A seismic survey is used to investigate Earth’s subterranean structure and is mainly used for oil and gas exploration. This method uses the principles of reflective seismology to acquire and interpret seismic data, which permits the estimation of the Earth’s composition. The technique is similar to that used by computerized axial tomography (CAT) scans, involving the analysis of seismic waves that travel through the Earth.
When a seismic survey is conducted, seismic waves are usually generated by dynamite or by special vibrators mounted on trucks. These vibroseis trucks, as they are known, are often used if the use of dynamite could cause serious damage to the surroundings, such as in cavernous areas. The trucks use heavy metal plates weighing in at over 3 tons (2.72 metric tons) that are placed in contact with the ground, and which are then struck with heavy hammers.
As the seismic waves pass through the Earth and encounter different materials, some of their energy is reflected off the boundaries between the different strata while other waves will pass through. The reflected energy returns to the surface, where its speed and strength is measured by special detectors, known as geophones. The geophones convert the movement of the ground into electrical signals, which are then digitized by seismometers. These signals are then processed by computers; the more complex the geology of the area being studied, the more computing power required to process the massive quantities of data.
The speed and strength of the reflected waves depends on the density of the strata they encounter. Rock becomes denser the deeper underground it is located, but pockets of gas and oil have a much lower density. The denser the material is, the faster the waves will be reflected. As the seismic waves encounter lower density materials, more of their energy passes through. By analyzing the time it takes for the waves to be reflected, geologists can build an accurate image of the subsoil by using 3D seismic interpretation.
When used in gas and oil exploration, a seismic survey can reveal pockets of lower density material and their location. This does not necessarily guarantee that these pockets contain oil or gas, as it could indicate any other lower density material, such as water. Even so, the wide variety and reliability of the collected data is invaluable and includes soil composition and solidity, depth to bedrock and water tables, rock structure and much more. This information has both academic as well as commercial value.
In addition to oil and gas exploration, a seismic survey may be used in other commercial applications. These can include the search for precious metals and stones, such as alluvial gold, diamonds, or platinum, or other resources, such as gravel, sand, and quarry sites. Seismic surveys are also used to help preventing oil spills by surveying ground stability where pipelines will be built.