How Many Drillholes Is Enough? Drill Spacing Explained

Let’s look at three common approaches to drill spacing.

The first is wide-spaced drilling, which is typically used during the early stages of exploration.

At this stage, the objective is to confirm whether mineralization is present, estimate the potential scale of the target, and keep initial exploration costs under control.

For example, during regional exploration, we may initially be trying to answer one basic question:

Is there mineralization here?

In this situation, relatively wide drill spacing may be appropriate, sometimes in the range of 400 to 800 metres, depending on the deposit type, target geometry, and available geological information.

The main advantages are lower initial costs and the ability to test a large area relatively quickly.

The disadvantage is a high level of geological uncertainty. With widely spaced holes, there is always a risk of missing narrow, discontinuous, or structurally controlled mineralized bodies.

The second approach is progressive drilling.

Once a promising target has been identified, the distance between drillholes is gradually reduced.

The program may move from initial target testing to defining the geometry and continuity of the mineralized body, and eventually to infill drilling.

At the more advanced stages, drill spacing may be reduced to approximately 10 to 50 metres, although the appropriate distance will always depend on the geology and the project objectives.

Each stage provides additional information, improves confidence in the geological interpretation, and reduces uncertainty.

This progressive approach is one of the most widely used exploration strategies.

Rather than drilling the entire project area on a dense grid from the beginning, teams start with wider spacing, evaluate the results, update the geological model, and then determine where additional drilling will provide the greatest value.

This allows the exploration budget to be used more efficiently and can support the staged collection of information required for resource estimation and classification under reporting frameworks such as JORC and NI 43-101.

The third approach is targeted drilling.

Regular drilling grids are no longer the only option available to exploration teams.

Increasingly, drillholes are being concentrated in the most prospective areas rather than being distributed evenly across the entire project.

These priority areas may be identified through geological interpretation, geophysical anomalies, geochemical results, structural analysis, historical drilling, and three-dimensional geological models.

Targeted drilling can be particularly useful during early-stage exploration, when teams need to test specific geological concepts or anomalies without committing to a full drilling grid.

It can also be applied during later stages to test extensions, investigate structural controls, and close important gaps in the geological model.

In practice, many successful exploration programs combine all three approaches: wide-spaced drilling to identify potential, progressive drilling to improve geological confidence, and targeted drilling to focus resources where they are most likely to generate valuable information.

Let’s look at three additional approaches to drill planning.

The first is structurally oriented drilling.

For structurally controlled deposits, including many gold systems, drillhole orientation can be even more important than drill spacing.

In these cases, drillholes are designed to intersect faults, shear zones, mineralized veins, or other controlling structures at the most informative angle.

Ideally, the drillhole should intersect the target as close to perpendicular as possible.

This helps geologists determine the true thickness, geometry, and continuity of the mineralized zone more accurately.

A single, well-positioned and correctly oriented drillhole can provide significantly more useful information than several holes drilled in the wrong direction.

The second approach is grid-based drilling.

Traditionally, many exploration projects have relied on regular drilling grids because they are relatively easy to design, manage, and interpret.

For example, drillholes may be positioned on a uniform grid of 400 by 400 metres during early-stage exploration or 50 by 50 metres during more detailed drilling.

This method can work well for large, relatively continuous deposits, such as stratiform mineralization or some porphyry systems.

However, a regular grid does not always reflect the true complexity of the geology.

It may result in unnecessary drilling in low-priority areas while leaving important structural or geological questions unresolved.

Modern digital technologies now allow exploration teams to use much more flexible drilling strategies.

This brings us to adaptive drill planning.

Modern geological software allows the geological model to be updated continuously as new drilling results become available.

Instead of following a fixed drilling pattern throughout the entire campaign, the program can be adjusted after each stage of drilling.

This is what we mean by adaptive drill planning.

Following each drilling phase, the team updates the geological model, evaluates the new information, identifies areas of remaining uncertainty, and determines where the next drillholes will have the greatest impact.

The drilling campaign therefore becomes a continuous learning process rather than a fixed plan developed several months in advance.

Each new drillhole provides information that influences the location, orientation, and priority of the drillholes that follow.

This approach has become much more practical through specialized drill-planning tools within modern geological software.

These tools allow teams to rapidly test alternative drilling scenarios, update proposed trajectories, and redirect the exploration program toward the areas where additional drilling is most likely to improve geological confidence and project value.

Let’s now look at two closely related approaches: risk-based drilling and value-driven drilling.

First, risk-based drilling.

This approach focuses on areas where geological uncertainty has the greatest potential impact on the project.

Teams may use uncertainty analysis, probabilistic models, scenario testing, geostatistics, and sensitivity analysis to understand where the geological model is least reliable and where additional information is most important.

The objective is not simply to drill the areas with the highest uncertainty.

It is to identify the uncertainties that create the greatest technical risk or economic risk and to design drillholes that can reduce those risks most effectively.

In this way, each new drillhole is selected based on the value of the information it is expected to provide.

Value-driven drilling takes this concept one step further.

Under this approach, every proposed drillhole should make a meaningful contribution to the overall value of the project.

Before a drillhole is approved, the team may consider several key questions:

How much will it reduce geological uncertainty?

Could it support a change in resource classification?

Will it materially improve the geological or structural interpretation?

Could the result influence the project’s economic model?

And is the expected value of the information sufficient to justify the cost of drilling?

Instead of continuing to drill areas that are already well understood, teams focus on the parts of the deposit where new information could have the greatest impact.

This may include testing an uncertain extension of the mineralized body, resolving a structural interpretation, improving confidence in a high-value area, or collecting the information needed to support future resource conversion.

Some of the most advanced exploration projects are increasingly moving toward this value-of-information approach.

And this brings us to the key message of this section:

There is no single “correct” drill spacing.

The optimal spacing is the one that provides the required level of geological confidence while minimizing unnecessary drilling.

That is why modern digital workflows have become such an important part of exploration planning.

They help teams quantify uncertainty, compare alternative drilling scenarios, prioritize the drillholes with the greatest potential impact, and use exploration budgets more effectively.

Ultimately, successful drill planning is not about drilling more holes.

It is about ensuring that every new drillhole answers an important geological question and contributes meaningful value to the project.