Earlier a
*Snell* wave was defined
as a plane wave that has become nonplanar
by moving into a velocity-stratified medium *v*=*v*(*z*).
A plane wave keeps its angle of propagation constant,
while a Snell wave keeps its stepout *dt*/*dx* a constant
function of *z*.
Figure 1 shows a Snell wave incident on the earth's surface.

Figure 1

The wavefronts at successive times are not parallel to each other;
they are horizontal translations of one another.
The
*slowness*
of horizontal motion is called the
*stepout.*
It is measured in units of inverse velocity
and is given as milliseconds per meter or as seconds per kilometer.
The slowness, denoted as *p*, is also called the
*ray*
*parameter*
or the
*Snell*
*parameter:*

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10/31/1997