Fan Shot Modeling Explanation
                   Bill Menke, 9 April 97

This is a approximate 2D model wgich assumes that the plane
of the rays is flat and horizontal.  This is a pretty good
approximation, atleast for heterogeneities in the NVZ.  The
RTMOD mesh file fan.msh is 80x120, with the top surface a
circle centered on (40,120).  Thus all points on the sufrace
are equidistant from the fan shot at (40, 120).  Thus the
effecive coordinate system is rotated about 4 degrees CCW
from north and east, because a station at x=40 actually has
a backazimuth of N86E to the fan, not 90 degrees as in this
model.  The background velocity is 6.5, and the velocity
anomalies range between 6.5 and 7.0.

RTMOD Controls should be set to:
	source (40, 0, 120)
        angles of incidence -160 to -200

The traveltime data file fan.tt is drawn from the last
column of east.tt, except that a constant time delay
has been added (simulating propagation through the
background medium).

The traveltime plot window should have settings
	data set east.tt
	T vs X
	ranges (0, 80) with ticks every 10 km
	time interval of (17, 19) with ticks every 1 s
	reduction velocity of 0