Exploration Drilling: Methods, Costs and How to Plan Holes

Exploration drilling is the stage of a mineral project where holes are drilled to test whether a target seen at surface, in geophysics or in satellite data actually carries mineralisation at depth. It produces the only direct samples of the rock below cover: chips from rotary methods, or solid cylinders of core from diamond drilling. Everything that follows, from an exploration target to a resource estimate to a mine plan, rests on what those samples show and how carefully they were collected.

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Drilling is also expensive, slow to mobilise and noisy on the ground, so the method you choose and the order you use it in matter as much as the number of metres. A company that drills diamond core on a target it has not yet tested with cheaper methods can spend a season’s budget on a handful of holes. One that skips core altogether can end up with grades nobody will finance because the geology was never seen intact. Our guide to junior and major gold exploration companies lists the red flags investors look for.

“Diamond drilling is the most common type of drilling conducted in Ontario, according to the province’s exploration permit guide.”

Types of drilling in mineral exploration, compared

The JORC Code’s Table 1 asks every public report to state the drill type used and lists the common ones: core, reverse circulation, open-hole hammer, rotary air blast, auger, Bangka and sonic. Investors reading an announcement should find that information in the first few lines of the sampling section. Here is what each method does and where it fits in a program of mineral exploration drilling.

Rotary air blast (RAB)

RAB uses a rotating blade bit on single-tube rods, with compressed air lifting the cuttings up the outside of the rods. Because the returning chips travel up the open hole, they can pick up material from higher levels. Novo Resources’ drilling glossary describes RAB sample quality as generally poorer because of that downhole contamination risk, and quotes a base rate of $12–20 per metre. It is an early-stage tool for geochemical sampling of the weathered profile, not for grade.

Aircore (AC)

Aircore uses a three-blade bit and a dual-tube system: air goes down between the tubes and the sample comes back up the inner tube, which keeps it cleaner than RAB. It only cuts unconsolidated or weathered material, so it stops at fresh rock. That makes it ideal for mapping bedrock geochemistry beneath sand, laterite or transported cover, at a quoted base rate of $18–30 per metre in the same glossary.

Reverse circulation (RC)

RC drives a hammer bit with compressed air sent down the outer tube, and the broken chips return through the inner tube to a cyclone and splitter. It cuts fresh rock quickly and generally gives cleaner samples than RAB or aircore. Novo quotes $30–55 per metre. RC is the workhorse of exploration drilling for gold and base metals in dry terrain: fast enough to test a target properly, cheap enough to drill many holes.

Diamond core (DD)

Diamond drilling uses a hollow bit set with industrial diamonds to cut an intact cylinder of rock, which is lifted to surface and laid in core trays. It is the slowest and dearest method (Novo quotes $110–250 per metre) but the only one that preserves the rock fabric: veins, contacts, fault orientations and textures. Ontario’s permit guide notes that a diamond-drill hole can run from a few metres to several thousand metres, with core diameters from under 2 cm to about 15 cm. Our guide to diamond drilling costs per metre, underground and surface breaks the rates down further.

Sonic, auger and other specialist methods

Sonic rigs vibrate the drill string to recover continuous samples from soft, unconsolidated ground such as tailings, gravels and mineral sands. Auger drilling is used for soils and shallow cover; Ontario’s guide lists it for geotechnical soil sampling and mentions overburden drilling for glacial deposits. Directional and wedged diamond holes branch off an existing hole to hit a target from another angle, saving the cost of drilling the upper section again.

Those are base rates. The same glossary warns that consumables, slow penetration, standby, mobilisation and water cartage often bring the final all-inclusive figure to double the quoted rate. Treat any single per-metre number as a starting point for a budget, never the budget itself.

Core drilling in mineral exploration: sizes, orientation and logging

Core drilling in mineral exploration is judged on recovery, size and what the geologist does with the core afterwards. Core size is set by the wireline bit series. The standard Q-series sizes run from AQ (27 mm core) through BQ, NQ and HQ to PQ (85 mm core). Novo’s glossary gives the same 27 to 85 mm span and says about 50 mm is the common size for exploration.

Why core size is a sampling decision

The CIM Mineral Exploration Best Practice Guidelines say the hole diameter should provide enough representative material for geological description, geotechnical work, analysis and a stored reference. For fine-grained, evenly spread mineralisation, NQ is often enough. For coarse gold, where a few particles carry the grade, a larger diameter reduces the chance that a sample misses or over-counts them. JORC Table 1 flags coarse gold as a case with “inherent sampling problems” that needs extra explanation.

Oriented core and structure

Many deposits are controlled by veins, shears or faults. The CIM guidelines note that for structurally controlled deposits, oriented core gives a better spatial description of structures and improves the predictive value of models built from them (see our comparison of 3D geological modelling software). Logging records the angle of each structure to the core axis. Without orientation, you know a vein is there but not which way it dips, which makes the next hole a guess.

What a good core log contains

• Lithology, alteration and mineralisation, each with type and intensity, as the CIM guidelines list.
• Structure, with angles to the core axis and, where oriented, true orientation.
• Geotechnical data such as recovery and fracture counts.
• Photographs of every tray before the core is split, kept for later review.
• Sample intervals marked in the trays by the logging geologist.

Ontario’s guide describes the standard practice after logging: the sampled core is cut in half lengthwise, one half goes to the laboratory and the other is stored as a reference. That retained half is what an auditor, a buyer or a future resource geologist will want to see. The CIM guidelines ask that the reason be documented any time drill material is not kept. What the laboratory then does with the core is covered in our guide to XRF, fusion and assay methods.

Planning a mineral exploration drilling program

A drill program is a set of questions. Is the anomaly real? How wide is the zone? Does it continue down dip? Each hole should answer one of them, and the plan should say which. The CIM guidelines encourage preparing a working cross section that shows the expected target location for every hole before it is drilled, and updating those sections as results come in.

From target to collar

Drill locations are chosen from earlier work. Ontario’s guide lists prospecting, geological mapping and geophysical surveys as the usual sources of targets. Our guide to geological mapping methods and scales covers the mapping side, and the geophysics hub for mineral exploration covers magnetics, IP and EM surveys that often sit between mapping and the first hole. Satellite analysis now adds an earlier, cheaper screening step at licence scale.

Collar surveys and downhole deviation

Holes wander. They bend with rock layering, bit pressure and angle. The CIM guidelines recommend deviation surveys at regular intervals down the hole, using instruments suited to the hole size, angle, length and the magnetic nature of the host rock. Collar positions from a handheld GPS may be fine at the exploration stage, but more precise surveying is expected once there is a discovery to delineate.

Sample intervals and assays

Reverse circulation and other rotary holes are usually sampled at a constant length, commonly one metre. Core intervals can follow geology instead, breaking at vein margins and contacts. JORC’s own example of a plain “industry standard” disclosure reads: reverse circulation drilling used to obtain 1 m samples, from which 3 kg was pulverised to produce a 30 g charge for fire assay. That single sentence tells a reader the method, the interval, the mass and the assay route.

The CIM guidelines ask for a complete, uninterrupted run of samples through the expected mineralised interval, plus enough unmineralised samples on either side to close it off. Samples should not straddle different rock types or very different styles of mineralisation. Quality control means external blanks, certified reference materials, duplicates and check assays at a second laboratory, all reported, not only the flattering ones.

What exploration drilling costs, and who helps pay for it

The per-metre rate is only the most visible number. A realistic budget adds mobilisation, pads and access, water, standby, downhole surveys, core handling, sample preparation, assays and the geologist’s time. Newfoundland and Labrador’s maximum allowable cost schedule for its incentive program gives a rare public benchmark: $200 per metre for diamond drilling, all-in including standby, moves and consumables; $25 per metre for downhole geophysics; $70 per geochemical analysis; and $900 per day for a professional geologist. Those are caps for rebates, not market prices, and the schedule is undated, so check the current version.

Co-funding in Western Australia

Some governments share the cost of drilling to encourage discovery. Western Australia’s Co-funded Exploration Drilling Program offers up to a 50% refund on direct drilling costs for innovative projects, with applications opening on the first Monday of February and August for 26 days. The department’s program sheet caps the refund at $220,000 for its Deep Drill category (one or two diamond holes planned to 600 m or more, which may include an RC precollar), $180,000 for the Universal category and $40,000 for prospectors. Confirm the caps for the round you apply to.

• Mobilisation and demobilisation: often a fixed charge, large for remote or helicopter-supported sites.
• Standby: hours the rig waits for water, access, weather or the geologist.
• Consumables: bits, reaming shells, casing, drilling fluids.
• Downhole surveys and geophysics at regular intervals.
• Sample handling, preparation and assays, which the estimator above helps size.
• ⚠ Rehabilitation of pads, sumps and tracks, which permits require.

Permits, pads and footprint: drilling and exploration on the ground

Drilling and exploration permits vary by jurisdiction (for Morocco, the DRC, the Sahel and Québec, see our guide to the permis de recherche, in French), and most regulators treat mechanised drilling as a step up in disturbance. Ontario is a useful, clearly documented example. Its early exploration permit guide says an exploration permit is required to use a drill that weighs more than 150 kilograms. The same guide requires permits for mechanised stripping over 100 square metres and bedrock excavation over three cubic metres within a 200-metre radius.

What a drill pad involves

Ontario’s guide describes a typical pad as 20 to 40 metres across, cleared so a rig can operate safely, with helicopter-supported pads typically 40 to 50 metres across. Rigs are often skid-mounted and dragged in by bulldozer or skidder, or flown in pieces to remote sites. Diamond rigs need pumped water to cool the bit and clear cuttings, drawn from sumps, lakes or streams, and casing through overburden is often left capped and marked so the hole can be deepened or surveyed later.

Portable exploration core drills

That 150 kg threshold is one reason small, man-portable rigs exist. Portable exploration core drills can be carried to steep or roadless ground and cut short holes with far less clearing. They trade depth and core size for access, so they suit early prospect testing, outcrop-scale sampling and teaching, rather than resource drilling. Check whether your jurisdiction sets a weight or activity threshold before assuming a small rig needs no permit.

Gold exploration drilling and underground exploration drilling

Gold exploration drilling has its own traps. Gold often sits in narrow veins and as coarse particles, so two halves of the same core can assay very differently. JORC Table 1 asks companies to report whether sample recovery relates to grade, and whether preferential loss or gain of fine or coarse material has biased results. In RC holes, wet samples and poor recovery are warning signs. In core, broken ground around veins can wash out exactly the material that carries the gold.

Practical responses include larger core, whole-core rather than half-core assays in high-nugget zones, bigger assay charges or screen-fire methods, and more duplicates. None of them removes the variability; they measure it. Our complete guide to mineral exploration covers where drilling fits in the wider sequence from target to resource.

Underground exploration drilling

Underground exploration drilling uses compact diamond rigs set up in drives or drill cuddies to reach extensions of a known orebody from inside the mine. Large contractors list underground coring as a core service; Major Drilling’s company history, for example, describes its move toward specialised drilling for difficult-to-access orebodies. Underground holes are usually shorter and fan out from one station, so collar and deviation surveys matter even more. For choosing a driller, see our guide to picking an exploration drilling contractor.

Geologic drilling beyond minerals

The same rigs and logging discipline serve other geologic drilling: geotechnical holes for pit walls and foundations, water bores, and scientific coring. Mineral exploration has the most demanding sampling and chain-of-custody requirements, because the results feed public disclosure and investment decisions.

Where satellite targeting fits before the first hole

Drilling is the only way to prove what is at depth, but it is a poor way to decide where to look. The cheapest metre is the one you never drill because the target was ruled out beforehand. That is the job satellite mineral detection does in an exploration drilling program: it screens the whole licence and ranks where the rig should go first.

Our satellite-based mineral detection analyses multispectral and hyperspectral imagery over your boundary to flag alteration halos, faults and fractures, and deposit-associated patterns, with no ground disturbance. You send coordinates, a KML/KMZ file or a polygon, plus the country and target mineral. We deliver in 5 to 20 business days depending on area and mineral complexity.

• Premium mineral-intelligence report: high-potential zones, prospectivity heatmaps, estimated location and depth ranges, indicative quantity and geological interpretation, as PDF plus georeferenced GIS files.
• Premium+ with TargetMax™ Drilling Intelligence: drilling-angle recommendations aimed at a higher ore-intersection probability, plus interactive 3D subsurface models of vein structures.
• Timeline and cost: early exploration from months to days, at up to 80–85% lower early-exploration cost.
• Track record: 100,000+ hectares scanned for 20+ mineral types in 25+ countries.
• ⚠ Limits: satellite targets are exploration targets, not resources. Drilling and assays confirm or reject them.

See how targets carry into 3D in our satellite-driven 3D mineral prospectivity mapping sample, draw your licence on mining.farmonaut.com: Map Your Mining Site, or request pricing through the mining query form.

A pre-drilling checklist

1. Confirm tenure and the permits your drill type and pad size require.
2. Draw a cross section for each planned hole with the expected target depth.
3. Choose the method and core size for the question, not by habit.
4. Agree sample intervals, QA/QC insertion rates and the laboratory before drilling.
5. Set up core photography, storage and a drill-hole database from day one.
6. Plan rehabilitation of every pad, sump and track.

“All-in drilling costs often reach double the quoted base rate once standby, moves and consumables are added, Novo Resources notes.”

Guides in this series

Five guides go deeper on the steps around exploration drilling.

• Precision exploration drilling contractors: how to compare drilling companies and read their bids.
• Diamond drilling, underground vs surface: what each setting costs per metre and when core is worth it.
• Geochemical survey design: soil, stream-sediment and plant sampling before the rig arrives.
• Geochemical analysis methods compared: portable and lab XRF, fusion, acid digestions and fire assay.
• Channel sampling in mining: when to cut a channel, take chips or sample a panel.

Frequently asked questions

Reviewed September 2026 against the JORC Code (2012) Table 1, the CIM Mineral Exploration Best Practice Guidelines (2018), Ontario’s Early Exploration Permit Activity Information guide, Newfoundland and Labrador’s maximum allowable cost schedule, Western Australia’s Co-funded Exploration Drilling Program pages and caps sheet, Novo Resources’ drilling techniques and costs glossary, and the standard Q-series core size reference.

Cost figures are published benchmarks and caps, not quotes, and can change; confirm with contractors and program administrators before budgeting. Satellite targets are exploration targets, not mineral resources.