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<< Click to Display Table of Contents >> Navigation: Velocity > Interval Velocity from RMS (3D data Dix 1D inversion) |
This module converts a 3D RMS (root-mean-square) velocity volume into interval velocity, using either the classic Dix formula or an iterative inversion approach. The input is a gather containing one RMS velocity trace per bin, covering the full 3D survey area. The module outputs two velocity volumes: interval velocity in the time domain, and interval velocity converted to the depth domain.
Before applying the conversion, the module constructs a regular 3D grid from the input traces using kriging interpolation. Optional median and smoothing filters help suppress noise and instabilities in the RMS velocity field before inversion. The resulting interval velocity volume is clipped against user-defined minimum and maximum bounds defined both globally and via a time-varying velocity function table. Use this module to prepare interval velocity models for depth conversion, depth migration, or geological interpretation of velocity anomalies.
The standard sequence input data item. Connect this to the output of the preceding module in your processing flow. The module reads a gather of RMS velocity traces — one trace per 3D bin (inline/crossline location) — which forms the input velocity volume for the inversion.
The 3D RMS velocity gather to be converted. Each trace in the gather represents the RMS velocity function at one bin location. The module uses the bin geometry (inline, crossline, and coordinate fields) to construct the 3D velocity grid. Ensure that the input gather has complete and correct geometry before connecting it here.
The reference elevation datum (in meters) to which the depth velocity volume is shifted. The section above the datum elevation is filled with the Replacement Velocity value (unless Fill replacement velocity from topography is enabled). If this field is left at its default value of -99999.9, the module automatically uses the maximum receiver elevation found in the trace headers as the datum. Set this explicitly when your project requires a fixed floating or fixed datum for consistency with other processing steps.
The velocity value (in m/s) used to fill the near-surface zone between the topographic surface and the datum level in the depth domain output. Default: 1500 m/s. This parameter is only active when Fill replacement velocity from topography is disabled. Set this to the near-surface velocity appropriate for your area — typically the velocity of the weathering layer or water (1500 m/s for marine surveys).
The vertical sample interval (in meters) used for the output depth-domain interval velocity volume. Default: 5 m. Together with Max depth, this determines the number of depth samples in the output. Use a finer interval for high-resolution depth models, but be aware of the increased memory and computation requirements.
The maximum depth (in meters) of the output depth-domain interval velocity volume. Default: 1 m (must be set by the user to a meaningful value). Set this to match the deepest target of interest in your project. The number of depth samples equals Max depth divided by dz.
The global lower bound for the output interval velocity (in m/s). Any computed interval velocity below this value is clipped to this minimum. Default: 1500 m/s. This guard prevents physically impossible or numerically unstable low velocities from appearing in the output. It also acts as an absolute floor for the time-varying bounds defined in the Interval Velocity-Time table.
The global upper bound for the output interval velocity (in m/s). Any computed interval velocity exceeding this value is clipped to this maximum. Default: 7500 m/s. Adjust this if your geology includes high-velocity carbonates or basement rocks that exceed the default ceiling.
A time-varying table that defines the acceptable range of interval velocities at different two-way travel times. Each row contains a time (in seconds), a minimum velocity, and a maximum velocity. The bounds are linearly interpolated between rows and applied sample-by-sample to clip the inversion result. At least one row must be provided — the module will report an error if the table is empty. Times must be in strictly increasing order, and the minimum velocity must be less than the maximum velocity in each row. Use this table to enforce geologically reasonable velocity bounds that vary with depth, for example allowing higher velocities at deeper times.
The minimum acceptable RMS velocity (in m/s) in the input data. Default: 1500 m/s. Input RMS velocities below this threshold are treated as invalid and excluded from the inversion. Use this to discard erroneous picks or header values that fall below physically realistic levels.
The maximum acceptable RMS velocity (in m/s) in the input data. Default: 7500 m/s. Input RMS velocities exceeding this threshold are treated as invalid and excluded from the inversion. Together with Min RMS Velocity, this pair guards against outlier picks corrupting the inversion.
Enables a median filter applied to the RMS velocity volume before inversion. Default: On. The median filter suppresses spike-like outliers in the velocity field without blurring lateral contrasts as severely as a mean filter. Use the Trace window and Time window parameters to control the filter aperture. Disable this if your input velocity field is already smooth and clean.
Enables a smoothing filter applied to the velocity volume. Default: On. When using the Dix method, smoothing is applied both before and after the inversion to reduce high-frequency noise in the interval velocity result. When using the Inversion method, smoothing is applied only to the input RMS field before inversion. The filter aperture is controlled by the Trace window and Time window parameters. Disable this when you need to preserve sharp lateral velocity contrasts.
The vertical half-aperture of the smoothing filter, expressed in seconds. Default: 0.1 s. This controls how many time samples are averaged in the vertical direction during smoothing. Increase this value to produce smoother interval velocities; decrease it to preserve finer vertical velocity detail. This parameter is active when either Use Media Filter or Use Smooth Filter is enabled.
The lateral half-aperture of the smoothing filter, expressed as a number of traces (grid nodes) in both the inline and crossline directions. Default: 100. Larger values produce greater lateral smoothing, which is useful for suppressing acquisition footprint or fill gaps between sparse velocity picks. Reduce this value if you need to preserve strong lateral velocity gradients associated with geological structure.
Controls the inline decimation step used during the Inversion method. Default: 10. The inversion is performed on a coarser sub-grid (every N-th inline), which reduces computation time. The result is then interpolated back to the full grid. This parameter has no effect when the Dix method is selected. Reduce this value to improve lateral resolution of the inversion output at the cost of longer processing time.
Controls the crossline decimation step used during the Inversion method. Default: 10. Works together with Inline skip to define the coarsened inversion grid. Has no effect when the Dix method is selected.
Selects the algorithm used to convert RMS velocity to interval velocity. Two options are available:
Dix — applies the Dix formula directly and independently at each trace location. This is the fastest option and is suitable when the RMS velocity field is smooth. The Calculation step parameter is disabled in this mode.
Inversion — performs an iterative least-squares inversion on a decimated grid (controlled by Inline skip / Crossline skip), then interpolates the result back to the full grid. This approach is more robust against noise and RMS velocity inconsistencies. Use Inversion when the Dix method produces erratic or physically implausible interval velocities. Default: Dix.
The vertical step size (in ms) used by the iterative inversion algorithm. This parameter is only active when the Creation method is set to Inversion — it is grayed out when Dix is selected. The step is automatically rounded to the nearest multiple of the input data sample interval, and is enforced to be at least three times the sample interval. A larger step speeds up computation but may reduce the resolution of the inverted velocity function. Leave this at a value close to your RMS velocity pick interval for best results.
When enabled, constrains the interval velocity from decreasing too rapidly with time. Default: Off. Rapid velocity decreases (velocity inversions) can be artifacts of noisy or inconsistent RMS velocity picks, and the Dix formula amplifies such anomalies. Enabling this option applies a monotonicity constraint that limits how much the interval velocity can drop between consecutive samples. The degree of the constraint is controlled by the Fraction of decrease parameter, which becomes active when this option is turned on.
Controls how strongly velocity decreases are suppressed when Limit decrease of VINT is enabled. Valid range: 0.01 to 1.0. Default: 0.99. A value close to 1.0 allows the interval velocity to decrease almost freely (the constraint is very weak). A value close to 0.01 prevents nearly any decrease in interval velocity (strong monotonicity enforcement). This parameter is grayed out unless Limit decrease of VINT is enabled.