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<< Click to Display Table of Contents >> Navigation: Velocity > VTI analysis |
Note: This module is deprecated. It is retained for compatibility with existing workflows.
The VTI Analysis module provides an interactive tool for picking the anisotropy parameter eta (η) from pre-stack seismic gathers. For each CMP location, the module first removes the isotropic NMO correction using the loaded Vrms model, then computes a semblance spectrum over a user-defined range of eta values. The resulting eta-semblance panel is displayed as a velocity spectrum, allowing the interpreter to pick the eta value that maximises semblance at each two-way time. The picked eta curve is applied to re-correct the gather using the Alkhalifah–Tsvankin VTI NMO equation, producing a flat gather output that accounts for both velocity and anisotropy effects.
This module is used after conventional velocity analysis to determine the effective anisotropy of layered sedimentary sequences, particularly where long-offset gathers show residual moveout that cannot be explained by a single isotropic velocity. The semblance spectra and picked eta values can be saved to and loaded from disk to allow multi-session interpretation workflows.
The file name or identifier of the velocity picking session associated with this VTI analysis. This read-only field links the module to the project velocity picking object so that picked eta values are stored and retrieved correctly across sessions.
The input CMP gather on which the VTI analysis is performed. This gather should have NMO already applied using the isotropic Vrms model. The module first reverses the NMO correction, then reapplies the VTI NMO with the picked eta value. Ensure that the gather has been sorted by offset before connecting it here.
The root-mean-square velocity model used to perform and reverse the isotropic NMO correction. This model must cover the full time range of the input gather. It is typically the result of a standard velocity analysis step applied to the same dataset. The accuracy of the Vrms model directly affects the quality of the eta semblance spectrum — poorly picked Vrms values will smear the semblance peaks and bias the eta estimates.
The project picking item that stores the eta picks made in this module. Connecting this item links the interactive display to the project database so that picks are saved persistently. This item is also used by downstream modules that apply the VTI correction using the stored eta curve.
The minimum eta value in the semblance scan range. The eta parameter quantifies the effective anisotropy of the medium; isotropic media have eta = 0. Negative eta values are physically possible but uncommon in sedimentary basins. A value of −0.2 is a reasonable lower bound for most exploration targets. Narrow the range if you already have a prior estimate of the expected anisotropy to improve semblance resolution.
Default: −0.2.
The maximum eta value in the semblance scan range. Clastic sedimentary sequences typically exhibit eta values between 0 and 0.3. A value of 0.2 is appropriate for most surveys; increase it to 0.4 or higher if you expect strongly anisotropic shales or carbonates in the section.
Default: 0.2.
The eta increment between successive semblance panels. Smaller steps provide finer eta resolution and smoother semblance spectra but increase computation time because more NMO corrections must be computed. A step of 0.01 gives 41 panels across the default range from −0.2 to 0.2, which is sufficient for most interpretation purposes. Reduce the step to 0.005 or smaller when picking in thin-layer sequences with rapidly changing anisotropy.
Default: 0.01.
The length of the time window, in seconds, over which semblance values are averaged during spectrum computation. A longer window produces smoother semblance peaks that are easier to pick visually but reduces the temporal resolution of the eta estimates, potentially blending picks across geological boundaries. Use a value of 40–100 ms for typical exploration data. Shorten the window if you need to resolve thin anisotropic layers.
Default: 0.05 s (50 ms).
The NMO stretch mute threshold applied before semblance computation, expressed as a fraction (0–1). Samples in a trace that have been stretched by more than this factor relative to their zero-offset position are excluded from the semblance calculation. This prevents the heavily stretched far-offset samples at shallow times from dominating the semblance spectrum and biasing eta picks toward spurious values. A value of 0.5 (50%) excludes samples that have been stretched by more than half their original waveform length, which is the standard mute criterion for conventional NMO.
Default: 0.5 (50%).
The time window, in seconds, used to normalise the semblance spectrum so that deep events with lower amplitudes are not overwhelmed by strong shallow reflectors. Semblance values within each window of this length are normalised by the local maximum, making the spectrum amplitude-balanced across the time axis. Use a value comparable to the typical inter-reflector spacing in your section (50–200 ms is typical for exploration data).
Default: 0.1 s (100 ms).
When enabled, the VTI NMO correction is recomputed and applied to the gather display in real time as you move the eta pick cursor on the semblance panel. This provides immediate visual feedback on the effect of each eta value on gather flatness, helping you judge the quality of your picks interactively. When disabled, the correction is applied only after a pick is confirmed, which reduces computational load during interpretation on large gathers.
Default: Disabled.
This group contains advanced parameters for handling the near-surface velocity estimation when topographic effects are significant.
When enabled (default), the module uses a surface velocity model derived from the topographic elevation data to improve the NMO correction near the surface. This mode is recommended when the survey area has significant elevation variation and the near-surface velocity is poorly constrained. When disabled, only the standard Vrms model is used for NMO without any near-surface velocity adjustment.
Default: Enabled.
The P-wave velocity assigned to the near-surface layer when Use velocity estimation from topography is enabled, in m/s. This value represents the average velocity of the weathering or replacement layer used for the surface-to-datum correction. Set it to the velocity estimated from uphole surveys, refraction analysis, or the known replacement velocity for the survey area.
Default: 1800 m/s.
Removes all eta picks from the current session, resetting the eta curve to an unpicked state. Use this action to start a fresh picking pass after reviewing the initial results, or to discard picks made in error for the currently displayed CMP location.
Imports a previously saved set of eta picks from a .corr file on disk. Use this action to resume an interrupted picking session or to apply an eta model picked on a representative line to another dataset with similar anisotropy characteristics.
Exports the current eta picks to a .corr file on disk. Save your picks regularly during a long picking session to prevent data loss. The saved file can be reloaded using the Load picking action or shared with other project members for quality control.