Example Rayleigh Wave Processing: Measuring Dispersion

The next step is to use the program bvax to measure a dispersion curve from the waveform data. Here, we are using the synthetic data of Figure 45. The input parameters for a command line execution are show below:

{bvaxhelp}
      bvax infile xmin xmax vmin vmax nvel . . .
           fmin fmax delf bwd iskp ivscn

 infile  =input file name 
 xmin    =minimum offset (float) 
 xmax    =maximum offset (float) 
 vmin    =minimum velocity 
 vmax    =maximum velocity 
 nvel    =number of velocity increments
 fmin    =minimum frequency Hz
 fmax    =maximum frequency Hz 
 delf    =frequency increment Hz 
 bwd     =filter bandwidth Hz 
 iskp    =skip filtering (if files already exist)
           1=YES  0=NO (-1=NO and delete when done)
 ivscn   =output velocity scan data sets 
           1=YES  0=NO

bvaxhelp

bvax wavV.seg .5 50. 50. 500. 200 6. 50. 1 .25 -1 0

The choice of frequency increment and bandwidth will depend on your data record length. For example, for 0.5 second signals, set $delf=2$ Hz increments. For 2.0 second signals, set $delf=0.5$ Hz. The longer the temporal aperture, the finer the spectral resolution possible. The program determines phase velocity using cross correlations of extremely narrow band signals (one can't pick arrivals, that would be appropriate for a group velocity procedure). At each frequency selected, trial velocities are applied as static shifts, and semblance computed to determine the degree of alignment. A peak in the semblance is picked by a Golden Section search, and the hope is that the picked peak is for the fundamental mode. The number of trial velocities needs to be large enough to provide good image files, but need not be large if your interest is primarily in the picks of semblance peaks (output file bvax.his).