SUVELAN_NSEL - compute stacking VELocity panel for cdp gathers
using the Normalized 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=100.0 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:
Normalized 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 normalization is
achieved in much the same way of normalizing the conventional
crosscorrelation sum.
Each crosscorrelation is divided by the geometric mean
of the energy of the traces involved, and the multiplying by a constant to
achieve maximum amplitude of unity. The constant is just the inverse of the
total number of crosscorrelations included in the sum. 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