Scientists often search for oil and gas reserves by transmitting waves of energy through the earth’s crust, and recording how they are reflected back. Called seismic exploration, this process often involves using explosives, vibrating trucks, or underwater air guns. The returning energy is typically detected by acoustical instruments, and then analyzed with powerful computers. Different layers in the ground can reflect the energy differently, so scientists often use seismic exploration to find areas that might have oil, gas, or valuable minerals.
Seismology is generally based on the composition of rock layers, in the earth’s crust, that affects how energy interacts with underground materials. Energy waves usually move through the rock and then reflect back toward where they came from. The direction in which they return can give an idea of what properties the rocks have. Data on the returning seismic waves are typically analyzed by supercomputers and three-dimensional imaging software. Engineers can use this information to locate the best sites to start drilling.
On land, dynamite and other explosives are sometimes drilled into the ground at various places and detonated. The explosions typically generate seismic waves, smaller but similar to those in an earthquake, which hit rocks below the surface and bounce off of them. Devices called geophones can be placed throughout an area to detect the returning energy. Vibrating trucks are sometimes used during seismic exploration, which lift up on a pole and shake the ground. These generally do not cause as many disruptions as explosives and are more often used in populated areas.
With underwater seismic exploration, compressed air bubbles can be ejected, which hit the rocks at the bottom. Energy is reflected by rock layers below the ocean floor and is often picked up by instruments called hydrophones. These are usually attached to ships. In choosing a drilling site based on the data received, engineers can determine if fluids, faults, or other formations underground can interfere with the project.
Seismic exploration is generally more useful in finding evidence of gas. It often helps in determining the shape and size of an underground reservoir, while measurements of electrical resistance are usually better for oil exploration. Seismic methods can be used for both, and can help engineers decide the best way to get to the reserve. This technique is often part of geophysical testing to find hydrocarbons. In many places, it is regulated by local and regional agencies where the environment is of concern, such near the artic icecaps and many offshore locations.