Description

This is an interactive residual moveout (RMO) correction module. It removes normal moveout using the input Vrms velocity model, computes a velocity/semblance panel around that model so you can pick small residual velocity corrections where reflections are still not perfectly flat, then adds those picks back onto the background velocity and re-applies moveout with the corrected model. The net effect is a fine correction of gathers that are almost, but not quite, flat after the initial velocity analysis — typically used as a refinement step before stacking or further offset-domain processing.

Input data

Input gather

The CDP/offset gather that the residual moveout correction is applied to.

Input super gather

An optional wider-aperture gather (for example neighbouring CDPs combined) used only to build a more stable velocity/semblance panel for picking. If not supplied, the main input gather is used instead. Supplying a super gather is recommended where fold is low, since it gives a cleaner picture on which to pick residual moveout.

Vrms model

The background RMS velocity model (from a prior velocity analysis) used as the starting point for the residual moveout correction. Both the initial moveout removal and the semblance panel are built around this model.

Velocity picking

Stores the interactive residual-velocity picks you make on the panel (one curve of velocity-versus-time deviations per location). If no picking exists yet for a given gather, an empty picking curve is created automatically.

Parameters

Velocity window

Half-width of the velocity range displayed around the background Vrms model in the residual-moveout semblance panel. This defines how far above and below the input velocity model you can pick a residual correction.

Default: 500 m/s. Minimum 1. Widen it if reflections show larger residual moveout than fits in the default window; narrow it for a higher-resolution panel around a well-trusted background model.

Step velocity

Velocity increment (sampling step) of the semblance panel within the velocity window. A smaller step gives finer velocity resolution on the panel at the cost of more computation.

Default: 50 m/s. Minimum 1.

Semblance smooth window

Time window over which the semblance panel is smoothed. A larger window produces a cleaner, less noisy panel but blurs the vertical (time) resolution of picked events.

Default: 0.05 ms. Minimum 0.

Min velocity

Absolute lower bound of the velocity axis on the panel, regardless of what the velocity window would otherwise allow. Prevents the panel from extending into geologically implausible, very low velocities.

Default: 1000 m/s. Minimum 1.

Max velocity

Absolute upper bound of the velocity axis on the panel, regardless of what the velocity window would otherwise allow. Prevents the panel from extending into unrealistically high velocities.

Default: 6000 m/s. Minimum 1.

Normalization window

Time window used to normalize trace amplitudes when computing the semblance panel, reducing the influence of strong-amplitude zones so weaker but still coherent reflections remain visible on the panel.

Default: 0.1 ms. Minimum 0.

Weighted semblance

Selects an alternative semblance-weighting scheme to sharpen the velocity/semblance panel. "None" uses conventional semblance. "Weighted" and "Similarity-weighted" apply additional emphasis (controlled by "Weight power" below) that can make true coherent trends stand out more clearly from background noise, at the cost of being more sensitive to the chosen weighting power.

Default: None. Try "Weighted" or "Similarity-weighted" if the standard panel is too noisy to pick confidently.

Weight power

Exponent controlling the strength of the weighted-semblance scheme selected above. Larger magnitudes sharpen the panel more aggressively toward the strongest coherent trends; a value of 0 has no effect.

Default: 0. Range -5 to 5. Only meaningful when "Weighted semblance" is not set to "None".

Surface velocity V0

Near-surface (replacement/datum) velocity used as the reference for the moveout equations, both when flattening the gather to build the panel and when re-applying moveout with the corrected velocity. Set it to match the replacement velocity used elsewhere in your velocity model for consistency.

Default: 1800 m/s. Minimum 0.

Stretching parameters

Stretch factor

Maximum allowable NMO stretch, expressed as a percentage, before the affected samples are muted during the moveout removal and re-application steps. Lower values mute more aggressively near the mute zone (shallow, far-offset samples), protecting against wavelet stretch distortion at the cost of losing more near-surface, far-offset data; higher values retain more data but allow more stretch distortion to remain.

Default: 1 (100%). Minimum 0.

Smoothing parameter Y velocity

Vertical (time) smoothing, in samples, applied to the velocity function used by the moveout operators. Smoothing avoids abrupt velocity changes between samples that would otherwise produce artifacts in the moveout-corrected gather.

Default: 10 samples.

Output data

Output gather

The input gather after residual moveout correction: moveout removed with the original Vrms model, then re-applied using the model corrected by your interactive residual-velocity picks.

Gather Vrms out

The corrected velocity function at this location: the original background Vrms plus the residual delta-velocity derived from your picks. This is the velocity model that was actually used to reconstruct the output gather.

Gather delta Vrms

Just the residual velocity correction curve derived from your interactive picks, as a function of time — useful for QC of the magnitude and consistency of the residual moveout correction being applied.

Nearest Vrms input

The background Vrms velocity trace extracted from the input Vrms model at this gather's location, provided for reference so it can be compared against the corrected velocity output.

References

Al-Yahya, K. M., 1989, Velocity analysis by iterative profile migration: Geophysics, 54, 718-729.