Reading a large no of source files as external source time function

[padesh]

@SPECFEM admins,

I am working on seismic interferometry modeling at an oil and gas reservoir scale for an offshore setup. The ambient field is due to gravity wave action at the seafloor. Thus, my ambient sources are distributed at the seabed. I cannot use the built-in interferometry modeling setup of Specfem3d cartesian due to:

1. With the built-in setup, you can only have noise sources at the free surface and not at any other depth (as in my case, where I need them at the seabed).
2. It is not implemented for acoustic yet, and in my case, I have an acoustic-elastic coupled setup.

I have created a custom workflow to achieve what I want to do following the Tromp 2010 paper:

1. Inject a point force source at the virtual source location (main receiver).
2. Record the wavefield at noise locations (place receivers at the seabed in my case).
3. Time reverse the recorded wavefield and inject it back.
4. Record it at the other stations to get a cross-correlation field.

For step 2, there are receivers at all the grid points at the seafloor (around 20,000) that become sources at step 3. So, I make them as external source files and inject them using extern STF flag of Parfile.

The problem I am facing is it's taking a lot of time (around 15-20 minutes) (both on GPUs and CPUs) to read these many numbers of external source files, while the rest of the tasks (mesher, database generation, solver) take about 4 minutes.

Is there any way to incorporate parallel file reading for external STFs or speed up the file reading? Any suggestions would be greatly appreciated.