A seismic source is a device that generates controlled
seismic
Seismology (; from Ancient Greek σεισμός (''seismós'') meaning "earthquake" and -λογία (''-logía'') meaning "study of") is the scientific study of earthquakes and the propagation of elastic waves through the Earth or through other ...
energy
In physics, energy (from Ancient Greek: ἐνέργεια, ''enérgeia'', “activity”) is the quantitative property that is transferred to a body or to a physical system, recognizable in the performance of work and in the form of heat a ...
used to perform both
reflection Reflection or reflexion may refer to:
Science and technology
* Reflection (physics), a common wave phenomenon
** Specular reflection, reflection from a smooth surface
*** Mirror image, a reflection in a mirror or in water
** Signal reflection, in s ...
and
refraction
In physics, refraction is the redirection of a wave as it passes from one medium to another. The redirection can be caused by the wave's change in speed or by a change in the medium. Refraction of light is the most commonly observed phenomeno ...
seismic surveys. A seismic source can be simple, such as
dynamite
Dynamite is an explosive made of nitroglycerin, sorbents (such as powdered shells or clay), and Stabilizer (chemistry), stabilizers. It was invented by the Swedish people, Swedish chemist and engineer Alfred Nobel in Geesthacht, Northern Germa ...
, or it can use more sophisticated technology, such as a specialized air gun. Seismic sources can provide single pulses or continuous sweeps of energy, generating
seismic wave
A seismic wave is a wave of acoustic energy that travels through the Earth. It can result from an earthquake, volcanic eruption, magma movement, a large landslide, and a large man-made explosion that produces low-frequency acoustic energy. S ...
s, which travel through a
medium
Medium may refer to:
Science and technology
Aviation
*Medium bomber, a class of war plane
* Tecma Medium, a French hang glider design
Communication
* Media (communication), tools used to store and deliver information or data
* Medium of ...
such as
water
Water (chemical formula ) is an inorganic, transparent, tasteless, odorless, and nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the fluids of all known living organisms (in which it acts as a ...
or layers of
rock
Rock most often refers to:
* Rock (geology), a naturally occurring solid aggregate of minerals or mineraloids
* Rock music, a genre of popular music
Rock or Rocks may also refer to:
Places United Kingdom
* Rock, Caerphilly, a location in Wales ...
s. Some of the waves then
reflect and
refract
In physics, refraction is the redirection of a wave as it passes from one medium to another. The redirection can be caused by the wave's change in speed or by a change in the medium. Refraction of light is the most commonly observed phenomeno ...
and are recorded by receivers, such as
geophone
A geophone is a device that converts ground movement (velocity) into voltage, which may be recorded at a recording station. The deviation of this measured voltage from the base line is called the seismic response and is analyzed for structure of ...
s or
hydrophone
A hydrophone ( grc, ὕδωρ + φωνή, , water + sound) is a microphone designed to be used underwater for recording or listening to underwater sound. Most hydrophones are based on a piezoelectric transducer that generates an electric potenti ...
s.
Seismic sources may be used to investigate shallow
subsoil
Subsoil is the layer of soil under the topsoil on the surface of the ground. Like topsoil, it is composed of a variable mixture of small particles such as sand, silt and clay, but with a much lower percentage of organic matter and humus, and it ...
structure, for engineering site characterization, or to study deeper structures, either in the search for petroleum and mineral deposits, or to map subsurface faults or for other scientific investigations. The returning signals from the sources are detected by seismic sensors (
geophone
A geophone is a device that converts ground movement (velocity) into voltage, which may be recorded at a recording station. The deviation of this measured voltage from the base line is called the seismic response and is analyzed for structure of ...
s or
hydrophone
A hydrophone ( grc, ὕδωρ + φωνή, , water + sound) is a microphone designed to be used underwater for recording or listening to underwater sound. Most hydrophones are based on a piezoelectric transducer that generates an electric potenti ...
s) in known locations relative to the position of the source. The recorded signals are then subjected to specialist processing and interpretation to yield comprehensible information about the subsurface.
Source model
A seismic source signal has the following characteristics:
# Generates an
impulse
Impulse or Impulsive may refer to:
Science
* Impulse (physics), in mechanics, the change of momentum of an object; the integral of a force with respect to time
* Impulse noise (disambiguation)
* Specific impulse, the change in momentum per uni ...
signal
# Band-limited
# The generated waves are time-varying
The generalized equation that shows all above properties is:
:
where
is the maximum frequency component of the generated waveform.
Types of sources
Sledgehammer
The most basic seismic source is a
sledgehammer
A sledgehammer is a tool with a large, flat, often metal head, attached to a long handle. The long handle combined with a heavy head allows the sledgehammer to gather momentum during a swing and apply a large force compared to hammers designed t ...
. A seismic energy is generated either by striking the ground directly, or more commonly striking a metal or polyethylene plate on the ground. Typically applied for near-surface
seismic refraction
Seismic refraction is a geophysical principle governed by Snell's Law of refraction. The seismic refraction method utilizes the refraction of seismic waves by rock or soil layers to characterize the subsurface geologic conditions and geologic str ...
surveys. Impact of sledgehammer contact with the surface can provide sufficient seismic energy for interface depths up to 30 m or more, depending on geological conditions and physical properties.
Explosives
Explosive
An explosive (or explosive material) is a reactive substance that contains a great amount of potential energy that can produce an explosion if released suddenly, usually accompanied by the production of light, heat, sound, and pressure. An expl ...
s most widely used as seismic sources are known as gelatin
dynamite
Dynamite is an explosive made of nitroglycerin, sorbents (such as powdered shells or clay), and Stabilizer (chemistry), stabilizers. It was invented by the Swedish people, Swedish chemist and engineer Alfred Nobel in Geesthacht, Northern Germa ...
s. These dynamites are placed into three subcategories, straight gelatins in which
nitroglycerin
Nitroglycerin (NG), (alternative spelling of nitroglycerine) also known as trinitroglycerin (TNG), nitro, glyceryl trinitrate (GTN), or 1,2,3-trinitroxypropane, is a dense, colorless, oily, explosive liquid most commonly produced by nitrating g ...
, also known as glyceryl trinitrate with the chemical formula C
3H
5(ONO
2)
3 is the active component, ammonia gelatins in which ammonia nitrite with chemical formula as the active component, and semi gelatins in which the composition consists mostly of nitroglycerin.
Upon detonation, explosives release large volumes of expanding gas very quickly, forcing great pressure to the surroundings in the form of seismic waves.
Using explosives as seismic sources has been in practice for decades because of the reliability and energy efficiency they provide.
Such sources are most commonly used on land and swampy environments because of high thickness in sediments. Typical charge sizes used in the field for reflection surveys are 0.25 kg to 100 kg for single hole sources, 0.25 kg to 250 kg or more for multiple hole sources, and may reach 2500 kg or more for refraction surveys.
Though dynamites and other explosives are efficient seismic sources because of their reduced costs, ease of transport in difficult terrains, and lack of regular maintenance compared to other sources, the use of explosives is becoming restricted in certain areas, causing decline and increasing popularity for alternative seismic sources.
For instance,
hexanitrostilbene
Hexanitrostilbene (HNS), also called JD-X, is an organic compound with the formula O2N)3C6H2CHsub>2. It is a yellow-orange solid.Jacques Boileau, Claude Fauquignon, Bernard Hueber and Hans H. Meyer "Explosives" in ''Ullmann's Encyclopedia of In ...
was the main explosive fill in the ''thumper''
mortar round canisters used as part of the
Apollo Lunar Active Seismic Experiments. Generally, the explosive charges are placed between below ground, in a hole that is drilled with dedicated drilling equipment for this purpose. This type of seismic drilling is often referred to as "Shot Hole Drilling". A common drill rig used for "Shot Hole Drilling" is the ARDCO C-1000 drill mounted on an ARDCO K 4X4 buggy. These drill rigs often use water or air to assist the drilling.
Air gun
![Seismologist with air gun array](https://upload.wikimedia.org/wikipedia/commons/4/45/Seismologist_with_air_gun_array.jpg)
An air gun is used for marine
reflection Reflection or reflexion may refer to:
Science and technology
* Reflection (physics), a common wave phenomenon
** Specular reflection, reflection from a smooth surface
*** Mirror image, a reflection in a mirror or in water
** Signal reflection, in s ...
and refraction surveys. It consists of one or more
pneumatic
Pneumatics (from Greek ‘wind, breath’) is a branch of engineering that makes use of gas or pressurized air.
Pneumatic systems used in Industrial sector, industry are commonly powered by compressed air or compressed inert gases. A central ...
chambers that are pressurized with compressed air at pressures from 14 to 21 MPa (2000 to 3000 lbf/in
2). Air guns are submerged below the water surface and towed behind a seismic ship. When an air gun is fired, a solenoid is triggered which releases high pressure air from one chamber to the back of a shuttle that is normally held in balance between the two equally pressurised chambers. The instant lowering of air pressure in the first chamber allows the shuttle to move rapidly into the first chamber, releasing a high pressure air reservoir that is behind the shuttle in the second chamber through ports directly into the sea producing a pulse of
acoustic energy. Air gun arrays may consist of up to 48 individual air guns with different size chambers or certain air guns volumes may be clustered together. The firing of all of the array is controlled by gun controller and is usually done to within a ± 1 or 2 millisecond tolerance, the aim being to create the optimum initial shock wave followed by the minimum reverberation of the air bubble(s). Since the shuttle is magnetised, the rapid movement into the first chamber on releasing the solenoid value provides a small current that is in effect a timing signal for the firing gun that is returned to the gun controller. A near-field
hydrophone
A hydrophone ( grc, ὕδωρ + φωνή, , water + sound) is a microphone designed to be used underwater for recording or listening to underwater sound. Most hydrophones are based on a piezoelectric transducer that generates an electric potenti ...
located at a known measured distance from the gun port can also be used to time the first break signal into the hydrophone for accurate gun timing verification.
Air gun maintenance is important as guns can misfire; the worst case scenario being an auto-fire where the gun actually fires repeatedly out of synch because of a defect in the gun itself such as a damaged solenoid valve or a leaking gun O-ring. A single auto-firing gun can result in the total array bubble signature becoming corrupted and if undetected, can result in many seismic lines being re-shot just for one auto-firing gun when the fault is found during initial data processing.
During normal handling for deployment and recovery, air guns must never be fully pressurised to their optimum working pressure on deck and it is normal practice to air down guns to 500 psi to prevent water ingress on deployment and recovery. It is also a poor and dangerous practice to test fire guns on deck in the air at pressure. There must also be an isolation system in place to prevent the accidental firing of guns on deck by observers or navigators by mistake. High pressure air releases on deck can amputate fingers and also result in a
high pressure injection injury
A high pressure injection injury is an injury caused by high-pressure injection of oil, grease, diesel fuel, gasoline, solvents, water, or even air, into the body. The most common causes are accidents with grease guns, paint sprayers, and pressu ...
through the skin, an almost untreatable and deadly injury in a seismic environment. Gunners should wear the required
personal protective equipment
Personal protective equipment (PPE) is protective clothing, helmets, goggles, or other garments or equipment designed to protect the wearer's body from injury or infection. The hazards addressed by protective equipment include physical, elec ...
to protect their eyes and their hearing and minimise exposure of uncovered skin.
Air guns are made from the highest grades of corrosion resistant stainless steel. Large chambers (i.e., greater than 1 L or 70 cu in) tend to give low frequency signals, and the small chambers (less than 1 L) give higher frequency signals.
Plasma sound source
A plasma sound source (PSS), otherwise called a spark gap sound source, or simply a sparker, is a means of making a very low frequency
sonar
Sonar (sound navigation and ranging or sonic navigation and ranging) is a technique that uses sound propagation (usually underwater, as in submarine navigation) to navigation, navigate, measure distances (ranging), communicate with or detect o ...
pulse underwater. For each firing, electric charge is stored in a large high-voltage bank of
capacitor
A capacitor is a device that stores electrical energy in an electric field by virtue of accumulating electric charges on two close surfaces insulated from each other. It is a passive electronic component with two terminals.
The effect of ...
s, and then released 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, useful for both
seismic
Seismology (; from Ancient Greek σεισμός (''seismós'') meaning "earthquake" and -λογία (''-logía'') meaning "study of") is the scientific study of earthquakes and the propagation of elastic waves through the Earth or through other ...
and
sonar
Sonar (sound navigation and ranging or sonic navigation and ranging) is a technique that uses sound propagation (usually underwater, as in submarine navigation) to navigation, navigate, measure distances (ranging), communicate with or detect o ...
applications.
There are also plans to use PSS as a
non-lethal weapon against submerged divers.
Thumper truck
![Thumper trucks, Noble Energy](https://upload.wikimedia.org/wikipedia/commons/0/0c/Thumper_trucks%2C_Noble_Energy.jpg)
In 1953, the weight dropping thumper technique was introduced as an alternative to dynamite sources.
![Vibroseis 2](https://upload.wikimedia.org/wikipedia/commons/7/7a/Vibroseis_2.jpg)
A thumper truck (or weight-drop) truck is a vehicle-mounted ground impact system which can be used to provide a seismic source. A heavy weight is raised by a hoist at the back of the truck and dropped, generally about three meters, 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.
More advanced thumpers use a technology called "Accelerated Weight Drop" (AWD), where a high pressure gas (min 7 MPa (1000 lbf/in
2)) is used to accelerate a heavy weight hammer () to hit a base plate coupled to the ground from a distance of . Several thumps are stacked to enhance signal to noise ratio. AWD allows both more energy and more control of the source than gravitational weight-drop, providing better depth penetration, control of signal frequency content.
Thumping may be less damaging to the environment than firing explosives in shot-holes, though a heavily thumped seismic line with transverse ridges every few meters might create long-lasting disturbance of the soil. An advantage of the thumper (later shared with Vibroseis), especially in politically unstable areas, is that no explosives are required.
Electromagnetic Pulse Energy Source (Non-Explosive)
EMP sources based on the electrodynamic and electromagnetic principles.
Seismic vibrator
A
seismic vibrator
A seismic vibrator is a truck-mounted or buggy-mounted device that is capable of injecting low-frequency vibrations into the earth. It is one of a number of seismic sources used in reflection seismology. The ‘Vibroseis’ exploration technique ...
propagates energy signals into the
Earth
Earth is the third planet from the Sun and the only astronomical object known to harbor life. While large volumes of water can be found throughout the Solar System, only Earth sustains liquid surface water. About 71% of Earth's surfa ...
over 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
In engineering, electromechanics combines processes and procedures drawn from electrical engineering and mechanical engineering. Electromechanics focuses on the interaction of electrical and mechanical systems as a whole and how the two systems ...
versions have also been developed.
The "Vibroseis" exploration technique was developed by the
Continental Oil Company (Conoco) during the 1950s and was a trademark until the company's
patent
A patent is a type of intellectual property that gives its owner the legal right to exclude others from making, using, or selling an invention for a limited period of time in exchange for publishing an enabling disclosure of the invention."A p ...
lapsed.
Boomer sources
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. Similar to the plasma source, a boomer source 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 3,000 V, 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.
Noise sources
Correlation-based processing techniques also enable seismologists to image the interior of the Earth at multiple scales using natural (e.g., the oceanic microseism) or artificial (e.g., urban) background noise as a seismic source.
[R.E. Sheriff (2002) p295] For example, under ideal conditions of uniform seismic illumination, the correlation of the noise signals between two seismographs provides an estimate of the bidirectional seismic
impulse response
In signal processing and control theory, the impulse response, or impulse response function (IRF), of a dynamic system is its output when presented with a brief input signal, called an Dirac delta function, impulse (). More generally, an impulse ...
.
See also
*
Reflection seismology
Reflection seismology (or seismic reflection) is a method of exploration geophysics that uses the principles of seismology to estimate the properties of the Earth's subsurface from reflected seismic waves. The method requires a controlled seismi ...
*
Seismic refraction
Seismic refraction is a geophysical principle governed by Snell's Law of refraction. The seismic refraction method utilizes the refraction of seismic waves by rock or soil layers to characterize the subsurface geologic conditions and geologic str ...
References
Bibliography
* Crawford, J. M., Doty, W. E. N. and Lee, M. R., 1960, Continuous signal seismograph: Geophysics, Society of Exploration Geophysicists, 25, 95–105.
* Robert E. Sheriff, Encyclopedic Dictionary of Applied Geophysics (Geophysical References No. 13) 4th Edition, 2002, 429 pag. .
*
* Seismic Wave Propagation Modeling and Inversion, Phil Bordin
* Derivation of Seismic wave equation can be found here
External links
Photos of Thumper trucks in actionNon-Lethal Swimmer Neutralization Study by The University of Texas, May 2002page 42
Vibroseis, Omnilaw International, a Texas CorporationIllustration of Vibroseis in 3D land seismic acquisition
{{DEFAULTSORT:Seismic Source
Sonar
Seismology measurement