SUFRAC -- take general (fractional) time derivative or integral of
data, plus a phase shift. Input is TIME DOMAIN data.

sufrac power= [optional parameters] <indata >outdata

Optional parameters:
power=0 exponent of (-i*omega)
=0 ==> phase shift only
>0 ==> differentiation
<0 ==> integration

sign=-1 sign in front of i * omega
dt=(from header) time sample interval (in seconds)
phasefac=0 phase shift by phase=phasefac*PI

Examples:
preprocess to correct 3D data for 2.5D migration
sufrac < sudata power=.5 sign=1 | ...
preprocess to correct susynlv, susynvxz, etc. (2D data) for 2D migration
sufrac < sudata phasefac=.25 | ...
The filter is applied in frequency domain.
if dt is not set in header, then dt is mandatory

Algorithm:
g(t) = Re[INVFTT{ ( (sign) iw)^power FFT(f)}]
Caveat:
Large amplitude errors will result if the data set has too few points.


Credits:
CWP: Chris Liner, Jack K. Cohen, Dave Hale (pfas)
CWP: Zhenyue Liu and John Stockwell added phase shift option

Trace header fields accessed: ns, dt, trid