Seismic data is used to visualize the Earth's sub surface. This is achieved by measuring travel times of reflections from pressure waves propagating through the ground. Seismic data and interpretations of seismic data will therefore often have z values in travel time, not depth. In order to get a depth value for a travel time value, we need to do a depth conversion. A depth conversion combines the velocity of the pressure wave with the travel time to get a depth value.
...
In the Layer Velocity model , you may use seismic horizons as interval velocity boundaries. The horizons may be interpreted lines from 2D seismic (IHorizon or PolyData) or grids (StructuredPoints). You may set the interval velocity of each interval using a table. In the table you also need to set the top horizon of each interval. In addition you need to set the water velocity as input.
When going from top to bottom, the layer velocity model will initially use the water velocity as interval velocity until it meets the top horizon. From the top horizon , it will use the top horizon velocity as interval velocity until it meets the next horizon, and so on. continuing this pairwise matching for the rest of the horizons.
During depth conversion , the interval velocities are used to calculate an average velocity.
...