C6 · Publication Volume 16

Core Recovery, RQD and Orientation

recovery, orientation lines and structural confidence

Learning objectives

This lesson explains three related but non-equivalent evidence controls. The learner should be able to reconcile run and recovered lengths, calculate recovery and rock-quality designation, distinguish natural from mechanical breaks, evaluate orientation marks and transfers, assign confidence and identify intervals where geological or structural claims exceed the physical evidence.

Core is not automatically continuous because it appears in ordered boxes. Missing, expanded, ground, swapped or rotated material can create a visually plausible but false sequence. Depth blocks, run sheets, recovery measurements and handling observations are part of the core evidence.

Run-depth reconciliation and recovery

Record drilling runs with from-depth, to-depth, run length, core-barrel configuration, recovered length, loss or gain, recovery percentage, drilling conditions and container allocation. Reconcile cumulative run depth with driller depth and box depth. Any overlap, gap or unexplained gain requires resolution or an uncertainty flag.

Recovery is recovered length divided by run length. Measure recovered pieces along the core axis without simply measuring empty tray length. Fragmented material may require a declared reconstruction rule. Values above 100% can arise from depth error, expansion, caving, inclusion of material from another run or measurement practice; they are not physically self-validating.

Recovery is a quantity, not a diagnosis. Low recovery may preferentially remove weak, weathered, fractured, clay-rich, soluble or mineralised material. The observed remainder can therefore be biased toward competent rock. Record where loss occurred if it can be constrained, but do not distribute missing length evenly without identifying that as an interpretation.

Rock-quality designation

Rock-quality designation is the percentage of a core run made up of eligible sound pieces at least 0.10\,\mathrm m long, measured along the core centreline. Use the actual run length in the denominator. The measure depends on core size, drilling quality, timing and break classification, so it must be accompanied by method metadata.

Mechanical breaks created by drilling or handling may be fitted together and treated as one piece under a declared rule. Natural discontinuities remain separate. Criteria such as fresh matching surfaces, geometry, mineral coating, weathering and roughness help classification, but confidence should be recorded. Do not manufacture a high designation by refitting uncertain breaks.

The measure describes fracture condition at core scale; it does not directly equal rock-mass quality, strength, permeability or excavation performance. Orientation, discontinuity sets, infill, persistence and stress context remain necessary. A high value can coexist with one critical continuous weak plane.

Core orientation, handling and depth evidence

Core orientation attempts to establish a reference direction around the core axis. A bottom-of-hole mark or equivalent observation is transferred into a continuous reference line across successive pieces. The evidence chain includes tool observation, run identity, mark quality, transfer steps, fit between pieces, line continuity and confidence by interval.

An orientation line should never be extended across rubble, missing core or an uncertain fit as if the orientation remained measured. Use states such as measured, transferred with high confidence, transferred with qualification and not oriented. Record reversal or polarity conventions clearly; a line drawn on the wrong side produces systematic structural error.

Compare independent orientation cues where available: repeated marks, penetrative fabric, known contacts, borehole imagery or geometric consistency among structures. Agreement supports but does not prove correctness; shared transfer error can make several features agree.

Handling, breakage and depth placement

Preserve top and bottom direction from extraction through boxing. Mark run boundaries, depth blocks, core loss and reconstructed fragments before logging or cutting. A piece placed backwards reverses apparent sequence and can corrupt orientation. A piece assigned to the wrong run moves every later interval unless depth is reconciled.

Distinguish drilling-induced breaks, handling breaks, saw breaks and natural discontinuities. Photograph core before destructive sampling and, where practical, before extensive handling changes its condition. Record washing, drying, slaking, oxidation and preservation because the material can change after recovery.

If soft material expands or contracts, record its condition and both measured and adopted length where needed. Do not force all material to fit painted tray marks. The tray layout is a storage representation, not the depth authority.

Uncertainty and quality controls

Recovery uncertainty includes run-depth accuracy, length measurement, fragment reconstruction, expansion and material attribution. Rock-quality designation adds piece eligibility and break-origin classification. Orientation adds tool accuracy, mark quality and transfer uncertainty. Report these separately.

Controls include independent recovery checks, run-to-box reconciliation, photographs before disturbance, explicit loss markers, duplicate designation measurements, orientation mark quality codes, discontinuity classification rules and periodic review between logging roles. Plot recovery, designation and orientation confidence together; a structural dataset from low-confidence orientation intervals should be filtered or weighted accordingly.

Do not hide missing evidence in a single “quality” score. A run can have high recovery, low rock-quality designation and no orientation. Each dimension affects different decisions.

Synthetic worked example

A synthetic run extends from 84.0 to 87.0\,\mathrm m, so L_r=3.0\,\mathrm m. Recovered core measures 2.70\,\mathrm m, giving R=90\%. Eligible sound pieces after confident refitting of two mechanical breaks measure 0.42, 0.36, 0.31, 0.24, 0.18 and 0.11\,\mathrm m. Their sum is 1.62\,\mathrm m, so RQD=54\%.

The remaining recovered material consists of pieces shorter than 0.10\,\mathrm m and soft fragments. The orientation mark is clear over the first 0.9\,\mathrm m, uncertain through 0.6\,\mathrm m of rubble, and re-established by a second mark for the final 0.8\,\mathrm m. The log therefore stores orientation confidence by interval, not one “oriented” flag for the run.

The result is not “90% good core.” It is 90% length recovery, 54% designation under a specified rule, and discontinuous orientation evidence. A structural measurement inside the uncertain segment cannot inherit confidence from the rest of the run.

Practice and review checklist

  • Do run from- and to-depths reconcile with drilled depth?
  • Is recovered length measured by a declared method?
  • Are gains above 100% investigated?
  • Is likely position of loss recorded without false precision?
  • Does designation use the correct run denominator and centreline lengths?
  • Are mechanical and natural breaks distinguished with confidence?
  • Are top, bottom, run boundaries and container sequence preserved?
  • Does the orientation line have provenance and interval-level quality?
  • Are gaps, rubble and uncertain transfers visibly interrupted?
  • Are recovery, designation and orientation quality available to downstream users separately?

Any structural interpretation that ignores an orientation gap, or any sampling interval that treats missing material as representative, should be returned for revision.

Decision implications and integration

Recovery informs whether the recovered material plausibly represents the drilled interval. Rock-quality designation informs one aspect of fracture condition. Orientation confidence determines whether planar and linear features can be transformed into geographic coordinates. None substitutes for the others.

The integrated dataset should retain run, box, recovery, designation, orientation and geological intervals as separate tables linked by hole and depth. Intersections can then inherit explicit quality attributes. Reviewers should be able to mask low recovery, low orientation confidence or ambiguous break classification and see whether the geological conclusion changes.

Recovery, rock-quality designation and orientation confidence are separate measurements linked to each core run.
Recovery, rock-quality designation and orientation confidence are separate measurements linked to each core run.

Sources