# Snell’s law

Series | Geophysical References Series |
---|---|

Title | Digital Imaging and Deconvolution: The ABCs of Seismic Exploration and Processing |

Author | Enders A. Robinson and Sven Treitel |

Chapter | 2 |

DOI | http://dx.doi.org/10.1190/1.9781560801610 |

ISBN | 9781560801481 |

Store | SEG Online Store |

Suppose we have two homogeneous isotropic media separated by a straight horizontal interface (Figure 10). We plot the depth axis *y* in the upward direction. Let . The low-velocity surface layer (layer 1, with constant slowness ) is in the range . The high-velocity deep layer (layer 2, with constant slowness ) is in the range . Ray *EFG* starts in a vertical direction. It strikes the horizontal interface at right angles. A particle on the ray speeds up when it passes from the low-velocity layer 1 into the high-velocity layer 2. The ray does not bend at the interface and travels in layer 2 in the same vertical direction. Ray *ABC* starts in a direction oblique to the interface. A particle on ray *ABC* speeds up when it passes from the low-velocity layer 1 into the high-velocity layer 2, just as the particle did for ray *EFG*. The particles on ray *EFG* and on ray *ABC* experience exactly the same contrast in velocity. However, ray *ABC* bends at the interface. Why?

The particle is like a little air-plane. The fuselage goes along the raypath. The airplane is steered by the wings, which extend at right angles to each side. The left wing enters the high-velocity layer first, whereas the right wing is still in the low-velocity layer. This swings the plane to the right, and the raypath bends at the interface. How much does it bend?

Ray vector *AB* in layer 1 has length and makes angle with the vertical. Ray vector *BC* in layer 2 has length and makes angle with the vertical. By Snell’s law, we have . This equation says that the horizontal projections of the two ray vectors are the same. We expect this result because the slowness does not change in the horizontal direction. The slowness does *E* change at the interface in the vertical direction. The shallow medium has slowness , and the deep medium has a smaller slowness . Because the interface is level, it follows that slowness is constant along any horizontal line, so . However, slowness does change at the interface. The difference of the slowness at the interface is , which is negative because . The gradient of slowness at the interface is proportional to . This vector points straight into the low-velocity layer. The difference vector *BD* = *BC* – *AB* must be proportional to the gradient of the slowness, and hence vector *BD* must point straight down. Thus, , which says that the raypath bends to the right at the interface. In this section, we have given a preview of the ray equation.

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Wave Motion | Visualization |

## Also in this chapter

- Reflection seismology
- Digital processing
- Signal enhancement
- Migration
- Interpretation
- Rays
- The unit tangent vector
- Traveltime
- The gradient
- The directional derivative
- The principle of least time
- The eikonal equation
- Ray equation
- Ray equation for velocity linear with depth
- Raypath for velocity linear with depth
- Traveltime for velocity linear with depth
- Point of maximum depth
- Wavefront for velocity linear with depth
- Two orthogonal sets of circles
- Migration in the case of constant velocity
- Implementation of migration
- Appendix B: Exercises