The Earth formed some 4.6 billion years ago from a swirling mass of nebular material. This material coalesced and accreted into a solid mass. The temperature of early Earth steadily increased due to the high velocity impact of nebular debris and the decay of radioactive material. This early period of heating resulted in the chemical and physical differentiation of Earth establishing the Earth’s concentric layers.
Through the use of seismic (sound wave) data, scientists are able to accurately map out Earth’s internal structure. There are thousands of earthquakes that occur every day around the world. Earthquakes are shaking motions and vibrations of the Earth caused by large releases of energy that accompany volcanic eruptions, explosions, and the movement of the plates along its boundaries. There are currently over 3,000 active seismic recording stations located around the world. Each of these stations utilizes a device called a seismograph that generates a written record as a seismogram of the seismic event.
Most earthquakes are associated with energy being released along fault boundaries. All faults have some type of stress associated with them, compression, tension, and/or shear stress. This stress being exerted on a body of rock is stored as elastic energy. A body of rock can only withstand a certain amount of stress without permanent change in size or shape; this is that body of rocks elastic limit. When the stress on the body of rock exceeds its elastic limit the body of rock will rupture, brittle fracture occurs, and elastic energy stored in the body of rock is suddenly released as seismic energy; the seismic energy released causes the shaking of the ground.
Seismic energy is suddenly, without warning, released along a fault boundary. This energy is carried outward in all directions by various seismic waves, some of which can reach the opposite side of the Earth in about twenty minutes. Seismic waves radiate outward from the initial point of rupture along the fault line at the hypocenter (focus). Seismic waves generated from the focus, called body waves, consist of both Primary waves (P waves) and Secondary waves (S waves). Located directly above the focus at the surface of the Earth is the Epicenter. Seismic waves, called surface waves, radiate outward in all directions at the surface from the epicenter. These surface waves are identified as Rayleigh and Love waves.
As seismic waves pass through the Earth they encounter boundaries of different density within the interior of the Earth. These boundaries of different densities will cause the seismic waves to reflect or refract (bend) as the seismic waves pass through a boundary of different density. This reflection and refraction of the seismic wave is caused by a change in speed of the seismic waves as they encounter materials with different density. Based on the arrival times of the P and S waves around the Earth scientists can generate an internal image of the Earth’s structure.
The use of seismic technologies also dominates the field for the exploration of deep underground reservoirs of hydrocarbons. Just as geologist use seismic data from earthquakes to depict an image of the interior of the Earth, geophysicists use reflection seismology to explore specific regions for hydrocarbons. Geophysicists deploy some type of acceptable energy at the surface to generate seismic waves that radiate out from the source station as a three-dimensional wave. As these waves propagate down through the Earths layers a portion of the wave is reflected back to the Earth’s surface and is recorded by a device called a geophone. The geophones are distributed in specific geometries at the surface to produce a very detailed image of the geology of the area of interest.