Exploration drilling is the moment of truth in any mineral exploration programme. After months of desktop research, geological mapping, geochemical sampling, and geophysical surveying, drilling is the only way to physically confirm what lies beneath the surface. A well-planned drilling campaign can transform a promising anomaly into a bankable resource estimate. A poorly conceived one can waste millions of dollars and months of valuable time.
Choosing the right drilling method is therefore one of the most consequential decisions an exploration team will make. The three principal exploration drilling techniques — Diamond Core Drilling, Reverse Circulation (RC) Drilling, and Rotary Air Blast (RAB) Drilling — each offer distinct advantages and limitations. Understanding these differences is essential for anyone involved in mineral exploration, whether you are a geologist planning a programme, an investor evaluating a project, or a mining company executive allocating exploration budgets.
This article provides a thorough comparison of all three methods, covering their mechanics, optimal applications, depth capabilities, cost profiles, and the quality of geological data they deliver. We also draw on ALOM's direct field experience in East Africa to illustrate how these methods are applied in practice.
Diamond Core Drilling: The Gold Standard of Exploration Data
Diamond core drilling is the most technically sophisticated and data-rich of the three methods. It uses a hollow drill bit studded with industrial diamonds to cut a continuous cylindrical core of rock from the subsurface. This core — typically 47.6 mm (NQ), 63.5 mm (HQ), or 85.0 mm (PQ) in diameter — is brought to the surface intact, preserving the original geological relationships, structures, alteration patterns, and mineralisation in their undisturbed context.
How It Works
The diamond drill bit is attached to the end of a rotating drill string. As the bit advances, it cuts an annular groove in the rock, and the resulting core feeds into a core barrel behind the bit. When the core barrel is full — usually after 1.5 to 3 metres of advance — the drill string is lifted, and the core barrel is retrieved to the surface using a wireline system. The core is then carefully extracted, placed in labelled core trays, and logged by geologists before being sampled and sent to a laboratory for assay.
Diamond drilling rigs range from compact, man-portable units capable of operating in rugged terrain to large truck-mounted rigs designed for deep holes in accessible locations. The rigs require a supply of water mixed with drilling additives to cool the bit and flush cuttings from the hole. Holes can be drilled vertically or at any angle, allowing geologists to intersect steeply dipping ore bodies at optimal angles.
Strengths of Diamond Core Drilling
- Unmatched geological detail. The continuous core provides a complete record of every rock type, structure, vein, and alteration zone intersected by the hole. Geologists can observe textures, measure structural orientations, and identify mineralogical relationships that are invisible in drill chips or cuttings.
- Structural and geotechnical data. Oriented core techniques allow measurement of the true orientation of faults, fractures, bedding planes, and foliation — critical data for geological modelling and mine design.
- Deep drilling capability. Diamond drilling can reach depths exceeding 3,000 metres, far beyond the practical limits of RC or RAB methods.
- Compliance with reporting standards. Resource estimates prepared for public reporting under JORC, NI 43-101, or SAMREC standards typically require diamond core data, particularly for higher-confidence resource categories (Indicated and Measured).
- Sample integrity. The core provides bulk samples that can be used for metallurgical testing, density measurements, and detailed petrographic studies.
Limitations
- Cost. Diamond drilling is the most expensive exploration drilling method. Costs typically range from USD 80 to 250 per metre depending on hole diameter, depth, rock hardness, and location. In remote areas of East Africa, mobilisation costs alone can add tens of thousands of dollars to a programme.
- Speed. Advance rates are slower than RC or RAB drilling, typically 15 to 40 metres per shift in hard rock. Deep holes or difficult ground conditions can reduce this further.
- Water requirement. Diamond drilling requires a continuous supply of clean water, which can be a logistical challenge in arid or remote locations.
When to Use Diamond Core Drilling
Diamond core drilling is best employed for detailed resource definition after initial targets have been identified and tested by less expensive methods. It is essential for resource estimation, feasibility studies, metallurgical sampling, and geotechnical assessment. It is also the method of choice when drilling through complex geology where preserving structural relationships is critical.
For a comprehensive overview of how drilling fits within the broader exploration workflow, see our guide on the mineral exploration process.
Reverse Circulation (RC) Drilling: Speed and Reliability
Reverse Circulation drilling is the workhorse of modern mineral exploration. It strikes a practical balance between the high data quality of diamond core drilling and the speed and cost efficiency needed for systematic target testing and resource delineation.
How It Works
RC drilling uses a pneumatic hammer at the bottom of the hole to pulverise the rock, while compressed air is forced down the annular space between an outer casing and an inner tube. The air lifts the rock cuttings (chips) back to the surface through the inner tube — hence the name "reverse circulation," as the sample travels in the opposite direction to the air flow. This closed system ensures that the cuttings from each metre of drilling are recovered separately and are representative of the rock at that specific depth.
At the surface, the cuttings pass through a cyclone and splitter system that separates a representative sample (typically 2 to 3 kilograms per metre) from the bulk reject. The samples are collected in labelled bags and sent to a laboratory for assay. The bulk reject cuttings can be retained as a backup or used for additional analysis if needed.
RC rigs are large, truck-mounted units equipped with powerful air compressors capable of generating pressures of 350 to 500 psi. They are significantly faster than diamond rigs, typically advancing 60 to 150 metres per shift depending on the rock type and hole diameter.
Strengths of RC Drilling
- Speed. RC drilling is two to four times faster than diamond drilling in comparable ground conditions, allowing more metres to be drilled within a given budget and timeframe.
- Cost efficiency. Costs typically range from USD 40 to 120 per metre — roughly half the cost of diamond drilling.
- Good sample quality. The reverse circulation system produces uncontaminated, representative samples from each metre interval, making RC data suitable for resource estimation in most geological settings.
- Depth capability. Modern RC rigs can comfortably drill to 300 to 500 metres, with some capable of reaching 700 metres or more in favourable conditions.
- Dry drilling. RC drilling does not require water, making it well suited to arid environments or locations where water supply is limited.
Limitations
- No continuous core. RC drilling produces chips, not core. While these chips provide reliable grade data, they do not preserve the structural relationships, textures, or geological context that core provides. Geologists must interpret the geology from small rock fragments rather than intact samples.
- Sample quality at depth. In very deep holes or below the water table, the reverse circulation system can lose efficiency, leading to wet samples, poor recovery, or contamination between intervals.
- Limited geotechnical data. RC drilling does not provide the structural or geotechnical information needed for detailed mine design.
When to Use RC Drilling
RC drilling is ideal for systematic first-pass drilling of geochemical or geophysical targets, infill drilling to tighten resource estimation, and grade control drilling during mine production. It is often used in combination with diamond core drilling — RC for broad coverage and grade estimation, with selected diamond holes for geological detail and structural data.
Rotary Air Blast (RAB) Drilling: Rapid Reconnaissance at Low Cost
Rotary Air Blast drilling is the simplest, fastest, and least expensive of the three exploration drilling methods. It is designed for rapid, shallow reconnaissance drilling to test large areas quickly and cheaply, identifying targets that warrant more detailed follow-up with RC or diamond drilling.
How It Works
RAB drilling uses a drag bit or blade bit attached to a rotating drill string, with compressed air blown down the outside of the rods and back up through the bit face. The air lifts rock cuttings to the surface through the annular space between the rods and the borehole wall — the opposite of the RC system. Because the cuttings travel through the open annulus rather than an enclosed inner tube, they can be contaminated by material from the borehole walls as they travel upward.
At the surface, the cuttings are collected in piles or bags at regular intervals (typically every 1 to 3 metres). RAB rigs are lighter and more manoeuvrable than RC rigs, often mounted on smaller trucks or even tracked carriers. They can drill quickly and can be repositioned rapidly between holes, making them ideal for programmes that require a large number of shallow holes across a broad area.
Strengths of RAB Drilling
- Low cost. RAB drilling costs typically range from USD 15 to 40 per metre, making it by far the most economical drilling method.
- High productivity. RAB rigs can complete multiple shallow holes per day, covering large areas in a short timeframe.
- Mobility. The lighter equipment can access sites that are difficult for larger RC or diamond rigs to reach.
- Effective for shallow targets. RAB drilling is well suited to testing geochemical anomalies, mapping the base of weathering, and sampling shallow mineralisation in oxide zones.
Limitations
- Shallow depth range. RAB drilling is typically limited to 30 to 80 metres in weathered material and soft rock. It struggles to penetrate hard, fresh bedrock.
- Lower sample quality. The open circulation system means that samples may be contaminated by material from shallower horizons, reducing confidence in the assay results. RAB data is generally not accepted for resource estimation under JORC or NI 43-101 reporting standards.
- Limited geological information. The small, potentially contaminated cuttings provide less geological detail than RC chips or diamond core.
When to Use RAB Drilling
RAB drilling is best used during the early stages of exploration when the objective is to test a large number of targets quickly and inexpensively. It is commonly employed for reconnaissance drilling over broad licence areas, testing soil geochemistry anomalies at shallow depth, and identifying zones of interest for follow-up with RC or diamond drilling. It is a screening tool, not a resource definition tool.
ALOM's RAB Drilling Experience in Northern Uganda
ALOM Mining & Geohydro Services has deployed RAB drilling programmes across East Africa. A notable example is our work for Acholi Resources Limited in the Kitgum District of northern Uganda, where ALOM conducted a systematic RAB drilling programme designed to test multiple geochemical targets across a large exploration licence. The programme successfully identified several zones of anomalous mineralisation that were subsequently prioritised for follow-up RC and diamond drilling. This project demonstrated the value of RAB drilling as a cost-effective first-pass tool in a region with extensive laterite cover that obscures the underlying geology.
Our mineral exploration services include the full spectrum of drilling methods, with programme design tailored to the geological setting, project stage, and client budget.
Choosing the Right Method: A Practical Decision Framework
Selecting the appropriate drilling method — or combination of methods — requires balancing several factors.
Project Stage
- Reconnaissance and target generation: RAB drilling is the default choice for early-stage exploration, offering maximum ground coverage per dollar spent.
- Target testing and initial resource estimation: RC drilling provides the optimal balance of cost, speed, and data quality for converting targets into drill-tested prospects.
- Resource definition and feasibility: Diamond core drilling is essential for generating the high-confidence geological, grade, structural, and geotechnical data needed for bankable feasibility studies.
Geological Setting
In deeply weathered tropical terrains — such as those found across much of Uganda — RAB drilling is highly effective in the oxide zone but cannot penetrate the underlying fresh rock. RC drilling handles the transition from weathered to fresh rock well, while diamond drilling is required for holes that must intersect hard crystalline basement at significant depth.
Regulatory and Reporting Requirements
If the objective is to generate data for a resource estimate that complies with international reporting standards, the drilling programme must include sufficient diamond core holes to meet the standard's requirements for geological confidence. RAB data alone will not support a reportable resource.
Budget and Logistics
In remote areas with difficult access, the choice of drilling method may be constrained by the available logistics. RAB and RC rigs require different levels of infrastructure support, and the cost of mobilisation can be a significant portion of the total programme budget in locations such as northern Uganda or the Karamoja sub-region.
Cost Comparison Summary
| Factor | Diamond Core | Reverse Circulation (RC) | Rotary Air Blast (RAB) | |---|---|---|---| | Cost per metre (USD) | 80 - 250 | 40 - 120 | 15 - 40 | | Typical depth range | 50 - 3,000+ m | 50 - 500 m | 10 - 80 m | | Advance rate per shift | 15 - 40 m | 60 - 150 m | 100 - 300 m | | Sample type | Continuous core | Rock chips | Rock cuttings | | Geological detail | Excellent | Moderate | Low | | Resource estimation use | All categories | All categories | Not accepted | | Water requirement | High | None to low | None |
Integrating Drilling with the Broader Exploration Programme
Drilling does not operate in isolation. The value of every dollar spent on drilling is maximised when the programme is informed by thorough pre-drilling work — geological mapping, geochemical surveys, and geophysical surveys — that identifies the highest-priority targets and reduces the risk of drilling unproductive holes.
Conversely, drilling results feed back into the geological model, refining interpretations and guiding subsequent phases of work. A staged approach — using RAB or RC drilling to test initial targets, then following up with diamond drilling on the most promising results — is the most efficient use of exploration capital.
At ALOM Mining & Geohydro Services, we design integrated exploration programmes that match the drilling method to the project objectives, geological setting, and available budget. From reconnaissance RAB programmes to detailed diamond core drilling campaigns, our experienced team delivers reliable results across East Africa.
Conclusion
The choice between Diamond Core, Reverse Circulation, and Rotary Air Blast drilling is not a matter of which method is "best" in absolute terms — it is about which method is best suited to the specific objectives, geology, stage, and budget of the project at hand. RAB drilling offers unmatched efficiency for early-stage reconnaissance. RC drilling delivers the speed and data quality needed for systematic target testing and resource estimation. Diamond core drilling provides the definitive geological and grade data required for feasibility studies and mine design.
Most successful exploration programmes use a combination of all three methods, deployed strategically across the project lifecycle. The key is to match the tool to the task, invest in thorough pre-drilling work to guide target selection, and engage experienced drilling professionals who understand the geological challenges of the operating environment.
If you are planning an exploration drilling programme in Uganda or East Africa, contact ALOM Mining & Geohydro Services to discuss how we can design a drilling strategy that meets your technical objectives and delivers maximum value from your exploration investment.