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In practice, horizontally going waves are easy to recognize
because their travel time is a linear function
of the offset distance between shot and receiver.
There are two kinds of horizontally going waves,
one where the traveltime line goes through the origin,
and the other where it does not.
When the line goes through the origin,
it means the ray path is always near the earth's surface
where the sound source and the receivers are located.
(Such waves are called ``ground roll'' on land
or ``guided waves'' at sea;
sometimes they are just called ``direct arrivals''.)
When the traveltime line does not pass through the origin
it means parts of the ray path plunge into the earth.
This is usually explained by
the unlikely looking rays shown in Figure 1
which frequently occur in practice.
headray
Figure 1
Rays associated with head waves.
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Later in this chapter we will see that Snell's law
predicts these rays in a model of the earth with two layers,
where the deeper layer is faster and the ray bottom
is along the interface between the slow medium and the fast medium.
Meanwhile, however, notice that these ray paths
imply data with a linear travel time versus distance
corresponding to increasing ray length along the ray bottom.
Where the ray is horizontal in the lower medium,
its wavefronts are vertical.
These waves are called ``head waves,''
perhaps because they are typically fast
and arrive ahead of other waves.
Next: Amplitudes
Up: Waves in strata
Previous: Vertical exaggeration
Stanford Exploration Project
12/26/2000