IP Geophysical Survey: Cost, Depth and Chargeability

Electrical resistivity geophysics measures how hard it is for electric current to flow through the ground. Dry, solid rock resists; water-filled fractures, clay and connected sulphides let current through. Map those differences along a line or across a grid and you can see faults, weathering depth, alteration zones and, with the induced polarisation (IP) add-on, the scattered sulphide grains that often travel with copper and gold.

Unlike magnetics or airborne EM, these are contact methods: someone has to walk the line and plant electrodes. That makes them slower and more expensive per kilometre, and it is why they are usually aimed at targets that other methods have already found. Get the array and spacing right and a single IP line can tell you more about a prospect than a month of guesswork. Our guides to geomagnetic survey specifications, TEM, VTEM and CSAMT electromagnetic surveys and airborne geophysical survey platforms cover those other methods.

“EM detects massive sulphide only if its grains touch; IP picks up the disseminated ones.” (USGS, paraphrased)

How electrical resistivity geophysics works

The US EPA’s electrical resistivity page describes the basic set-up. Current is injected through two current electrodes, the potential difference is measured across two potential electrodes, and a resistivity meter records the ratio of voltage to current. A single measurement needs four electrodes coupled to the ground. Because resistance depends on the shape of the measured volume as well as the material, results are converted to apparent resistivity in ohm-metres using a geometric factor that depends on the electrode layout.

Apparent resistivity is the value a uniform half-space would give for that layout. Real ground is layered and faulted, so each reading is a weighted average of a large volume, with the material nearest the electrodes counting most. That is why modern electrical resistivity geophysics relies on inversion: software finds a resistivity model of the ground that reproduces all the readings along a line (a 2D section, or electrical resistivity tomography) or across a grid (3D).

What controls resistivity in rock

The EPA notes that mineral grains in soil and rock are essentially non-conductive, so resistivity in most ground depends on the amount of pore and fracture water and how salty it is. Resistivity falls with certain ore minerals, fine-grained clays and high temperature. Metallic sulphides and graphite are the exception to the “rock is resistive” rule: University of Alberta geophysics notes give sulphide resistivities of about 10⁻⁵ to 10⁻³ ohm-m, graphite about 8 × 10⁻⁶ ohm-m and pure diamond above 10¹⁰ ohm-m.

What electrical resistivity geophysics is used for

The EPA’s list of applications is broad: groundwater and aquifer studies, salt-water and contamination mapping, fault and fracture zones, karst and voids, archaeology and mineral exploration. In an exploration programme, electrical resistivity geophysics usually does three jobs. It maps the thickness of cover and weathering so that drilling and sampling are planned sensibly. It traces faults and shears, which are often more conductive than the rock around them because they hold water and clay. And it outlines resistive silicified zones or conductive sulphide-rich zones that may relate to mineralisation.

Electrode arrays and depth: designing a geophysical resistivity survey

The layout of the four electrodes is called the array, and it decides depth, resolution and signal strength. The EPA lists Wenner, Schlumberger, reverse-Schlumberger, gradient and dipole-dipole as the most common, and gives a rule of thumb for depth of investigation in uniform ground: roughly 30% of the current-electrode separation for Wenner, 25% for dipole-dipole and 20% for Schlumberger. It also notes that resolution is generally about half the electrode spacing, so widening the spread to see deeper costs you detail.

From pseudosection to inverted model

Raw readings are first plotted as a pseudosection. The EPA explains that each apparent resistivity value is plotted where 45° lines drawn from the midpoints of the current and potential pairs meet. Pseudosections are useful for checking data but have limited value for interpretation, so electrical resistivity geophysics data are normally inverted into 2D sections or 3D volumes. The EPA also warns about equivalence: different resistivity structures can produce similar readings, so an inverted model is one reasonable answer, not the only one. Geological control from mapping, pits and drill holes is what narrows it down.

Wenner, Schlumberger and when to use each

M.H. Loke’s widely used tutorial on electrical imaging surveys compares the arrays in practical terms. The Wenner array has the strongest signal of the common arrays because its geometric factor, 2πa, is the smallest, which helps in noisy areas. It resolves vertical changes (flat layers) well but is poorer at narrow vertical structures. Its median depth of investigation is about 0.52 times the electrode spacing “a”, or about 0.17 times the total array length. The EPA’s older resistivity guidance adds that Schlumberger soundings are faster in the field, because only the outer electrodes move between readings.

Dipole-dipole: the IP workhorse

Dipole-dipole separates the current pair from the potential pair and steps the potential dipole outward by multiples “n” of the dipole length. Loke describes it as very sensitive to horizontal changes, so good at mapping steep structures such as dykes, veins and alteration pipes, and it gives better horizontal coverage in 2D surveys. The catch is signal strength: voltage falls roughly with the cube of n, so for the same current it drops about 200 times from n = 1 to n = 6. Its median depth rises with n, as the chart shows.

Pole-dipole: the compromise many IP crews choose

Loke describes pole-dipole as an attractive alternative for IP. It keeps good horizontal coverage, has a stronger signal than dipole-dipole, and has lower electromagnetic coupling than the Wenner-type arrays because the current and potential circuits are kept apart. Its signal falls with the square of n rather than the cube, though Loke still advises against n values above about 8 to 10. The price is a remote current electrode placed far off the line: if that electrode sits more than five times the largest C1 to P1 distance away, the error from ignoring it is under 5%. Because the array is asymmetric, crews often read it in both forward and reverse directions and combine the two, which removes the bias it can put into the inverted model.

In practice the choice comes down to the target and the site. Steep, narrow bodies favour dipole-dipole or pole-dipole. Flat layers such as cover sequences favour Wenner or Wenner-Schlumberger. Noisy sites near power lines favour arrays with stronger signal. Running more than one array on the same electrode layout and inverting the data together is another option worth pricing, since the extra readings cost little once the cable is on the ground.

Real surveys show how the n-levels are chosen. The Max-K2 pole-dipole survey used 50 m dipoles and n = 1 to 6. At Eldorado the dipole-dipole survey used a 100 m dipole and n = 0.5 to 12.5. Gnawed Mountain ran a 100 m pole-dipole and dipole-pole survey from n = 0.5 to 19.5.

IP geophysics: induced polarisation and chargeability

Induced polarisation was discovered during ordinary DC resistivity work. The EPA’s IP and complex resistivity page explains what happens: after the current is switched off, the measured voltage does not drop to zero at once but decays over time, because the ground stores charge like a capacitor. The effect happens at interfaces between pore fluid and mineral grains. The EPA also notes that IP was historically used mainly for prospecting disseminated ore, and that IP and resistivity data are usually collected together, although IP needs specialised meters and quality data are harder to get.

Time-domain and frequency-domain IP

In a time-domain IP geophysical survey, the receiver integrates the decaying voltage over a set window and divides by the primary voltage. The result is apparent (integral) chargeability, reported in millivolts per volt (mV/V) or as a percentage. The EPA notes that values depend on the timing and are commonly converted to the Newmont standard of three-second on and off times with a one-second integration window. Frequency-domain IP instead injects current at two or more frequencies, typically between 0.1 and 10 Hz, and compares the responses; spectral IP, or complex resistivity, measures magnitude and phase across several frequencies.

What chargeability geophysics picks up

UBC’s Geophysics for Practicing Geoscientists lists the most chargeable materials as sulphide minerals (massive and disseminated), clay-rich materials and graphite, and calls sulphide exploration “unquestionably” the most common application of IP. It also warns that there is no standard unit for chargeability: milliseconds, percent or milliradians depending on the system, which makes comparing surveys tricky.

The key advantage over EM is disseminated mineralisation. The USGS chapter on VMS geophysics says IP is very effective at detecting disseminated sulphide bodies, which often sit in the altered halo around a massive sulphide lens, and that where sulphide grains are not electrically connected (as in stockwork below the lens), IP can succeed where EM does not. Porphyry copper halos and gold systems with disseminated sulphides are classic IP targets for the same reason.

• High chargeability, low resistivity: could be abundant sulphides or graphite, or clay-rich ground; check geology and magnetics.
• High chargeability, high resistivity: disseminated sulphides in silicified rock, a pattern many explorers look for.
• Low chargeability, low resistivity: usually saturated clay, saline water or weathered rock.
• ⚠ Clay trap: clays polarise too. A chargeability high in a clay-altered zone is not proof of sulphides.

IP survey for gold exploration: what it shows and where it fails

No IP survey measures gold. It maps the sulphide, clay and silica changes that gold systems often leave around them. The White and Hedenquist review of epithermal gold exploration says that, at prospect scale, IP, electromagnetic and resistivity surveys can locate pyritisation, silicification and clay alteration, and that exploration succeeds when several techniques are combined (SEG Newsletter, 1995).

• A mixed result: the Canadian Malartic bulk-tonnage gold deposit in Quebec. Osisko’s own account says earlier pole-dipole and dipole-dipole IP surveys gave inconclusive results, and that the deposit did not respond to ground magnetic, EM or IP surveys (Wares and Burzynski).
• A target, not an answer: the Eldorado gold property in Oregon. A dipole-dipole IP survey in July 2024 found a chargeability high with low resistivity. The interpreter said that pattern is typical of graphite-bearing sediment, but that the structures bounding it could have carried gold-bearing fluids. The report asked for ground checks first and a drill test on one area (Provenance Gold, 2024 IP report).
• A drill result from IP: in Zambia, the Chimiwungo North copper orebody was found in 2004 by drill-testing an IP chargeability anomaly (USGS, sediment-hosted copper model).

IP is worth pricing when gold is expected with disseminated sulphide in resistive rock, and when geology can separate clay or graphite from sulphide.

Self potential geophysical survey: the cheapest electrical method

Self-potential (SP) needs no transmitter at all. The EPA’s self-potential page describes it as a passive technique that, from the early twentieth century, was mainly used to prospect for metallic sulphide ore. Natural voltages arise from groundwater flow and from electrochemical reactions. The ore-related one, called mineralisation potential, forms when a metallic body connects zones of different redox potential and acts like a natural battery, with the most negative reading over its near-surface end. The EPA notes this is commonly indicative of sulphide, oxide or graphite bodies.

Field work is simple. A self potential geophysical survey uses non-polarising porous-pot electrodes, for example a copper rod in copper sulphate solution, one fixed at a base station and one roving, joined by insulated wire to a voltmeter. The roving electrode should revisit the base regularly to catch drift. The limits are just as clear: the EPA says depth is only roughly estimated from an anomaly’s half-width, the method is unlikely to see zones deeper than about 30 m, and it is no longer a modern ore-prospecting tool because most shallow ores have already been found. It still earns a place as a quick, cheap check over shallow conductors.

“SP is unlikely to be sensitive to zones deeper than about 30 metres.” (US EPA)

Running an IP geophysical survey that answers the question

Most failed IP surveys, and most disappointing electrical resistivity geophysics jobs in general, fail in design rather than in the field. Before a crew mobilises, write down the question the survey must answer: is there a chargeable body under this soil anomaly? How deep is the weathering? Does the alteration continue beneath cover? Then set the layout to match.

1. Lines across strike of the target, long enough that the deepest readings reach the target depth under the middle of the line.
2. Array to suit geometry: dipole-dipole or pole-dipole for steep bodies; Wenner or Schlumberger styles for flat layering.
3. Good electrode contact: Loke flags poor ground contact as a common problem, and in dry, resistive ground it means noisy data. Check contact resistance before reading.
4. Repeat readings and reciprocals to measure noise; ask for the error estimates in the data.
5. Joint inversion of resistivity and chargeability, with depth-of-investigation estimates shown on the sections.
6. Drill-test the model, not the pseudosection: the EPA notes pseudosections have limited value compared with inverted sections.

Reading an inverted section

Start at the edges of the model. The EPA recommends site-specific depth-of-investigation estimates during inversion, which show where the data stop constraining the model; anything below that line is the software’s guess. Then compare resistivity and chargeability side by side, and look for bodies that cut across layering rather than following it. Finally, check whether the anomaly is wider than half the electrode spacing, since smaller features are below the survey’s resolution. A good section from electrical resistivity geophysics comes with these limits drawn on it.

For how resistivity and IP sit alongside magnetics, gravity and EM in a full programme, see our overview of non-intrusive geophysics. For the mapping that should come first, see our guide to geological mapping methods and scales.

What an IP geophysical survey costs and what the crew delivers

Three British Columbia assessment reports filed with the provincial geological survey show the range. At Gnawed Mountain (2015) a five-person crew with a truck billed C$3,850 per day, and the filed total for about 12 line-km of pole-dipole work was C$23,876. (BC assessment report 35646). On the Max-K2 property (surveyed in 2010) a six- or seven-person crew billed C$3,200 to C$3,500 per day; with line cutting, mobilisation, camp and reporting the total was C$97,350 for about 20.5 line-km (BC assessment report 32790). The Copau survey (2012) filed C$112,132 for 10 lines and 11.5 line-km, with 3D inversion models (BC assessment report 33292).

That is roughly C$2,000, C$4,750 and C$9,750 per line-km. IP needs specialised meters, and the US EPA notes that good data are harder to get than for resistivity alone. Scopes differ, so ask each contractor what the rate includes: line cutting, camp, mobilisation, inversion and the report.

Ask for these in the quote:

• Line spacing, dipole length and highest n-level. The 2024 Eldorado survey used four lines 200 m apart (6.2 line-km, 13 days in the field), a 100 m dipole and n = 0.5 to 12.5; its model reached 300 to 400 m.
• Raw readings plus inverted resistivity and chargeability sections, with units and timing window stated.
• Depth slices and GIS or Google Earth files, as the Eldorado report supplied.

If you are still choosing a contractor, our guide to geophysical survey companies compares ground and airborne providers, and our note on sequencing a first-season IP survey covers the order of work. Try your own block size and rates below.

▶ Explains how satellite targets are ranked into a drill order before field work. · 2:19

Where satellite targeting fits before electrical resistivity geophysics

Because electrical resistivity geophysics and IP are slow per kilometre, the biggest saving is in deciding where the lines go. Satellite remote sensing maps surface alteration (clays, sericite, iron oxides) and structure across a whole licence from reflected light. Lines laid across the best alteration and structure intersections test the most likely ground first, and electrical resistivity geophysics then tells you what lies beneath.

IP coverage is small compared with a licence. The filed surveys above covered 6 to 20 line-km, so a block of a few hundred hectares gets only about 4 to 10 lines at 200 m spacing. Satellite alteration and structure maps help choose where those lines go, and they come before geophysics, not instead of it. Farmonaut returns exploration targets, not resources, and the IP lines then test them below the surface.

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.

• 📊 Faster screening: exploration timelines cut from months to days at the targeting stage.
• 📊 Lower cost: up to 80–85% lower early-exploration cost.
• Deliverables: prospectivity heatmaps, estimated location and depth ranges, geological interpretation, and GIS files to plan IP lines around.
• 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: satellites see the surface; targets are exploration targets, not resources, and need geophysics, sampling 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. Our mineral exploration guide sets out the full sequence.

▶ Shows how Farmonaut Mining Pro ranks where to drill first on a licence from satellite and geophysical layers. · 1:46

Frequently asked questions

Depth rules of thumb assume uniform ground and are no substitute for inversion and site-specific design. Satellite targets are exploration targets, not mineral resources, and must be confirmed by geophysics, sampling and drilling.