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PROCESS VELAN (VELOCITY ANALYSIS)
chars nrp opath refrac staper stretc
ttaper type vels vtuple winlen
Document Date: 31 October 1995
A velocity analysis is an aid in determining the stacking velocities to
use in normal moveout. There are currently 6 types of analysis
available in this package, the first 3 for reflectors and the last 3
1) constant velocity gathers
2) constant velocity stack
3) semblance spectra
4) constant p tau-p gathers
5) tau-sum or slant stack
6) tau-p semblance spectra
Constant Velocity Gathers
-------- -------- -------
A constant velocity gather velocity analysis usually consists of all
the traces of a single rp (reflection point) moved-out with a number of
different velocities that do not vary in time.
This method of analysis is useful for examining exactly how the data
aligns at different velocities. The velocity is written into every
SEG-Y trace header in short integer word 46 (where 1 is the first).
Example: for 24 fold data a single rp with vels 5000 200 7000 would
output the first 24 traces moved out with a velocity of 5000. The next
24 traces would be moved-out with a velocity of 5500. The following 24
traces would be moved-out with a velocity of 6000, etc.
Constant Velocity Stack
-------- -------- -----
A constant velocity stack is a suite of stacked gathers, usually several
adjacent rps, moved-out with velocities that do not vary with time.
This method of velocity analysis is useful because it shows the actual
stack of the data using the normal nmo and stack routines. The velocity
is written into every SEG-Y trace header in short integer word 46
(where 1 is the first).
To obtain a constant velocity stack from this software package, process
stack must follow process velan and the CVEL option must be used. By
doing this, the constant velocity gathers made by velan will be stacked.
procs input velan stack output end
velan type cvel nrp 6 vels 1800 50 2000 100 2500 end
will produce the following output:
Traces 1-6, stack of the 6 rps at 1800 m/sec.
Traces 7-12, stack of the 6 rps at 1850.
Traces 13-18, stack of the 6 rps at 1900.
Traces 19-24, stack of the 6 rps at 1950.
Traces 25-30, stack of the 6 rps at 2000.
Traces 31-36, stack of the 6 rps at 2100.
Traces 37-42, stack of the 6 rps at 2200.
Traces 43-48, stack of the 6 rps at 2300.
Traces 49-54, stack of the 6 rps at 2400.
Traces 55-60, stack of the 6 rps at 2500.
The output is normally displayed with a space between the traces with
The velocity is plaved in SEG-Y trace header short integer word 46
A semblance velocity spectra is the printer plot of the semblance of
trace windows moved-out with constant velocities. Semblance is the
ratio of the energy of the output trace (the stacked trace) and the mean
energy of the input traces. The stacked trace is obtained by summing
all the traces of the rp after moving them out. The move-out is a little
different from conventional NMO since the entire window receives the
same NMO as the center of the window. The mean energy is simply the
sum of the energies of all the corresponding input windows. Successive
windows are spaced half a window away, thus the windows overlap.
More than one rp may be included in the spectra by using the parameter
NRP. In this case, all the input rps (traces) are treated as one larger
rp, as if smearing the subsurface. This may increase the signal to
noise ratio in areas of negligible dip.
Velocity spectra are discussed in a paper by Terry Tanner of
Seiscom-Delta in Geophysics, December 1969. Semblance is discussed in a
paper by Tanner and Neidell of Seiscom in Geophysics, June 1971.
Constant p Gathers
-------- - -------
A constant p gather is analogous to the constant velocity gather except
that the data is 'moved-out' using the linear time shift equation of a
refracted ray (t0=tx-x/v or tau=t-px where p=1/v, the slowness). The
parameter REFRAC must be used in order to obtain the refraction equation.
A slant stack is obtained by stacking the constant p gathers. This is
analogous to constant velocity stack except that the 'move-out' is for
refracted arrivals rather than reflected arrivals. The velan parameter
must be used.
Tau-p Semblance Spectra
----- --------- -------
The tau-p semblance spectra is analogous to the reflection semblance
spectra except that the linear refraction equation is used.
THE PARAMETER DICTIONARY
--- --------- ----------
Parameters Needed by Both Constant Velocity and Velocity Spectra:
VELS - The list of velocity-velocity increments to be included in the
analysis. The first and last entry must be velocities.
e.g. vels 1800 50 2200 100 3000 will produce an analysis with
velocities 1800 1850 1900 1950 2000 2050 2100 2150 2200
2300 2400 2500 2600 2700 280 2900 3000.
Required. Up to 21 entries may be given
NRP - The number of rps to be included in each analysis.
TYPE - The type of velocity analysis to perform.
=CVEL, constant velocity moveout will be done.
=SPEC, the velocity spectra velocity analysis will be done.
REFRAC - Refraction event analysis (t0=tx-x/v) will be performed when
REFRAC is set non-zero.
Preset=0. e.g. refrac 1
STAPER - The number of traces to weight in order to form a spatial
taper. A spatial taper or window reduces the edge effects due
to there not being an infinite number of traces in the
analysis. The taper is applied to STAPER traces at the
beginning and the end of each rp in the analysis. The taper
is a Bartlet window or linear ramp. e.g. staper 3 will weight
the first and last trace by .25, the 2nd and second to last by
.5, and the third and third to last by .75.
Preset=0 for normal incidence
Preset=3 for refraction
TTAPER - The length of a linear taper, in seconds, to apply to the ends
of the data in order to prevent boundary problems in time.
Each trace will be tapered from either the delay or mute time
for TTAPER seconds. Likewise, each trace will be tapered for
TTAPER seconds at the end of data. A linear taper is also
called a Bartlett window.
Preset=0. For normal incidence data
Preset=.2 For refraction (tau-p) data
STRETC - The amount of stretch (nmo), in seconds, permissible. Data
with NMO exceeding stretch will be muted.
Velocity Spectra Parameters
-------- ------- ----------
WINLEN - The window length, in seconds, of the window to use in the
semblance spectra. The window length should include a full
Preset=.100. Example, winlen .080
VTUPLE - The velocity (horizontal) scale to use on the printer plot of
the velocity spectra. The tuple is comprised of the minimum
velocity to plot, the maximum velocity to plot, and the number
of columns to use between the minimum and the maximum
Preset = 0 0 0 example: vtuple 5000 20000 101
CHARS - The character set to use on the line printer semblance spectra
plot. The semblance values are normally divided into 10
intervals, 0. -1., 1. -2., 2. -3.,....., 9. -10. The semblance
value is a real number which is converted to integer by
truncating. This integer is then used as an index to the
character array used in the plot. The characters in the CHARS
list must be separated by a blank. A blank character in the
CHARS list is represented by any two characters without a blank
separator. Up to 50 characters may be given.
PRESET= 0 1 2 3 4 5 6 7 8 9 e.g. chars AA 1 2 3 4 5 6 7 8 9
OPATH - The pathname of an output file containing the semblance values
in full floating point (before being truncated to integer).
The output file format is ASCII, one semblance value per line
and grouped by velocity, unless the last characters of
the filename are "segy" or "mat" for SEG-Y or MATLAB.
= SEGY These files are constructed as a CMP gather with each
or MAT velocity as a trace within the gather. The semblance values
are resampled (spline interpolated) so that the semblance
sample interval is the same as the time domain sample interval,
and the semblance values are in host floating point, which
allows seismic picking and plotting programs to function
easily (e.g. script VPICK for SIOSEIS/MATLAB picking). The
SEGY format contains the velocity in the SEG-Y header word 10
(range) and the trace values are the modified SEG-Y format 5,
host floating point.
Preset = none e.g. opath velan.1234
END - Terminates each parameter list.
Written and copyrighted by:
Paul Henkart, Scripps Institution of Oceanography, 26 March 1981
All Rights Reserved.
Go to the list of seismic processes.
Go to SIOSEIS introduction.