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This module prepares the input parameter set required by the Stereo Tomography solver. It reads a depth velocity model, a delta (anisotropy or perturbation) model, and a constraint gather, then performs kinematic ray tracing through the velocity model to construct tomographic pick structures at user-specified offsets. The resulting output items are passed directly to the Stereo Tomography or Stereo Tomography 3D modules to update the velocity model.
The module uses a Runge-Kutta ray tracer to shoot rays through the depth velocity model at each specified source-receiver offset and emergence angle. For every ray that successfully returns to the surface, it records the two-way traveltime, the dip angle, and the reflection point location. These parameters form the stereo tomographic data (tomo picks) that the inversion solver uses to update interval velocities and reflector geometry simultaneously.
Run this module before each iteration of Stereo Tomography. The quality of the tomo picks — and therefore of the final velocity update — depends on choosing appropriate depth ranges, offset sets, and ray-tracing grid resolutions that match your model complexity and data fold.
The interval velocity model in the depth domain. This is typically the output of a depth velocity conversion or a previous tomographic update. Traces are organized along a line or by inline/crossline, with depth as the sample axis. Units are m/s per depth sample. The model is used both as the medium for ray tracing and as the background for computing traveltimes.
A depth-domain gather containing the Thomsen delta parameter, which describes the short-spread anisotropy of the medium. This gather has the same spatial and depth sampling as the depth velocity model. Providing accurate delta values improves the accuracy of ray paths and traveltimes in anisotropic (VTI) media. If the medium is isotropic, this input may be set to a zero-filled gather.
A depth-domain gather (template or pattern gather) that defines where tomographic picks are permitted to be placed. Non-zero values at a given depth indicate that picks are allowed at that location. This item is used when the option According to pattern/fixed gathers is enabled; it restricts pick placement to geologically meaningful zones and prevents spurious picks in noise-dominated regions.
A horizon picking item containing one or more interpreted horizons in depth. These horizons define the layer boundaries used when the pick-search zone is constrained by the Layer from and Layer to options. When provided, only reflection points that fall within the specified layer interval are accepted as tomo picks. This input is optional when pick search is depth-based only.
When enabled (default: on), activates the Version 1 line selection scheme. In this mode you manually specify the set of inlines and crosslines to process using the parameters in the Version 1 parameters group below. Disabling this option switches to a different internal processing path that derives lines automatically from the input data geometry.
Selects which line directions are processed. Options: IL's (inlines only), XL's (crosslines only), or IL's and XL's (both directions). Default: IL's. For 2D surveys use IL's. For 3D surveys, processing both directions yields a more isotropic constraint on the velocity model and is recommended when the acquisition geometry supports it.
The first and last inline numbers to include in the computation. Defaults: start = 10, stop = 999999 (effectively all inlines). Set these to limit processing to a sub-area of the survey — for example, a pilot zone for quality control before a full-volume run.
The first and last crossline numbers to include. Defaults: start = 10, stop = 999999. Use in combination with the inline range to define a rectangular processing window within the 3D survey.
The decimation step applied to inline and crossline indices. Default: 25 for both. A step of 25 means that only every 25th line is processed. Increase the step to speed up computation on large 3D surveys; reduce it to achieve denser spatial coverage of tomo picks. The step should be chosen so that the sampled lines still adequately represent the lateral velocity variation in the model.
Controls how inline and crossline constraints are combined when both are active. Logical AND (default): a bin is processed only if it falls within both the inline and crossline range. Logical OR: a bin is processed if it falls within either range. AND produces a rectangular mask; OR produces a cross-shaped or union mask.
Specifies whether the inline/crossline start, stop, and step values are interpreted as sequential trace indices (Seq, default) or as real-world bin numbers (Real). Use Seq when your inline/crossline numbering is contiguous and starts from 1; use Real when your survey has non-sequential or irregular bin numbering.
When enabled (default: on), the module operates in 3D mode, reading the velocity model and producing tomo picks in full 3D space. Disable this option for 2D line processing, where the velocity model is a single 2D section and picks are confined to a single vertical plane.
The sample interval of the output tomo item in the time domain, in seconds. Default: 0.004 s (4 ms). This determines the temporal resolution of the output tomo pick structures. Smaller values increase resolution but also increase memory consumption and computation time. Match this value to the sample rate of your seismic data.
The total time length of the output tomo item, in seconds. Default: 5 s. Set this to cover the full two-way traveltime range of your reflection targets. The module will not create tomo picks at traveltimes beyond this limit, so ensure the value is at least as large as the maximum two-way traveltime in your data.
The search grid step in the inline (X) direction used when locating reflection points, in metres. Default: 500 m. This defines the lateral resolution of the location search grid. Smaller values find reflection points more precisely but increase computation time. For typical marine surveys a value of 250–500 m is sufficient; for land surveys with finer structures, reduce to 100–200 m.
The search grid step in the crossline (Y) direction used when locating reflection points, in metres. Default: 500 m. Applies the same logic as Find step X grid but in the perpendicular lateral direction. For azimuthally anisotropic surveys, you may wish to set X and Y steps independently to match the acquisition geometry.
The search grid step in the depth (Z) direction used when locating reflection points, in metres. Default: 250 m. This controls the vertical resolution of the reflection point search. Reduce this value when working in areas with closely spaced reflectors or steep velocity gradients.
The lateral padding (buffer zone) added around the velocity model during ray tracing, in metres. Default: 3500 m. This extra margin prevents ray paths near the edges of the model from failing due to boundary effects. Increase this value if you observe anomalous picks near survey edges; reduce it to save memory on very large models.
The reference near-surface velocity used during ray initialization, in m/s. Default: 1500 m/s. This value sets the starting velocity for rays emerging from the surface before they enter the depth velocity model. For marine surveys 1500 m/s (water velocity) is the appropriate choice. For land surveys, set this to the average near-surface velocity in your area (typically 1500–2500 m/s depending on lithology).
The list of source-receiver offsets (in metres) at which tomo picks will be generated. Default offsets: 100, 300, 600, 1000, 1500, 2300, 3000, 4000, 5000 m. The module shoots pairs of rays (one from the source, one from the receiver) for each offset. Including a wider range of offsets improves the angular coverage of the velocity update, which helps resolve velocity-depth ambiguity. Remove very large offsets where the data quality is poor or where the assumption of a 2D ray geometry breaks down.
An optional label applied to the output tomo item to identify the processing line or run. Leave blank to use the default naming. This field is useful when processing multiple lines in a project to keep output items distinguishable in the project tree.
Determines how candidate reflection points are generated for ray tracing. Options:
Grid and horizons (default): candidate points are placed on a regular depth grid AND on the interpreted horizons simultaneously. This is the most comprehensive option and is recommended for the first iteration.
Grid: candidate points are placed on the regular depth grid only, without any horizon guidance. Use this option when no reliable horizons are available.
Horizons: candidate points are placed only on the interpreted horizons. Use this when you have well-constrained horizons and want to focus the tomographic update on specific reflectors.
The angular half-aperture (in degrees) used during the ray search. Default: 2.5°, range: 1°–45°. The ray tracer searches for the emergence angle that produces a ray landing at the target reflection point within this angular tolerance. Larger values make the search more tolerant of model errors and steep dips but may allow spurious picks. Smaller values enforce a tighter geometric constraint. For flat-layered models 2°–5° is appropriate; for steeply dipping structures increase to 5°–15°.
The lateral step of the grid used internally by the Runge-Kutta ray tracer in the inline (X) direction, in metres. Default: 25 m. This controls the accuracy of the ray path calculation in the horizontal plane. Smaller values produce more accurate ray paths through complex velocity structures but increase computation time proportionally. Values of 10–50 m are typical for depth imaging applications.
The vertical step of the grid used internally by the ray tracer in the depth (Z) direction, in metres. Default: 15 m. This controls the vertical accuracy of the Runge-Kutta integration. A finer step captures thin-layer velocity variations more accurately. Balance this against computation time: halving the step approximately doubles the run time per ray.
The depth interval within which reflection points are searched, in metres. Default: 50 m (from) to 10000 m (to). Only reflection points whose depth falls within this range will be accepted as tomo picks. Use this to exclude the very shallow section (where the velocity model may be unreliable) and to limit processing to the depth of interest. Restricting the depth range also reduces computation time on deep models.
When Fixed constraints horizons are provided, these dropdown lists select the upper and lower horizon boundaries of the search zone. Only reflection points falling between the selected horizons will be accepted. Layer names are populated automatically from the horizon picking item. Use this to confine the tomographic update to a specific stratigraphic interval — for example, between the water bottom and a target horizon.
When enabled (default: on), the module uses the Depth constraint gather to filter pick placement. Only depth samples where the constraint gather has non-zero amplitude are eligible for picks. This is the recommended setting when a good-quality constraint gather is available, as it focuses the tomo picks on geologically significant reflectors. Disable this option to allow picks anywhere within the specified depth and layer bounds.
When enabled (default: on), the module suppresses tomo pick generation inside regions defined as salt bodies (high-velocity anomalies). Ray paths passing through or reflecting off salt flanks are geometrically complex and can introduce erroneous constraints into the tomographic inversion. Enable this option when salt bodies are present in the model and you want to isolate the velocity update to the sedimentary section above, below, or beside the salt.
Controls how gaps (missing picks) in the output tomo item are handled. Don't fill (default): gaps are left empty and are ignored by the solver. Fill with zero dt: gaps are filled with picks having a zero traveltime perturbation, effectively asserting that the current model is correct in those zones. Use filling with caution — it can suppress legitimate velocity updates in poorly illuminated areas.
The time integration step used in the Runge-Kutta ray tracer, in seconds. Default: 0.002 s (2 ms), range: 0.00001–1 s. This controls the precision of the ray path integration in the time parameter. Smaller values give more accurate ray paths at the cost of longer computation. In most cases the default of 2 ms is a good balance; reduce to 0.5–1 ms only for very complex velocity models with strong lateral gradients.
A dimensionless correction factor applied to the computed ray emergence angles before they are stored in the tomo pick. Default: 1.0 (no correction), range: -4 to 4. A value of 1.0 means the angle is used as computed. Values other than 1.0 scale the angle to account for systematic bias in the ray tracer or data geometry. This parameter is an advanced tuning control; in most workflows it should remain at 1.0.
When enabled (default: on), a single uniform depth step is used for the reflection point search grid, ignoring the Depth-Step table. The step size in this mode is taken from Find step Z grid. Disable this option to activate the depth-variable step table below, which allows coarser sampling at shallow depths (where ray coverage is typically dense) and finer sampling at depth (where rays spread out).
Active only when Simple grid is disabled. This table defines a depth-variable vertical sampling interval for the reflection point search grid. Each row specifies a From depth (in metres) and the Depth step (in metres) that applies from that depth downward until the next row. Default rows: 50 m step from 0 m, 75 m step from 500 m, 200 m step from 1000 m, 500 m step from 3000 m. This scheme concentrates computation in the shallower section and reduces it in the deeper, more sparsely illuminated section. Add or modify rows to match the structure of your velocity model.
The primary output: a multi-line tomo item containing the full set of stereo tomographic picks computed across all processed inlines and crosslines. Each pick stores the two-way traveltime, the source and receiver ray angles, and the depth of the estimated reflection point. This item is the direct input to the Stereo tomography and Stereo tomography 3D solver modules.
A secondary tomo item containing the complete pick dataset in a single-line format. This output can be used for quality control, visualization of pick density, or as an alternative input format to the solver when multi-line organization is not required. It contains the same physical information as the multiline item but organized differently for compatibility with other workflow branches.