Normal Moveout Equation
Normal moveout is the extra travel time a reflection takes at offset compared to the vertical path, and correcting for it is what lets seismic traces stack.
The equation
ΔT = √(t0² + x²/V²) – t0
- t0 is the zero offset two way time in seconds
- x is the source to receiver offset in meters
- V is the NMO velocity in meters per second
Worked example
A reflector at t0 of 1.0 seconds with velocity 2000 meters per second shows a reflection at 1.118 seconds on a 1000 meter offset trace, a moveout of 118 milliseconds. Flattening that hyperbola across all offsets is the NMO correction.
When to use it
The hyperbolic form assumes a flat layered earth and small offsets relative to depth. At long offsets or under strong anisotropy the hyperbola bends, which is why higher order terms exist.
Related Calculators and Reference
- Seismic Wave Simulator: watch wavefronts obey these relations live.
- Seismic Velocity Converter: apply the Dix equation to your own picks.
- SEG Wiki: the standard reference for seismic methods.