Description

This module builds a synthetic prestack (offset) dataset from an existing stacked volume. For every trace you want in the output geometry, it takes the stacked trace from the nearest bin location in the input stack and stretches its reflection times, using an RMS velocity model, to the moveout that would be expected at that trace's specific offset. The result is a modeled prestack gather that reproduces the stack's reflection character at any chosen source-receiver geometry. This is useful for QC, for generating synthetic gathers for testing prestack workflows, or for producing a pseudo-prestack dataset from legacy data where only a stack is available.

Input data

Input stack

The stacked volume providing the reflection character to model from. Each trace should represent the stacked (effectively zero-offset) response at its bin location; this is the source data whose events are stretched to build each output offset trace.

Output trace headers

The desired geometry of the modeled prestack dataset: one header entry for every trace you want the module to produce, giving its position and offset (and any other header values you need). The module models one output trace for each entry, taking its reflection character from the nearest bin in the Input stack.

Vrms model

The RMS velocity model used to compute the offset-dependent moveout applied when stretching each stacked trace to the target output offset. The velocity function is sampled at every output trace's bin location.

Output file name

Base name for the new internal dataset that will hold the modeled prestack traces. The module writes results directly to this new dataset as it processes the output geometry, rather than producing a gather that flows to the next step in the current flow.

Default: none — a name must be supplied before the flow can run.

Parameters

Bulk trace size

Number of output traces modeled and written to the output dataset together in one batch. This is a performance setting only and does not change the modeled results. Lower it if the machine has limited memory or the output geometry is very large; raise it for faster throughput when ample memory is available.

Default: 10000 traces (minimum 1).

Shift type

Controls whether the modeled traces are left referenced to the flat processing datum or additionally shifted to match real surface topography. "Floating datum" outputs the modeled traces as computed, referenced to the flat datum used during processing. "Topography" applies an extra static correction after modeling so the output honors each trace's actual surface elevation, using the constant replacement velocity set in Velocity.

Default: Floating datum.

Velocity

Constant replacement velocity used only when Shift type is set to "Topography", to convert the datum static time shift into a depth/time correction that honors surface topography. Set this to a value representative of the near-surface velocity in the survey area; it has no effect when Shift type is "Floating datum".

Default: 1500 m/s (minimum 1 m/s).

Output data

Modeled prestack volume

A new dataset, named from Output file name, containing one modeled trace for every entry in Output trace headers. Each trace reproduces the reflection character of the nearest input stack trace, stretched to the moveout expected at that trace's offset, and optionally shifted to surface topography.

References

Taner, M. T. and Koehler, F., 1969, Velocity spectra-digital computer derivation and applications of velocity functions: Geophysics, 34, 859-881.