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Seismic surveys use reflection seismology to investigate the Earth’s structure, generating seismic waves through dynamite or vibroseis trucks. Geophones detect the waves, which are processed by computers to estimate the Earth’s composition. The method is used in oil and gas exploration, as well as prospecting for other resources and preventing oil spills.
A seismic survey is used to investigate the underground structure of the Earth and is mainly used for oil and gas exploration. This method uses the principles of reflection seismology to acquire and interpret seismic data, which enables the Earth’s composition to be estimated. The technique is similar to that used by computerized axial tomography (CAT), which involves analyzing seismic waves traveling through the Earth.
When a seismic survey is being conducted, seismic waves are usually generated by dynamite or special truck-mounted vibrators. These vibroseis trucks, as they are known, are often used if the use of dynamite would cause serious damage to their surroundings, such as in cavernous areas. The trucks use heavy metal plates weighing over 3 tons (2.72 metric tons) which are placed in contact with the ground and which are then struck with heavy hammers.
When seismic waves pass through the Earth and encounter different materials, part of their energy is reflected by the boundaries between the different layers while other waves pass through it. The reflected energy returns to the surface, where its speed and strength is measured by special detectors, called geophones. Geophones convert ground motion into electrical signals, which are then digitized by seismometers. These signals are then processed by computers; the more complex the geology of the area studied, the more computing power is required to process the huge amounts of data.
The speed and strength of the reflected waves depends on the density of the layers they encounter. Rock gets denser the further underground it is, but pockets of gas and oil have a much lower density. The denser the material, the faster the waves will be reflected. When 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 picture of the subsurface using 3D seismic interpretation.
When used in oil and gas exploration, a seismic survey can reveal pockets of low-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 data collected is invaluable and includes soil composition and soundness, bedrock and groundwater depth, rock structure and much more. This information has both academic and commercial value.
In addition to oil and gas exploration, a seismic survey can be used in other commercial applications. These may include prospecting 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 prevent oil spills by sensing the stability of the ground where pipelines will be built.
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