C6 · Publication Volume 16

Drilling, Core, Logging, Sampling, Assay and QA/QC

Covers the evidence chain from drilling objective and collar control through logging, sampling, assay and validation.

Purpose and boundary of this book

A drillhole is an engineered observation path through the subsurface. It is not a transparent window into the ground. The evidence recovered from that path depends on the question, drilling method, collar and trajectory control, recovery, material handling, logging vocabulary, structural orientation, sampling support, preparation, analytical method, quality controls, data transformations and record custody. Every stage can preserve information, blur it, bias it or break its connection to depth. A reported interval is therefore credible only when its full lineage can be examined.

This book develops a measurement-centred workflow from the reason for drilling to the release of a validated drillhole dataset. It covers diamond core, reverse-circulation, rotary air-blast and aircore methods at a conceptual level; collar setup; downhole surveys; core recovery; rock-quality designation; orientation; geological, structural, geotechnical and hydrogeological logging; interval and sample-support design; crushing, splitting, pulverising, digestion and analysis; standards, blanks and duplicates; photography; data integration; and program review. The objective is not to imitate one field procedure. It is to understand what each record means, which uncertainty it carries and what must be checked before it supports a geological decision.

This tutorial does not authorise drilling, prescribe a rig or tool, replace competent field supervision, define a statutory reporting code, certify a laboratory, estimate a resource or approve a safety, environmental, land-access or commercial decision. Drilling and sample preparation involve moving machinery, stored energy, pressure, rotating components, heavy loads, chemicals and site-specific hazards. Operational work must follow applicable law, permits, engineered controls, equipment instructions, risk assessment and competent supervision. The conceptual checks here are additions to, not substitutes for, those controls.

General and institution-neutral scope

This is a general, institution-neutral tutorial. It has no relationship to, affiliation with, sponsorship by, endorsement from or curriculum dependency on any company or individual. It is not written for a named owner, operator, contractor, consultancy, laboratory, university, public agency, software product, property, deposit, mine or private database. Every unnamed hole, collar, rig, sample, laboratory batch, assay value, geological interval, photograph and decision in an example is synthetic teaching material.

Names of public bodies, standards organisations and technical documents occur only in source notes when needed to identify evidence. A citation does not make a named person or organisation the author, publisher, sponsor, provider, partner, endorser, scientific authority or subject of this tutorial. The website carrying these pages is only a hosting and delivery surface. It is not the tutorial's author, publisher, sponsor, provider, owner, scientific authority or curriculum subject, and it asserts no institutional ownership of the curriculum.

Institutional neutrality is also a quality control. A logo cannot repair a swapped sample, an unexplained depth gap, a magnetic survey affected by steel, an orientation mark with uncertain transfer, an inappropriate digestion or an undocumented database overwrite. Evidence must stand on identifiers, measurements, controls, uncertainty and reproducible transformations. The same audit questions apply whether the records came from a small teaching exercise or a large program.

The drillhole evidence chain

Use the following chain throughout the book:

  1. State the decision, geological hypotheses and observation that drilling is expected to discriminate.
  2. Select a drilling method by evidence requirement, ground conditions, sample support, recovery, depth, geometry, safety and environmental constraints.
  3. Assign a persistent hole identifier and control the collar position, reference frame, planned azimuth and dip before drilling.
  4. Record actual setup, drilling events, run or sample intervals, losses, additives, delays and deviations without rewriting the original field record.
  5. Measure the downhole trajectory at declared depths, with instrument identity, calibration status, north reference and interference checks.
  6. Reconcile drilled length, recovered material and box or bag depth; preserve the distinction between natural and drilling-induced breakage.
  7. Log lithology, texture, alteration, mineralisation, structures, geotechnical condition and water observations using defined fields and confidence codes.
  8. Design sample intervals from the decision and material support, then preserve sample identity, residual material and chain of custody.
  9. Record every preparation and analytical transformation, including mass changes, splits, batches, methods, units, detection limits and quality controls.
  10. Validate identifiers, depths, intervals, units, statuses, duplicates, lineage and release state before geological integration.
  11. Review orientation bias, spatial coverage, missing support, safety and cost together before deciding what the program established and what remains unresolved.

The chain must also work backwards. Starting from an assay or interpreted contact, a reviewer should be able to find the analytical batch, preparation history, submitted sample, physical interval, core or cuttings container, original log, trajectory, collar and drilling objective. A value that cannot be traced is not rescued by numerical precision.

Learning outcomes

After completing the book, a learner should be able to:

  • translate a geological decision into method-selection and data-quality requirements;
  • compare drilling methods without treating cost, speed or sample quality as a single universal ranking;
  • define collar, azimuth, dip, depth and north-reference conventions unambiguously;
  • audit downhole survey stations for spacing, interference, repeatability and trajectory plausibility;
  • calculate and interpret core recovery and rock-quality designation without confusing them;
  • evaluate core-orientation marks and transfer confidence before using structural measurements;
  • design geological, structural, geotechnical and hydrogeological logs as observations rather than retrospective stories;
  • choose sample intervals and retained material that respect boundaries, support and representativity;
  • trace preparation and assay results through masses, splits, methods, units and detection limits;
  • design a quality-control system that distinguishes contamination, bias, imprecision, swaps and transcription failures;
  • produce depth-registered photographs and digital logs with durable provenance;
  • validate relational drillhole tables before desurveying or modelling; and
  • review a drill program against its objective, orientation bias, coverage, safety evidence and decision value.

Prerequisites and notation

The concept map places this book after quantitative foundations, geological observation and the relevant rock, structure, mineral-system and exploration-method volumes. Learners should be comfortable with units, coordinates, vectors, angles, probability, uncertainty, sampling support, geological contacts and competing hypotheses. Later volumes develop three-dimensional modelling, estimation and drillhole algorithms in more depth.

Measured depth along the hole is m and is not the same as true vertical depth. Collar coordinates are (E_0,N_0,Z_0) in a declared coordinate reference system. Azimuth A is measured clockwise from a declared north; dip D is negative downward in this book unless a record explicitly declares another convention. A trajectory station is (m,A,D) plus time, method, tool, quality status and uncertainty. Interval notation is half-open, [f,t), so an interval includes its from-depth f and excludes its to-depth t unless a source system documents otherwise.

For a drill run of length L_r and recovered core length L_c, recovery is

$R=100\frac{L_c}{L_r}\%.$

Rock-quality designation is

$RQD=100\frac{\sum_i L_i^{(\ge 0.10\,\mathrm m)}}{L_r}\%,$

where eligible sound pieces are measured along the core centreline and mechanical breaks are treated according to a declared rule. Recovery and rock-quality designation answer different questions. A run can have high recovery but low designation if the recovered material is naturally broken.

Concentration units, detection limits and qualifiers must be explicit. A reported value is never just a number: it has an analyte, method, unit, sample support, result status, laboratory batch and quality state. Use “not analysed,” “not received,” “below a stated limit,” “above a stated limit” and “invalid” as different states rather than one null code.

How to use the diagrams and synthetic cases

Each numbered lesson contains one purpose-built diagram. Hole traces, core trays, sample intervals, assay charts and identifiers are schematic. They do not depict a real property, program, rig, laboratory, database or organisation. Colours separate evidence states; they do not encode a commercial convention. Arrows show lineage or dependency, not a prescribed corporate workflow.

All unreferenced names, coordinates, depths, masses, recoveries, assay values, control limits and costs are synthetic. Their purpose is to expose calculations and failure modes, not to supply operating thresholds or expected performance. A synthetic case is complete only when it declares its assumptions, units, support, controls, uncertainty, alternative explanations and reversal condition. Learners should change the numbers and confirm that the reasoning still works.

Do not copy a synthetic acceptance limit into a real program. Limits must be derived from the decision, method capability, material heterogeneity, risk and approved quality plan. Do not infer a safety procedure from a diagram. Field execution belongs under local controls and competent authority.

Reproducible drillhole records

Treat original observations and received analytical files as immutable records. Corrections create new versions linked to the superseded value, reason, evidence, authorising role and timestamp. A reproducible drillhole package includes the program and hole objectives; identifier rules; coordinate reference system and vertical datum; planned and actual collar; survey stations with instrument and north reference; drilling method and diameter by depth; run, recovery and loss records; orientation quality; core or sample-container register; controlled logging vocabularies; interval tables; sample dispatch and custody events; preparation and analytical methods; result units and qualifiers; quality-control samples and decisions; photographs with scale, colour control and depth registration; validation reports; release status; and checksums for exchanged files.

Identifiers should be opaque and stable. Do not encode mutable geology, contractor, year or status into the only identity key. Preserve source identifiers even when a warehouse assigns surrogate keys. Every interval needs a hole identifier, from-depth, to-depth, unit, interval type, observation or method identity and record status. Every sample needs a unique identity that cannot be confused with a bag number, dispatch position or analytical row number.

Provenance should record entities, activities and responsibility roles without turning a person or organisation into the scientific basis for a claim. For example, a result entity was generated by an analytical activity that used a prepared pulp entity derived from a split entity derived from a drill interval entity. The claim is testable because the transformation and controls are recorded, not because a name appears beside it.

Assessment and completion standard

Completion requires a synthetic, institution-neutral drillhole review package containing:

  1. a decision, competing geological hypotheses and explicit drilling objectives;
  2. a method-selection matrix covering evidence quality, ground, depth, sample support, safety and environmental constraints;
  3. collar and downhole survey records with conventions, calibration and interference checks;
  4. reconciled drilling, recovery, orientation and container-depth records;
  5. geological, structural, geotechnical and hydrogeological logs with vocabularies and confidence;
  6. a sampling plan showing boundaries, support, retained material and representativity risks;
  7. preparation, assay and quality-control records linked by sample and batch;
  8. depth-registered photographs with image provenance;
  9. an automated validation report for identifiers, intervals, units, statuses and lineage; and
  10. a program review separating observation, interpretation, limitation, residual risk and next decision.

The package passes when another reviewer can reconstruct the trajectory, locate every interval and sample, reproduce key ratios and quality-control plots, identify excluded or uncertain support, recover the original records and explain why the resulting evidence is adequate or inadequate for the stated decision. A polished section or three-dimensional trace without a recoverable evidence chain does not meet the standard.

Core sources