Module performs smoothing of 3D velocity field.

Theory

Velocity smoothing in 3D is the process of making a velocity volume (inline × crossline × time/depth) laterally and vertically consistent by removing short-wavelength, non-geological variations while preserving true geological velocity trends.

In 3D, velocities must vary smoothly in both inline and crossline directions, not just along CMPs as in 2D.

1) Large volume of velocity picks - Smoothing enforces global consistency

• 3D surveys contain thousands of CMPs
• Manual velocity picking introduces inconsistencies
• Small picking errors propagate across the volume

2) Suppression of acquisition footprint - Smoothing removes non-geological lineations

• 3D data often show inline/crossline striping
• These patterns leak into velocity picks
• Un-smooth velocities amplify footprint during migration

3) Geological realism

• Subsurface velocity changes gradually, not abruptly
• Sharp velocity jumps between adjacent CMPs are unrealistic
• Smoothing restores physically meaningful velocity trends

4) Stability of later processing - Smoothing ensures numerical stability

• NMO, stacking, and migration are velocity-sensitive
• Small velocity fluctuations cause large imaging errors

What happens if 3D velocities are NOT smoothed?

In NMO correction

• Events flatten in one inline but not the next
• Residual moveout remains after correction
• Far-offset stretching increases

In stacking

• Poor trace alignment across offsets
• Destructive interference during summation
• Reduced signal-to-noise ratio

In migration

• Migration smiles and frowns appear
• Faults and reflectors lose continuity
• Structural closures become unreliable

In 3D volumes

• Inline/crossline striping
• Velocity footprint dominates the image
• Interpreter confidence reduces

How velocity smoothing is performed in 3D? Velocity smoothing in 3D is applied in three directions:

1) Inline smoothing

• Enforces consistency along survey lines
• Removes line-to-line picking noise

2) Crossline smoothing

• Eliminates striping and footprint
• Ensures lateral continuity between lines

3) Vertical (time/depth) smoothing

• Removes rapid velocity oscillations with time
• Preserves compaction trend

In practice, horizontal smoothing is stronger than vertical smoothing to avoid destroying layering.

Common 3D velocity smoothing methods.

Fast (moving average) smoothing

Gaussian smoothing

• Distance-weighted averaging
• Preferred for final velocity volumes

Structure-oriented smoothing

• Follows reflectors
• Preserves faults and dips (advanced Workflows)

Effect of velocity smoothing on NMO

Provides consistent flattening across inlines and crosslines

• Minimizes residual moveout
• Reduces NMO stretch at far offsets
• Produces stable and repeatable stacks

Effect of velocity smoothing on migration

• Stabilizes ray propagation
• Improves focusing of diffracted energy
• Enhances fault definition and reflector continuity
• Reduces migration artefacts caused by velocity noise

What velocity smoothing removes vs preserves

Removes

• Random picking noise
• Inline/crossline acquisition footprint
• Non-physical velocity jumps

Preserves

• Long-wavelength velocity trends
• Structural velocity variations
• Compaction and lithological effects

Input data

Input DataItem

Use gathers or seismic file { Gather in - gather out, File in - file out }

there are two options for the user to choose from the drop down menu.

Use gathers or seismic file - Gather in - gather out

Input gather

connect/reference to the velocity gather that needs to be smoothed.

Use gathers or seismic file - File in - file out

Input SEG-Y data handle

connect/reference to Output SEG-Y data handle.

Input trace headers

connect/reference to the output trace headers.

Output filename

specify the output file name. This generates the smoothed velocity model as an output.

Use slope gathers

by default, FALSE. If checked, the user must provide the inline and crossline slope gathers.

Use slope gathers - true

Input gather slope inline

Input gather slope crossline

Parameters

Gather domain { TIME, DEPTH, FREQUENCY }

it automatically detects the input gather domain. By default, TIME. In case the user wants to select the appropriate domain, choose the gather domain from the drop down menu.

Type of smooth { Fast, Gaussian }

there are two types of smoothing options are available. Depending on the requirements of the operation, the user can choose the smoothing type.

Type of smooth - Fast

this is a simple averaging of velocities in a fixed window. Each velocity value is replaced by the mean of nearby samples.

Work mode { Constant, Variant }

select the work mode from the drop down menu.

Work mode - Constant

in this mode, it will be constant values of the horizontal and vertical windows.

Smooth grid x direction(radius)

number of inlines to be considered in the X direction for smoothing

Smooth grid y direction(radius)

number of crosslines to be considered in the Y direction for smoothing

Smooth grid z direction(radius)

number of time/depth samples to be considered for smoothing in the Z direction

Work mode - Variant

this mode works with variable values of both horizontal and vertical windows.

Variant values table

click on the icon and it will pop-up a new window. Define the respective parameter values in the table.

Use fractional window

by default, FALSE (Unchecked). If checked, it will use

Use dynamic window

by default, FALSE (Unchecked). This option is available when the Type of smoothing is "Fast"

Use dynamic window - true

Max aperture

by default, 5. Maximal number of iterations to find a value that differs from the ignored value.

Apply horizons constrain

by default, FALSE (Unchecked). This will give more control to the user to smooth the velocity model based on the horizon.

Apply horizons constrain - true

If the horizon constraint is TRUE, the user should provide the horizon.

Horizons

connect/reference to the input horizon to control the velocity smoothing.

Horizon taper size

this controls the velocity smoothing. Based on the user specified taper value, it will apply the velocity smoothing below or above the horizon.

Type of smooth - Gaussian

this is a weighted smoothed method by using Gaussian bell shaped function. In this method, nearby samples contribute more than the far samples.

UNSAFE

Ignore value mode

in this section, the user has the option to ignore some of the velocity values.

Fill ignore value places

by default, FALSE (Unchecked). If TRUE (Checked), replace the sample with smoothed value.

Ignore value

specify the value that should be ignored. Value that will not participate in smoothing algorithm, this value remains as is without changes.

Settings

Auto-connection

By default, TRUE(Checked).It will automatically connects to the next module. To avoid auto-connect, the user should uncheck this option.

Number of threads

One less than total no of nodes/threads to execute a job in multi-thread mode. Limit number of threads on main machine.

Skip

By default, FALSE(Unchecked). This option helps to bypass the module from the workflow.

Output data

Output DataItem

Output SEG-Y data handle

Output trace headers

Output gather

outputs the smoothed velocity gather as an output gather.

Information

Calculated inlines count

displays the total number of calculated inlines

Calculated crosslines count

displays the total number of calculated crosslines

Examples

In this example, we've connected/referenced a single inline Vrms velocity as an input to Smooth 3D module. This input is taken from Stack Imaging module.

In case the user wants to smooth the velocity model based on the horizon constraints, then provide the horizon and execute the smooth 3D module. It is necessary to inform to the user that it may take longer time to complete the operation with horizon constraint.

Actions

There are no action items available for this module.

Video lesson

YouTube video lesson, click here to open [VIDEO IN PROCESS...]

Related articles

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

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