There are 3 parts to this demo.

First, a ray plot and shot record is generated for 4 primaries from
a flat layered model.  (See param1.1).  Notice how a gap is specified
in the geometry file (geometry8).

Next, we get a ray plot and data for a direct wave and 4 headwaves.
(See param1.2.)

Finally, direct wave, primaries and head waves are generated in one
run. (See param1.2.)


For head waves, you may see where the ray hits the interface at the
critical angle but there is no receiver segment to the raypath.  This
is because the receiver segment emerges beyond the limits of the line.


In the last trace plot it is difficult to distinguish between head wave
wavelets and primaries.  This is because the waves are all arriving at
the about the same time.  The wavelets you see in this example are
probably from the primaries (the head wave amplitude falls off more
quickly with distance from the shot).
