HORIZONTALLY MOVING WAVES

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.

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.