many helicopter EM systems record magnetics on the same flight, which removes the choice.

▶ Shows how a licence boundary becomes ranked satellite targets before drilling. · 1:46

Where satellite targeting fits before an EM programme

EM is excellent at answering “is there a conductor here?” but expensive to use as a blanket search. Satellite remote sensing answers a different, earlier question: where does the surface show alteration, iron oxides and structures that fit the deposit model? Screening a licence that way decides where an electromagnetic survey is worth flying tightly. Our guide to remote sensing for mineral exploration covers the spectral methods in detail.

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

• 📊 Speed: early exploration 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 PDF plus georeferenced GIS files to overlay on EM conductor picks.
• Premium+: TargetMax™ Drilling Intelligence with drilling-angle recommendations and 3D subsurface models.
• Scale: 100,000+ hectares scanned for 20+ mineral types across 25+ countries.
• Limits: satellites read the surface. Targets are exploration targets, not resources, and conductors still need geophysics and drilling.

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. The full stage-by-stage workflow is in our mineral exploration guide.

A sensible EM sequence

1. Screen the licence with satellite alteration mapping and public magnetics.
2. Forward-model your target for two or three candidate systems.
3. Fly an airborne electromagnetic survey over the best ground, with lines across strike.
4. Rank conductors against geology, magnetics and cover.
5. Follow up on the ground with TEM loops, and IP where disseminated sulphides are expected.
6. Drill the conductors that survive, and consider downhole EM in the holes you drill.

For a claim you are thinking of buying or optioning, the order matters because AEM is priced per line-kilometre. A satellite screen of surface alteration and structure, read alongside public magnetics, can narrow a licence to the blocks where a tight-spaced EM grid is worth the money. It does not replace the survey, because satellites cannot see conductors below the surface.

▶ Shows how satellite targets are ranked into a drill order. · 2:19

Frequently asked questions

System names are given for illustration and are not an endorsement; specifications change, so confirm them with the contractor. Satellite targets are exploration targets, not mineral resources, and must be confirmed by geophysics, sampling and drilling.