Auto-tracking: when it wins, when it fails

How auto-tracking actually works

Strip away the marketing and production auto-trackers do something very simple:

• Read the seed pick's amplitude sign — call it positive (peak) or negative (trough).
• Move to the next crossline.
• Within a small time window around the seed's time, search for the nearest local extremum of the SAME sign.
• If one is found AND the time jump from the seed is within a dip tolerance AND the amplitude is above a noise threshold, place a pick there.
• Use that new pick as the seed for the next crossline. Repeat.
• When any condition fails (no extremum, time jump too big, amplitude too small), stop.

That's the whole algorithm. No machine learning, no pattern recognition in the modern sense. Its power comes from the fact that real reflectors are laterally continuous in amplitude, polarity, and time — so a 1-D continuity follower tracks them well. Its limits come from the cases where that continuity breaks.

Where auto-tracking wins

• High signal-to-noise zones. When the target reflector is an order of magnitude brighter than the noise, the nearest-extremum search finds the right event every time.
• Continuous, gently-dipping reflectors. Normal sedimentary reflectors with no tectonic complications are the bread-and-butter case. Auto-track thousands of traces in seconds, then QC the map for anomalies.
• Dense, regular survey geometry. When consecutive traces are close in space (10 m to 25 m bin size) the reflector barely moves between traces and the tracker's small search window succeeds.
• Zero-phase data. The peak sits on the reflector, so snap-to-peak behaves. Phase-rotated or minimum-phase data confuses the tracker because "the peak" may not be on the reflector.

Where auto-tracking fails

• Faults. The reflector on the other side of the fault sits at a different time. The tracker, which only knows about local continuity, either drops the pick (if the time jump exceeds the dip tolerance) or walks onto a DIFFERENT reflector on the other side of the fault that happens to be near the tracker's last time estimate. The second case is especially dangerous — the tracker keeps going, confidently picking the wrong reflector across the entire downthrown block.
• Stratigraphic pinchouts. The reflector gets progressively dimmer as the bed thins below tuning. The tracker's amplitude threshold eventually rejects the pick and the horizon stops — often correctly, but sometimes in a way that hides a pinchout that is still geologically real.
• Low S/N or attenuation zones. In deep, attenuated parts of the volume, the "brightest extremum" may be noise rather than the reflector. The tracker picks noise.
• Overlapping reflectors of similar brightness. When two reflectors are near each other and comparably bright, the tracker flips between them, producing a "noisy" horizon that is really a hybrid of two different surfaces.
• Lateral facies change. A sand grading into a shale changes the reflector's sign or strength. The tracker stops (polarity flip) or walks onto a different event.

Exercise

• Seed. Click once on the brightest continuous reflector near the middle of the section, placing a single high-quality seed pick.
• Auto-track the inline. Press Auto-track from seed. Picks propagate outward along the full inline. Inspect them — do they all sit on the same reflector? If there are stragglers on sidelobes, shift-click to delete them.
• Show the time map. Press Time map. Your picks appear as a single vertical band on the IL×XL plan view, coloured by time.
• Propagate across inlines. Press Propagate → next inline repeatedly. Watch the time map fill in as you cross the volume.
• Observe what happens at the fault. The synthetic has a fault at inline 1031 (16 ms throw). When you propagate across that boundary, the picker either drops picks (if the jump exceeds the tolerance) or transfers them with a visible time jump in the map. Either way, you see the fault directly on the time map as a sharp colour discontinuity.
• Grade. Press Grade my picks. The RMS now aggregates across hundreds of picks spanning many inlines. A good auto-track on this synthetic lands in Excellent or Good.

The QC discipline

Auto-tracked picks are never final until they have been inspected. The time map is your primary QC tool. What to look for:

• Smooth colour gradients. Sedimentary reflectors produce maps that look like smooth topography. Sudden spots of very different colour in the middle of a smooth field are almost always pick errors.
• Sharp linear discontinuities. These may be faults (real) or tracker failures (not real). Cross-reference with your structural picks from §2.5 (when we build the fault picker) and with coherence attributes (Part 6). A lineament on the time map that has no corresponding fault on the structural interpretation is a bug, not a feature.
• Areas where picks drop out. The tracker gave up. Usually because the reflector became too dim or the dip too steep for the tolerance. You may need to re-seed in that area manually, or widen the tolerance and re-track.
• Near-well consistency. If you have wells, check that the auto-tracked horizon matches the well marker at each well within your expected depth uncertainty. Any systematic offset at wells means the tracker walked onto the wrong reflector somewhere between the seed and that well.

A working rhythm

The professional workflow combines manual and automatic:

• Seed manually at wells (highest-confidence anchor points).
• Seed manually at a few widely-spaced other inlines to form a skeleton.
• Auto-track from each seed, filling the volume.
• QC on the time map. Fix problem zones (delete bad picks, re-seed, re-track with adjusted tolerances).
• Cross-check at wells one more time.
• Ship the horizon to the downstream workflow (depth conversion, attribute extraction, volumetric calculations).

The step that junior interpreters skip is step 4 — the QC. The interpreters whose maps get drilled through without nasty surprises are the ones who treat step 4 as the most important step, not the last one.