You are here: OpendTect 5.0 dGB Plugins Doc > Velocity Model Building > Vertical Velocity Analysis



Vertical Velocity Analysis

From the VMB toolbar, launch the vertical velocity update window by pressing the corresponding icon .


The window is composed by a Common Image Gather from a single trace on the left, and the corresponding semblance plot on the right:



The display is annotated by separate display settings for both 2D viewers. The 2D viewer settings are similar to the standard 2D viewer, except that the semblance cannot be displayed using wiggles. Furthermore the semblance panel features one or two additional curves: The migration velocity (always shown) and the picked velocity, based on the velocity picks and migration velocity. Thus this second curve is shown once a velocity pick is made. If horizons are loaded in the main scene they will be shown as well in this vertical analysis window as horizontal coloured segments.


The velocity picks, mute definitions and tomography events are editable objects. As such they can be edited using the same tools as for the picksets and polygons: left click to add a new, hold left click to drag and drop an existing pick, ctrl+left click to remove. In all cases you must be in edit mode and make sure that the object you would like to edit is highlighted/selected in the list of editable objects to apply those actions. Selection and removal tools are available to quickly remove a large number of pick on either the CIG (for mute and pre-stack events) or the semblance gather (for RMOs). You will first need to make a square or polygonal selection before to press the trash icon , that will remove the selected picks. Before removing a large number of picks you might want to use the undo and/or redo buttons.


Back in the view mode you are able to zoom in and out on the panels, that always remain synchronized.


When moving the cursor over the semblance panel you will see an overview of the RMO correction that is linked to your cursor position.

In the two viewer you can jump to any position using this toolbar icon , that launches a CDP selection window:



It is highly recommended to browse at start-up through the settings before picking a mute, RMO or tomography events. Those settings will be presented here below:


Settings: (available via File--> Settings...)

The following settings define the length of the time/depth gate used for the computation of the semblance:

Additional VMB-specific pre-processing method are available, like Velocity correction:

This pre-processing method will apply the velocity correction defined by the gridded velocity picks to the pre-stack data.


Semblance annotations Annotations for the semblance display can be set and saved as user default settings in the following window:

Those settings can be retrieved from the Velocity picks menu with the option Properties:


The seeds appear larger than the tracked positions, although both types are editable (movable or removable).

Futhermore there is always one active prestack event displayed using a thicker bold line, except if the cursor is outside of the search window of all existing seeds of the CIG. Any new pick will be append to the active pre-stack event if the cursor is within the search window of an existing pick, except if the keys control+shift are pressed when picking. In that case a new pre-stack event will be created.


On a new pre-stack gather the pre-stack may be picked and tracked or autotracked . The autotracking is to be used at a new locations as it will become unavailable if one or more pre-stack events are picked. On the contrary at least one seed must exist before pressing the track option.


The autotracking option is meant to use either the calculated semblance or the post-stack horizons to create seeds for the autotracking of the pre-stack events. This option must be set in the tracking settings before using the corresponding icons in the velocity analysis windows. All autotracked events can be edited afterwards.

Those settings can be retrieved from the Prestack event menu with the option Properties:

Those settings can be retrieved from this icon in the toolbar: