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<< Click to Display Table of Contents >> Navigation: Velocity > Create Dip Corrected Vrms 3D |
This module computes dip-corrected root-mean-square (Vrms) velocities for 3D seismic surveys. Standard velocity analysis assumes flat, horizontal reflectors; when reflectors are dipping, the measured Vrms is distorted by the dip component. This module corrects that distortion by combining the input Vrms volume with independently derived dip information (azimuth and angle of dip), yielding a Vrms volume that more accurately represents the true subsurface interval velocities.
The correction uses semblance weights to suppress unreliable velocity picks: bins with semblance below a user-defined threshold are treated as null values in the output. The spatial aperture controls how far the algorithm searches in the inline and crossline directions when interpolating and smoothing the corrected velocity field. The resulting dip-corrected Vrms volume is written to a SEG-Y file and can be used as input for subsequent depth conversion, migration velocity model building, or time-to-depth conversion workflows.
Use this module when your survey area has significant structural dip and you need geologically consistent Vrms velocities prior to interval velocity derivation or depth migration.
The virtual geometry trace header set describing the output grid. This defines the inline and crossline layout into which the corrected Vrms values will be written. It must match the spatial coverage of the output SEG-Y file.
The 3D bin grid definition for the survey. The module reads the inline and crossline bin spacing (in metres) and the total number of inline and crossline bins from this item. These spatial dimensions are used internally to compute dip-correction geometry and to correctly orient the aperture search relative to the actual bin dimensions on the ground.
The trace header set for the input seismic data geometry. This provides the inline and crossline addressing used to index into the input Vrms, semblance, fold, and dip volumes during processing.
The input root-mean-square velocity volume (SEG-Y format), typically produced by 3D semblance-based velocity analysis. Each trace contains a time-varying Vrms function for one inline-crossline bin. This is the velocity field that will be corrected for structural dip effects.
The semblance (coherence) volume associated with the velocity analysis, in SEG-Y format. Semblance values range from 0 to 1 and indicate the reliability of each velocity pick. Bins where semblance falls below the Minimal semblance threshold are excluded from the dip-correction calculation and output as null values. Higher semblance values carry proportionally more weight in the correction.
An optional fold volume (SEG-Y format) providing the CMP fold count for each bin. When provided, fold information can be used as an additional quality weighting factor during velocity correction. This input is optional; if no fold volume is connected, the correction proceeds without fold-based weighting.
A SEG-Y volume containing the azimuth direction of reflector dip at each bin, expressed in degrees measured clockwise from north (or from the inline direction, depending on the dip computation convention used upstream). Together with the angle of dip volume, this defines the full 3D orientation of each reflector. Both azimuth and angle of dip volumes are required inputs.
A SEG-Y volume containing the magnitude of reflector dip at each bin, expressed in degrees from horizontal. Small angles indicate gently dipping or flat reflectors; larger angles indicate steeply dipping structure. The dip angle determines the magnitude of the velocity correction applied at each sample. This is a required input alongside the azimuth of dip volume.
The full file path and name for the output dip-corrected Vrms SEG-Y file (.sgy). The output volume has the same trace count, sample count, and sample interval as the input Vrms volume. Bins excluded by the semblance threshold are written as null traces. Ensure the destination directory exists and has sufficient disk space for a full 3D velocity volume.
The near-surface reference velocity used to initialise the dip correction for the shallow part of the section, in metres per second. This value is applied where the time-Vrms function is not yet well constrained by semblance picks (typically near time zero). The default value is 1500 m/s, which is appropriate for marine surveys or water-saturated near-surface sediments. For land surveys with slower near-surface velocities, reduce this value to match the expected weathering layer or replacement velocity. The minimum allowed value is 1000 m/s.
The minimum semblance value required for a velocity pick to be included in the dip-correction calculation. Picks with semblance below this threshold are treated as unreliable and are written as null values in the output file. The default value is 0.01. Increase this threshold (for example, to 0.05–0.10) in areas of poor data quality or high noise to avoid propagating unreliable velocities into the corrected volume. Decrease it only if your data is high quality and you need maximum spatial coverage of the output.
The lateral search radius used during the dip-correction computation, in metres. The algorithm searches within this aperture around each output bin to gather neighbouring velocity and dip information for the correction. The default value is 3000 m. A larger aperture produces a spatially smoother corrected velocity field by incorporating information from a wider neighbourhood, which is beneficial in areas of sparse velocity picks. A smaller aperture preserves more lateral velocity detail but may be more sensitive to pick noise. Set this value to roughly the expected lateral distance over which the velocity field varies smoothly — typically one to several bin spacings up to a few kilometres, depending on survey size and structural complexity. The minimum allowed value is 0 m.