Geological Core Logging

Geological core logging records the characteristics and relationships visible in recovered drill core, together with the depth and evidence needed to interpret them. It provides a basis for correlating units, understanding mineralisation, selecting samples and informing engineering investigations. A useful log separates observations, provisional identifications and later interpretations instead of presenting all three as equally certain facts.^[1][2]

Logging is not a transcription of assay results, nor a substitute for the driller's operational record. Geological descriptions, drilling events, geotechnical measurements and laboratory analyses are complementary datasets. Their identifiers and depth references must connect, but each retains its own purpose and measurement basis.

Establishing Purpose and Depth Control

The logging scheme should reflect the geological questions and intended engineering uses. A description designed only to distinguish broad rock units may not preserve the alteration, structural or mineralogical detail needed for later interpretations. Conversely, detailed terminology is of little value when important contacts, uncertainty or missing material are omitted. Reclamation's engineering-geology guidance emphasizes recording characteristics relevant to the investigation rather than descriptive detail for its own sake.^[2]

Before interpreting the core, the logger needs a reliable sequence. Hole identification, box order, run markers, depth units, core condition and known removals should agree with the drilling and sample records. An unresolved discrepancy should remain visible rather than being hidden by adjusting interval lengths to fit the box.^[2]

The depth reference also needs to be explicit. Distance along the borehole is not necessarily vertical depth. Logging normally identifies material by along-hole intervals; its position in a section or model depends on the collar and trajectory data applied later. Descriptions should not acquire false spatial precision because they are displayed in a three-dimensional model.^[1]

Describing Material and Relationships

A geological description commonly addresses composition, grain size, texture, colour, fabric, mineralisation, alteration and weathering. The combination matters: a mineral name alone does not describe whether it occurs as disseminated grains, veins or replacement material, or how it relates to contacts and structures.^[1]

Descriptions should distinguish a visual estimate from an analytical result. A tentative mineral identification may require petrography or laboratory testing. A visual estimate of a mineral’s volume fraction is not an elemental assay reported by mass: the constituent and measurement basis differ. Abundance estimates need their method and confidence retained. A laboratory result can refine interpretation without rewriting what was visible when the core was logged.^[3][1]

The USGS logging method published for the Idaho National Laboratory in 2026 illustrates a structured approach to recording lithology, texture, composition, alteration and interval boundaries. Its rock types and descriptive conventions were developed for that archive. They provide an example of consistent evidence recording, not a universal classification scheme for every mineral deposit. Even a lithological name can incorporate interpretation; the textures and components supporting that name should remain available for review.^[3]

Structural observations need similar care. Natural fractures, drilling-induced breaks and unresolved breakage should be distinguished where the evidence permits. Measurements relative to the core axis are not automatically geographical dip measurements. The physical reference and conversion method must be known before structural data are used as oriented observations.^[2]

Choosing Intervals and Preserving Missing Information

Run, geological and sampling intervals serve different purposes. A geological boundary may occur partway through a drilling run, while an assay sample may cover a different length. Logging should preserve the observed boundary rather than move it to match a convenient run or sampling grid. Links between these datasets can be established without forcing their boundaries to coincide.^[1]

An interval record should explain what is included and how its boundaries were selected. Sharp contacts, gradual transitions and uncertain boundaries warrant different descriptions. The USGS method explicitly discusses interval selection and missing-core records; it illustrates why apparently continuous table entries should not conceal gaps in the physical evidence.^[3]

Several apparently empty entries have different meanings. Material may not have been recovered, a feature may not have been examined, or the logger may have examined it without being able to identify it confidently. These states should not be converted into zero abundance or absence. A useful database design preserves the distinction in its codes and comments.

Where core loss crosses a suspected contact, the boundary may be constrained to an interval but not located precisely. Moving fragments or a loss marker within the box cannot supply the missing observation. A model may require an interpreted position; its uncertainty and evidence should remain attached rather than replace the record that the contact was not recovered.^[2]

Consistency, Codes and Review

Common vocabularies, definitions and reference examples help loggers describe comparable material consistently. CIM recommends retaining reference examples of important rock and mineralisation types. As a practical check, comparing descriptions of the same material and documenting terminology changes can reveal inconsistent code use. A shared list alone cannot ensure that different loggers apply a code to equivalent observations.^[1]

Research on harmonising geological logs shows that legacy datasets contain inconsistent terminology, abbreviations and descriptive styles. Translating them into common categories can improve usability, but the original text and the translation method need to remain available. Automated classification or a database lookup does not independently validate the geology.^[4]

Checks should also test interval logic: reversed depths, unintended overlaps, unexplained gaps, duplicate identifiers and inconsistent units. These are data-integrity checks, not proof that a rock identification is correct. Geological review must still compare the records with physical core, photographs and relevant supporting information.

Re-logging should have a defined purpose and version history. It may resolve earlier uncertainty, use a more suitable classification or interpret new laboratory evidence. Retaining the earlier record allows reviewers to distinguish newly observed features from a change in interpretation or terminology.

Linking Logs to Samples and Models

The log is part of a chain connecting recovered material to laboratory samples and spatial interpretation. Samples need traceable source intervals, while results need their analytical method and units. Photographs taken before splitting can help document the original arrangement, but they cannot replace all observations or tests on the physical material.^[1]

Geotechnical users may require measurements beyond a geological description, including recovery, fracture characteristics or other parameters appropriate to their assessment. Recording them in linked tables avoids pretending that a lithological code alone establishes rock-mass behaviour. Conversely, an engineering classification should not erase the geological observations on which it relies.^[2]

Model building introduces another interpretation step. Correlating similar logged intervals between holes requires a geological explanation, not merely matching codes. Their relationship may be affected by faulting, changes in facies or altered material that resembles another unit. Where competing correlations affect the mine design, the alternatives and supporting evidence should be assessed explicitly.

Using New Evidence Without Losing Traceability

Consider a hypothetical interval where later assays identify elevated metal content but the original log records no visible mineralisation. The result may justify re-examination or additional mineralogical work. It does not, by itself, establish that a visible lithological contact exists at the assay boundary.

The appropriate update preserves the original description, attaches the analytical evidence and records any revised interpretation. This approach makes the log useful beyond the immediate campaign: later users can determine what was seen, what was measured, what changed and which conclusions still depend on assumptions. The goal is a coherent, reviewable account of the core rather than a record made artificially certain after the event.^[1][4]