A geophysical survey measures a physical property of rock — its magnetism, its ability to hold or conduct electrical charge, or its density — from the surface or the air, without drilling. Exploration geologists run these surveys to see through soil, water, and rock cover and map structures and mineralisation styles that would otherwise stay hidden until a drill bit reached them.
This guide covers the four geophysical methods that show up most often in mineral exploration news and technical reports — magnetics, induced polarization (IP), electromagnetics (EM), and gravity — explaining what each one actually measures, what it’s good at finding, and how the four are typically combined into a single exploration picture.
Why run geophysics before drilling?
Drilling is the only way to directly confirm what’s underground, but it is also the slowest and most expensive step in an exploration program, frequently running into the hundreds of dollars per metre once mobilisation, core recovery, logging, and assay costs are added up. A single drill hole also only samples a narrow cylinder of rock a few centimetres wide — everything between holes is inferred, not measured.
Geophysical surveys fill that gap. By measuring how rock responds to natural or induced physical fields — magnetic, electrical, or gravitational — geophysicists can build a continuous, area-wide picture of the subsurface at a fraction of the cost of drilling. That picture doesn’t tell you the grade or the exact rock type, but it tells you where the underlying geology changes, where fluids have altered the rock, and where sulphide minerals might be concentrated — in other words, where to drill first.
Geophysics rarely stands alone. It is almost always used together with geological mapping and geochemical sampling (soil, stream sediment, or rock chip surveys) to build and rank a target before a single metre is drilled. For more on how those methods fit together, see Exploration Methods Explained: Mapping, Geochem, Geophysics, Drilling.
How do magnetic surveys work?
Magnetic surveys measure small variations in the Earth’s magnetic field caused by magnetic minerals in the rock below — chiefly magnetite, and to a lesser extent titanomagnetite, pyrrhotite, and a few other iron oxide and iron sulphide minerals. Rocks with more of these minerals produce a locally stronger magnetic signature; rocks with less produce a weaker one.
Because different rock types carry different amounts of magnetic minerals, a magnetic survey effectively maps geology — contacts between rock units, faults, folds, and intrusions — even where all of it is buried under soil, cover, or water. Airborne magnetic surveys, flown by fixed-wing aircraft or helicopter typically 60–100 metres above ground with a sensor mounted on the aircraft or towed behind it, can cover hundreds of line-kilometres a day and are one of the most common first-pass tools used to map regional geology before any other exploration work begins. Ground magnetic surveys, walked or driven with a handheld or cart-mounted magnetometer, provide tighter-spaced, higher-resolution data over a specific target once one has been identified.
What magnetics is good at finding: mapping geological structure and lithology (faults, intrusive contacts, folds); direct detection of magnetite-rich deposits (banded iron formations, some skarns, magnetite-associated porphyry systems); and — counterintuitively — locating some hydrothermal alteration zones indirectly, because certain alteration styles destroy magnetite and show up as a magnetic “low” or “hole” against the surrounding rock.
What it’s not good at: magnetics responds to magnetic mineral content, not metal grade directly, so a strong magnetic anomaly does not by itself mean an economic deposit — many magnetic highs are simply unmineralised magnetite-rich rock.
What is an induced polarization (IP) survey, and why does it matter for sulphides?
Induced polarization measures how strongly the ground holds an electrical charge after a current is switched off — a property called chargeability. Two current electrodes push an electrical current into the ground; two separate potential electrodes measure the voltage. When the current is turned off, the voltage doesn’t drop to zero instantly — it decays over a period of seconds as the ground releases the charge it briefly stored. Rock with a lot of disseminated sulphide minerals or clay content holds a charge longer and shows higher chargeability than barren host rock.
This makes IP the workhorse geophysical method for sulphide-hosted deposits. Because disseminated sulphide grains behave like tiny capacitors scattered through the rock, IP can detect low-grade, fine-grained sulphide mineralisation — including the kind spread through a large porphyry copper system — even when the sulphide content is too fine and too diffuse to show up clearly in other methods. IP surveys typically also record resistivity (how easily current flows through the ground) as a byproduct of the same electrode array, giving geologists a second, complementary dataset from the same field program.
What IP is good at finding: disseminated and massive sulphide mineralisation — the primary reason it’s run so often over porphyry copper-gold, epithermal, and volcanogenic massive sulphide (VMS) targets — and mapping clay-rich alteration zones, which also show elevated chargeability.
What it’s not good at: IP surveys are relatively slow and expensive to run at close electrode spacing over large areas compared with airborne methods, and chargeability alone can’t distinguish valuable sulphides (chalcopyrite, pyrite hosting gold) from barren pyrite — that distinction still needs geological logging and assay data.
How do electromagnetic (EM) surveys detect ore bodies?
Electromagnetic surveys work by inducing an alternating electromagnetic field into the ground — either from a loop on an aircraft, a ground-based transmitter loop, or a moving platform — and measuring the secondary field that bounces back. If a conductive body sits underground, the primary field induces swirling “eddy currents” within it, and those eddy currents generate their own secondary magnetic field that a receiver picks up. The strength and shape of that secondary response indicates how conductive, how large, and roughly how deep the target is.
Massive sulphide minerals — particularly pyrrhotite, chalcopyrite, and pyrite in sufficient concentration — are excellent electrical conductors compared with the silicate rock that typically hosts them, which is why EM is the go-to method for direct detection of massive (rather than disseminated) sulphide bodies, especially volcanogenic massive sulphide (VMS) deposits. Airborne EM systems, often flown together with a magnetometer on the same aircraft, can screen huge areas quickly for conductive anomalies; ground EM surveys then follow up individual anomalies with tighter, more detailed measurements to define their size, depth, and orientation before a hole is drilled.
What EM is good at finding: massive sulphide bodies with strong conductivity contrast against host rock, which is why it’s central to VMS and some nickel-sulphide exploration; it also works well through shallow conductive cover in many terrains.
What it’s not good at: EM responses can be masked or complicated by conductive overburden (thick clay, saline groundwater, graphitic shale), which can produce “false” conductors unrelated to any sulphide mineralisation — a classic source of drill-target disappointment that follow-up geological work is needed to rule out.
What does a gravity survey measure, and when is it used?
Gravity surveys measure tiny variations in the Earth’s gravitational field caused by differences in the density of rock below the survey point. Denser rock — a massive sulphide lens, a chromite layer, a body of dense mafic or ultramafic rock — exerts a very slightly stronger gravitational pull than the lighter rock around it, and precision instruments called gravimeters can detect that difference. Typical density contrasts run from around 1.2–1.5 (unconsolidated sediment) up to 2.5–3.5 for typical igneous or metamorphic rock, and 3–5 for massive metallic sulphide or oxide minerals — a large enough spread that a dense ore body produces a measurable, if often subtle, gravity anomaly.
Because a gravity survey requires a precisely levelled reading at each station along with careful corrections for elevation, latitude, and terrain, ground gravity surveys are typically slower and more labour-intensive per station than airborne magnetic or EM work, though airborne and marine gravity gradiometry systems now allow wider area coverage. Gravity is prized less for speed than for what it uniquely reveals: high-density targets that magnetics and EM can miss entirely, such as non-magnetic massive sulphide lenses or buried mafic intrusions.
What gravity is good at finding: massive sulphide bodies, magnetite-poor iron formations, chromite layers, some skarn systems, and mapping the shape of buried intrusions — anywhere a meaningful density contrast exists between target and host rock.
What it’s not good at: subtle or low-density-contrast targets (most disseminated, low-grade systems) produce little to no gravity response, and survey costs and processing complexity are typically higher per data point than for magnetic or EM methods.
Comparing the four methods
| Method | What it measures | Best at detecting | Typical deployment | Limitation |
|---|---|---|---|---|
| Magnetics | Magnetic mineral content (mostly magnetite) | Geological structure, contacts, magnetite-rich deposits, some alteration zones | Airborne (regional) or ground (detail) | Doesn’t measure grade; many anomalies are barren |
| Induced polarization (IP) | Chargeability (charge retention) | Disseminated & massive sulphides, clay alteration | Ground, tighter grid, often paired with resistivity | Slower/costlier per area; can’t distinguish ore vs. barren sulphide alone |
| Electromagnetics (EM) | Electrical conductivity | Massive sulphide bodies (VMS, Ni-sulphide) | Airborne (regional screening) or ground (follow-up) | Conductive overburden causes false anomalies |
| Gravity | Rock density | Dense massive sulphides, chromite, mafic intrusions | Ground (detailed) or airborne/marine gradiometry (regional) | Weak response to low-density-contrast targets; slower per station |
In practice, most exploration programs run two or more of these methods together — commonly magnetics and EM flown on the same airborne survey, followed by ground IP over the most promising conductive or structural anomalies — because each method is sensitive to a different physical property, and a target that shows up consistently across multiple methods is a far stronger drill candidate than one flagged by a single technique.
How do geologists turn a geophysical anomaly into a drill target?
A raw geophysical anomaly — a magnetic high, an IP chargeability spike, a conductive EM plate, a gravity high — is a hypothesis, not a discovery. Geophysicists process the raw survey data into grids and maps, then apply modelling and inversion software to estimate the size, depth, and shape of the body producing the anomaly. That model is then tested against everything else known about the area: surface geological mapping, existing drill data, geochemical results, and the deposit model the team believes applies to the target.
Only anomalies that are geologically plausible — that sit in the right rock type, the right structural setting, and align with other supporting data — get ranked and prioritised for drilling. This is also where the connection back to core logging and data management becomes concrete: once a hole is drilled to test a geophysical target, the lithology, alteration, structural, and mineralisation logging from that hole is what either confirms or rules out the geophysical model, and that logged data needs to be captured consistently and tied back to the same coordinate and depth reference the geophysical model used. Programs that keep drillhole data, collar coordinates, and downhole logging in one structured, exportable database — which is exactly what Blue Butterfly’s browser-based core logging platform is built for — make it far easier to overlay drill results directly against a geophysical model and see quickly whether the target held up.
FAQ
Do I need all four geophysical methods for every exploration program? No. The choice of method depends on the deposit type being targeted and what physical property is likely to contrast with the host rock. A program targeting a magnetite-rich skarn will lean on magnetics and gravity; a program targeting a VMS deposit will lean more heavily on EM and IP. Most programs still combine at least two methods to cross-check results.
Can geophysics find a deposit on its own, without drilling? No. Geophysics narrows down where to drill by highlighting anomalies consistent with a target deposit model, but it cannot directly confirm grade, true rock type, or economic viability. Only drilling, sampling, and assaying can do that.
Why do exploration news releases mention “IP chargeability anomalies” or “EM conductors”? Companies report these results because a strong, well-defined geophysical anomaly is often the basis for planning a drill program, and disclosing it gives investors a sense of what’s being tested and why. This is standard technical disclosure, not a confirmed discovery — the anomaly still has to be drill-tested to know if mineralisation is actually present. This section is educational and not financial or investment advice.
What’s the difference between airborne and ground geophysical surveys? Airborne surveys (fixed-wing or helicopter) cover large areas quickly and are typically used for regional screening; ground surveys are slower and more labour-intensive but provide tighter spacing and higher resolution over a specific target, and are usually run as a follow-up once an airborne anomaly or geological target has been identified.
How deep can geophysical surveys see? It depends heavily on the method, the instrument, and the conductivity/density contrast involved. Some airborne EM and magnetic surveys can resolve features hundreds of metres deep, while others (especially high-resolution ground IP) are more effective in the near-surface few hundred metres. Depth of investigation is always a key question to ask when interpreting any survey result.
Sources
- U.S. Geological Survey — Geophysical Mapping (Earth MRI): https://www.usgs.gov/special-topics/earth-mri/geophysical-mapping
- U.S. Geological Survey — Geophysical Methods in Exploration and Mineral Environmental Investigations: https://pubs.usgs.gov/of/1995/ofr-95-0831/CHAP3.pdf
- Advanced Geosciences, Inc. (AGI) — An Introduction to Induced Polarization (IP) Surveying: https://www.agiusa.com/introduction-to-induced-polarization-surveying
- University of British Columbia, EOSC 350 — Introduction to Induced Polarization Surveying: https://www.eoas.ubc.ca/courses/eosc350/content/methods/meth_2/ip.pdf
- Wikipedia — Induced polarization (cross-checked against AGI/UBC technical sources): https://en.wikipedia.org/wiki/Induced_polarization
- Wikipedia — Exploration geophysics (general method overview, cross-checked against USGS sources): https://en.wikipedia.org/wiki/Exploration_geophysics
- GeologyScience — Electromagnetic (EM) Methods: https://geologyscience.com/geology-branches/geophysics/electromagnetic-em-methods/
- GeologyScience — Gravity Surveys: Basic Principles, Applications: https://geologyscience.com/geology-branches/geophysics/gravity-surveys/
- Britannica — Magnetic survey: https://www.britannica.com/science/magnetic-survey
This content is educational and does not constitute financial or investment advice.