Picking horizons: methodology and practice

What a "horizon" is and is not

A seismic horizon is a locus of picks — one pick per trace (or per picked trace) — chosen so they all follow the same reflector. The horizon is, in principle, a surface. In practice it is a cloud of picks that you later interpolate into a surface.

Important: a horizon is not the reflector itself. The reflector is a physical boundary in the earth; the horizon is your interpretation of where that reflector sits at every location on your survey. The horizon can be wrong in many ways (picked the wrong peak, followed a different reflector across a fault, picked noise where there is no reflector). Treating the horizon as ground truth rather than as your best interpretation is a recipe for overconfidence.

The four-stage picking workflow

• Choose your reflector. Before picking, decide which reflector you want to trace. Usually this is determined by a well tie: the well log says "Top Reservoir is at 2340 m", the synthetic seismogram computed from the log says "this corresponds to a strong peak at 1870 ms TWT", so that's the event you follow. Without a well tie, you can still pick horizons, but the name attached to the horizon ("top carbonate") is a geologic hypothesis, not a fact.
• Seed a few key inlines. Place picks on 3–5 widely-spaced inlines manually — ideally inlines you understand well (near wells, across obvious structures). This is your "skeleton". Every subsequent pick gets cross-checked against the skeleton for consistency.
• Propagate. Fill in the gaps between seed inlines. This is where auto-tracking helps (§2.4 covers it). For now, do it manually — walk through inlines at regular spacing and make sure your picks stay on the same reflector as the skeleton.
• QC and fix. Display the horizon as a time map (shading by the pick time across the survey). Geologic surfaces should look smooth, with coherent structural patterns. If your horizon map has isolated bright or dark spots that don't match the reflectors on nearby slices, those are pick errors. Go back, inspect, and fix. §2.6 covers QC in detail.

The discipline is: never skip from stage 1 to stage 3 without seeding. A horizon picked by scrolling from one end of the survey to the other, clicking on "the brightest reflector at each location," almost certainly drifts off the intended surface somewhere along the way because the "brightest" reflector changes as you cross faults or go through a stratigraphic pinchout.

Snap-to-peak and its limits

Every interpretation tool provides snap-to-peak: you click near a reflector and the software refines your click to the nearest local extremum (peak or trough) within a small window. This does two things: it removes click-position noise (turning ±5 ms scatter into ±1 ms consistency), and it enforces that you are always picking an extremum, which is what a reflector event actually is.

When snap-to-peak fails: (1) in low signal-to-noise zones where there IS no clear extremum near your click; (2) when two reflectors are closer than your snap window, causing the snap to jump between them unpredictably; (3) when the wavelet is not zero-phase and the "extremum" does not sit at the actual reflector position. If you see a run of picks with large time scatter, or the picks bouncing between two cycles, snap-to-peak is confusing you rather than helping. Turn it off in those zones and pick manually.

Common picking mistakes, and how to catch them

• Cycle skip. You pick the reflector on one inline, then on the next inline you accidentally pick the peak one cycle shallower or deeper. The horizon map shows a step. Fix: display the map, scroll through the steps, re-pick the offending inlines.
• Crossing a fault. Your reflector is offset by a fault; you didn't notice, and the picks walk from one side of the fault onto a reflector on the other side. Fix: display coherence or dip attributes (Part 6) which highlight fault planes and let you stop your picks at fault boundaries.
• Following sidelobes. The wavelet has sidelobes on either side of its main peak. Your pick drifts onto a sidelobe because of local amplitude variation, then keeps following the sidelobe. Fix: a well tie will reveal a consistent time bias; re-pick with reference to the well.
• Noise masquerading as reflector. In low-SNR zones you can pick "reflectors" that are actually processing artifacts. Fix: check the horizon continuity against known wells; if a pick in a low-SNR zone disagrees with the nearest well, it is almost certainly noise.

Exercise

• Start on the default inline and find the brightest continuous reflector near the middle of the section. That is the target horizon.
• Place 6–8 picks across the current inline, following that reflector. Use snap-to-peak. Space your picks across the whole inline, not clustered.
• Scroll to a different inline (use the Inline slider). Place picks on the same reflector there too.
• Repeat on 3–4 more inlines across the volume. Notice the fault offset when you reach it — pick both sides.
• Toggle Show reference to see the expert pick. Compare your picks to it.
• Press Grade my picks. The widget reports your RMS error and a quality band.

Aim for "Good" or "Excellent". "Fair" usually means you picked a sidelobe or a different cycle for part of the volume; "Check your work" usually means you picked a different reflector entirely. Tip: if your first grade is poor, use Show reference to see where you diverged, fix the bad picks (shift-click them to delete), re-pick, and regrade.