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The Vrms to angle module converts NMO-corrected offset gathers into angle gathers. Starting from a gather in the offset domain, the module uses an input RMS velocity model to compute the reflection angle at each time sample and offset, then resamples the data onto a regular angle axis. Angle gathers are the preferred input for AVO analysis and amplitude-versus-angle (AVA) attribute extraction because angle is directly related to the physics of reflection and transmission at geological interfaces, whereas the relationship between offset and angle varies with depth and velocity.
The angle computation uses either the RMS velocity model directly (straight-ray approximation) or can optionally incorporate an interval velocity model for a more accurate ray-parameter-based calculation. The module generates a synthetic offset axis covering the specified offset range and step, computes the incidence angle at each time sample and offset using the selected velocity model, and maps the input amplitudes onto the output angle axis. An optional smoothing window can be applied along the time axis to stabilise the angle values.
The input NMO-corrected seismic gather in the offset domain. The module maps offset traces to angle traces using the velocity model. The gather must be NMO-corrected before connecting to this input so that the amplitudes are aligned to the correct reflection time.
The input RMS velocity model used to convert offset to angle. This is the primary velocity input and must always be connected. The velocity values are used to compute the reflection angle at each time sample via the relationship sin(theta) = offset / (2 * Vrms * t) for the straight-ray approximation, or via the equivalent ray-parameter formula when interval velocity is also provided.
The length of the smoothing window applied along the time axis to the computed angle values before remapping, in seconds. Default: 0.01 s (10 ms). Smoothing stabilises the angle field where the velocity model is noisy or sparsely sampled. Use a longer window (e.g. 50–100 ms) when working with a coarsely sampled velocity model, and a shorter window when the velocity model is dense and reliable.
The maximum offset (in metres) considered when building the angle axis. Default: 15000 m. Set this to the maximum usable offset in the input data. Traces at offsets beyond this value will not contribute to the output angle gather.
The minimum offset (in metres) considered. Default: -15000 m. For standard positive-offset surveys set this to 0 or to the minimum usable offset in the data. Negative values are used for datasets where negative offsets are present (for example, from reciprocal geometry).
The spacing between synthetic offset samples used when mapping offset to angle, in metres. Default: 25 m. A smaller step produces a more densely sampled angle axis and can improve accuracy at shallow times where angle changes rapidly with offset, but increases computation time. A typical value of 25–50 m is sufficient for most surveys.
Selects the velocity input mode. Set to V0 to use a single constant near-surface velocity (V0) for all times — useful for a quick test or when no velocity model is available. Set to V Interval to use the connected interval velocity model for a more accurate angle calculation based on the ray parameter through the layered velocity field.
An optional interval velocity model used when V input type is set to V Interval. When connected, the module uses the layer-by-layer interval velocity to trace the ray path and compute incidence angles more accurately than the straight-ray approximation. This input is not required when using V0 mode.
The constant velocity (in m/s) used for angle computation when V input type is set to V0. Default: 1800 m/s. This parameter is only active when V input type is V0. Set it to a representative average velocity for the target zone if no model is available.