Horizons and gridding: turning picks into a map
Learning objectives
- Seed a horizon and read the tracker's stop reasons as information, not failure
- Build a framework by picking a horizon on a coarse grid of lines
- Explain why inverse distance weighting produces bullseyes and cannot extrapolate a trend
- Recognise when the error in a map is set by pick density rather than by the algorithm
Now the surfaces. In the Section viewport choose PICK HORIZON and click a strong reflector. The seed snaps to the nearest peak or trough and a tracker carries it both ways along the line, exactly as the auto-trackers of Part 2 did, with the same four stop conditions: no extremum in the search window, a time jump beyond the dip tolerance, amplitude collapse, waveform decorrelation. When it stops, the status bar says why. That sentence is the most useful thing an auto-tracker produces; on the far side of a fault, re-seed and let it run again.
In the workstation
Pick your target horizon on every eighth inline or so, the way a working interpreter actually does, because picking every line on a real survey takes a week. Then open the Map viewport in TIME STRUCTURE: your line picks grid into a surface, and the amber dots show exactly where the map is evidence and where it is invention.
The two algorithms behave completely differently
Switch the method and watch. INVERSE DISTANCE takes a weighted average of nearby picks: easy to explain, and visibly wrong. Because it is an average it can never produce a value outside the range of the data, so it cannot extrapolate a trend, and because the weight blows up at a control point every picked line becomes a ridge of bullseyes. MINIMUM CURVATURE solves a different problem: the smoothest surface that passes through every pick, which is what a geologist does with a pencil. The TENSION knob interpolates between two physical models, a thin elastic plate at zero and a soap film pulled taut at one. Real gridding packages expose exactly this knob, usually without saying what it does.
And the limit that no algorithm escapes: a map built from every eighth line cannot contain structure narrower than eight lines. Where your picks are sparse, the surface between them is smooth because the gridder made it smooth, not because the earth is. The fix is more picks, not a better algorithm, and the place to spend them is wherever the map is about to be used to make a decision.
The STATISTICS tab keeps score as you pick: coverage, why the tracker stopped and how often, and the line-to-line mistie, which is the honesty check no answer key can give you. The 3D tab renders every horizon surface you have mapped, with holes where you have not.