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<< Click to Display Table of Contents >> Navigation: Migration > Kirchhoff PreSTDeMigration - file in/out - migration |
The Kirchhoff PreSTDeMigration module performs pre-stack time demigration using a Kirchhoff summation algorithm. Demigration is the inverse of migration: it transforms migrated seismic data back into the unmigrated (shot-record or common-offset) domain. This operation is essential in velocity model building workflows where demigrating and re-migrating data with updated velocities is used to iteratively improve the subsurface velocity model.
The module reads migrated pre-stack data from file, applies a rho (derivative) filter to the data, and then uses an RMS velocity model to compute Kirchhoff demigration operators. It outputs Common Image Gathers (CIGs) in the offset domain, written directly to an output file in the g-Platform seismic data format. Processing supports both 2D and 3D geometries, and can be distributed across multiple CPUs or GPUs for large datasets.
An optional sub-procedure (Input data processing) can be inserted in the workflow to apply additional trace conditioning (for example, muting or gain) to the input data before demigration is performed.
Connect the SEG-Y file handle that provides the migrated pre-stack input seismic data. This is typically a pre-stack time-migrated dataset in the form of Common Image Gathers or Common Offset sections. The module reads seismic traces from this handle in batch form and applies the rho filter and Kirchhoff demigration operator to produce demigrated output.
Connect the trace header collection (geometry) describing the input migrated data. These headers supply the source and receiver coordinates, offset values, and bin locations for every input trace. The module uses this geometry to organize input traces into spatial bins and to select traces within the migration aperture for each output bin.
Connect the RMS velocity model used to compute travel-time move-out for the demigration operator. The module queries this velocity model at every output bin location and for every time sample to calculate the Kirchhoff diffraction curve. Providing an accurate Vrms model is critical for correct demigration — errors in the velocity model will cause incorrectly repositioned events in the output data.
Connect the trace header collection describing the output geometry — that is, the set of bins into which demigrated data will be written. These headers define which spatial positions are produced in the output file. This allows the output grid to differ from the input grid, for example when demigrating to a different line spacing or a subset of the survey area.
Specify the path and file name for the output demigrated dataset (GSD format). The output file stores Common Image Gathers organized by offset classes defined by the Minimum offset CIG, Maximum offset CIG, and Offset increment parameters. If the file already exists, the module will ask whether to overwrite it (in Direct mode) or will append new results (in Append mode).
Select the dimensionality of the input data: 2D or 3D. This setting controls the rho filter applied to the data prior to demigration — the 2D case uses a half-derivative filter (pi/4 phase rotation), while the 3D case uses a full derivative filter (pi/2 phase rotation). Choose 2D for 2D seismic lines and 3D for full 3D pre-stack volumes. Default: 2D.
The minimum source-receiver offset (in meters) for the output Common Image Gathers. Input traces with absolute offset values below this threshold are excluded from the demigration. Set this value to exclude near-offset traces that may be contaminated by guided waves or direct arrivals. Default: 0 m.
The maximum source-receiver offset (in meters) for the output Common Image Gathers. Input traces with absolute offset values above this threshold are excluded. Together with Minimum offset CIG and Offset increment, this parameter defines the offset axis of the output CIGs. Set this to cover the full useful offset range of your acquisition. Default: 3000 m.
The offset class interval (in meters) for binning input traces into output CIG offset panels. Traces with offsets in the range [N * increment, (N+1) * increment] are summed into the corresponding output panel. Smaller values produce more offset panels with finer offset resolution, useful for velocity analysis, but increase output file size. Default: 50 m.
A dimensionless scaling factor applied uniformly to all velocity values from the Vrms model before computing the demigration operator. A value of 1.0 uses the model velocities as-is. Values greater than 1.0 increase the effective velocity (stretching diffraction curves), while values less than 1.0 decrease it. This parameter provides a quick way to test the sensitivity of the demigration result to velocity perturbations without modifying the velocity model itself. Default: 1.0.
When enabled, the demigration operator uses picket-based (along-line coordinate) distances instead of Cartesian X/Y distances for computing travel times. This is appropriate for 2D crooked-line surveys or any survey where the acquisition geometry is defined in terms of picket (chainage) coordinates rather than map coordinates. Default: disabled.
The lateral search radius (in meters) within which input traces are included in the Kirchhoff summation for each output bin. Only traces whose bin location falls within this distance from the output bin centre are used. A larger aperture captures more traces and is needed to image steeply dipping structures, but increases computation time. A smaller aperture limits the spatial contribution and improves efficiency, but may under-sample the diffraction curve. This parameter is active when Aperture type is set to Constant. Default: 3000 m.
Controls whether the aperture is constant with time or varies as a function of two-way travel time. Select Constant to use a single fixed aperture value for all time samples (set via the Aperture parameter). Select Time variant to define a table of (time, aperture) pairs — useful for surveys where shallow reflectors need a smaller aperture and deeper reflectors need a larger one. When Time variant is selected, the Time-aperture factors table becomes active and the constant Aperture field is hidden. Default: Constant.
When enabled, applies a stretch mute to the demigrated traces based on the Stretching factor value. The stretch mute suppresses far-offset traces where the normal move-out correction would cause excessive wavelet stretching, which degrades the frequency content at large offsets. Enable this option when demigrating data with long offsets to avoid stretched (low-frequency) events contaminating near-offset panels. Default: disabled.
The maximum allowable wavelet stretch, expressed as a percentage of the zero-offset wavelet period. Traces where the NMO-induced stretch exceeds this threshold are muted. For example, a value of 33% means that any sample for which the stretched wavelet is more than 33% longer than the original will be zeroed out. This parameter is only active when Use stretching factor is enabled. Typical values range from 20% to 50%. Default: 0%.
The near-surface replacement (datum) velocity in m/s used in the Kirchhoff operator computation. This velocity is used to substitute for zero or negative velocity values that may occur in the model, and is incorporated into the computation of Kirchhoff diffraction radii. It should correspond to the velocity used during the original datum correction (static shifts) applied to the data. Typical values are 1500 m/s (water velocity for marine data) or the near-surface refractor velocity for land data. Default: 1500 m/s.
A coefficient that controls the anti-aliasing filter applied during Kirchhoff summation. Anti-aliasing prevents spatial aliasing artefacts that occur when the Kirchhoff operator is sampled too coarsely relative to the trace spacing. Higher values result in stronger anti-aliasing filtering. A value of zero disables the anti-aliasing filter. In most cases the default value (as set in the velocity model) provides an adequate balance between aliasing suppression and resolution. Adjust this value if steep-dip aliasing artefacts are visible in the output.
The maximum frequency (in Hz) passed through the rho (derivative) filter applied to the input data before demigration. The rho filter is required by the Kirchhoff integral and amplifies high frequencies; setting a maximum frequency prevents noise amplification above the useful signal band. Set this to match the highest signal frequency in your data. Values above the Nyquist frequency of the data have no effect. Default: 120 Hz.
The maximum emergence angle (in degrees) at which contributions are included in the Kirchhoff summation. Traces whose ray parameter exceeds the angle corresponding to this limit are excluded. Restricting the maximum angle reduces contributions from very shallow ray paths (large offsets relative to depth) that may be noisy or physically unreliable. The value 90 degrees includes all arrivals regardless of angle. Reduce this value (for example to 60 degrees) to suppress shallow-angle artefacts when processing deep targets. Default: 90 degrees.
Controls how results are written to the output file. Direct mode overwrites the output file from scratch each time the module runs. Append mode resumes a previously interrupted run, writing only the output bins not yet marked as complete in the file. Append mode is particularly valuable for large 3D surveys where the job may be interrupted — it avoids re-computing bins that are already done and allows continuation from where processing stopped. Default: Direct.
An integer quality/oversampling factor (range 1–50) that controls the accuracy of the Kirchhoff operator by increasing the density of summation points along the diffraction curve. Higher values reduce operator aliasing and improve the quality of the demigrated result, especially for steep dips, but linearly increase computation time. A value of 1 uses the standard single-fold summation. Increase this value (for example to 2 or 4) when the output shows operator noise or aliasing artefacts. Default: 1.
When enabled, applies an amplitude normalization based on the local RMS velocity values. This corrects for the amplitude scaling introduced by the velocity-dependent Kirchhoff demigration operator, helping to preserve relative amplitudes in the output gathers. Disable this option only if you intend to apply your own amplitude correction in a subsequent step. Default: enabled.
This container holds a research and development parameter (Clear file name) intended for advanced testing workflows. Under normal processing conditions, this group should be left at its default (empty) setting.
An optional path to a text file listing inline/crossline bin pairs (one per line) whose output status in the output file should be reset to 'uncalculated' before processing begins. This allows targeted reprocessing of specific bins when using Append mode without re-running the entire survey. Under typical production workflows, this field should be left empty.