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                    PROCESS VELAN  (VELOCITY ANALYSIS)
                    ------- -----

Parameters, alphabetically:
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 
for refractors.
          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.

Example: 
  procs input velan stack output end 
  velan nrp 6 vels 1800 50 2000 100 2500 end 
  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
different velocities.

Semblance Spectra
--------- -------

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.


Slant Stack
----- -----

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.
         Preset=1.

TYPE   - The type of velocity analysis to perform.
       =CVEL,  constant velocity moveout will be done.
       =SPEC,  the velocity spectra velocity analysis will be done.
         Preset=spec

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


CVEL Parameters
---- ----------

STRETC - The amount of stretch (nmo), in seconds, permissible.  Data
         with NMO exceeding stretch will be muted.
         Preset=1.


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
         wavelength.
         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 
         (inclusive).
         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.