Velocity model is the key component in the seismic imaging. Be it a time imaging or depth imaging. To achieve the best velocity model, geophysical community around the world uses various methods like tomography (grid tomography, stereo tomography etc), FWI (Full Wave Inversion) etc.
To start with, we need a good initial velocity model. From the time imaging, we take Vrms velocity model and convert that into Vinterval depth velocity model. To perform this within g-Platform, the user should use either Grid tomography 2D/3D module or Dix TV module to generate the Vinterval depth velocity model from the initial Vrms velocity model.
In the above image, we read the initial Vrms velocity model by using "Read seismic traces" module. We connect the initial Vrms velocity model to Grid tomography 2D/3D module. As per our data we need to set up our parameters. In this example we are showing the parameters for a 2D case.
Few parameters are worth mentioning here. Unlike the time imaging where the migration runs from the topography, depth imaging runs from the datum. So the user should provide the datum value in the respective field.
Parameters:
Tomography deminsion Choose the dimension of the data i.e. 2D/3D
Delta time(meter) Specify the delta time
Inversion parameters
Max number of global iterations
Number of local iterations
Sigma V
Epsilon Z
Advanced inversion parameters
sigPxix
sigPxiy
sigXix
sigXiy
Sigma M
Sigma T
Grid parameters
Use constant gradient initial model By default TRUE. If the user prefers a constant gradient keep this option as checked and specify the gradient value in the "Initial model velocity gradient" parameter.
Initial depth velocity model
Initial model velocity gradient Specify the velocity gradient value if "Use constant gradient initial model" option is checked.
Replacement velocity (m/s) Mention the near surface/replacement velocity of the surface.
Keep constant V0 By default TRUE.
Z start (meter) Define the starting depth value of the interval velocity model
Z end (meter) Define the ending depth value of the interval velocity model
X start (meter)
X end (meter)
Y start (meter)
Y end (meter)
Resolution factor
Number of rays per knot
Additional bottom rays
Initial X grid step (meter) Specify the initial grid step size in X direction
Initial Y grid step (meter) Specify the initial grid step size in Y direction.
Initial Z grid step (meter) Specify the initial grid step size in Z direction
Final X grid step (meter) Specify the final grid step size in X direction
Final Y grid step (meter) Specify the final grid step size in Y direction
Final Z grid step (meter) Specify the final grid step size in Z direction
Datum Provide the datum value.
Output velocity parameters
Create velocity gather By default is it FALSE. If the user checks this option, it will generate the output Depth interval velocity model.
Depth Specify the total length of the output velocity model in terms of depth.
Delta Z (meter) Define the depth sample interval.