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<< Click to Display Table of Contents >> Navigation: Diffraction > Edge Diffraction Geometry Input Data By Azimuth |
This module prepares the input trace geometry required for edge diffraction imaging. It scans the available pre-stack seismic data, identifies all traces that fall within a defined spatial aperture around each output diffraction bin, and then organizes those traces into azimuth sectors. Each azimuth sector spans a user-defined angular range, stepped from a first to a last azimuth angle. This selective, azimuth-based geometry preparation is essential for illuminating subsurface diffracting edges from specific source-receiver directions, which in turn enables coherent summation along diffraction traveltime curves and suppression of specular reflections.
The module produces two output gathers: a Unique Gather containing one copy of each contributing trace (for efficient processing), and a Duplicate Gather in which each trace is replicated once per output diffraction bin it contributes to (when per-bin output is enabled). Use this module as the geometry input stage immediately before the Edge Diffraction imaging engine.
Connect the SEG-Y file handle for the pre-stack seismic dataset to be used in diffraction imaging. This handle provides access to the actual amplitude data that will be read for each selected trace. The data must be in SEG-Y format and accessible through the project's data management system. This is the same SEG-Y dataset whose geometry is described by the sorted headers input below.
Connect the sorted trace header index for the input seismic data. This index allows the module to spatially query which traces fall within the aperture of each output diffraction bin without reading all trace amplitudes. The index must have been pre-computed and sorted by inline/crossline position. Typically this is generated by a binning or geometry setup module earlier in the processing flow.
Connect the trace vector (output bin geometry) that defines the set of diffraction image points to be computed. Each bin in this collection becomes a candidate output diffraction point. The module searches the input data for traces that lie within the aperture of each such bin. This geometry item is typically generated by the diffraction point definition or binning stage. It must not be empty — if the bin collection contains no traces, the module will report an error asking you to run the binning step first.
The starting azimuth angle (in degrees) for the first azimuth sector. Azimuth is measured clockwise from north (0° = north, 90° = east). The module will create the first azimuth bin centered at this angle. Default: 10°. Set this to the smallest source-receiver azimuth you want to include in the diffraction analysis. For near-full azimuth datasets, values near 0° are typical. Increasing this value excludes low-azimuth (near-north) source-receiver pairs from the analysis.
The ending azimuth angle (in degrees) for the last azimuth sector. The module steps from FirstAzimuth to LastAzimuth in increments of 2 × HalfStepAzimuth, creating one azimuth sector per step. Default: 170°. Together with FirstAzimuth, this defines the full azimuth range covered by the geometry preparation. Sectors beyond LastAzimuth are not computed. For typical land 3D surveys covering all azimuths, a range of 10° to 170° (combined with 180° symmetry of source-receiver geometry) provides nearly full azimuth coverage.
The half-width (in degrees) of each azimuth sector, which also controls the stepping interval between sectors. Each sector covers the angular range from (center − HalfStepAzimuth) to (center + HalfStepAzimuth), and sector centers are spaced 2 × HalfStepAzimuth apart. Default: 10° (giving 20°-wide sectors stepping every 20°). Smaller values create narrower, more numerous azimuth sectors and improve azimuth resolution at the cost of lower fold per sector. Larger values broaden each sector and increase fold but reduce azimuth discrimination.
An angular tolerance (in degrees) applied near zero-offset traces to control their inclusion in azimuth sectors. Near-zero-offset traces have poorly defined azimuths (the source-receiver direction is ambiguous when the offset is very small), so this parameter sets a cone around zero offset within which traces are accepted regardless of their nominal azimuth. Default: 2°. Increase this value if near-offset traces are being incorrectly excluded from azimuth sectors due to azimuth instability at small offsets.
The half-width of the spatial aperture (in metres) used to select contributing input traces for each output diffraction bin. The module computes how many inline and crossline bins fit within this radius (using the CMP interval parameters), then gathers all input traces whose bin position falls within that rectangular aperture. Default: 3000 m. A larger aperture increases the number of contributing traces (higher fold, better imaging of deep or laterally extensive features) but also increases computation time and memory use. Set this to cover the expected lateral extent of the diffraction operator at your target depth.
The cross-azimuth aperture (in metres) used in the trace selection step within each azimuth sector. This controls how far from the main azimuth direction traces can be laterally and still be included. It acts as an additional spatial constraint perpendicular to the azimuth sweep direction. When set to a small value, only traces very close to the azimuth direction are selected; larger values allow traces from a wider swath perpendicular to the azimuth. If not explicitly set, the behaviour defaults to a value derived from the main Aperture parameter. Adjust this when your acquisition geometry results in uneven trace density across azimuths.
When enabled, the full CMP aperture is used for trace selection rather than the narrower azimuth-constrained aperture. This means all traces within the main Aperture radius are considered for every azimuth sector, bypassing the cross-azimuth restriction. Default: disabled. Enable this option when your dataset has sparse azimuth sampling and you need to maximize fold per sector, accepting some azimuth contamination in exchange for better signal-to-noise ratio in the output gathers.
An additional lateral aperture (in metres) applied to the sides of the azimuth sector footprint. This extends trace selection to include traces at the edges of the main aperture that might otherwise be cut off by the azimuth angle boundaries. Increasing this value widens the selection footprint at the sector margins and is useful when the geometry has irregular sampling near the aperture boundary or when you want to avoid sharp edge effects in the output gathers.
The CMP bin size along the inline direction (in metres). The module uses this value to convert the Aperture distance into a number of inline bins to search. Default: 12.5 m. Set this to match the actual inline bin spacing of your seismic survey. An incorrect value will cause the spatial aperture to be either too narrow or too wide along the inline direction, leading to incorrect trace selection. This parameter should be consistent with the bin grid definition used in the rest of the diffraction processing flow.
The CMP bin size along the crossline direction (in metres). Together with Cmp Interval Along Inline, this converts the Aperture radius from metres into a bin count for the crossline axis. Default: 25 m. For surveys where the crossline bin spacing differs from the inline spacing (which is common in land 3D surveys), set this independently to reflect the actual crossline bin interval. Mismatching this value will distort the effective aperture shape in the crossline direction.
When enabled, the module reads and assembles a separate amplitude gather for each individual output diffraction bin, producing the Duplicate Gather output in which traces can appear multiple times (once per bin they contribute to). Default: disabled. When disabled, only the Unique Gather output (one copy of each trace regardless of how many bins it contributes to) is populated, which is more memory-efficient. Enable this option when the downstream diffraction imaging engine requires per-bin gathers with explicitly duplicated traces for its summation algorithm.
This group of parameters controls whether and how the number of contributing traces (fold) per azimuth sector and diffraction bin is constrained. Fold limitation is useful for balancing coverage across sectors when the acquisition geometry produces highly uneven trace density in different azimuths.
Enables fold limitation so that the Min Fold and Max Fold thresholds are applied during trace selection. Default: disabled. When disabled, all traces within the aperture and azimuth sector are included regardless of total fold count. Enable this option when certain azimuth sectors are heavily oversampled and you want to cap the maximum number of contributing traces to reduce computation time, or when sectors with too few traces (below Min Fold) should be excluded to avoid imaging noise from low-fold bins.
The maximum number of traces allowed per output diffraction bin and azimuth sector when fold limitation is active. If more traces qualify within the aperture and azimuth window, only up to this number are retained. Default: 1,000,000 (effectively no limit). Reduce this value when aperture overlap between neighbouring bins causes some bins to accumulate very large numbers of traces, which would make the imaging step impractically slow or memory-intensive.
The minimum number of traces required per output diffraction bin and azimuth sector when fold limitation is active. Bins with fewer contributing traces than this threshold are excluded from the output to avoid unreliable imaging results driven by insufficient data. Default: 5. Increase this value to enforce a higher minimum data quality standard, which will suppress output in areas of sparse coverage (such as at the edges of the survey or in low-azimuth-fold sectors). Decrease it (minimum 1) if you need to retain output even in poorly sampled areas.
A gather in which each input trace appears once for every output diffraction bin it contributes to. This output is only populated when Create Gather For Each Bin is enabled. It provides per-bin amplitude data needed by diffraction imaging engines that require explicit, bin-separated input gathers. Each trace in this gather has its CDP header set to the index of the corresponding output diffraction bin.
A gather containing exactly one copy of each contributing trace, regardless of how many output diffraction bins it overlaps. This is the memory-efficient alternative to the Duplicate Gather and is always computed. It provides a compact representation of all seismic data within the aperture of the current azimuth sector, suitable for diffraction imaging engines that manage their own bin-to-trace mapping internally.