Seismic data is used to visualize the ground. This is achieved by measuring travel times of reflections of pressure waves traveling 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 the travel times, 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.
Data sets may be depth converted in Geocap using the command "Velocity Model Depth Convert". This command uses a "Velocity Model" as input in order to determine the velocity of a the pressure wave.
Velocity Model
A velocity model is stored as an item your Project. I represents the seismic velocities in an area, and is used as input to the "Velocity Model Depth Convert" command. In order to create a velocity model, set a "Velocity models" schema on a generic folder, and run the command "Generate Velocity model". In order to see or edit the parameters of a velocity model, right click the velocity model item in the project and select "View/Edit".
Geocap currently supports these types of velocity models:
Constant Ground Velocity
The constant ground velocity model is a single layer velocity model. It uses one single ground velocity. Input to the velocity model is Water velocity, the seabed surface in time, and the ground velocity.
Layer Velocity model
In the Layer Velocity model, you may use seismic horizons as interval velocity boundaries. The horizons may be interpreted lines from 2D seismic or grids. 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. You also need to set the water velocity as input.
When going from top to bottom, the layer velocity model will use the water velocity as interval velocity until it meets the top horizon. From the top horizon, it will use the top horizon velocity until it meets the next horizon, and so on.
RMS velocity (Stacing velocities)
The RMS velocitiy velocity model uses RMS velocity datasets as input. In some cases Stacking velocities may be used as RMS velocities. The velocity model computes the velocity of a point, it will use the RMS velocity points within a search radius of the point to compute the velocity for the point. The RMS dataset, the interpolation method, and the search radius are input to the RMS velocity model.
In order to generate an average velocity, the velocity model will run Dix formula on the RMS velocity to generate interval velocities. An average velocity is computed from the interval velocities.
Average velocity cube
An average velocity cube is a 3D raster which holds the average velocity at the cube nodes. Such a cube may be generated in Geocap, or it may be imported from other software. Such a cube may be used as input for the Average Velocity cube velocity model. You may also specify a seabed surface and a water velocity which will be used above the seabed surface. The cube will be used to look up velocities below the seabed surface. The velocity model will clamp the bottom velocity in the velocity cube as velocity below the cube.