Description

Imaging - Velocity Selector (3D) is an interactive velocity analysis tool for Kirchhoff Prestack Time Migration (KPTM) of 3D surveys. It reads the family of pre-migrated stacks produced by the 3D KPTM engine, one stack for every constant velocity-scaling factor in a velocity fan, and lets you review orthogonal inline and crossline stack panels together with the full prestack Common Image Gather (CIG) at any bin to judge which velocity factor best focuses reflectors and diffractors.

You can pick the best velocity factor by hand at selected locations, or let the module search for it automatically using semblance (stacking coherency) analysis across the whole 3D volume, then interpolate the picks into a smooth, continuous velocity-factor field. Unlike the 2D Velocity Selector, which works along a single line, this module operates on the full survey bin grid: it shows and lets you pick on two intersecting panels at once (an inline panel and a crossline panel through the current bin) and builds a complete 3D interpolated RMS velocity volume covering every inline and crossline, ready to drive the final migration pass.

Custom Actions

The module exposes the following actions, available from the flow's action/verb menu, in addition to the parameters below.

Clear all picking

Deletes all manually picked velocity-factor points, after asking for confirmation. Use this to start hand-picking over from a clean slate.

Load picking

Opens a saved velocity-picking file (.velpick) and loads its manually picked points into the current session.

Save picking

Saves your current manual velocity-factor picks to a .velpick file so you can reload or share them later.

Recreate stacks for selected lines

Rebuilds the migration stacks for only the currently displayed inline and crossline, after confirmation. Use this after upstream data or parameters change and only the current pair of lines needs refreshing.

Recreate stacks for cube

Rebuilds the migration stacks for every inline and crossline in the whole 3D volume, after confirmation. This can take a long time on large surveys, so reserve it for cases where the entire cube genuinely needs refreshing.

Convert picking

Converts an older or alternate picking-corridor file (.corr) into the current picking format, writing the result next to the original file. Use this when importing legacy picking files from earlier projects.

Clear mute

Deletes the current mute picking, after confirmation, removing any top-mute curves picked on the CIG.

Load mute

Loads a previously saved mute picking file (.corr) and applies it as the current top mute.

Interpolate factors

Rebuilds the interpolated velocity-factor cube from the current picks (manual points, or the automatic semblance results in Advanced Mode), after confirmation. Run this whenever you add or adjust picks and want to refresh the interpolated field shown on screen.

Calculate Max And Sparse By Time Semblance (Current InLine and CrossLine)

Runs the automatic, semblance-based velocity search (Advanced Mode only) for just the currently displayed inline and crossline. Use it to preview and QC the automatic picking behavior on a single pair of lines before committing to the whole survey.

Calculate Max And Sparse By Time Semblance (Whole Cube)

Runs the same automatic semblance search across the entire 3D volume (Advanced Mode only). This reads the full migration storage and may take a long time on large surveys, so a confirmation warning is shown before it starts.

Save Debug Info

Writes internal diagnostic gathers (original and interpolated velocities, semblance data and related information) to the folder set in the Debug Info Path parameter. Intended for troubleshooting picking or interpolation problems, not for routine use.

Calculate Interpolated Stack (Current InLine and CrossLine)

Recomputes the interpolated stack panels (Advanced Mode only) for the current inline and crossline from the current velocity-factor field, so you can immediately review the effect of your latest picks on the stacked image.

Input data

Storage file

The multi-velocity Kirchhoff PSTM migration storage (.kdb) produced earlier by the 3D KPTM engine. For every inline and crossline in the survey it holds a family of image stacks, one for each velocity-scaling factor in the velocity fan, together with the base RMS velocity model used as the reference. This module reads that storage to build the interactive inline, crossline and CIG panels used for picking.

Output velocity file name

The destination path for the final interpolated 3D RMS velocity volume (.gsd) produced once picking is complete. It combines the base velocity model from the storage file with your picked and interpolated velocity-factor field, and is normally used to drive the final Kirchhoff PSTM migration or a later re-migration/QC pass.

Parameters

Use RAM to Create

When enabled, the module loads the entire multi-velocity migration cube into memory while writing the final interpolated output volume, instead of reading it from disk in smaller pieces.

Default: off. Enable only on machines with enough RAM for your survey size; it can speed up the final write step, but with a very large cube (many inlines and crosslines times many velocity factors) it can exhaust available memory. Leave it off for large projects.

Velocity Number

Selects which velocity-factor stack, one of the pre-computed migrated volumes in the constant-factor velocity fan stored in the .kdb file, is currently displayed in the inline and crossline stack panels and the CIG display.

Default: automatically set to 1 the first time a storage file is opened; valid values are clamped to 1 through the number of velocity factors in the storage. Step through this value, or scroll the mouse wheel over a stack panel, to flip between the constant-velocity-factor stacks and judge which factor best focuses reflectors and diffractors at each location.

Mute Taper Window

Length of the smooth taper ramp applied at the edge of the picked mute, so the transition from muted to un-muted samples is gradual instead of an abrupt cut.

Default: 0 ms (a hard, un-tapered mute edge). Increase this if a sharp mute edge introduces visible edge effects or ringing into the CIG or stack panels used for picking.

Sparse Window Along Crossline

Size, in bins, of the coarse grid used along the crossline direction when interpolating the picked velocity factors across the volume, used both for the on-screen interpolated stack panels and when building the final factor field.

Default: 5 bins, minimum 1. Larger values give a coarser, smoother interpolation (faster, more spatial averaging); smaller values follow local picks more closely but take longer to compute.

Sparse Window Along Inline

Same as Sparse Window Along Crossline, but sets the coarse grid size, in bins, along the inline direction.

Default: 5 bins, minimum 1. Keep the inline and crossline sparse windows consistent with each other unless your survey has a strongly asymmetric bin spacing.

Sparse Window by Time

Sets the time-domain decimation window used when building the coarse velocity-factor grid before full interpolation: picks and semblance results are grouped into windows of this length along time before being turned into a continuous curve.

Default: 200 ms, minimum about 1 ms. Use a shorter window where the velocity field changes quickly with depth (near surface), and a longer window for smoothly varying deep sections to reduce computation and interpolation noise.

Magnet (Snap) Mode

Controls how a location you click on the map or line view snaps to the survey's bin grid. Cross snaps to the nearest grid intersection (an inline/crossline crossing point); Single snaps to the single nearest bin center regardless of grid alignment; No magnet uses your click position exactly as given, without snapping.

Default: Cross. Keep Cross for normal navigation along inline/crossline intersections; switch to No magnet only if you need to select an arbitrary, off-grid position.

Manual Interpolation Type

When Use Advanced Mode is off and you are picking velocity factors by hand, this selects how your scattered hand picks are turned into a continuous 3D factor field. Laplace solves a smoothing surface through the picks for a very smooth result; Triangulation builds a triangulated surface that honors the picks more locally and exactly.

Default: Triangulation. This setting has no effect when Use Advanced Mode is enabled, since Advanced Mode always uses its own automatic gridding.

Advanced Mode

Use Advanced Mode

Switches the module between two working modes. Enabled, it turns on the automatic semblance-driven workflow: for every bin, the module searches the velocity-factor fan for the factor that maximizes stacking semblance (coherency) within the time and factor limits set below, effectively performing automatic velocity analysis across the whole cube. Disabled, it switches to a fully manual workflow where you pick velocity-factor control points by hand on the inline, crossline and CIG panels, and the field between your picks is filled using the Manual Interpolation Type parameter above.

Default: on. Leave Advanced Mode on for fast, first-pass velocity fields on large 3D surveys; turn it off when you need full manual control over problem areas.

Semblance Auto-Pick Settings

These settings only take effect when Use Advanced Mode is enabled; they control how the automatic semblance search finds and accepts the best velocity factor at each bin.

Semblance Normalization Window

Length of the time window used to normalize the semblance (coherency) calculation, compensating for amplitude variations with time so the automatic picker is not biased toward strong-amplitude zones.

Default: 500 ms, minimum 0 (normalization off). Increase for a more stable, amplitude-independent semblance measure; set to 0 to disable normalization entirely.

Semblance Smooth Window

Length of the smoothing window applied to the raw semblance curve before its maximum is picked, suppressing spurious local peaks caused by noise.

Default: 50 ms, minimum about 1 ms. Increase this if the automatic picker jumps erratically between adjacent velocity factors on noisy data; decrease it to preserve sharper velocity changes with time.

Semblance Threshold

Minimum normalized semblance (stacking coherency) value, from 0 to 1, that an automatic pick must reach before it is accepted.

Default: 0.1, valid range 0 to 1. Raise this threshold to keep only the most confident, well-focused automatic picks; lower it to accept more picks in areas of weaker coherency, at the risk of more noise-driven mispicks.

Minimum Time

Excludes the automatic semblance search from times earlier (shallower) than this value.

Default: 0 ms (no shallow limit). Raise this if the shallow section is dominated by noise, multiples or direct arrivals that should not drive the automatic velocity search.

Maximum Time

Excludes the automatic semblance search from times later (deeper) than this value, working together with Minimum Time to bound the search window.

Default: 1000 ms. Raise this to cover the full length of your section or target zone; the automatic picker only searches for velocity factors within the Minimum Time to Maximum Time window you define here.

Minimum Velocity Factor

Lower bound of the velocity-scaling-factor range the automatic picker is allowed to choose from, expressed so that a factor of 1 equals 100% of the base (reference) velocity, for example 0.8 equals 80% and 1.2 equals 120%.

Default: 0 (effectively unrestricted, since it lies below any realistic factor). Raise this toward a realistic lower bound, for example 0.7 to 0.9, to prevent the automatic picker from selecting implausibly slow velocity factors.

Maximum Velocity Factor

Upper bound of the velocity-scaling-factor range the automatic picker is allowed to choose from, on the same scale where a factor of 1 equals 100% of the base velocity.

Default: 100 (effectively unrestricted, since real factors rarely exceed about 1.5 to 2). Lower this toward a realistic upper bound to keep the automatic search away from implausibly fast velocity factors that noise might otherwise favor.

Additional Parameters (Diagnostics)

These are secondary controls for tuning and troubleshooting the automatic semblance analysis on difficult data; leave them at their defaults for standard processing.

Normalize Semblance Fluctuation

Enables an extra normalization step that compensates for local fluctuations in the semblance curve.

Default: off. Leave off for standard processing; try enabling it if automatic picks still look erratic after adjusting the Semblance Smooth Window.

Show Filtered CIG Result on Screen

When enabled, displays the CIG after the AGC, Hilbert and band-pass diagnostic filters have been applied, so you can visually confirm what the automatic picker actually analyzes.

Default: off. Turn on temporarily while tuning the diagnostic filters, then turn off again for normal picking.

Use Hilbert Transform for CIG

Applies a Hilbert transform to the CIG before the semblance calculation, which can make weak or phase-shifted events easier for the automatic picker to detect.

Default: off. Consider enabling it on data with strong polarity or phase variation across velocity factors.

Use AGC for CIG

Applies Automatic Gain Control to the CIG before semblance analysis, balancing amplitudes with time so that deeper, naturally weaker events contribute to the automatic picking on equal footing with shallow, stronger events.

Default: off. Enable on data where deep targets are being outweighed by strong shallow amplitude during automatic picking.

AGC Window Time for CIG

Length of the sliding time window used by the AGC above. Only takes effect when Use AGC for CIG is enabled.

Default: 100 ms, minimum about 1 ms. Shorter windows give more aggressive gain balancing; longer windows preserve more of the natural amplitude trend.

Debug Info Path

Optional folder where the module writes internal diagnostic files, such as original and interpolated velocity gathers and semblance data, when you run the Save Debug Info action or during processing when debug output is enabled.

Default: empty (debug output disabled). Set this to a valid, existing folder only when investigating a picking or interpolation problem; leave it empty for normal use.

Visualization

Datum

Reference elevation used to statically shift the displayed inline and crossline stacks and the CIG when Shift to Datum is enabled, so the panels line up with a chosen datum instead of the natural, as-recorded elevation.

Default: 0. This affects only the on-screen display used for picking, not the final output velocity volume.

Shift to Datum

When enabled, applies a static time shift to the displayed panels so they appear referenced to the Datum elevation above, using the base velocity from the storage file for the conversion.

Default: off. Use for display consistency when comparing lines with different surface elevations; it is a display-only convenience and does not alter the stored data.

Load CIG

Controls whether the full prestack Common Image Gather is loaded and displayed for the current bin, in addition to the inline and crossline stack panels.

Default: on. Turning this off skips loading the often-large CIG data, speeding up navigation across the survey when you only need the stack panels; turn it back on when you need to inspect individual offsets or angles, or edit the mute.

Show Interpolated Vrms

When enabled, computes and displays the fully interpolated RMS velocity curve (base velocity combined with your picked and interpolated factor field) on the inline and crossline panels, so you can compare it directly against the original base velocity.

Default: off. Turn this on to visually QC how your picking is modifying the velocity field before committing to the final output.

Mute

Apply Mute

When enabled, applies the mute curve you have picked or loaded (see the Load mute and Clear mute actions) to the CIG before it is used for display and for the automatic semblance calculation, removing near-offset noise, direct arrivals or other unwanted energy from the analysis.

Default: off. Turn this on whenever your CIGs contain contamination that would otherwise bias the velocity or semblance analysis.

Patch (Manual Interpolation Smoothing)

Laplace Error

Convergence tolerance used by the Laplace-equation solver when it fits a smooth surface through your hand-picked velocity-factor points, used both by the Laplace Manual Interpolation Type and internally by the Advanced Mode gridding.

Default: 0.05. Smaller values force a tighter, more exact fit to your picks but can be less smooth and slower to converge; larger values allow a smoother, more relaxed surface that may deviate more from individual picks.

Create Stack Area

Defines the inline and crossline subset of the survey used by the area-level actions (such as recreating stacks or running the whole-cube semblance search over a limited zone), so heavy calculations can be restricted to an area of interest instead of the entire volume.

First Inline

First inline number to include in the calculation area.

Default: -1, meaning no lower limit (starts from the first inline in the survey). Set to a specific inline number to restrict heavy calculations to a sub-area instead of the whole 3D volume.

Last Inline

Last inline number to include in the calculation area.

Default: -1, meaning no upper limit (extends to the last inline in the survey). Combine with First Inline to define a bounded inline range.

First Crossline

First crossline number to include in the calculation area, working the same way as First Inline but along the crossline direction.

Default: -1, meaning no lower limit.

Last Crossline

Last crossline number to include in the calculation area.

Default: -1, meaning no upper limit.

Inline Step

Decimation interval, in number of inlines, used when scanning the calculation area.

Default: 1, minimum 1, meaning every inline is used. Increase this to thin out a QC pass over a very large area for a faster, coarser preview.

Crossline Step

Decimation interval, in number of crosslines, used when scanning the calculation area, analogous to Inline Step above.

Default: 1, minimum 1, meaning every crossline is used.

Map (Current Position)

Inline

The inline number currently displayed in the Inline stack and CIG panels. Type a value here, or click on the map or line panels, to jump directly to that inline.

Default: taken from the storage file when it is opened.

Crossline

The crossline number currently displayed in the Crossline stack and CIG panels. Type a value here, or click on the map, to jump directly to that crossline.

Default: taken from the storage file when it is opened.

Band-pass Filter (Display Only)

This filter changes only what is shown on the inline, crossline and CIG panels while picking. It never modifies the stored migration data or the final output velocity volume.

Use Band-pass Filter

When enabled, applies a four-corner (trapezoidal) band-pass filter to the displayed panels, to make reflectors or diffractors easier to see and pick.

Default: off.

Frequency 1 (Low Cut Start)

Low end of the band-pass filter's low-cut ramp: frequencies below this value are fully removed from the display.

Default: 1 Hz, minimum 0 Hz.

Frequency 2 (Low Cut End)

Frequency above which the filter passes energy at full amplitude on the low side; the ramp between Frequency 1 and Frequency 2 tapers energy in smoothly.

Default: 5 Hz, minimum 0 Hz.

Frequency 3 (High Cut Start)

Frequency below which the filter still passes energy at full amplitude on the high side; above this the high-cut ramp begins.

Default: 100 Hz, minimum 0 Hz.

Frequency 4 (High Cut End)

High end of the band-pass filter's high-cut ramp: frequencies above this value are fully removed from the display. Together, Frequency 1 to Frequency 4 define the trapezoidal display pass-band; adjust them to match the useful signal bandwidth of your migrated data.

Default: 105 Hz, minimum 0 Hz.

Output data

Mute Picking

Stores the interactively picked top-mute curve or curves used to blank out unwanted energy, such as direct arrivals, refractions or near-surface noise, on the displayed CIG before velocity and semblance analysis. It is saved with the flow so the mute can be reloaded, edited or reused later.

Number of Calculated Bins

A read-only counter reporting how many bins in the survey, or within the limited Create Stack Area, already have a completed migration or semblance calculation. Use it to track QC progress as you work through the volume.

References

Yilmaz, O., 1987, Seismic data processing: Society of Exploration Geophysicists.

Al-Yahya, K. M., 1989, Velocity analysis by iterative profile migration: Geophysics, 54, 718-729.