Passive seismic
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Passive seismic - listening of the natural low frequency earth movements, usually with the purpose to guess geological structure and/or to locate the underground source of oil, gas or other valuable resources. Usually the data listening is done in multiple measurement points that are separated by several hundred meters, measuring from several hours till several days, using portable seismometers. The conclusions about the geological structure are based on the spectral analysis or on the mathematical reconstruction of the propagation and possible sources of the observed seismic waves.
Passive seismic usually focuses on a low frequency signals (0..10 Hz) and is sometimes called the "low frequency" seismology. The seismometers record movements in all 3 possible directions independently (such devices also have other application areas like long-term measurement stations). When needed, data acquisition is also done under water, using waterproof devices that measure earth movements at the bottom of the sea. Geophones are almost never used due to their limited sensitivity.
Passive seismic is much cheaper than well drilling. It is also more cheaper and environment friendly than active seismic which requires the strong source of the seismic waves (like underground explosion) to predict the structure. In some cases it may be the only method for which the land access is granted by the land owner. However despite the method has been successfully applied in many parts of the world, this approach is currently less reliable as the scientific methods are still largerly under development.
[edit] Literature
- Summary of the theoretical background of the passive seismic.
