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<< Click to Display Table of Contents >> Navigation: Velocity > Create Dip Corrected Vrms (2D) |
This module applies dip correction to a root-mean-square velocity (Vrms) field in 2D. In the presence of structural dip, conventional velocity analysis produces apparent velocities that deviate from the true layer velocities. By incorporating a dip angle field derived from the seismic data, this module adjusts the Vrms values at each time-trace location to compensate for the dip effect, resulting in a geologically more accurate velocity model.
The correction is constrained by the semblance quality: only velocity samples with semblance values above a user-defined minimum threshold are considered reliable and used in the calculation. Locations where reliable data is insufficient are treated as holes and filled using one of three available methods. Use this module after standard velocity analysis when dipping reflectors are present, and before depth conversion or migration to improve imaging accuracy.
The input RMS velocity field (Vrms), typically produced by semblance-based velocity analysis or velocity interpolation. This is a 2D gather-style data item with traces along the horizontal axis and two-way time along the vertical axis. All three input data items must have the same number of traces and the same number of time samples.
A 2D attribute field containing the local structural dip angle (in degrees) at each time-trace location, co-registered with the Input Vrms field. This angle field drives the dip correction calculation: larger dip angles produce larger velocity corrections. The angle field is typically derived from dip estimation or structural attribute analysis applied to the seismic data.
A 2D semblance (coherence) field co-registered with the Input Vrms field. Semblance values indicate how reliable the velocity pick is at each time-trace location. Only samples where semblance exceeds the Minimal Semblance threshold are included in the correction; all other locations are treated as holes and filled according to the selected Hole Filling Method.
Selects the strategy used to fill regions of the output velocity field where reliable corrected velocity values could not be computed (holes). Three options are available:
Fill With Previous Values (default) — propagates the last valid corrected velocity value downward in time to fill any gaps. This is the simplest and fastest option, appropriate when the velocity field is relatively smooth and holes are isolated.
Mapping — fills holes using a Laplace-equation-based smoothing (diffusion) approach, which blends surrounding valid velocity values into the gap. This produces smoother transitions across holes and is better suited for large or irregularly shaped gaps where simple propagation would leave visible artefacts.
Neighbourhood — fills each hole location by averaging valid corrected velocity values found within a user-defined spatial and temporal search window (see Lateral Neighborhood and Vertical Neighborhood parameters below). This method activates its own dedicated processing path and is well suited when you want direct control over the search radius used during infilling.
Controls the spatial interpolation scheme used when resampling or reconstructing the velocity field on the output grid. Two options are available:
CCBS (default) — Cubic Constrained B-Spline interpolation. Produces smooth, continuous velocity surfaces with minimal ringing artefacts. This is the recommended choice for most datasets.
GridInterpolation — a grid-based linear or bilinear interpolation scheme. Computationally faster than CCBS but may produce less smooth results. Use this option when processing speed is a priority or as a quick-look alternative.
The near-surface reference velocity (in m/s) used as a starting constraint in the dip correction calculation. This value anchors the velocity model at the surface (zero time) and is used in the iterative computation of dip-corrected velocities. Set this to the best estimate of the near-surface P-wave velocity at your survey location, such as the velocity obtained from refraction analysis or a first-break tomography result. The default value is 1500 m/s, with a minimum of 1000 m/s.
The half-width of the local calculation box in the lateral (CMP) direction, measured in CMP points. The dip correction at each location is computed using velocity and angle values from within this window around the current trace. A larger box incorporates more lateral context and can produce a smoother correction, but may smear local lateral velocity variations. The default value is 40 CMP points, with a minimum of 1.
The half-height of the local calculation box in the vertical (time) direction, measured in samples. Together with Calculation box X, this defines the 2D analysis window used for the dip correction at each output point. Increase this value when the velocity field varies slowly in time and more temporal averaging is beneficial. The default value is 40 samples, with a minimum of 1.
The minimum semblance value (dimensionless, range 0 to 0.5) that a velocity sample must have to be considered a reliable pick and included in the dip correction. Samples with semblance below this threshold are excluded and treated as holes to be filled. Raise this threshold to use only the highest-quality velocity picks in the correction, which is advisable in noisy datasets. Lower it to retain more samples when the semblance panel has generally weak energy. The default value is 0.01.
The lateral search radius (in metres) used when the Hole Filling Method is set to Neighbourhood. For each hole location, the module searches within this radius in the CMP direction for valid corrected velocity values to use in the infill. A larger radius finds more candidates but may blend velocities from structurally different areas. This parameter has no effect when Mapping or Fill With Previous Values is selected. The default value is 200 m, with a minimum of 20 m.
The vertical search radius (in seconds of two-way time) used when the Hole Filling Method is set to Neighbourhood. For each hole location, the module searches within this time window in the vertical direction for valid corrected velocity values to use in the infill. Increase this value when holes span a large time interval. This parameter has no effect when Mapping or Fill With Previous Values is selected. The default value is 0.05 s (50 ms), with a minimum of 0.008 s (8 ms).
The primary output: the dip-corrected RMS velocity field in the same time-trace grid as the input. Each velocity value has been adjusted for the local structural dip angle and constrained by semblance quality. Holes where no reliable correction could be computed have been filled using the selected Hole Filling Method. This output is ready for use in NMO correction, depth conversion, or pre-stack time migration velocity model building.
A diagnostic output field that flags the locations where the module could not compute a reliable dip-corrected velocity from the input data — either because semblance was below the Minimal Semblance threshold or because data was missing. Inspecting this field helps you assess the coverage quality of the correction and decide whether to adjust the Minimal Semblance threshold, the Hole Filling Method, or the calculation box sizes.