Drilling survey from the physics and the public standards — what the numbers mean, and where they go wrong. Two modules are live; three more are being written, and self-check questions are coming to all of them.
True, grid and magnetic north; convergence and declination; and why an azimuth without a stated reference is not a number. The signs are where careers go wrong.
Why a position needs a coordinate reference system before it means anything — and why depth is worse than position. MD, TVD and TVDSS are three different axes.
What is measured at a station and what is merely computed. Minimum curvature derived, dogleg severity, and what a chord between two stations costs you.
Position is an ellipse, not a point. The separation factor, the travelling cylinder, and the difference between avoiding a collision and mitigating one.
The sensor reads the bent steel, not the hole. Where the correction comes from, what it costs you in TVD, and what the model needs from you to be worth anything.
Three of these are still being written. They are drafted, but every worked example in them is recomputed independently before it goes up, and this batch did not survive that: sixty numbers did not reproduce. A course that teaches a wrong sign is worse than no course, so they wait. The two below are live now.
The questions, when they arrive, will each name the tempting wrong answer and take it apart — knowing why a plausible answer fails is worth more than knowing the right one. Nothing will be scored against you and nothing stored; the answers live in the page.
A survey error is rarely a slip. It is a sign convention applied twice, a depth quoted from the wrong datum, an azimuth compared against a different north, or a chord measured where an arc was drilled. Each of those is invisible in the numbers and obvious once you know the shape of it — which is what these modules are for.
Ultimate QC checks all of it automatically, on every station, as it lands. But a check you do not understand is a check you will eventually override, so the reasoning is here in full and free. See what the live service does, or run the free sag check on your own BHA.
