(1) |

where is the angle of the ray with respect to the vertical, *v*
is the velocity of the shallow subsurface, *t _{0}* is the time at which
the wave is incident on the surface and

Discussion of plane waves in media of spatially variant velocity,
such as the Earth, is at best a crude approximation. For the special case
of a media of stratified velocities, a model which can be considered
in many cases a good approximation of the Earth, plane waves have
a simple derivative. A plane wave in a media of constant
velocity incident at an acute angle to a media of stratified velocities
will be distorted upon entering the region of stratified velocities. In
the stratified media, the former plane wave will retain
memory of its former existence and maintain a distinctive character, one
important characteristic being the value of the velocity
of the wavefront along a horizontal plane, such as the Earths surface.
Waves with these characteristics are known as Snells waves. The inverse
of the horizontal velocity of the wavefront is known as Snells
parameter, or the slowness, which is the linear shift factor encounter
in the slant stack transformation equation, , where *p* is Snells
parameter, is the retarded time, *t* is the travel time in the shot
gather and *h* is the offset in the shot gather.
The important characteristic of Snell waves to practitioners is not
only that the Snells parameter is constant but also that it is an
observableClaerbout (1984).

Shot gathers are an observation of a propagating wave event and can be modeled directly using the wave equation. Slant stacks applied to shot gathers reveal information about a the physical state of the propagating wave field for values of Snells parameter and time. Unfortunately slant stacks of common shot gathers can be plagued by aliasing effects when applied to sparsely sampled data due to the presence of conflicting dips. On the other hand, common midpoint gathers are much less prone to having severely conflicting dips because all the hyperbolic events are aligned along the zero offset axis. Common midpoint gathers are not a coherent representation of a wave field, rather a cmp gather is a fragmentation of many separate shot events and cannot be used to evaluate characteristics of the wavefield such as Snells parameter.

11/12/1997