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<< Click to Display Table of Contents >> Navigation: MultiFocusing > MFParams to time arrivals |
Note: This module is deprecated. It is retained for compatibility with existing workflows.
This module converts multi-focusing (MF) parameter gathers into tomographic time arrivals for use in velocity model building. It reads the zero-offset stack gather, the multi-focusing semblance (correlation) gather, the emergence angle gather, the CRE (Common Reflection Element) gather, and optionally the gamma gather from a 3D dataset. For each reflector detected in the semblance panel, the module reconstructs source–receiver ray pairs at multiple opening angles, computes the associated travel times, and writes the results as tomographic picks to an output tomo item.
Use this module to generate first-arrival and reflection tomography constraints from an existing multi-focusing search result. The output tomo item can be fed into a tomographic velocity inversion workflow to update the near-surface or subsurface velocity model.
The zero-offset (or near-offset) stack gather produced by the multi-focusing search engine. This gather provides the amplitude and time information at each reflector position used to detect and rank reflection events.
The multi-focusing semblance (correlation) gather. This is the primary quality indicator used to identify reflection events. Samples with semblance above the Semblance Threshold are considered candidate reflectors.
The emergence angle gather from the multi-focusing search engine. Each sample contains the estimated normal-ray emergence angle (radians) at the corresponding midpoint and time position. These angles are used to reconstruct the source and receiver ray directions for each detected reflector.
The CRE (Common Reflection Element) radius gather from the multi-focusing search engine. Each sample contains the estimated CRE radius (m), which together with the emergence angle defines the subsurface reflection point position used for ray reconstruction.
Optional. The gamma parameter gather from a 3D multi-focusing search. When connected, detected reflectors with gamma values exceeding the Gamma Threshold are excluded from the output. Leave unconnected for 2D datasets.
Selects the reflector detection algorithm. Fast (default) scans the semblance gather using a sliding window and picks only the maximum semblance value within each window cell, producing a reduced but quickly computed set of picks. Long performs a more exhaustive search over all sample positions, finding all local maxima above threshold and de-duplicating results; this is more accurate but significantly slower on large datasets.
The half-width of the spatial search window (in traces) used when detecting local semblance maxima. The default is 10 traces. Increase this value for datasets with large midpoint spacing or smoothly varying reflectors to ensure each reflector is detected at least once. Reduce it for densely sampled data to improve lateral resolution of the picks.
The half-width of the time search window (in samples) used when detecting local semblance maxima. The default is 50 samples. Increase this value to merge closely spaced reflectors into a single pick. Reduce it to resolve thin layers with closely spaced reflection events.
The angular step (degrees) used to generate multiple ray pairs around each detected reflector. The default is 15 degrees. For each reflector, rays are launched at increments of this angle up to the maximum opening half-angle. A smaller value generates more ray pairs per reflector and provides finer angular coverage for tomographic inversion, at the cost of increased output volume.
The near-surface (replacement) velocity (m/s) used to compute travel times along reconstructed ray paths. The default is 1500 m/s. Set this to the best available estimate of the near-surface velocity in the survey area. This value is used directly in the travel-time formula for each generated ray pair.
The minimum semblance value (dimensionless, 0–1) a candidate reflector must exceed in order to be included in the output tomographic picks. The default is 0.05. Increase this value to restrict picks to high-confidence reflectors and reduce noise-driven false picks. Decrease it if too few picks are generated for the tomographic inversion.
The minimum absolute amplitude value a candidate reflector must exceed in the stack gather to be included in the output. The default is 0.005. This filter removes low-amplitude noise events that may pass the semblance threshold but do not correspond to real reflection arrivals.
The maximum allowed absolute gamma value for a candidate reflector when the Gamma input gather is connected (3D datasets). The default is 0.1. Reflectors with gamma exceeding this threshold are excluded, as large gamma values indicate non-hyperbolic moveout or poor MF focusing quality. This parameter is not used when the Gamma input is not connected.
The maximum source–receiver offset (m) allowed for the reconstructed ray pairs written to the tomographic output. The default is 5000 m. Ray pairs with a reconstructed offset greater than this value are discarded. Reduce this value to limit the tomographic inversion to near-offset rays, which are less sensitive to anisotropy and velocity model errors at depth.