How to Read a Drill Log

HOW-TO · AUG 2026 · 3 MIN READ

A drill log is a deposition under oath from the only witness who's actually been down there. Most people skim the assay column and skip the testimony. Here's how to read the whole document — and catch the logs that are quietly lying.

01Start at the header, not the rock

Collar coordinates and datum, total depth, azimuth/dip, drilling method (core vs. RC vs. air rotary), rig and date, and who logged it. A log without survey-grade collar data and a named logger is a story, not data — and mixed datums between old and new holes have wrecked more block models than bad assays.

02Read recovery and RQD before anything else

Core recovery (% of interval actually retrieved) and RQD (% of core in sound sticks >4 inches) are the credibility columns. Recovery below ~90% means part of the story is missing — and the missing part is usually the weak, altered, or washed-out zone that matters most. Low RQD zones drive slope design, dilution, and water: flag every one.

03Walk the lithology column with the depths

Good logging reads like a section: rock type, color, grain size, weathering, alteration, structure angles to core axis. Watch the contacts — sharp vs. gradational tells you about continuity between holes. If the descriptions suddenly get lazy ('gray rock') mid-hole, note the depth: logger fatigue is real and it clusters.

04Mark water, voids, and loss

Water strikes, loss of circulation, rods dropping, mud rings — these hydro notes predict pumping costs, blasthole loading products, and karst surprises. In carbonate country, a 'void 42.5–44.0 ft' scribble is worth more than the whole assay table.

05Read assays as intervals, not points

Check sample intervals against lithology breaks — samples spanning a contact smear grade across two different rocks. Note the lab, method, and detection limits, and look for QA/QC: standards, blanks, duplicates. A beautiful grade intersection with no QA/QC line items is an unverified rumor.

06Cross-examine adjacent holes

One log is an anecdote. Lay two or three neighbors side by side at the same datum and see whether contacts, grades, and RQD zones correlate. Continuity between holes is what turns intercepts into a resource — and its absence is what due diligence exists to find.

The TrapReading only the mineralized interval. The overburden thickness, the water table, the RQD of the hanging wall — the 'boring' parts of the log — are where mining cost lives. Every log has two stories: what you'll sell, and what you'll suffer to get it. Buyers who read only the first one overpay.

Rule of ThumbHeader, recovery, rock, water, grade — in that order. And low recovery in the ore zone means the grade is a guess wearing a lab certificate.

ShotRock reads drill logs for a living — geotech reviews, resource audits, and the due-diligence log-by-log grind that finds what summaries smooth over.

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Figures are approximate, compiled from public sources as of mid-2026, and rounded for readability. Rankings shift with markets, mergers, and new discoveries. Nothing here is investment, legal, or engineering advice for a specific site — for that, hire an engineer. We know one.