SUVELAN_USEL - compute stacking velocity panel for cdp gathers
using the UnNormalized Selective CrossCorrelation Sum
suvelan_usel stdout [optional parameters]
Optional Parameters:
nx=tr.cdpt number of traces in cdp
dx=tr.d2 offset increment
nv=50 number of velocities
dv=50.0 velocity sampling interval
fv=1500.0 first velocity
tau=0.5 threshold for significance values
smute=1.5 samples with NMO stretch exceeding smute are zeroed
dtratio=5 ratio of output to input time sampling intervals
nsmooth=dtratio*2+1 length of smoothing window
verbose=0 =1 for diagnostic print on stderr
pwr=1.0 semblance value to the power
Notes:
UnNormalized Selective CrossCorrelation sum: is based on the coherence
measure known as crosscorrelation sum. The difference is that the selective
approach sum only crosscorrelation pairs with relatively large differential
moveout, thus increasing the resolving power in the velocity spectra
compared to that achieved by conventional methods. The selection is made
using a parabolic approximation of the differential moveout and imposing a
threshold for those differential moveouts.
That threshold is the parameter tau in this program, which varies between
0 to 1. A value of tau=0, means conventional crosscorrelation sum is
applied implying that all crosscorrelations are included in the sum. In
contrast, a value of tau=1 (not recomended) means that only the
crosscorrelation formed by the trace pair involving the shortest and longest
offset is included in the sum. Intermediate values will produce percentages
of the crosscorrelations included in the sum that will be shown in the
screen before computing the velocity spectra. Typical values for tau are
between 0.2 and 0.6, producing approximated percentages of crosscorrelations
summed between 60% and 20%. The higher the value of tau the lower the
percentage and higher the increase in the resolving power of velocity
spectra.
Keeping the percentage of crosscorrelations included in the sum between 20%
and 60% will increase resolution and avoid the precense of artifacts in the
results. In data contaminated by random noise or statics distortions is
recomended to mantaing the percentage of crosscorrelations included in the
sum above 25%. After computing the velocity spectra one might want to
adjust the level and number of contours before velocity picking.
Credits: CWP: Valmore Celis, Sept 2002
Based on the original code: suvelan_.c
Colorado School of Mines: Dave Hale c. 1989
References:
Neidell, N.S., and Taner, M.T., 1971, Semblance and other coherency
measures for multichannel data: Geophysics, 36, 498-509.
Celis, V. T., 2002, Selective-correlation velocity analysis: CSM thesis.
Trace header fields accessed: ns, dt, delrt, offset, cdp
Trace header fields modified: ns, dt, offset, cdp