1. The crust and the Earth’s interior
Most of the material making up the Earth’s interior is not available for analysis. Some material is brought up to the surface by volcanism and deformation from depths of several 100 kms but represents a very small fraction of the Earth.

2. Propagation of seismic waves
As seismic body waves travel through the Earth along various paths, their velocity varies as a function of the properties of the material they encounter. If all the Earth was made up of the same material, the velocity of body waves would change smoothly with depth as pressure and, in turn, the density and rigidity of the material increases  readily predict arrival times, but ...

3. Seismic wave reflection and refraction
Much like light rays are reflected or bounce of the surface of water and/or refracted (velocity and path is modified) upon entering the water, seismic waves can be reflected (bounce off a surface) or refracted (path and velocity are modified upon entering a new medium) when they encounter the interface between phases of different density.

4. C_06.jpg
1909 –Andrija Mohorivicic – first convincing evidence of layering.

5. C_09.jpg P-wave travel paths Core-mantle boundary
Because they are reflected and refracted at the core-mantle boundary (CMB), none of the P-waves emerge at the surface between 103° and 143° from the epicenter.

6. S-wave travel paths Core-mantle boundary (CMB)
The core-mantle boundary casts an even more pronounced shadow for the S-waves, between 103° and 180°, from the epicenter.

7. Reflection of P-wave at core/mantle and outer/inner core boundaries
Just as a sound wave bounced off the bottom of a lake or a school of fish can be used to determine its depth or the position of the fish in the water column, the round-trip travel time for a reflected P-wave can be used to determine the depth of various boundaries  CMB = 2900 km.
Seismograph
5100 km
2900 km
1216 km
The presence of a solid inner core was first predicted in 1936 by the discovery of weak reflections of P-waves from a boundary within the core. Later, a Danish seismologist observed that P-waves accelerate below a depth of about ~5100km, but it was not before the early 1960's that the actual size of the inner core was accurately calculated after underground nuclear tests were conducted in Nevada.

8. Based on the velocity of seismic waves through the mantle, we know that the density increases slowly from 3.3 g/cm3 to 5.5 g/cm3 from the top to the bottom of the mantle. We also know that the mean density of the Earth is 5.5g/cm3. To make up for the difference, the core must be composed of material with a density of at least 10 to 11 g/cm3 – iron.
Mass of the Earth = 5.98 x 1024 kg
Density of the Earth = 5.52 g/cc
Density of rock at the Earth’s surface = ~2.67 g/cc
Density of the ocean crust and upper mantle= 3.3 g/cc

9. Velocity-versus-depth curve
From a composite of the data obtained from seismographic recordings of earthquakes or man-made explosions and their analysis, seismologists have constructed a map of the Earth’s interior and how seismic waves travel through each layer. No trivial task …

10. Tomographic images Subducting slab Whole Earth
In recent years, sophisticated algorithms have been used to compile global seismic data and create a three-dimensional image of seismic-wave velocities (reflecting temperature variations) within the Earth.
Subducting slab
Whole Earth

11. Convection in the mantle
The upwelling regions, depicted in yellow, consist of rising hot mantle, and the downwelling regions, depicted in blue, consist of sinking cooler mantle. The red sphere inside is the surface of the outer core.

12. Convection in the mantle
Seismic tomography has allowed seismologists to better refine conceptual models of the dynamics of Earth’s interior.

13. Velocity-versus-depth curve (Based on the velocity of P-waves in the mantle and the analysis of the few rocks found near the surface, believed to have originated from the mantle, the mantle would be composed of rocks that are rich in dense minerals such as olivine, pyroxene, and garnet.)
Asthenosphere
Mesosphere

14. Boom trucks for seismic surveys
Seismic techniques also allow us to fine-tune our image of the crust and explore for mineral and energy resources.

15. C_13d.jpg Seismic surveys at sea
By using dynamite or releasing bursts of compressed air in the ground (boom trucks) or at sea, geologists create artificial seismic waves that propagate down into the earth and reflect off the boundaries between different layers of rock in the crust.

16. Seismic-reflection profile (a cross-sectional view of the crust)
This image defines the depths at which specific strata occur and reveals the presence of subsurface features such as folds, faults, mineral, gas and oil deposits.