C6 · Publication Volume 16

Drill Program Review

orientation bias, spacing, coverage, safety and cost

Learning objectives

This lesson reviews a drilling program as a decision experiment. The learner should be able to compare achieved evidence with objectives, assess orientation bias, spacing and coverage, aggregate quality without hiding failures, examine safety and environmental evidence, separate sunk activity from future value and recommend continuation, redesign, follow-up or stop conditions transparently.

The review is not a celebration of metres, samples or completed holes. It asks what uncertainty was reduced, what new uncertainty appeared, which observations remain credible and which next action has the best decision value under real constraints.

Objective-to-evidence matrix

Return to the pre-drill hypotheses. For every objective, list predicted observations, achieved coverage, usable evidence, excluded evidence, result, confidence and disconfirming outcome. A hole that misses its planned target because of deviation may still test another hypothesis, but that reinterpretation must be explicit.

Separate operational completion from scientific success. Metrics such as metres, rate, recovery, samples submitted and turnaround describe execution. Scientific metrics describe discrimination between hypotheses, spatial constraint, uncertainty reduction, validation and fitness for the next decision. Both matter, but they answer different questions.

Classify outcomes as supported within sampled scope, weakened, unresolved, contradicted or not tested. Avoid “negative hole” as a universal label. Absence of observed mineralisation can mean true absence, target miss, inadequate method, poor recovery, wrong scale or an incorrect hypothesis. The evidence chain determines which interpretations remain viable.

Orientation bias, spacing and coverage

Drillholes sample anisotropically. Apparent thickness depends on intersection angle; structures parallel to holes can be poorly constrained; cuttings and core have different support. Review the distribution of hole azimuths and dips relative to interpreted features, not only plan-view spacing.

Spacing should be evaluated in three dimensions and by decision. Equal collar spacing does not guarantee equal subsurface coverage when holes diverge, curve or terminate early. Map uncertainty envelopes, unsurveyed tails, recovery gaps and excluded intervals. Coverage includes quality, not just geometric reach.

Look for clustered evidence that shares one bias: the same drilling direction, season, method, logging scheme, laboratory batch or interpretation assumption. Additional holes of the same orientation may increase sample count without resolving structural ambiguity. An orthogonal or otherwise discriminating observation can have greater value.

Completeness, quality and residual risk

Summarise quality by evidence dimension: collar, trajectory, recovery, orientation, geological reproducibility, sampling support, preparation, analytical controls, photography, data validation and provenance. Do not average them into a score that allows a severe identity failure to be cancelled by high recovery.

For each unresolved issue, state scope, mechanism, consequence, detectability, recoverability and decision impact. A missing end-of-hole survey may be remediable; destroyed unretained core is not. A held assay batch can block interpretation even when all other records are complete.

Audit missingness. Which holes, depths, materials or concentration ranges are absent, and why? Failure-related missingness is often informative and cannot be treated as random. Report excluded support quantitatively and spatially.

Safety, environmental and cost evidence

Review whether work was executed within applicable controls, whether incidents, changed conditions and stop-work decisions were recorded, and whether rehabilitation and waste obligations have evidence. This tutorial does not judge legal compliance or operational adequacy; those determinations belong to authorised competent roles. The scientific review records dependencies and unresolved restrictions.

Cost review should connect expenditure to evidence and decisions rather than rank methods by unit rate alone. Separate mobilisation, setup, drilling, surveys, recovery controls, logging, sampling, analysis, rework, delay and data management where records allow. Cheap metres that do not test the objective have low decision value; expensive controls may be justified when they prevent an irreversible ambiguity.

Use prospective value, not sunk cost, for the next decision. Ask what observation would most change the action, whether existing retained material can provide it, and whether a new hole, alternate orientation, survey, relog, re-preparation or different analysis is required.

Synthetic worked example

A synthetic five-hole program tested a steep tabular target. Four holes used similar azimuths; one was abandoned early. Recovery is generally high, but orientation confidence falls across the most deformed interval in two holes. A batch covering one central hole is held for contamination investigation. The interpreted target appears continuous in section, yet its dip is largely constrained by contacts observed from one direction.

The review does not claim full success from four completed holes. It marks the central assays unavailable, structural dip as orientation-biased and the abandoned sector as untested. It also identifies retained pulp and core that can resolve the batch and alteration questions without immediate new drilling.

The next decision is staged: resolve the held batch from retained material, remeasure selected structures where orientation evidence supports it, then consider one hole with a substantially different intersection direction. The reversal condition is explicit: if corrected assays remove the central anomaly or alternate orientation disproves continuity, the next hole is not justified.

Practice and review checklist

  • Are original objectives and competing hypotheses still visible?
  • Is achieved evidence separated from completed activity?
  • Are target misses distinguished from geological absence?
  • Are hole directions, curvature and feature orientations compared?
  • Does coverage include uncertainty and excluded intervals?
  • Are shared method, batch and interpretation biases identified?
  • Are severe failures reported separately rather than averaged away?
  • Are safety and environmental dependencies evidenced by authorised records?
  • Are future actions compared by information gained and decision changed?
  • Does each recommendation have a stop or reversal condition?

A review should be returned when it reports only metres, grades and attractive sections, or when it recommends more of the same evidence without explaining which uncertainty that evidence will reduce.

Decision, release and completion

The final review package contains the objective-to-evidence matrix, adopted data release, quality findings, three-dimensional coverage, orientation assessment, excluded support, unresolved risks, safety and environmental dependencies, cost categories, retained-material options and recommended decision gates. Separate factual observations, interpretations and recommendations.

The program is complete for a decision when the remaining uncertainty is understood and acceptable for that decision—not when every geological question is answered. It may be scientifically correct to stop, redesign or defer. A transparent inconclusive result is stronger than a confident conclusion built on untraceable evidence.

Program review compares objectives with usable evidence, orientation and coverage, unresolved risk and reversible next decisions.
Program review compares objectives with usable evidence, orientation and coverage, unresolved risk and reversible next decisions.

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