Velocity Pushdown
Learning objectives
- Explain that a time section maps traveltime, not depth
- See a flat reflector sag beneath a slow (gas) velocity anomaly
- Recognise velocity pushdown as an artefact, not real structure
- Know that only correct-velocity depth conversion removes it
The Clock, Not the Depth
A seismic time section is a map of traveltime, and traveltime equals depth only when velocity is constant. Where the overburden velocity varies from place to place, the section stops being a faithful picture of structure, and the classic offender is a slow body: a gas cloud.
Put a gas cloud, where velocity drops sharply, above a perfectly flat reflector. Waves crawl through the slow gas, so the reflector directly beneath it takes longer to image and appears pushed down in time. The reflector has not moved a millimetre; only the clock reading it has slowed. This is a velocity sag or pull-down.
An Artefact That Drills Dry Holes
The dashed line marks where the reflector actually is, flat, while the imaged event sags beneath the gas. Slow the gas and the sag deepens; raise its velocity toward the background and the sag fades, because with no velocity contrast there is no pull-down. This is not a subtle effect: a strong gas sag can mimic a structural low, and the steep edges of the sag can mimic faults. Interpreters have mapped these artefacts as real closures and drilled them, only to find flat, undisturbed rock.
The cure is depth conversion with the correct velocity, which straightens the reflector back to flat, or the discipline of remembering that time is not depth. A convolutional synthetic is blind to this entirely: with no lateral velocity in its model, it never bends the reflector and so never even poses the question. The wave equation makes the sag because it is the honest consequence of a wave moving slower through slow rock. Reading it correctly, sag as velocity rather than structure, is one of the most valuable habits an interpreter has.