Computer Programs in Seismology

rfile and cpulse96

Tian Mengnam noted a problem when using cpulse96 with an external waveform file. The program did not work. The source file has since been corrected. This note provides an opportunity to demonstrate the use of the external file and to offer an alternative, perhaps better method to apply a user provided source pulse.

The shell script DOIT runs the code and also creates the user provided rfile. In addition, a FORTRAN program rfile2sac.f was created to convert the text rfile to a Sac file named rfile.sac. To use this FORTRAN program, compile it by the command

gfortran rfilesac.f

DOCMP

Download this shell script DOCMP and then execute everything:

chmod +x DOIT
chmod +x DOCMP
DOIT
DOCMP

DOCMP runs the CPS codes but also uses the ImageMagick package to convert EPS to PNG. If you do not have ImageMagick installed, you can view the comparison with the command:

plotxvig < P001.PLT
plotxvig < P002.PLT

Discussion

DOCMP creates some Sac files and then plots them for comparison. These files, int he order shown in the figures, are as follow:

• rfile.sac - This is the sac file created in the script DOIT. Note that for everything to work properly the sample interval for the synthetics and the rfile must be the same., in addition the rfile cannot have more than 1024 points, a constraint built into cpulse96.f and rfile2sac.f. Note that the use did not create a pulse with unit area, which is a requirement if one is interested in absolute amplitudes.
• ZEXpar.sac - This is the ZEX Green's function resulting from the command cpulse96 -V -p -l 1. Since the parabolic pulse is an approximation to an impulse, the trace would be the displacement for an impulsive source, or equivalently, the velocity for a step-like source.
• ZEXparconv.sac - The result of convolving the rfile.sac with the ZEXpar.sac using the gsac convolve command.
• ZEXrfileV.sac - This is the result of applying the rfile directly with the command cpulse96 -V -F rfile
• ZEXparconvd.sac - This is the time derivative of ZEXparconv.sac obtained using the dif command of gsac
• ZEXrfileA.sac - This is the result of applying the rfile directory using the command cpulse96 -A -F rfile

Output

This has shown that the external user supplied pulse can be applied either directory using cpulse96 or by convolving with gsac. I prefer the latter since the advantage of the cpulse96 -V -p l 1 is that the signal spectrum is zero at the Nyquist frequency, thus yielding a signal without a Gibb's phenomena.

The next figure shows the result of lpwpass filtering all signals at 1.0 Hz. Note the similarity of the second and third traces, and the fourth and fifth traces.

Low-pass filtered traces

Summary

This example demonstrates the the -F rfile command works for this program.