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Drilling is the most expensive part of most exploration and investigation programmes. Core logging is how that investment is converted into usable information. A cylinder of rock pulled from hundreds of metres below ground contains the evidence of what is down there, but only if geologists record it carefully, consistently, and accurately.
What Is Core Logging?
Core logging is the systematic description and measurement of drill core by a geologist or geotechnical engineer. It records rock type, texture, mineralogy, alteration, structure, mineralisation, and physical properties, and it identifies intervals for sampling. The log becomes the primary record of the hole and underpins geological models, resource estimates, and engineering designs.
Types of Core Logging
Geological logging. Describes lithology, colour, grain size, texture, weathering, alteration, veining, sulphide content, and visible mineralisation. Standardised codes and legends keep descriptions consistent across geologists and holes.
Structural logging. Measures the orientation and character of faults, fractures, veins, and foliation. When core is oriented, true strike and dip can be calculated, which is critical for understanding mineralised structures.
Geotechnical logging. Records Total Core Recovery (TCR), Solid Core Recovery (SCR), Rock Quality Designation (RQD), fracture frequency, joint condition, and rock strength estimates. These parameters feed rock mass classification systems used in slope design, underground support, and foundations.
Geochemical and sampling logging. Marks sample intervals, sample types, and quality control insertions such as standards, blanks, and duplicates.
Hydrogeological logging. In water projects, notes fractures, weathering, and water strikes to understand aquifer zones.
From the Drill Rig to the Core Shed
Reliable logging starts at the rig. Good practice includes:
- Careful core handling: Placing core in labelled trays with depth blocks marking each run.
- Depth checks: Reconciling driller’s depth records with core lengths.
- Photographing core: Taking wet and dry photographs of each tray before sampling.
- Orientation marks: Recording orientation lines where orientation tools are used.
- Secure storage: Protecting trays from weather, theft, and mixing.
At the core shed, trays are laid out, cleaned, and checked before logging begins.
Key Logging Parameters Explained
Recovery. The percentage of the drilled interval actually recovered as core. Low recovery can bias sample results and signals fractured or weak ground.
RQD. The percentage of core pieces longer than 10 cm in a run. It is a simple indicator of rock mass quality.
Alteration. Chemical and mineral changes caused by fluids, such as silicification, sericitisation, chloritisation, and carbonatisation. Alteration patterns often vector toward mineralisation.
Veins. Type, thickness, orientation, mineral content, and crosscutting relationships help establish paragenesis and structural controls.
Mineralisation. Style, mineral species, and intensity are recorded, supported by assays rather than estimated by eye alone.
Digital Logging and Data Management
Modern programmes use digital logging software or structured spreadsheets with dropdown lists. This reduces transcription errors, enforces consistent codes, and speeds validation. Data are stored in a central database linked to collar, survey, assay, and geotechnical tables. Backups and version control are essential, since logs and assays are long-lived project assets.
Sampling and QA/QC
After logging, core is marked for cutting. Half core is typically sent for assay, with the remaining half retained as a permanent record. Insertion of certified reference materials, blank samples, and duplicates monitors laboratory accuracy and contamination. Chain-of-custody procedures protect sample integrity from the core shed to the laboratory.
Applications in Different Projects
- Mineral exploration: Understanding host rocks, alteration, and structural controls to refine targets.
- Resource definition: Providing the geological interpretation behind modelling.
- Geotechnical design: Supplying rock quality data for pit slopes, tunnels, and foundations.
- Geothermal and hydrogeology: Identifying fractures, alteration, and permeable zones.
- Quarry and aggregate assessment: Evaluating rock strength and quality.
Common Problems
- Inconsistent codes between geologists, leading to unreliable data.
- Poor core recovery that is not recorded or interpreted.
- Lost or mislabelled trays.
- Logging only what appears obviously mineralised and ignoring context.
- Insufficient photography and documentation.
- Neglecting core storage, causing deterioration.
Selecting a Core Logging Service Provider
Look for experienced geologists familiar with your deposit type, clear logging procedures and codes, use of digital tools, geotechnical capability, and strict QA/QC discipline. Ask to see sample logs and core photography standards. Training and supervision matter, since logging quality depends on the people.
Conclusion
Core is the most valuable physical evidence in a drilling programme. Skilled, consistent logging protects that investment and provides defensible data for decisions on exploration, mining, and engineering.
Innercore Geoconsultants Services Limited provides geological, structural, and geotechnical core logging services across Kenya, supported by rigorous sampling and QA/QC procedures. Contact us to support your next drilling campaign.
