notes

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                DEMO04 - Ray Plot - Direct Wave

This demo goes quite a bit further and makes a plot of rays
corresponding to the direct wave.  There are a lot more entries
in PARAM1 for this run....

syncline                            :model file
4                                   :#interfaces in model
plotcolors                          :model colors file
m                                   :first plot descriptor (mq)
rwell                               :receiver well coordinates
wg                                  :receiver plot descriptor (wgq)
d                                   :shooting mode (dg)
swell1                              :source well coordinates
don't care                          :file containing coords. of sources
ws                                  :source plot descriptor (wsq)
r                                   :job descriptor (rlt)
don't care                          :output file name
-90.  90.                           :range of takeoff angles 
1.  .1   3.                         :coarse, fine angle increment; ray density
4000.  6000.  7000.  7500. 12000.   :velocities
y                                   :direct wave?
n                                   :primaries?
n                                   :head waves?

First, Record 10 has been changed to ws (plot the source well, plot
the source locations, do not quit).
Record 11 is job descriptor.  To get a ray plot enter r in Record 11.
To get a listing of some ray information enter l.  To get time sections
(ie, wiggle traces - built later by program CSHOT2) enter t.
To get all three at once, enter rlt, and so on.
See Demo07 for an example of generating listings.
The first wiggle traces are produced in Demo08.

When generating a listing or time data, the program creates output files.
The first part of the name given to these files comes from Record 12.
Appended to this name are .listing for the listing file, and
.shot for the time section file.  No output files are generated
when producing only ray plots, so Record 12 is not used here.

Record 13 is the range of takeoff angles over which rays will be 
shot out from the source.
The program rotates the range of angles you specify in Record 13
depending on the direction of the target.  For example, for upgoing rays
(aimed at a target reflector above the source location) the angles are
measured from the upward pointing vertical, as follows


                            0
                           ^
                           |
                           |
               -90 <------   -----> +90



Downgoing rays are measured from the downward pointing vertical

               -90 <------   -----> +90
                           |
                           |
                            0



The coordinate system for direct waves depends on whether the source
is to the left or right of the receiver well.  For example, for a
source to the left of the receiver well the angles are measured as
                           
                                        r
              -90 |                     e                   
                  |                     c
           source |---------- 0         e
                  |                     i
              +90 |                     v
                                        e
                                        r
                                        s

The program rotates the coordinate system automatically.
A range of -180 degrees --> +180 degrees covers all possible takeoff
angles; however, a more restricted range results in a faster run


Record 14 contains three entries: coarse change in takeoff angle, fine
change in takeoff angle, and ray density.
Beginning with the starting takeoff angle (given in Record 13) the program
shoots rays out from the source until a ray intersects the well.
The program uses the coarse change in takeoff angle for this search.
Once a ray that intersects the well has been found, the program
backs up to the previous takeoff angle and approaches the well more carefully
using the fine change in takeoff angle.  The aim is to find a ray that
intersects the well near to the well top (or the bottom, depending on the
type of ray).  The reason for this
is that the program uses interpolation to obtain traveltimes and amplitudes
at receiver locations.  We want rays to intersect the well all the
way along its length (and not miss large sections at the ends).
When the well is located for the second time (using the fine change in
takeoff angle), the program starts to monitor the change in end point
of the rays that intersect the well.  This is where the ray density takes
over: the program now tries to adjust the change in takeoff angle to satisfy
the ray density parameter.  Ray density is defined here as
the number of rays that hit the well between each pair
of receivers.  For example, given a ray density of 3 the program
will try to find 3 rays between each pair of receivers (it is not 
possible to guarantee this).  
Larger values for the ray density may result in 
more accurate traveltime and amplitude calculations in the 
interpolation procedure.  

The program will continue to search for rays that intersect the well
until it reaches the final takeoff specified in Record 14.


Record 15 of PARAM1 contains the layer velocities (shallowest layer
first).  The program expects to find one more velocity than the
number of interfaces specified in Record 2.

Next come the event specifications.  To generate a direct wave
enter y in Record 16; else enter n.
To generate primary reflections from all interfaces in the model
enter y in Record 17; else enter n.
To generate head wave events from all interfaces in the model
enter y in Record 18; else enter n.