IWAVE Structure and Basic Use Cases

Movie from Initial Data.

Figure 2 displays a Gaussian initial field, input as the initial potential field uc. Since up is not input, it remains at its default value of zero. The resulting IWAVE solution is not convergent to the solution of any particular Cauchy problem with full convergence order (2), but produces a reasonable-looking movie. Another (minor) application would produce a correction to be added to the up field to restore 2nd order convergence.

gauss
Figure 2. Gaussian initial datum

The data depicted in Figures 1 and 2 is input to the simulation, so clearly must exist prior to simulation. However the output must also exist: IWAVE I/O, both reads and writes, is driven by the target data structure. Therefore the movie output file must be constructed before the simulation fills it with data. The SConstruct script contains an invocation of the sfmakevel command which creates a 3D rsf file movieinit.rsf. On completion of the command, this file holds the movie output.

Perusal of this command reveals some customization of the rsf file format, as compared to its standard use (Fomel et al. (2013)). The duration of the movie determines the duration of the simulation: the initial simulation time is the time of the initial movie frame, and similarly for the final time. Thus IWAVE must be able to determine which axis specified in movieinit.rsf is the time axis. Three additional header word categories, beyond those of the rsf standard, make this feat possible:

• dim = spatial dimension - should be same as spatial dimension of the primary static file (keyword csq in this case).
• gdim = global dimension - of the data array, at least dim
• id1, id2, id3,...: identification tags of axes 1, 2, 3,...

By IWAVE convention, axis with tag = dim is the time axis, and axes 0, 1,..., dim-1 are the spatial axes. More than dim axes are permitted - they are used to represent other acquisition or modeling parameters, see section on multi-shot simulation, below.

In this example, the space dimension is 2, so id3=2 indicates that the 3rd axis is the time axis.

After propagating 5.12 s and interacting with both the reflecting (Dirichlet) boundaries and the interfaces in the model, the potential field becomes that depicted in Figure 3.

frameinit
Figure 3. Acoustic potential field at 5.12 s, resulting from Gaussian initial data

Parameters passed to the command acd.x included

           csq = ../csq_4layer.rsf
         initc = ../init.rsf
         movie = ../movieinit.rsf

Keywords data and source were ignored. Many other paramters were required; a brief description of these is included in the self-doc of the command acd.x.

Note that the pathnames refer to the directory level above the working directory. IWAVE produces various diagnostic output at runtime, switched by various flags passed as parameters. These outputs, and possibly other auxiliary outputs of commands built upon IWAVE (eg. the data residual in an inversion) vary with application and data, so are inconvenient to specify individually as cleanup targets. Instead, the SConstruct script creates a working subdirectory and executes (and dumps its auxiliary output) there. The entire directory is cleaned up by scons -c. So the correct parameter specification for archival input and output files is one directory level up.

IWAVE Structure and Basic Use Cases