When seismic waves reach the boundary between different rock types, several key phenomena occur due to changes in the physical properties of the rocks, primarily their density and seismic wave velocity:
Behavior of Seismic Waves at Rock Boundaries
- Reflection : Part of the seismic wave energy is reflected back into the original medium. This happens because of an impedance contrast, which is the product of rock density and seismic wave velocity. A significant difference in seismic impedance between two rock layers causes a portion of the wave to bounce back toward the surface
- Refraction : The transmitted part of the wave bends (refracts) as it passes into the new rock layer. The direction and speed of the refracted wave depend on the relative seismic velocities of the two media. If the wave moves from a slower to a faster medium, it bends toward the boundary; if from faster to slower, it bends away from the boundary
- Change in Velocity and Amplitude : Seismic wave velocity changes when crossing boundaries due to differences in rock rigidity (shear modulus) and density. For example, waves travel faster in denser, more rigid igneous rocks and slower in less rigid sedimentary rocks. When waves slow down in softer materials like sedimentary rocks, their amplitude increases because the material deforms more easily, allowing larger oscillations
- Energy Partitioning : The incident wave energy is split between the reflected and refracted waves, with the proportions depending on the angle of incidence and the contrast in material properties at the boundary
- Surface and Shadow Zones : At certain boundaries, such as the core-mantle boundary, P-waves are refracted sharply due to a sudden decrease in velocity, creating shadow zones where direct waves do not arrive. S-waves cannot pass through liquid layers like the outer core, leading to S-wave shadow zones
In summary, seismic waves encountering a boundary between different rock types are partly reflected back and partly refracted into the new medium, with changes in speed and amplitude governed by the contrast in density and rigidity of the rocks. This behavior is fundamental to seismic imaging and understanding Earth's internal structure
