- Seismic source
:"This article is about artificial seismic sources. For natural seismic sources, see
Earthquake, Volcano, and related articles."A seismic source generates controlled seismic energythat is used in both reflection and refraction seismic surveys. A seismic source can be simple, such as dynamite, or it can use more sophisticated technology, such as a specialized air gun. Seismic sources can provide single pulses or continuous pulses of energy that generates seismic waves, which travel through a medium such as wateror layers of rocks. Some of the waves then reflect and refract to receivers, such as geophones or hydrophones.
Seismic exploration using sound sources may be used to investigate shallow subsoil structure, for engineering site work, or deeper structures, usually in the search for oil, or for scientific investigation. The returning signals from the sources are detected by geophones, laid in known locations relative to the position of the source. The recorded signals are then subjected to specialist analysis and processing to yield comprehensible data.
A seismic source signal has the following characteristics:
# generated as an impulsive source
# the generated waves are time-varyingThe generalized equation that shows all above properties is::where is the maximum frequency component of the generated waveform. [Seismic Wave Propagation Modeling and Inversion, Phil Bording [http://www.ipp.mpg.de/de/for/bereiche/stellarator/Comp_sci/CompScience/csep/csep1.phy.ornl.gov/sw/sw.html] ]
Types of sources
Explosives, such as dynamite, can be used as crude but effective sources of seismic energy.
An air gun is used for marine reflection and refraction surveys. It consists of one or more
pneumaticchambers that are pressurized with air. The air gun array is submerged below the water surface, and is towed behind a ship. When the air gun is fired, a bolt is retracted, allowing the air to escape the chamber and to produce a pulse of acoustic energy.
Plasma sound source
A plasma sound source (PSS), otherwise called a spark gap sound source, is a means of making very low frequency sonar pulse underwater.
For each firing, it stores electric charge in a large high-voltage bank of
capacitors, and then releases all the stored energy in an arc across electrodes in the water. The underwater spark discharge produces a high-pressure plasma and vapor bubble, which expands and collapses, making a loud sound. Most of the sound produced is between 20 and 200 Hz.
The PSS has also been used for
sonar. There are also plans to use PSS as a non-lethal weapon against submerged divers.
A Thumper truck (or weight-drop) truck is a vehicle mounted ground impact which can used to provide the seismic source. A heavy weight is raised by a hoist at the back of the truck and dropped, possibly about three metres, to impact (or "Thump") the ground. To augment the signal, the weight may be dropped more than once at the same spot, the signal may also be increased by thumping at several nearby places in an array whose dimensions may be chosen to enhance the seismic signal by spatial filtering.
Thumping might be less damaging to the environment than firing explosives in shot-holes, though a heavily thumped seismic line with transverse ridges every few metres might create long-lasting disturbance of the soil. An advantage of the Thumper (later shared with Vibroseis) especially in politically unstable areas, was that no explosives were required.
Vibroseis is a method used to propagate energy signals into the
earthover an extended period of time as opposed to the near instantaneous energy provided by impulsive sources. The data recorded in this way must be "correlated" to convert the extended source signal into an impulse. The source signal using this method was originally generated by a servo-controlled hydraulic vibrator or "shaker unit" mounted on a mobile base unit, but electro-mechanical versions have also been developed.
Vibroseis was developed by the Continental Oil Company (Conoco) during the 1950s and was a trademark until the company's
Boomer sound sources are used for shallow water seismic surveys, mostly for engineering survey applications. Boomers are towed in a floating sled behind a survey vessel. Similarly to the plasma source, it stores energy in capacitors, but it discharges through a flat spiral coil instead of generating a spark. A copper plate adjacent to the coil flexes away from the coil as the capacitors are discharged. This flexing is transmitted into the water as the seismic pulse.
Originally the storage capacitors were placed in a steel container (the bang box) on the survey vessel. The high Voltages used, typically 3,000V, required heavy cables and strong safety containers. Recently, low Voltage boomers have become available. These use capacitors on the towed sled, allowing efficient energy recovery, lower Voltage power supplies and lighter cables. The low Voltage systems are generally easier to deploy and have fewer safety concerns.
* Crawford, J. M., Doty, W. E. N. and Lee, M. R., 1960, Continuous signal seismograph: Geophysics, Society of Exploration Geophysicists, 25, 95-105.
* Seismic Wave Propagation Modeling and Inversion, Phil Bording [http://www.ipp.mpg.de/de/for/bereiche/stellarator/Comp_sci/CompScience/csep/csep1.phy.ornl.gov/sw/sw.html]
* Derivation of Seismic wave equation can be found here. [http://www.ees.nmt.edu/Geop/Classes/GEOP523/Docs/waveeq.pdf]
* [http://www.naturalgas.org/naturalgas/exploration.asp Photos of Thumper trucks in action]
* [http://arctic.fws.gov/seismic.htm Arctic Refuge thumper trails]
* [http://www.sierraclub.org/ut/careforutah/p/DSCN4683.html Utah thumper trails]
* [http://www.spawar.navy.mil/sti/publications/pubs/td/3138/td3138cond.pdf Non-Lethal Swimmer Neutralization Study by The University of Texas, May 2002] page 42
* [http://www.vibroseis.com/ Vibroseis, Omnilaw International, a Texas Corporation]
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