Exploration drilling methods fall into two families: those that recover an intact cylinder of rock (diamond and sonic) and those that return broken chips (RC, RAB, and auger). They trade cost against sample quality and depth — auger and RAB are cheap and shallow for first-pass sampling, RC balances cost and quality, and diamond core gives the highest-fidelity data for defining a deposit.
There is no single “best” drill. Each method wins on a particular mix of price, depth, ground conditions, and sample quality. This guide explains how the five main methods work, what each one samples, what it costs, and how an exploration team decides which rig to put on which hole.
How do explorers choose a drilling method?
Choosing a drill comes down to four questions, traded off against each other on every program: How deep is the target? How hard is the ground? How good does the sample need to be? And how much can the budget stand?
Those questions pull in opposite directions. The cheap, fast methods are shallow and produce rough samples; the methods that give pristine, deep, fully interpretable rock are slow and expensive. So drilling tends to run as a funnel, mirroring the wider exploration sequence: a team uses cheap, shallow drilling to screen a lot of ground quickly, then steps up to more expensive methods only on the targets that survive.
The single most important distinction between methods is what they bring to surface:
- Chip samples (RAB, air core, RC) — the rock is broken up at the bit and blown to surface as fragments. You learn the chemistry and rough geology of an interval, but the rock’s original texture and structure are destroyed.
- Core samples (diamond, sonic) — the method recovers a continuous, intact cylinder of rock. You can see textures, contacts, veins, and structures exactly as they sit in the ground, and measure their angles.
That difference — chips versus core — drives almost everything else: cost, speed, and how much geology you can actually read from the hole.
Auger drilling: the lightest, cheapest start
Auger drilling is the smallest of the exploration methods. A helical screw is rotated into the ground and the soil rides up the blade of the screw to surface, much like a drill bit pulling up wood shavings. Rigs range from handheld units to small vehicle-mounted machines.
Auger only works in soft ground — soil, sand, and weathered material — and it is shallow, typically reaching only around 25 metres at most. What it offers is speed and very low cost. That makes it ideal for geochemical reconnaissance: collecting soil and saprolite samples across a wide area to map near-surface metal anomalies. In practice, auger holes are often used to find the good spots to put bigger, more expensive rigs.
It does not penetrate fresh rock and returns no usable structural information, so it never defines a deposit on its own. It is a search tool, not a measurement tool.
RAB (rotary air blast): fast, shallow reconnaissance
Rotary air blast (RAB) is the most common shallow drilling method. A piston-driven “hammer” drives a tungsten-steel bit into the rock, breaking it into chips that compressed air lifts to surface. RAB can punch many holes quickly and reaches deeper than auger — roughly up to 100–150 metres, though it is usually run much shallower.
The catch is sample quality. In RAB, the chips travel up the outside of the drill rods (the annulus), so they scrape past everything above and can be contaminated by material that has caved or smeared from shallower in the hole. For that reason, RAB samples are treated as indicative only, and RAB holes are typically excluded from formal Mineral Resource estimates.
RAB’s role is the same as auger’s, one size up: screening large areas cheaply to find anomalies worth following with a better method.
A close cousin — air core (AC) drilling. Often preferred over RAB because it returns cleaner samples: a three-bladed bit on a hollow rod lets compressed air flush the cuttings up an inner tube rather than the outside of the rods, so they’re far less prone to contamination. Air core is slower and costlier than RAB and still can’t cut fresh rock, but it sits neatly between RAB and RC for first-pass work.
Reverse circulation (RC) drilling: the workhorse compromise
Reverse circulation (RC) is the workhorse of exploration drilling because it strikes the best balance between cost and sample quality. Like RAB, it uses a piston-driven hammer and a tungsten-steel bit, but larger rigs let it drill much deeper — depths of up to around 500 metres are routinely achieved.
The key to RC is in the name. The drill rods are dual-walled, and compressed air drives the dry rock chips up through an inner tube, sealed off from the walls of the hole. Because the sample never touches the sides on its way up, RC delivers essentially contaminant-free chips — clean enough to be used for resource-quality assays. That reliability also makes it the standard for grade control in operating mines, where it’s used to define the boundary between ore and waste.
RC is slower and more expensive than auger, RAB, or air core, but it is markedly cheaper than diamond drilling — commonly on the order of 25–40% less per metre. What you give up is the rock itself: RC returns a pile of chips for each interval, not an intact core, so you can read the chemistry and broad lithology but not fine textures, vein relationships, or structural angles.
Diamond core drilling: the highest-fidelity method
Diamond drilling is the most expensive method — and the most informative. A diamond-impregnated bit on the end of hollow drill rods grinds out a continuous cylinder of rock, the core, which is recovered intact and laid out in trays in the exact order and orientation it came from the ground.
That intact core is the gold standard of exploration data. Because the rock is preserved, geologists can log lithology, alteration, veining, and mineralisation in full detail; measure the angles of structures and contacts; photograph the core; and select precise intervals for assay. It is the only method that supports proper geotechnical logging (recovery, RQD, fracture frequency) and structural measurement, and resource estimates, mine designs, and feasibility studies ultimately rest on it.
Diamond drilling also goes where nothing else can: it penetrates hard rock and can reach several kilometres deep. The trade-offs are speed and cost. It is the slowest method and needs more equipment, fuel, and water, so a single deep diamond hole can run well into six figures before a sample is even assayed. Explorers therefore tend to bring in diamond rigs once they’ve found a mineralised system worth defining in detail, or when they need to test hard-rock structures at depth.
Sonic drilling: continuous core in difficult ground
Sonic drilling is the specialist of the group. The rig sends high-frequency resonant vibrations down the drill string to the bit, which fluidises the material right at the cutting face and lets a core barrel advance with very little friction. The result is near-continuous core recovery — close to 100% — even in the loose, saturated, layered, or mixed ground where other methods struggle to recover anything coherent.
That makes sonic the method of choice for unconsolidated material and overburden: thick soil and regolith profiles, alluvial and placer deposits, mineral sands, tailings, and geotechnical or environmental holes. In soft ground it is also fast — often three to five times quicker than conventional drilling — produces much less waste, and can run with little or no drilling fluid behind a fully cased hole, which matters in environmentally sensitive areas.
Its limits are depth and economics. Small commercial sonic rigs generally work to a few hundred metres (often quoted as under about 1,000 feet), and the method can be prohibitively expensive for routine early-stage hard-rock targets. Where the ground is soft and continuous core is essential, though, nothing else competes.
The exploration drilling methods at a glance
| Method | Sample | How it works | Typical depth | Relative cost | Best for |
|---|---|---|---|---|---|
| Auger | Soil / regolith | Helical screw lifts soft material to surface | ~25 m | Lowest | Geochemical soil reconnaissance |
| RAB | Chips (up outside of rods) | Air-hammer bit; chips blown up the annulus | ~100–150 m | Very low | Fast, shallow anomaly screening |
| Air core | Chips (up inner tube) | Bladed bit; cleaner chips up an inner tube | ~100 m+ | Low | First-pass sampling in soft/weathered ground |
| RC | Chips (up inner tube) | Air-hammer bit; clean dry chips up an inner tube | ~500 m | Moderate | Resource-grade sampling, grade control |
| Diamond | Intact core | Diamond bit cuts a continuous rock cylinder | Several km | Highest | Detailed logging, structure, geotech, deposit definition |
| Sonic | Intact core | Resonant vibration recovers continuous core | A few hundred m | Moderate–high | Unconsolidated ground, overburden, mineral sands |
Depth and cost figures are directional and vary widely with ground conditions, rig, and location; treat them as relative, not absolute.
Which method for which job?
Read as a sequence, the methods map onto the stages of a program:
- Earliest, widest search — auger and RAB (and air core) cover large areas cheaply, turning a whole licence into a handful of geochemical anomalies. Samples are rough, and that’s fine; the job is to rank ground, not measure a deposit.
- Target testing and resource-grade sampling — RC takes over once there’s a real target. It’s cheap enough to drill a lot of metres yet clean enough that the assays count, which is why so much of the world’s drilling is RC.
- Definition and detail — diamond core comes in when a discovery needs to be understood and measured: the structures that control it, the geotechnical character of the rock, and the precise grade and width that feed a resource model.
- Difficult ground — sonic is chosen by the material, not the stage. Whenever continuous core is needed through soft, wet, or unconsolidated cover, it’s the right tool regardless of where the project is in its life.
Many programs use several methods on the same prospect — for example, RC to test a target quickly, then a diamond “tail” on the same hole to core the mineralised zone for detailed logging and structural data.
What does the drill method signal about a project?
For investors reading drill-result announcements, the method itself carries information. A company reporting auger or RAB results is usually at the earliest, most speculative stage — hunting anomalies. RC results suggest a defined target being tested for grade. Diamond drilling generally signals a project advanced enough to justify the most expensive, most detailed work — defining a known mineralised system. The drill type is one clue, among many, to how mature a project really is.
This section is educational and is not investment advice.
From the rig to the database: where logging fits
Whichever method is used, the moment core or chips reach surface the data work begins — and the type of sample shapes how it’s recorded. Core (diamond, sonic) is logged as continuous depth intervals: lithology, alteration, structure, mineralisation, geotechnical measurements, and sample numbers, all tied to precise from–to depths and rendered as a strip log. Chips (RC, RAB) are logged interval by interval too, but as broken-rock descriptions and assay intervals rather than intact rock.
This is the first hard, direct data a program produces, and every later model is built on it — so it pays to capture it cleanly from the first metre. In Blue Butterfly, the drilling method and core diameter are standard collar fields on every hole, and the same browser-based platform handles both worlds: interval logging and depth-scaled strip logs for core, and structured interval and assay tables for RC chips — all validated at the point of entry and synced to a single cloud database, online or offline. Whatever you put down the hole, the data lands in one place, ready to model from.
FAQ
What are the main types of exploration drilling? The five most common are auger, RAB (rotary air blast), RC (reverse circulation), diamond core, and sonic. Auger and RAB are cheap, shallow methods for early geochemical sampling; RC is the mid-cost workhorse that returns clean chips; diamond drilling recovers intact core for detailed work; and sonic recovers continuous core in soft, unconsolidated ground. Air core is a close relative of RAB used for cleaner first-pass sampling.
What is the difference between RC and diamond drilling? RC drilling returns broken rock chips, blown up an inner tube by compressed air — fast, relatively cheap, and clean enough for resource-grade assays, but with no intact rock. Diamond drilling recovers a continuous, intact core that preserves texture, veins, and structure, so it gives far more geological detail. Diamond is slower and more expensive — often 25–40% more per metre than RC.
Why don’t explorers just use diamond drilling for everything? Cost and speed. Diamond drilling is the slowest, most expensive method, and a single deep hole can cost well into six figures. Spreading it across a whole licence would be wasteful, so cheaper methods (auger, RAB, RC) are used first to narrow the search down to the few targets that justify core drilling.
Which drilling method gives the best samples? Diamond core gives the highest-fidelity sample because it recovers intact rock you can log, measure, and photograph in full detail. Sonic also recovers continuous core and is unmatched in soft, unconsolidated ground. Chip methods (RC, air core, RAB) give progressively less reliable samples, with RAB the lowest quality because its cuttings can be contaminated on the way up.
How deep can each method drill? As a rough guide: auger to about 25 m, RAB to roughly 100–150 m, RC routinely to about 500 m, and diamond drilling to several kilometres. Sonic typically reaches a few hundred metres. Actual depths depend heavily on ground conditions, rig size, and budget.
Sources
- Stockhead — Drilling guide: here’s everything you need to know (auger, RAB, air core, RC, diamond — how each works and what it’s used for): https://stockhead.com.au/primers/drilling-guide-heres-everything-you-need-to-know/
- Mining.com.au — Mineral exploration drilling: a guide to AC, RC, and diamond methods (sample quality, RC depth, RAB resource-estimate exclusion): https://mining.com.au/mineral-exploration-drilling-a-guide-to-ac-rc-and-diamond-methods/
- NextInvestors — The difference between aircore, RC and diamond drilling (method roles and project-stage signalling): https://nextinvestors.com/learn-to-invest/mining/difference-between-aircore-rc-and-diamond-drilling/
- UndervaluedEquity — What distinguishes auger drilling from RAB, RC and diamond drilling (auger and RAB depth ranges; soft-ground use): https://undervaluedequity.com/drilling-rigs-what-distinguishes-auger-drilling-from-rab-drilling-rc-drilling-and-diamond-drilling/
- RCDrilling.com — Comparative costs of drilling (RC vs diamond cost per metre): https://www.rcdrilling.com/rc-drilling-guide/comparative-costs-of-drilling/
- Boart Longyear — Sonic drilling: a sound solution for frustrating formations (resonant mechanism; recovery in difficult ground): https://www.boartlongyear.com/insite/a-sound-solution-for-frustrating-formations/
- Royal Eijkelkamp — Understanding sonic drilling (continuous core recovery, speed, waste, fluidless cased-hole drilling): https://www.royaleijkelkamp.com/academy/knowledge-hub/understanding-sonic-drilling/
- CorePlan — Air core drilling: what it is and what’s good about it (air core sample quality and limitations): https://www.coreplan.io/blog/air-core-drilling-what-it-is-and-whats-good-about-it