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Seismic attributes are stored as children of the input seismic in the project tree. Seismic attributes for a Seismic Line are just datasets containing the attribute values. Seismic attributes for Seismic Brick Cubes on the other hand are complete Seismic Brick Cubes themselves.
In order to display a Seismic Attribute for a Seismic Line you must edit the Seismic Display command:
The Seismic Display command let you choose which Attribute to display (Envelope in this case). The original seismic is referred to as "Amplitudes".
Seismic Trace Attributes
Right-clicking on a Seismic Line or a Seismic Brick Cube and selecting "Seismic Trace Attributes..." pops up the following dialog:
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