2.3 Exploration Drilling & Core Logging

Exploration drilling obtains subsurface evidence along a physical path; core logging turns recovered material into descriptions that can be located, compared and interpreted. Their combined value depends on more than reaching a target or producing an assay. The investigation must connect the drilling method, recovered sample, depth record, geological observations and spatial position to the question being tested.^[1]

A hole can intersect mineralisation without adequately resolving its thickness, structural setting or engineering behaviour. Conversely, apparently unmineralised ground may provide important evidence about access routes, groundwater or excavation conditions. Planning drilling and logging together helps distinguish successful hole completion from successful investigation.^[2]

Defining the Evidence Required

An investigation should identify the uncertainty that matters: the position of a contact, continuity of mineralisation, rock condition around a proposed opening, or material needed for testing. The required evidence determines where holes are placed and which sampling, surveying and logging capabilities are needed. Engineering investigation guidance from the U.S. Bureau of Reclamation emphasizes purpose and geological conditions before equipment selection; its planning principles are useful here without treating its infrastructure-specific procedures as universal mining requirements.^[2]

Sampling for chemical analysis and investigating structural fabric are different objectives. Broken samples may support compositional observations, whereas recovered core can preserve textures and relationships within the material that survives drilling and handling. Neither cylindrical shape nor apparent intactness demonstrates complete or unbiased recovery. Core also does not automatically preserve geographical orientation. The investigation therefore needs an explicit link between the intended decision and the kind of evidence the drilling system can deliver.^[3][4]

Spatial coverage also matters. A detailed record from one intersection remains evidence along one path. Extending it into an orebody surface or a geotechnical domain requires interpretation between observations. Additional holes should address consequential gaps rather than simply increase the number of metres recorded.

Drilling, Recovery and Sample Condition

Different drilling systems produce different forms of evidence. Reverse-circulation and rotary-mud methods can provide chips or finer cuttings; core drilling recovers cylindrical samples that may retain texture, contacts and discontinuities. Selection depends on the target material and purpose, not on a fixed ranking of methods.^[3]

Drilling can also change the evidence. Weak material may be lost, core can break during retrieval, and fluids or caving material can complicate interpretation. The drilling record should therefore accompany the geological log: changes in equipment or method, loss of returns, unstable intervals and unusual drilling behaviour help explain the condition of what arrives at the core shed.^[5]

Core recovery measures material returned relative to the interval drilled under a stated measurement convention. It is not equivalent to rock quality or assay representativeness. Preferential loss of a particular component can affect the surviving sample even where the recovered material can be analysed precisely. Table 1 of the 2012 JORC Code treats recovery, logging and sampling quality as separate reporting considerations; it specifically asks whether recovery and grade may be related.^[6]

A missing interval is therefore an information gap, not a zero-grade result. Nor can its lithology be established simply by extending descriptions from the adjacent recovered core. An interpretation may be proposed, but its basis and uncertainty need to remain distinguishable from direct observation.

Logging Observations Before Building Interpretations

Core logging records lithology, texture, mineralisation, alteration, weathering and relevant structures at a scale suited to the investigation. Geological, geotechnical and sampling records may describe the same material differently because they answer different questions. Their depth references and identifiers need to agree, even when their interval boundaries do not coincide.^[1][5]

A drilling run is an operational interval; a geological interval follows an observed change or a justified descriptive boundary; an assay interval defines the material submitted for analysis. Treating these as interchangeable can conceal contacts or create false alignment. The original intervals should remain available when data are subsequently grouped or composited. A sample or composite interval must not be mistaken for an observed geological boundary.^[1]

Descriptions should identify what was observed and how confidently it was recognised. An uncertain mineral identification is different from a laboratory-confirmed result. Likewise, a broken surface may be a natural discontinuity, drilling damage or unresolved breakage. Recording that distinction is more useful than forcing every observation into a confident structural interpretation.^[5]

Common terminology improves comparability, but standardisation must preserve the original evidence. Research on geological-log harmonisation documents the effects of inconsistent nomenclature and subjective descriptions. A code conversion can make records easier to combine; it cannot establish geological equivalence that the source descriptions do not support.^[7]

Positioning Intersections and Orienting Structures

The collar gives a starting position. Downhole surveys constrain borehole direction at measured positions; logged depths are then located on a reconstructed trajectory. Interpolation between survey stations introduces assumptions, so the calculated trace is not a continuous direct observation of the hole. Planned orientation alone is inadequate where the hole deviates. Coordinate systems, elevation references, depth origins and angular conventions must remain associated with the survey data.^[1][8]

Core orientation answers another question: how the recovered cylinder was rotated about the borehole axis in the ground. A reliable orientation reference, measurements of features relative to the core and the local hole direction together allow structural orientations to be calculated. An accurate hole survey cannot recover a lost rotational reference by itself.^[4]

This distinction also affects thickness. A logged intersection length describes distance along the hole. Inferring the true thickness of a body requires information about its boundaries and their relationship to the hole; irregular or incompletely intersected geometry may prevent a unique answer. The reporting distinction between downhole lengths and true widths is explicit in JORC Table 1.^[6]

Preserving a Traceable Investigation Record

A usable record links hole and run identifiers, depth intervals, sample numbers, photographs, logs, surveys and analytical certificates. Corrections need an explanation and a retained history, particularly where a depth adjustment changes several datasets. Replacing a diagram without retaining the underlying survey or interval records makes later verification difficult.^[1]

Photographs preserve visible condition at a particular time; physical samples retain material for re-examination or testing, subject to deterioration and previous sampling. Photography, structural measurements and destructive sampling need a coordinated sequence so that one activity does not unknowingly remove evidence needed by another. Neither replaces the other. Records of missing material, destructive tests and storage location are necessary to explain differences between the original log and the archive subsequently inspected.^[5]

Using Results and Revising the Investigation

Review should connect data quality to the intended engineering use. A hole may provide adequate compositional evidence but insufficient structural orientation, or reliable geometry but poor recovery through a critical interval. These limitations call for different responses, such as checking depth records, reviewing sample loss, changing an investigation method or obtaining independent observations.^[1]

Consider a hypothetical weak zone that is largely absent from recovered core. Adjacent pieces, drilling behaviour and nearby holes may suggest an explanation, but none turns the missing material into a direct observation. Before using the interpretation to position an excavation, the team must assess whether plausible alternatives would change the decision. The useful outcome is not merely a completed log: it is an identifiable body of evidence whose strengths, limitations and unresolved questions remain visible to the next engineering discipline.