A velocity model featuring multiple constant velocity layers is a slightly more complex model than a single constant velocity.
Multiple layers can be created using constant Time or depth Z values, or horizons.
There are two tools that can be used for constructing constant multiple layer models.
The first method uses the volume maths process.\
Open the process tab and create a new volume maths process.
The name that you give the process will become the name of the velocity volume you create.
Select a seismic volume.
Although this is not required for 3D data, the input volume IL/CL range can be used to quickly define the output volume range, even though the amplitude values are not part of the process.
Let's assume that sea bed is a constant 250 milliseconds and that velocity changes at the sea bed.
In this example use the if command with the expression twt_ms to compare the output volume twt in milliseconds to a cut-off of 250.
If less than 250, set the output velocity value to 1500 meters per second,
else if more than 250, set the output velocity value to 2500 meters per second.
Set the output geometry to be at least 100m by 100m in the X/Y axis and 24ms in the Z axis to keep processing time efficient.
You need to set the volume class to Interval Velocity for it to display correctly.
Set the min and max range to match the class settings.
QC the model using a section view.
For additional layers, build a nested structure where the second variable becomes a new if statement.
The updated model will display in section view if you have the syntax correct.
You can add as many nested layers to the formula as required.
To make the model more structurally viable, swap the constants for Horizons.
Enable the horizons and display in map view and section view to check they are suitable for inclusion in the model.
Add the horizons to the process variables and change the horizon names.
Change the constant time values to the horizon variable symbols.
The model updates using the horizons as boundaries.
The same models can be used in 2D as in 3D.
Use the same procedure but select your 2D volume as the input to define the matching output geometry.
Display the 2D line for Quality Control.
Now lets build exactly the same model using a different method.
The second method uses the velocity from checkshot process.
Step one is to create a window model that defines the layers.
Models can be TWT or TVDSS as required.
It is possible to build the model using constant values as we did using volume maths.
For example 250, 1500, 2500 milliseconds.
For a structurally viable model, swap the constant values for horizons.
Once the window model is created, open the process tab and create a new velocity model from well checkshots process.
The name that you give the process will become the name of the velocity volume you create.
Select the window model you created previously to define the layers.
The output for this method is a 3D cube using an ILCL survey grid.
In this example, the ILCL separation is set to 200m in the XY axis
and 24ms in the Z axis to keep processing time efficient for this large regional model.
For this simple constant velocity example do not specify any wells to be included.
Change the per layer settings to constant values and type in the chosen velocities.
1500, 2500, 2750 and 3500 meters per second.
QC the results using a section view.
Here you can see a 2D line zoomed to its full extent.
The velocity values only display across the part of the line that intersects the smaller 3D survey.
Models are not extrapolated beyond the defined 3D ILCL geometry.
The 3D model can be applied to the 2D data if the IL/CL extents are increased to cover the 2D lines.
Once the ILCL extents match the 2D line XY range
use Insights amazing ability to automatically resample data across surveys, for QC of the 3D model in the 2D section.