moveout, Oil & Gas Glossary

Moveout is the systematic change in the arrival time of a seismic reflection as a function of the horizontal distance, called offset, between the seismic source and the receiver that records it. When a shot is fired at surface, the sound energy travels down to a reflecting rock boundary and back up to a spread of geophones or geophone groups laid out at increasing distances from the source. A receiver directly over the reflection point records the shortest, so-called zero-offset, two-way travel time, while receivers farther away record progressively longer travel times because the sound has to travel a longer slanted path. The most important form is normal moveout (NMO), the increase in reflection time with offset for a flat, horizontal layer, and its relationship is hyperbolic: the observed time squared equals the zero-offset time squared plus the offset squared divided by the stacking velocity squared. Processing geophysicists exploit this by applying an NMO correction, an offset-dependent time shift that removes the extra travel time and flattens the reflection hyperbola across a common midpoint (CMP) gather so that all traces line up at their zero-offset time. Once flattened, the traces are summed, or stacked, which cancels random noise and reinforces the true reflection, dramatically raising the signal-to-noise ratio of the final seismic section. The velocity that best flattens the hyperbola is the stacking velocity, and building a field of these velocities through velocity analysis is one of the central steps in seismic processing. A second form, dip moveout (DMO), corrects for the additional time shift introduced when the reflector is tilted, and a third, residual moveout, is the small leftover misalignment used in modern amplitude-versus-offset (AVO) and prestack depth migration workflows to refine velocities and detect fluid or lithology changes. In the Western Canadian Sedimentary Basin, accurate moveout correction is what turns raw shot records into interpretable images of Montney and Duvernay shale targets, Cardium and Viking sand pinchouts, and Leduc and Nisku carbonate reef edges. A wrong stacking velocity leaves reflections under- or over-corrected, smearing the very stratigraphic and structural traps that an operator like ARC Resources or Tourmaline pays several hundred thousand CAD per township to shoot, so moveout analysis sits at the heart of drilling-location risk reduction.

Key Takeaways

Time increases with offset: Moveout is the extra reflection travel time recorded at larger source-receiver distances because the raypath lengthens. For flat layers this normal moveout follows a hyperbola governed by zero-offset time and stacking velocity, the fundamental relationship every CMP-based processing flow relies on.
NMO correction flattens gathers: Applying an offset-dependent time shift aligns all traces in a common midpoint gather to their zero-offset time. Only after this flattening can traces be stacked to cancel noise; a WCSB Montney line unusable in raw form becomes a clean image of the reservoir after correct NMO.
Stacking velocity is the control: The velocity that best flattens the hyperbola is picked during velocity analysis on semblance panels. These stacking velocities feed migration and, via Dix inversion, interval velocities used for depth conversion of WCSB targets like Leduc reefs where a small velocity error moves a drilling location tens of metres.
NMO stretch is the trade-off: The correction stretches the wavelet at far offsets and shallow times, lowering frequency content, so processors apply a stretch mute to discard badly distorted data. Managing this mute protects far-offset amplitudes needed for AVO fluid detection in Cretaceous WCSB sands.
Dip and residual moveout refine the image: Dip moveout handles tilted reflectors before stack, and residual moveout on migrated gathers drives modern velocity model updates and AVO. Both are essential where WCSB structure is complex, such as thrust-faulted Foothills gas plays west of Calgary.

Velocity Analysis and Semblance Picking in Practice

To find the stacking velocity that flattens a gather, a processor scans a range of trial velocities and measures coherence, usually with a semblance calculation displayed as a colour panel of velocity against time. Bright semblance peaks mark the velocity that best aligns each reflection, and the interpreter picks a smooth velocity function through them. On a WCSB 3D survey this is repeated at thousands of locations to build a spatially varying velocity field. Picking too fast over-corrects the hyperbola into a frown; too slow leaves a smile. Good picks are what let a Montney horizontal well be landed within a one to two metre target window, while sloppy velocities can push the interpreted reservoir depth off by five to ten metres and miss the sweet spot entirely.

NMO Stretch, Muting, and Far-Offset Amplitudes

The NMO correction does not shift a trace uniformly; it stretches the waveform most at large offsets and early times, artificially lowering its frequency. If those stretched samples are stacked they blur the section, so processors apply a stretch mute that zeros data beyond a chosen distortion threshold, commonly around 20 to 30 percent stretch. The choice is a balance: an aggressive mute throws away far-offset traces that carry the amplitude-versus-offset information used to distinguish gas-charged from wet sand in Cretaceous WCSB reservoirs, while a lax mute lets stretch noise degrade resolution. Getting this balance right is central to reliable AVO analysis over Cardium and Viking targets.

Fast Facts

The hyperbolic moveout equation that underpins essentially all modern reflection seismic processing was formalized in the 1950s and 1960s as the common-midpoint method spread through the industry, and it remains so central that a single 3D survey over a WCSB play can involve millions of individual NMO corrections. The stacking velocities picked from moveout are not the true rock velocities; they are root-mean-square approximations, and converting them to interval velocities requires the Dix equation, published by C. Hewitt Dix in 1955.

Moveout is one link in the seismic imaging chain, so it connects to several related terms. Seismic reflection is the underlying method that generates the offset-dependent arrivals moveout describes, and stacking velocity is the parameter that quantifies the hyperbola and controls the correction. Common midpoint gathering is the trace-sorting scheme that makes NMO correction and stacking possible, while seismic migration is the follow-on step that repositions reflectors to their true subsurface location. Together they turn field shot records into the images that guide WCSB drilling.

Real-World WCSB Scenario: Mispicked Velocities on a Duvernay 3D Survey

An operator shoots a 120 square kilometre 3D seismic survey over a Duvernay shale lease near Fox Creek at a cost of roughly 3 to 5 million CAD to plan a series of horizontal wells at about 3,400 metres depth. During processing, an early velocity model uses stacking velocities picked too fast in a zone of poor semblance, over-correcting the moveout and pulling the interpreted Duvernay marker up by about eight metres relative to well control.

A reprocessing pass with careful semblance picking and residual moveout analysis on migrated gathers corrects the velocity field, restores the marker to its true depth, and reveals a subtle fault the first pass had smeared out. The refined image reroutes two planned horizontal legs away from the fault damage zone, avoiding an estimated 8 to 12 million CAD in lost production and remediation from wells that would otherwise have drilled into compromised rock.