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Core Logging Practitioner

How to Log Drill Core, Step by Step

Logging drill core means recording everything the core reveals at measured depths, in a fixed sequence. Here's the field procedure, step by step — from depth-checking the core to validating the data.

Logging drill core is the systematic, depth-by-depth recording of everything a core reveals — rock type, structure, alteration, mineralisation, and rock quality — at measured intervals down the hole. You do it in a fixed order: receive and depth-check the core, reconstruct and mark it, measure recovery and RQD, photograph it, log the geology interval by interval, then mark samples and validate the data. Follow that sequence and nothing irreversible happens before it’s recorded.

This is a practitioner’s walkthrough of that procedure — the actual order of operations a geologist follows at the logging table, and the reasons each step comes where it does. If you want the what and why of core logging as a discipline, start with our companion guide, What Is Core Logging? This page is the how.

One principle runs through all of it: measure and record before you destroy. Core can only be photographed whole once, recovery can only be measured before pieces are removed, and a depth error caught at the table costs minutes — caught in the resource model, it costs months.

Before you start: set up so the data is clean at source

Good logging is mostly decided before the first tray arrives. Three things should already exist:

  • A logging template and code dictionary. Agree the rock-type, alteration, mineralisation, and structural codes for the deposit before drilling, so every geologist writes GRP for the same granite porphyry rather than four spellings of it. The CIM Mineral Exploration Best Practice Guidelines are explicit that logs should use a standard geological legend consistent across the property, and that the logging software should capture information in a consistent, functional, and secure way for later stages of evaluation.
  • A logging station. A flat, well-lit table or rack; a fibreglass tape; a hand lens (10×); dilute (10%) HCl for carbonates; a magnet; a ruler or core-frame; an angle-iron or V-rail to lay broken core in; and a camera setup. The What Is Core Logging? guide has the full equipment list.
  • A device to log into. Paper sheet, tablet, or laptop. The choice shapes everything downstream — see Digital vs. Paper Core Logging.

With that in place, here is the sequence.

Step 1 — Receive the core and check it against the driller’s blocks

Core arrives from the rig in trays, laid out the way it came from the ground: top-left to bottom-right, shallow to deep, like reading a page. Between each drill run the driller has inserted a core block — a marker written with the end-of-run depth — and may have marked breaks they caused while handling the core.

Your first job is depth registration: confirm the depths on the blocks are sequential and sensible, and reconcile the driller’s measured depth with the core actually in the tray. The two never agree perfectly. A short stub of core can be left in the hole at the end of a run, so the driller’s depth (collar to bit) and the break you see in the tray can differ by centimetres. Resolve those discrepancies now, while the rig can still be asked, not three months later.

Why this is Step 1: every record you make afterwards is tied to a from–to depth. If the depth framework is wrong, every lithology contact, assay, and structural measurement is wrong by the same amount.

Step 2 — Reconstruct, orient, and mark the core

Fit the core back together as a continuous cylinder. Match the broken faces of successive pieces and push the run together along a V-rail so it approximates the volume of rock that was actually drilled. This step is also where you separate natural fractures (real geology) from mechanical breaks (caused by drilling or handling) — a distinction that drives the next two measurements.

If the hole was drilled with an orientation tool, transfer the bottom-of-hole reference onto the core now: draw a continuous bottom-of-hole (BOH) line down the core surface, joining the orientation marks run to run. This single line is what later lets you turn an angle measured on the core into a real dip and dip direction in the ground (Step 6).

Mark the top of each run with a reference line before any pieces are turned or removed. Clean the core — a light water spray is usually enough — so textures, contacts, and fine minerals are visible.

Step 3 — Measure core recovery (before any core is removed)

Core recovery is the proportion of rock you actually got back, relative to what was drilled:

Core recovery (%) = (length of core recovered ÷ length of the drill run) × 100

Measure it run by run, with the core fitted together, before anything is sampled or removed. Recovery, RQD, and fracture frequency are all measured at this point precisely because removing pieces for the lab makes them impossible to measure honestly afterwards.

Recovery below ~90% is worth a second look. It can mean genuinely weak or broken ground — useful information — or it can mean lost core from poor drilling, flushing, or a blockage. Distinguishing the two matters, because low recovery through a mineralised zone can quietly bias grade. Note the cause on the log; don’t just record the number.

Step 4 — Measure RQD and fracture frequency

Rock Quality Designation (RQD), introduced by Don Deere in 1967, is the single most-reported measure of rock-mass quality. It’s the percentage of a run made up of sound, intact pieces longer than 10 cm, measured along the core axis:

RQD (%) = (Σ lengths of intact core pieces ≥ 10 cm ÷ total length of the core run) × 100

Two rules keep RQD honest: measure piece lengths along the core centreline, and count only breaks at natural fractures — piece the mechanical/drilling breaks back together first (this is why Step 2 comes before this one). Record fracture frequency at the same time — the number of natural fractures per metre — which complements RQD where pieces cluster near the 10 cm threshold.

RQD maps to a standard quality scale:

RQD (%)Rock-mass quality
90–100Excellent
75–90Good
50–75Fair
25–50Poor
0–25Very poor

Deere classification. RQD was originally defined on NX-size core; the ≥ 10 cm threshold is the constant.

Because RQD overlaps with a topic already covered on the commercial blog, this guide keeps it to the working definition; the dedicated RQD Logging Explained module goes deeper.

Step 5 — How should you photograph the core?

Photograph every tray before it’s sampled or split — the photo is the permanent visual record, and after sampling the core is never whole again. Shoot each tray both dry and wet: wet core reveals colour, veining, and texture that dry core hides.

Industry photography guidance converges on a few standards:

  • One tray per frame, square to the tray, with the entire tray in shot — including the depth labels and tray ends, which carry data.
  • A scale with millimetre divisions and a colour/grey reference card in every frame, so images stay measurable and colour-consistent between trays.
  • Diffuse, off-axis lighting. Avoid a single bright source directly above or behind the camera — it throws reflections off wet core. A shroud and consistent, high-CRI light remove background variation.
  • A consistent setup tray to tray. The value of core photos is comparability down the hole and across the project; consistency beats artistry.

Large programs increasingly use automated core-scanning rigs, but a fixed camera, a shroud, and a scale card are the dependable baseline.

Step 6 — Log the geology, interval by interval

This is the heart of the job: working top-to-bottom through the hole, you break the core into intervals and describe each one. Define intervals at contacts — where the rock changes — not at round-number depths. A logging interval closes when the geology changes; the Depth To of one interval is the Depth From of the next.

Most programs log in passes, and the usual order is:

  1. Lithology — the foundation pass. Rock type, texture, grain size, colour (fresh and weathered), primary fabric (bedding, foliation), and the nature of each contact (sharp, gradational, faulted). Lithology defines the intervals the other passes hang off.
  2. Alteration — the mineralogical changes the host rock has undergone (potassic, phyllic, argillic, propylitic, silicification), usually logged with an intensity so you can vector toward the system.
  3. Mineralisation — ore and sulphide minerals, their abundance (visual %), and style (disseminated, stringer, vein-hosted, massive). This pass feeds your sampling decisions in Step 7.
  4. Structure — veins, faults, shears, foliations and their orientations.

For each coded field, pick from the agreed dictionary rather than free-typing — that’s what makes the data comparable across the team and across holes.

How do you record a structure on oriented core?

On oriented core you don’t record a dip directly — you record two internal angles against the BOH line you drew in Step 2, and the geometry is reconstructed later:

  • Alpha (α): the acute angle (0–90°) between the core axis and the long axis of the ellipse the structure makes where it cuts the core surface.
  • Beta (β): the angle (0–360°) measured clockwise around the core, looking down-hole, from the BOH line to the down-hole end of that ellipse.

Those two numbers, combined with the downhole survey, convert into a true dip and dip direction. Full method and the common pitfalls live in Structural Logging & Measuring Core Angles.

The most common logging error isn’t a wrong rock name — it’s an inconsistent one. The same unit logged as BX, bx, and breccia by three geologists fractures the dataset. A locked code dictionary prevents it at the keystroke. More in Common Core Logging Mistakes.

Step 7 — Mark and record sample intervals

With mineralisation logged, mark where samples will be cut. In mineral exploration you generally sample by half-core: the core is split lengthways (usually by diamond saw), half goes to the lab for assay and half is archived so the interval can be re-examined or re-assayed years later. Record each sample’s number and its from–to depth in the log as you mark it.

Two disciplines matter here:

  • Honour geological boundaries. Start and end samples at contacts you logged, and respect the program’s minimum and maximum sample lengths (commonly ~0.3–1.5 m). A sample that straddles a sharp ore/waste contact smears grade across it.
  • Insert QA/QC samples to plan. Certified reference materials (standards), blanks, and duplicates go into the sample stream at set frequencies so assay quality can be checked later. CIM best practice treats sample numbering, standards/blanks/duplicates, and chain of custody as core parts of the data chain — not optional extras. Detail lives in QAQC for Exploration Drilling.

Keeping the sample register inside the same system as the log — rather than a separate spreadsheet — is what stops the classic duplicated or skipped sample number.

Step 8 — Validate, sync, and hand the data off

A log isn’t finished when the last interval is described; it’s finished when the data is clean, backed up, and in one place. Before you leave the interval:

  • Validate. Check for overlapping or inverted intervals, depths outside the run, missing required fields, and codes that aren’t in the dictionary. On paper these surface weeks later in the office; caught at entry, they’re a five-second fix.
  • Reconcile sample intervals against logged geology and against recovery, so a mineralised interval with 60% recovery is flagged, not buried.
  • Back up / sync. The log is now the lasting record — the physical core will eventually be sampled out or lost. Get it off the single device it was typed on.

From here the validated intervals render as a strip log, feed the 3D interpretation, and export to modelling software. That hand-off is only as good as the consistency you enforced in Steps 1–7.

The procedure at a glance

#StepYou must do it before…
1Depth-check against driller’s blocks…recording any interval
2Reconstruct, orient, mark…measuring recovery/RQD
3Measure core recovery…removing any core
4Measure RQD & fracture frequency…removing any core
5Photograph (wet & dry)…splitting or sampling
6Log lithology → alteration → mineralisation → structure…marking samples
7Mark & record sample intervals (half-core + QA/QC)…cutting the core
8Validate, sync, hand off…the core leaves the shed

FAQ

What is the correct order for logging drill core? Receive and depth-check the core against the driller’s blocks; reconstruct, orient, and mark it; measure core recovery; measure RQD and fracture frequency; photograph it wet and dry; log the geology (lithology, then alteration, mineralisation, and structure); mark and record sample intervals; then validate and back up the data. The fixed rule is to measure and photograph before any core is removed or split.

How do you calculate core recovery and RQD? Core recovery (%) is the length of core recovered divided by the length drilled in that run, times 100. RQD (%) is the summed length of sound, intact core pieces longer than 10 cm — measured along the core axis and counting only natural fractures — divided by the total run length, times 100. Measure both before sampling.

Why measure recovery and RQD before sampling? Because removing pieces for the lab makes the run impossible to measure honestly. Recovery, RQD, and fracture frequency all depend on the complete, reassembled run being present. Once core is split or sent away, those numbers can’t be reconstructed.

What’s the difference between a natural fracture and a mechanical break? A natural fracture existed in the rock; a mechanical break was caused by drilling or handling. It matters because RQD counts only natural breaks — mechanical breaks are pieced back together first. Drillers mark breaks they cause for exactly this reason.

How long does it take to log a hole? It varies with complexity. An experienced geologist can log lithology and alteration through competent core at roughly 20–30 metres per hour, but heavily altered, fractured, or mineralised intervals — and any oriented structural logging — take considerably longer.

How Blue Butterfly handles this

Every step above produces data that has to land somewhere clean — and the failure point in most programs isn’t the geology, it’s the hand-off. Blue Butterfly is built so the procedure and the database are the same act:

  • Depth integrity (Steps 1–6): Depth From / Depth To are first-class fields, and the validation engine flags overlapping or inverted intervals as you type, not in the office.
  • Codes that stay consistent (Step 6): dropdown columns enforce the agreed dictionary, so the same unit can’t be logged three ways across the team.
  • Recovery, RQD & structure alongside the geology (Steps 3–6): geotechnical and structural measurements log as their own validated tables next to lithology, and render straight into a depth-scaled strip log.
  • Samples in the same system (Step 7): the sample register lives with the log, so numbers don’t get duplicated or skipped between a spreadsheet and the database.
  • Synced and safe (Step 8): records write to a cloud database automatically — online or offline — so the log is never trapped on one device, and it exports to your modelling stack without an Excel step in the middle.

The point isn’t more software in the core shed. It’s that the validated log is the deliverable — not a backlog of sheets waiting to be typed up.

Sources

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