Thursday, August 19, 2010

Seismic Refraction and Reflection

Today I am going to explain the nature of seismic reflection and refraction and then briefly how we use it to extract information pertaining to the structure of Earth's subsurface. Let me start of with explaining what seismic refraction is.

The speed at which a seismic wave travels through a particular material is strongly dependent on the density and elastic properties of that material. So seismic waves travel at different speeds through materials with different properties. Generally, the denser the material, the faster seismic waves travel through it. When a seismic wave travels from one material into another, it does not continue in the same direction, but will bend. This bending of the seismic wave path is known as seismic refraction. Seismic refraction is caused by the difference in seismic wave speed between the two materials and is characterized by Snell’s Law, which is illustrated in the figure to the right. Given an angle of incidence (the angle between approaching seismic wave path and the line perpendicular to the interface) and the seismic wave speed in each material, Snell’s Law dictates the angle of refraction (the angle between the departing seismic wave path the line perpendicular to the interface).

All waves(light, sound, etc.) undergo refraction when moving from one material to another. A good everyday example of refraction is when you look at a straw in a glass of water and the straw seems to bend where it enters the water. Well the straw is not actually bending but the path of the light traveling to your eyes from the submerged straw bends slightly as it enters the air from the water. This is because light travels at slightly different speeds through water and through air. It is this refraction (bending) of the path of the light from water to air that makes it appear that the straw is bent.

Now that we know what seismic refraction is, what is seismic reflection? The answer is that seismic reflection is a type of seismic refraction! When a seismic wave is travelling from a material with a lower seismic wave speed to a material with a higher seismic wave speed, the angle of refraction is larger than the angle of incidence. In this case, there exists an angle of incidence, where the angle of refraction is 90 degrees and the refracted seismic wave runs parallel to the interface between the two materials. The angle of incidence at which this occurs is known as the critical angle. When the angle of incidence is larger than the critical angle, the seismic wave is refracted back into material 1 and leaves the interface at the same angle as the incident seismic wave approached it. This post-critical refraction is call total internal reflection. These phenomena are illustrated in the figure just above, where the red wave path is critically refracted and the yellow wave path is reflected.

So how do we use seismic reflection and refraction to extract information about the structure of Earth’s subsurface? In general the density of material increases with depth in Earth, therefore the seismic wave speed increases with depth. As seismic waves travel down through the subsurface, they will see larger and larger velocity materials the deeper they go and according to Snell’s Law, will refract and reflect back up to the surface, where we can record them. Well by using controlled seismic sources (like the airguns described in Kai’s post) we can send seismic waves into Earth’s subsurface and record the reflections and refractions when the arrive. By measuring when these refractions and reflections arrive, we can determine where the interfaces between differing materials are in the subsurface, thus generating a cross-sectional view of the subsurface.

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