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<< Click to Display Table of Contents >> Navigation: Velocity > Interval Velocity Inversion (2D data) |
This module converts RMS (root-mean-square) velocity to interval velocity for 2D seismic data using an iterative inversion algorithm. The inversion solves for the true layer-by-layer velocity field that best reproduces the observed RMS velocity picks, making it more robust than a direct Dix conversion, particularly in the presence of noisy or irregularly sampled velocity data.
The module processes each trace of the input RMS velocity gather and outputs a corresponding interval velocity gather. It is designed for 2D velocity panels where velocities are represented as a gather (a collection of velocity-versus-time traces arranged spatially). The calculation step and trace skip parameters allow users to control the spatial density of the inversion, balancing computation time against lateral resolution. An optional constraint can be applied to prevent physically unrealistic decreases in interval velocity with depth.
Note: This module is deprecated. For new projects, consider using the current velocity inversion workflows available in the Velocities processing group.
The input RMS velocity gather for the 2D seismic line. Each trace in this gather represents a velocity-versus-time function at one lateral position (CMP or trace location) along the 2D profile. The gather must be a velocity-type dataset (displayed in the velocity visualization panel). The module reads the time sampling interval directly from this input to ensure consistent inversion spacing.
The output interval velocity gather for the 2D seismic line. Each trace contains the inverted interval velocity as a function of two-way travel time, corresponding to the same lateral positions as the input RMS velocity gather. This output can be used directly for depth conversion, geological interpretation, or as input to subsequent processing steps requiring interval velocities.
The vertical time interval (in milliseconds) at which the inversion solves for interval velocity values. The module rounds this value to the nearest multiple of the input data's sample interval and enforces a minimum of three samples. A smaller step produces a finer-resolution velocity function with more detail but increases computation time; a larger step yields a smoother result that is faster to compute. The default value is 100 ms. For typical seismic data sampled at 2–4 ms, values between 50 ms and 200 ms are common. Use smaller values only when the RMS velocity data is densely and accurately sampled in time.
The number of traces to step over between successive inversion calculations along the 2D profile. A value of 1 processes every trace; a value of 10 processes every 10th trace and interpolates in between. The default value is 10 traces. Increasing this value reduces computation time when the velocity field varies slowly along the line. Use smaller values (closer to 1) when rapid lateral velocity changes are expected, such as in areas with complex geology or near-surface heterogeneity. The minimum enforced value is 1.
When enabled, this option constrains the inversion so that interval velocity is not allowed to decrease with depth by more than the fraction specified in the Fraction of decrease parameter. In real geology, interval velocity can legitimately decrease with depth (e.g., overpressured zones, velocity inversions), but numerically unstable inversions can produce unrealistically large velocity decreases. Enabling this option helps suppress such artifacts. By default this constraint is disabled. When enabled, the Fraction of decrease parameter becomes editable.
Controls the maximum fraction by which interval velocity is permitted to decrease from one time sample to the next when Limit decrease of VINT is enabled. The value ranges from 0.01 to 1.0. A value close to 1.0 (e.g., the default of 0.99) allows only a very small decrease per sample, effectively enforcing a nearly monotonically increasing velocity trend. A smaller value (e.g., 0.8) allows larger decreases per step and is more permissive. This parameter is only active when Limit decrease of VINT is checked. Use values close to 0.99 when you want to strongly suppress velocity inversions in the output.