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:
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
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 Hz increments. For
2.0 second signals, set 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).
pm 2018-04-08