Every mineral discovery starts with a question: what lies beneath the surface? Diamond drilling answers that question by recovering cylindrical samples of bedrock, but the rock only becomes valuable information once it has been properly logged.
Core logging is the structured process by which geologists examine, describe, and record drill core to characterize the subsurface, guide exploration decisions, and build the geological model that supports resource estimation.
Whether the project targets gold, copper, lithium, or zinc, the principles are the same: read the rock carefully, record it consistently, and preserve it so the data can be verified years later. Doing core logging best practices, data collection becomes the most reliable input in any exploration database.
Done poorly, it propagates errors through every downstream calculation.
From the Rig to the Logging Shed
Core logging begins at the drill rig. The rig is positioned over a target and diamond drilling recovers rock in approximately 10-foot runs that are carefully transferred into core boxes and delivered to the geology team for detailed logging. From the moment the core leaves the barrel, depth control and orientation must be preserved — otherwise the relationship between sample and position in the borehole is lost.
At the logging shed, the geologist’s first job is housekeeping: confirm meterage blocks, mark drill runs, measure core recovery and RQD (Rock Quality Designation), and ensure that every interval in the core trays matches the driller’s log. Only then does descriptive logging begin.
What Geologists Actually Record
Core logging is interval-based. Each observation is tied to a “from–to” depth that corresponds with the start and end of the material being described. The categories typically include:
- Lithology: rock type, grain size, texture, color, and any sedimentary, igneous, or metamorphic features.
- Structure: bedding, foliation, faults, fractures, veins, and their orientations relative to the core axis.
- Alteration: the mineral assemblages that overprint the original rock — silicification, sericite, chlorite, argillic, potassic, propylitic — and their intensity.
- Mineralization: sulfide and oxide content, mode of occurrence (disseminated, vein-hosted, breccia), and visual estimates of grade.
- Geotechnical parameters: core recovery, RQD, fracture frequency, and rock strength estimates.
- Physical measurements: magnetic susceptibility, specific gravity, and, where available, portable XRF readings that provide an early indication of chemistry before the lab returns results.
Consistency across loggers is critical. Most projects rely on a written logging protocol, controlled codes, and regular calibration sessions where senior geologists review intervals alongside the team to keep descriptions comparable from hole to hole. These observations form the framework that geochemical assays will later confirm and quantify.
The geochemical signature of mineralization usually develops as a halo around the deposit, with elemental enrichment and depletion patterns that commonly exhibit systematic vertical and lateral zonation. Those patterns can be used to vector toward the mineralized center, sometimes from a few meters to several kilometers away, depending on the deposit type.
Identifying the start of that halo in the logging shed — before the assays return — is one of the highest-value skills in mineral exploration.
Why Core Boxes and Core Trays Matter
Logging is only as good as the physical condition of the core. A misplaced run, a mixed interval, or a broken stick that no one can put back together will compromise every downstream interpretation. That is why the choice of core boxes and core trays is a technical decision, not a logistical one.
Quality trays preserve the order and orientation of every meter recovered, withstand handling between the rig and the warehouse, and keep the core dry and undamaged through years of storage.
Because most projects sample only half of the core — sending one half to the lab for analysis while keeping the other for future observation — the retained half must remain identifiable and accessible long after the drill rig has left the property. Standardized, durable core trays make that long-term retrievability possible.
From Logs to Data
Once an interval has been logged and reviewed, the geologist marks sample intervals, typically at regular spacings or across geological contacts. The core is split, half goes to the laboratory, and the remaining half stays in the box. At the lab, samples are dried, crushed, split, pulverized, and dissolved according to the chosen analytical method, with QA/QC protocols verifying representativeness, accuracy, and precision at every stage of mass reduction.
Assay results return as a CSV file and are imported into the project database, where they sit alongside the logged descriptions for joint interpretation.
The combined dataset — descriptive logs plus geochemistry — is then visualized in 3D software such as Leapfrog or Geoscience Analyst. Continuous and categorized color schemes highlight high and low values, and interpolants like iso-shells help evaluate spatial continuity and volumes.
This is where the patient work of the logging shed pays off: every interval that was logged correctly contributes to a sharper, more defensible geological model.
How Core Logging Complements LWD
In oil and gas, Logging While Drilling (LWD) integrates sensors directly into the drill string to measure gamma radiation, resistivity, density, and porosity in real time, enabling immediate decisions about well trajectory and reservoir characterization.
In mineral exploration, the analogous workflow still relies primarily on physical core: the rock itself is the record. The two approaches share a goal — understanding the subsurface as efficiently as possible — but core logging remains the gold standard for the level of structural, mineralogical, and textural detail that mineral projects demand.
Core logging is where exploration data is born. The discipline rewards consistency, careful observation, and respect for the physical sample. With the right protocols, well-trained geologists, and durable storage in proper core boxes and core trays, every meter drilled becomes an asset — usable today, verifiable tomorrow, and defensible for the entire life of the project.
FAQs
What is the difference between core logging and assaying?
Core logging is the visual and descriptive characterization of the rock recovered from a drill hole — lithology, structure, alteration, and mineralization. Assaying is the laboratory measurement of specific ore elements and metals, typically with an economic focus. The two are complementary: logging tells you what the rock is, assaying tells you how much of an element it contains.
How long should drill core be preserved?
Best practice is to retain at least half of every sampled interval for the life of the project, and often well beyond. Future re-logging, re-sampling, and verification of historical data depend on access to the original core, which makes long-term storage in quality core trays essential.
Is core logging done on paper or digitally?
Most modern projects use digital logging tools with predefined codes for lithology, alteration, and structure, allowing the data to flow directly into the project database. Paper logs are still used as a backup or in remote conditions, but digital workflows reduce transcription errors and accelerate interpretation.
Can drill core data be used to vector toward a deposit?
Yes. Geochemical halos around mineralized centers usually display systematic vertical and lateral zonation. By analyzing both the logged features and the geochemical results, geologists can identify trends that point toward higher-grade mineralization, sometimes from several meters to several kilometers away.
How is core logging different from Logging While Drilling (LWD)?
LWD is an oil and gas technology that captures formation properties in real time through sensors built into the drill string. Core logging is the manual, interval-by-interval description of physical rock recovered by diamond drilling, and it is the dominant method in mineral exploration where detailed lithological and structural observation is critical.
