Core cutting and sampling discipline in diamond drilling: five habits that protect your grade

Wednesday 8 July 2026

Incorrect core cutting and sampling technique is one of the most common sources of grade bias in diamond drilling programs. The consequences extend well beyond the core shed. A poorly oriented cut or a biased half-core selection can shift a reported grade by 10-30% from the true value. That margin is enough to change a resource classification, alter an economic assessment, and mislead the investment decisions that follow.

The mechanics of core cutting are straightforward: a diamond saw splits the core lengthwise, one half goes to the laboratory for assay, and the other stays in the tray. The discipline around those mechanics is where programs succeed or fail. When technique is inconsistent, the bias compounds across hundreds of samples and embeds itself in the resource model without any single point of failure to identify.

These five habits address the most common sources of grade bias at the saw and the sample table.

1. Always cut the core on the same side of the orientation line

The orientation line, scribed during core orientation, marks a fixed reference on the circumference of the core. The saw cut should always run on the same side of that line, producing two halves with a consistent spatial relationship to the original downhole geometry.

When the cut drifts between left and right across a program, the two halves no longer represent the same geological cross-section from sample to sample. In mineralised zones where grade varies across the core diameter, particularly in vein-hosted or structurally controlled deposits, this inconsistency introduces a directional bias. The assay results may systematically overstate or understate the true grade depending on which side of the vein was preferentially sampled, and the bias is invisible in the assay data because each individual result looks plausible.

Consistency at the saw is the cheapest form of grade control in a drilling program.

2. Always sample the same half

Convention in most Australian programs is to sample the right-hand half when looking downhole. The remaining left-hand half stays in the tray for reference, assay verification, and potential re-sampling if QA/QC checks flag an issue.

The convention itself matters less than the consistency. Switching halves mid-program, whether through crew changeover, unclear procedure documentation, or simple inattention, introduces a systematic error that compounds across every interval. The competent person reviewing the program may never identify the switch from the assay data alone, because the grades will fall within plausible ranges. The error surfaces later, when re-sampling produces results that do not reconcile with the original assays, and the cause is traced back to inconsistent half-core selection.

Document the convention in the program's standard operating procedure and verify it at every crew handover.

3. Mark sample intervals before cutting

Pre-marking sample intervals on the core ensures that the intervals respect geological contacts rather than arbitrary metre marks. The geologist reviews the core, identifies lithological and mineralogical boundaries, and marks the sample breaks before the saw is switched on.

When intervals are marked after cutting, the pressure to fit samples to convenient round numbers increases, and geological contacts are more likely to fall in the middle of a sample rather than at the boundary. A sample that straddles a contact between mineralised and unmineralised rock dilutes the grade and misrepresents the geometry of the deposit. The resulting assay will return a blended value that does not reflect either rock type, making it harder to define the true extent of mineralisation in the resource model.

Pre-marking takes minutes. The grade resolution it preserves can change the shape of an ore body.

4. Never sample across a geological contact without flagging it

Even with pre-marked intervals, contacts can be missed or fall within a pre-defined sample length. When this happens, the correct procedure is to split the sample at the contact and log the reason for the split. The extra sample bag costs cents. The grade distortion from blending mineralised and unmineralised material costs confidence in the entire assay dataset.

Unflagged contact crossings are particularly damaging in narrow vein systems and deposits with sharp grade boundaries, where the difference between ore and waste can occur over centimetres. A single diluted sample across a contact can suppress the peak grade of an interval, lowering the reported grade-width product and potentially shifting the intercept below the cutoff used for resource estimation.

The geologist logging the core is the only person in the program with the geological context to make this call. The laboratory cannot correct for it after the fact.

5. Photograph each tray before and after cutting

The pre-cut photograph is the permanent visual record of the core in its original state: lithology, structure, alteration, mineralisation, and spatial relationships visible and undisturbed. Once the saw cuts, that record is gone. The post-cut photograph confirms which half was sampled, verifies the cut quality, and documents any core loss or mechanical damage from the cutting process.

When an auditor or competent person reviews the program months later, the photographic record is the primary evidence that the sampling procedure was followed correctly. Without it, the reviewer is relying on written logs alone, and written logs cannot capture the visual context that photographs preserve.

Photograph every tray, every time. The cost is negligible and the evidentiary value is permanent.

The bottom line

Core cutting and sampling discipline is one of those areas where the gap between a good program and a poor one is measured in small, repeatable habits rather than expensive equipment or complex procedures. The five habits above cost almost nothing to implement. The grade bias they prevent can shift a reported result by enough to change a resource classification, and once that bias is embedded in the dataset, it is rarely identified without a full re-sampling campaign.

Every program that takes sampling seriously starts with the people holding the saw and the sample bags. Their discipline is the foundation that the resource model, the economic assessment, and the investment decision are built on.

At Norwest Exploration and Mining Services, we place experienced geologists and field technicians who bring this kind of discipline to every program. Whether you need a single geologist for a short campaign or a full field crew for a multi-rig program, contact us to discuss your requirements.