1.1.1. Vel2Grid - velocity model description to 3D model grid

Given a velocity model description, Vel2Grid generates a 3D model Grid header and buffer files containing velocities, slownesses or other model specification.

1.1.1.1. Overview

The Vel2Grid program converts analytic or other velocity model specifications into a `3D Grid <./formats.html#_grid_>`__ file containing velocity or slowness values.

The Vel2Grid program uses a “flat earth”, rectangular, left-handed, x,y,z,t coordinate system (positive X = East, positive Y = North, positive Z = down). Distance units are kilometers.

1.1.1.2. Running the program - Input

Synopsis: Vel2Grid InputControlFile

The Vel2Grid program takes a single argument InputControlFile which specifies the complete path and filename for an Input Control File with certain required and optional statements specifying program parameters and input/output file names and locations. See the `Vel2Grid Statements section <./control.html#_Vel2Grid_>`__ of the Input Control File for more details. Note that to run Vel2Grid the `Generic Statements section <./control.html#_generic_>`__ of the Input Control File must contain the CONTROL and TRANS (Geographic Transformation) statements.

In addition, the Vel2Grid program requires a set of `Vel2Grid Statements <./control.html#_Vel2Grid_>`__ in the Input Control File that specify a layered model or a 3D velocity model. The velocity model can be specified in the control file by:

1. A set of `LAYER <./control.html#_Vel2Grid_layer_>`__ statements defining a horizontally layered model with constant or constant-gradient velocity and density in each layer.

2. A set of `VERTEX <./control.html#_Vel2Grid_vertex_>`__, `EDGE <./control.html#_Vel2Grid_edge_>`__, and `POLYGON2 <./control.html#_Vel2Grid_polygon2_>`__ statements defining a 2D polygon model and a `2DTO3DTRANS <./control.html#_Vel2Grid_2d3dtrans_>`__ statement to convert this 2D model into a 3D model. Optionally, there may be a set of `LAYER <./control.html#_Vel2Grid_layer_>`__ statements defining a horizontally layered background model. This background model must be defined if the transformed 2D polygon model does not completeley fill the requested 3D grid.

1.1.1.3. Output

The velocity or slowness values throughout the requested grid are written to a new `3D Grid File <formats.html#_grid_>`__. For a descrition of the naming convention for these grid files, see the `VGOUT <./control.html#_Vel2Grid_vgout_>`__ statement in the Vel2Grid Statements section of the Input Control File.

1.1.1.4. Processing and Display of results

The 3D model grids can be post-processed with the program Grid2GMT to produce a GMT command script for plotting with the GMT plotting package.