Seismometers are instruments that measure motions of the ground, including those of seismic waves Seismic waves are waves of force that travel through the Earth or other elastic bodies, for example as a result of an earthquake, explosion, or some other process that imparts forces. Seismic waves are studied by seismologists, and measured by a seismograph, which records the output of a seismometer, or geophone. For seismic studies of oil generated by earthquakes An earthquake is the result of a sudden release of energy in the Earth's crust that creates seismic waves. Earthquakes are measured with a seismometer; a device which also records is known as a seismograph. The moment magnitude (or the related and mostly obsolete Richter magnitude) of an earthquake is conventionally reported, with magnitude 3 or, nuclear explosions However, depending on the design of the weapon and the environment in which it is detonated the energy distributed to these categories can be increased or decreased. The blast effect is created by the coupling of immense amounts of energy, spanning the electromagnetic spectrum, with the surroundings. Locations such as submarine, surface, airburst,, and other seismic sources. Records of seismic waves allow seismologists Seismology is the scientific study of earthquakes and the propagation of elastic waves through the Earth. The field also includes studies of earthquake effects, such as tsunamis as well as diverse seismic sources such as volcanic, tectonic, oceanic, atmospheric, and artificial processes (such as explosions). A related field that uses geology to to map the interior of the Earth, and locate and measure the size of these different sources.

The word derives from the Greek Greek , an independent branch of the Indo-European family of languages, is the language of the Greeks. Native to the southern Balkans, it has the longest documented history of any Indo-European language, spanning 34 centuries of written records. In its ancient form, it is the language of classical ancient Greek literature and the New Testament of σεισμός, seismós, a shaking or quake, from the verb σείω, seíō, to shake; and μέτρον, métron, measure.

Seismograph is another Greek term from seismós and γράφω, gráphō, to draw. It is often used to mean seismometer, though it is more applicable to the older instruments in which the measuring and recording of ground motion were combined than to modern systems, in which these functions are separated.

Both types provide a continuous record of ground motion; this distinguishes them from seismoscopes, which merely indicate that motion has occurred, perhaps with some simple measure of how large it was.[1]

Strong Motion seismometer that measures acceleration In physics, and more specifically kinematics, acceleration is the change in velocity over time. Because velocity is a vector, it can change in two ways: a change in magnitude and/or a change in direction. In one dimension, i.e. a line, acceleration is the rate at which something speeds up. However, as a vector quantity, acceleration is also the. This model is a K2 made by Kinemetrics and part of the Pacific Northwest Seismograph Network The Pacific Northwest Seismic Network, or PNSN collects and studies ground motions from about 400 seismometers in Oregon and Washington to monitor volcanic and tectonic activity, and gives advice and information, and works to mitigate earthquake hazard.

Contents

Basic principles

Inertial seismometers have:

Any motion of the ground moves the frame. The mass tends not to move because of its inertia Inertia is the resistance of any physical object to a change in its state of motion. It is represented numerically by an object's mass. The principle of inertia is one of the fundamental principles of classical physics which are used to describe the motion of matter and how it is affected by applied forces. Inertia comes from the Latin word, ", and by measuring the motion between the frame and the mass the motion of the ground can be determined, even though the mass does move.

Early seismometers used optical levers or mechanical linkages to amplify the small motions involved, recording on soot-covered paper or photographic paper.

Modern instruments use electronics. In some systems, the mass is held nearly motionless relative to the frame by an electronic negative feedback loop. The motion of the mass relative to the frame is measured, and the feedback loop Feedback describes the situation when output from an event or phenomenon in the past will influence an occurrence or occurrences of the same (i.e. same defined) event / phenomenon (or the continuation / development of the original phenomenon) in the present or future. When an event is part of a chain of cause-and-effect that forms a circuit or applies a magnetic or electrostatic force to keep the mass nearly motionless. The voltage needed to produce this force is the output of the seismometer, which is recorded digitally. In other systems the weight is allowed to move, and its motion produces a voltage in a coil attached to the mass and moving through the magnetic field of a magnet attached to the frame. This design is often used in the geophones A geophone is a device which converts ground movement 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 the earth used in seismic surveys for oil and gas.

Professional seismic observatories usually have instruments measuring three axes: north-south, east-west, and the vertical. If only one axis can be measured, this is usually the vertical because it is less noisy and gives better records of some seismic waves.

The foundation of a seismic station is critical.[2] A professional station is sometimes mounted on bedrock In stratigraphy, bedrock is the native consolidated rock underlying the surface of a terrestrial planet, usually the Earth. Above the bedrock is usually an area of broken and weathered unconsolidated rock in the basal subsoil. The top of the bedrock is known as rockhead and identifying this, via excavations, drilling or geophysical methods, is an. The best mountings may be in deep boreholes, which avoid thermal effects, ground noise and tilting from weather and tides. Other instruments are often mounted in insulated enclosures on small buried piers of unreinforced concrete. Reinforcing rods and aggregates would distort the pier as the temperature changes. A site should always be surveyed for ground noise with a temporary installation before pouring the pier and laying conduit.

Zhang Heng's seismoscope

Replica of Zhang Heng Zhang Heng (CE 78–139) was a Chinese astronomer, mathematician, inventor, geographer, cartographer, artist, poet, statesman and literary scholar from Nanyang, Henan. He lived during the Eastern Han Dynasty (CE 25–220) of China. He was educated in the capital cities of Luoyang and Chang'an, and began his career as a minor civil servant in's seismoscope Houfeng Didong Yi See also: List of Chinese inventions China has been the source of many significant inventions, including the Four Great Inventions of ancient China: papermaking, the compass, gunpowder, and printing . The list below contains these and other inventions

In 132 CE, Zhang Heng Zhang Heng (CE 78–139) was a Chinese astronomer, mathematician, inventor, geographer, cartographer, artist, poet, statesman and literary scholar from Nanyang, Henan. He lived during the Eastern Han Dynasty (CE 25–220) of China. He was educated in the capital cities of Luoyang and Chang'an, and began his career as a minor civil servant in of China's Han dynasty The Han Dynasty was the second imperial dynasty of China, preceded by the Qin Dynasty (221–206 BCE) and succeeded by the Three Kingdoms (220–265 CE). It was founded by the peasant rebel leader Liu Bang, known posthumously as Emperor Gaozu of Han. It was briefly interrupted by the Xin Dynasty (9–23 CE) of the former regent Wang Mang. This invented the first seismoscope (by the definition above), which was called Houfeng Didong Yi (literally, "instrument for measuring the seasonal winds and the movements of the Earth"). The description we have, from the History of the Later Han Dynasty The History of the Later Han is one of the official Chinese historical works which was compiled by Fan Ye in the 5th century, using a number of earlier histories and documents as sources. It covers the history of Eastern Han from 25 to 220 CE, says that it was a large bronze vessel, about 2 meters in diameter; at eight points around the top were dragon's heads holding bronze balls. When there was an earthquake, one of the mouths would open and drop its ball into a bronze toad at the base, making a sound and supposedly showing the direction of the earthquake. On at least one occasion, probably at the time of a large earthquake in Gansu Gansu (simplified Chinese: 甘肃; traditional Chinese: 甘肅; pinyin: Gānsù; Wade-Giles: Kan-su, Kansu, Kan-suh) is a province located in the northwest of the People's Republic of China. It lies between Qinghai, Inner Mongolia, and the Huangtu plateaus, and borders Mongolia to the north and Xinjiang to the west. The Yellow River passes the in 143 CE, the seismoscope indicated an earthquake even though one was not felt. The available text says that inside the vessel was a central column that could move along eight tracks; this is thought to refer to a pendulum, though it is not known exactly how this was linked to a mechanism that would open only one dragon's mouth. The first ever earthquake recorded by this seismograph was supposedly somewhere in the east. Days later, a rider from the east reported this earthquake.[3][4]

An early example

The principle can be shown by an early special purpose seismometer. This consisted of a large stationary pendulum When a pendulum is displaced from its resting equilibrium position, it is subject to a restoring force due to gravity that will accelerate it back toward the equilibrium position. When released, the restoring force combined with the pendulum's mass causes it to oscillate about the equilibrium position, swinging back and forth. The time for one, with a stylus A stylus is a writing utensil. The word is also used for a computer accessory (PDAs). It usually refers to a narrow elongated staff, similar to a modern ballpoint pen. Many styluses are heavily curved to be held more easily. Another widely-used writing tool is the stylus used by blind users in conjunction with the slate for punching out the dots on the bottom. As the earth Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets. It is sometimes referred to as the World, the Blue Planet,[note 6] or by its Latin name, Terra.[note 7] starts to move, the heavy mass of the pendulum has the inertia Inertia is the resistance of any physical object to a change in its state of motion. It is represented numerically by an object's mass. The principle of inertia is one of the fundamental principles of classical physics which are used to describe the motion of matter and how it is affected by applied forces. Inertia comes from the Latin word, " to stay still in the non-earth frame of reference A frame of reference in physics, may refer to a coordinate system or set of axes within which to measure the position, orientation, and other properties of objects in it, or it may refer to an observational reference frame tied to the state of motion of an observer. It may also refer to both an observational reference frame and an attached. The result is that the stylus scratches a pattern corresponding with the Earth's movement. This type of strong motion seismometer recorded upon a smoked glass (glass with carbon soot Soot is a general term that refers to impure carbon particles resulting from the incomplete combustion of a hydrocarbon. It is more properly restricted to the product of the gas-phase combustion process but is commonly extended to include the residual pyrolyzed fuel particles such as cenospheres, charred wood, petroleum coke, etc. that may become). While not sensitive enough to detect distant earthquakes, this instrument could indicate the direction of the pressure waves and thus help find the epicenter of a local earthquake — such instruments were useful in the analysis of the 1906 San Francisco earthquake The San Francisco earthquake of 1906 was a major earthquake that struck San Francisco, California, and the coast of Northern California at 5:12 a.m. on Wednesday, April 18, 1906. The most widely accepted estimate for the magnitude of the earthquake is a moment magnitude of 7.9; however, other values have been proposed, from 7.7 to as high as 8.25. Further re-analysis was performed in the 1980s using these early recordings, enabling a more precise determination of the initial fault break location in Marin county and its subsequent progression, mostly to the south.

Early designs

After 1880, most seismometers were descended from those developed by the team of John Milne, James Alfred Ewing Sir James Alfred Ewing KCB FRS FRSE MInstitCE was a Scottish physicist and engineer, best known for his work on the magnetic properties of metals and, in particular, for his discovery of, and coinage of the word, hysteresis and Thomas Gray At the British-influenced Imperial College of Engineering in Tokyo, he helped John Milne and James Alfred Ewing develop the first modern seismometers in Japan from 1880 to 1895. All three men worked as a team on the invention and use of seismographs, though John Milne is generally credited with the invention of the first modern horizontal-pendulum, who worked in Japan Japan is an island country in East Asia. Located in the Pacific Ocean, it lies to the east of the Sea of Japan, China, North Korea, South Korea and Russia, stretching from the Sea of Okhotsk in the north to the East China Sea and Taiwan in the south. The characters that make up Japan's name mean "sun-origin", which is why Japan is from 1880-1895. These seismometers used damped horizontal pendulums. After World War II, these were adapted into the widely used Press-Ewing seismometer.

Later, professional suites of instruments for the world-wide standard seismographic network had one set of instruments tuned to oscillate at fifteen seconds, and the other at ninety seconds, each set measuring in three directions. Amateurs or observatories with limited means tuned their smaller, less sensitive instruments to ten seconds. The basic damped horizontal pendulum seismometer swings like the gate of a fence. A heavy weight is mounted on the point of a long (from 10 cm to several meters) triangle, hinged at its vertical edge. As the ground moves, the weight stays unmoving, swinging the "gate" on the hinge.

The advantage of a horizontal pendulum is that it achieves very low frequencies of oscillation in a compact instrument. The "gate" is slightly tilted, so the weight tends to slowly return to a central position. The pendulum is adjusted (before the damping is installed) to oscillate once per three seconds, or once per thirty seconds. The general-purpose instruments of small stations or amateurs usually oscillate once per ten seconds. A pan of oil is placed under the arm, and a small sheet of metal mounted on the underside of the arm drags in the oil to damp oscillations. The level of oil, position on the arm, and angle and size of sheet is adjusted until the damping is "critical," that is, almost having oscillation. The hinge is very low friction, often torsion wires, so the only friction is the internal friction of the wire. Small seismographs with low proof masses are placed in a vacuum to reduce disturbances from air currents.

Zollner described torsionally-suspended horizontal pendulums as early as 1869, but developed them for gravimetry rather than seismometry.

Early seismometers had an arrangement of levers on jeweled bearings, to scratch smoked glass or paper. Later, mirrors reflected a light beam to a direct-recording plate or roll of photographic paper. Briefly, some designs returned to mechanical movements to save money. In mid-twentieth-century systems, the light was reflected to a pair of differential electronic photosensors called a photomultiplier. The voltage generated in the photomultiplier was used to drive galvanometers which had a small mirror mounted on the axis. The moving reflected light beam would strike the surface of the turning drum, which was covered with photo-sensitive paper. The expense of developing photo sensitive paper caused many seismic observatories to switch to ink or thermal-sensitive paper.

Another relatively simple device was used in the late 19th and early 20th century. This consisted of a pendulum free to swing in any direction, with a scribe at the bottom touching a smoked glass plate. While not providing time information or information on distant earthquakes these did give accurate initial shock directions and proved useful in a late 20th century analysis of the 1906 San Francisco earthquake The San Francisco earthquake of 1906 was a major earthquake that struck San Francisco, California, and the coast of Northern California at 5:12 a.m. on Wednesday, April 18, 1906. The most widely accepted estimate for the magnitude of the earthquake is a moment magnitude of 7.9; however, other values have been proposed, from 7.7 to as high as 8.25.

Modern instruments

Modern instruments use electronic sensors, amplifiers, and recording devices. Most are broadband covering a wide range of frequencies. Some seismometers can measure motions with frequencies from 500 Hz (0.5 seconds per cycle) to 1/850 Hz (850 seconds per cycle). The mechanical suspension for horizontal instruments remains the garden-gate described above. Vertical instruments use some kind of constant-force suspension, such as the LaCoste suspension. The LaCoste suspension uses a zero-length spring In mechanics, and physics, Hooke's law of elasticity is an approximation that states that the extension of a spring is in direct proportion with the load added to it as long as this load does not exceed the elastic limit. Materials for which Hooke's law is a useful approximation are known as linear-elastic or "Hookean" materials. Hooke's to provide a long period (high sensitivity).[5] [6] Some modern instruments use a "triaxial" design, in which three identical motion sensors are set at the same angle to the vertical but 120 degrees apart on the horizontal. Vertical and horizontal motions can be computed from the outputs of the three sensors.

Seismometers unavoidably introduce some distortion into the signals they measure, but professionally-designed systems have carefully characterized frequency transforms.

Modern sensitivities come in three broad ranges: geophones A geophone is a device which converts ground movement 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 the earth, 50 to 750 V The volt is the SI derived unit of electromotive force, commonly called "voltage". It is also the unit for the related but slightly different quantity electric potential difference (also called "electrostatic potential difference"). It is named in honor of the Italian physicist Alessandro Volta (1745–1827), who invented the/m; local geologic seismographs, about 1,500 V/m; and teleseismographs, used for world survey, about 20,000 V/m. Instruments come in three main varieties: short period, long period and broadband. The short and long period measure velocity and are very sensitive, however they 'clip' the signal or go off-scale for ground motion that is strong enough to be felt by people. A 24-bit analog-to-digital conversion channel is commonplace. Practical devices are linear to roughly one part per million.

Delivered seismometers come with two styles of output: analog and digital. Analog seismographs require analog recording equipment, possibly including an analog-to-digital converter. The output of a digital seismographs can be simply input to a computer. They present the data in standard digital forms (often "SE2" over ethernet Ethernet is a family of frame-based computer networking technologies for local area networks . The name came from the physical concept of the ether. It defines a number of wiring and signaling standards for the Physical Layer of the OSI networking model as well as a common addressing format and Media Access Control at the Data Link Layer).

Teleseismometers

A low-frequency 3-direction ocean-bottom seismometer (cover removed). Two masses for x- and y-direction can be seen, the third one for z-direction is below. This model is a CMG-40TOBS, manufactured by Güralp Systems Ltd and is part of the Monterey Accelerated Research System.

The modern broadband seismograph can record a very broad range of frequencies Frequency is the number of occurrences of a repeating event per unit time. It is also referred to as temporal frequency. The period is the duration of one cycle in a repeating event, so the period is the reciprocal of the frequency. Loosely speaking, 1 year is the period of the Earth's orbit around the Sun, and the Earth's rotation on its axis has. It consists of a small "proof mass", confined by electrical forces, driven by sophisticated electronics Electronics is the branch of science and technology which makes use of the controlled motion of electrons through different media and vacuum. The ability to control electron flow is usually applied to information handling or device control. Electronics is distinct from electrical science and technology, which deals with the generation,. As the earth moves, the electronics attempt to hold the mass steady through a feedback Feedback describes the situation when output from an event or phenomenon in the past will influence an occurrence or occurrences of the same (i.e. same defined) event / phenomenon (or the continuation / development of the original phenomenon) in the present or future. When an event is part of a chain of cause-and-effect that forms a circuit or circuit. The amount of force necessary to achieve this is then recorded.

In most designs the electronics holds a mass motionless relative to the frame. This device is called a "force balance accelerometer". It measures acceleration instead of velocity of ground movement. Basically, the distance between the mass and some part of the frame is measured very precisely, by a linear variable differential transformer The linear variable differential transformer is a type of electrical transformer used for measuring linear displacement. The transformer has three solenoidal coils placed end-to-end around a tube. The centre coil is the primary, and the two outer coils are the secondaries. A cylindrical ferromagnetic core, attached to the object whose position is. Some instruments use a linear variable differential capacitor.

That measurement is then amplified by electronic amplifiers An electronic amplifier is a device for increasing the power of a signal. It does this by taking energy from a power supply and controlling the output to match the input signal shape but with a larger amplitude. In this sense, an amplifier may be considered as modulating the output of the power supply attached to parts of an electronic negative feedback loop. One of the amplified currents from the negative feedback loop drives a coil very like a loudspeaker A loudspeaker is an electroacoustic transducer that converts an electrical signal into sound. The speaker moves in accordance with the variations of an electrical signal and causes sound waves to propagate through a medium such as air or water, except that the coil is attached to the mass, and the magnet is mounted on the frame. The result is that the mass stays nearly motionless.

Most instruments measure directly the ground motion using the distance sensor. The voltage generated in a sense coil on the mass by the magnet directly measures the instantaneous velocity of the ground. The current to the drive coil provides a sensitive, accurate measurement of the force between the mass and frame, thus measuring directly the ground's acceleration (using f=ma where f=force, m=mass, a=acceleration).

One of the continuing problems with sensitive vertical seismographs is the buoyancy of their masses. The uneven changes in pressure caused by wind blowing on an open window can easily change the density of the air in a room enough to cause a vertical seismograph to show spurious signals. Therefore, most professional seismographs are sealed in rigid gas-tight enclosures. For example, this is why a common Streckheisen model has a thick glass base that must be glued to its pier without bubbles in the glue.

It might seem logical to make the heavy magnet serve as a mass, but that subjects the seismograph to errors when the Earth's magnetic field moves. This is also why seismograph's moving parts are constructed from a material that interacts minimally with magnetic fields. A seismograph is also sensitive to changes in temperature so many instruments are constructed from low expansion materials such as nonmagnetic invar Invar, also known generically as FeNi36 , is a nickel steel alloy notable for its uniquely low coefficient of thermal expansion (CTE or α). It was invented in 1896 by Swiss scientist Charles Édouard Guillaume. He received the Nobel Prize in Physics in 1920 for this discovery, which shows the importance of this alloy in scientific instruments. &.

The hinges on a seismograph are usually patented, and by the time the patent has expired, the design has been improved. The most successful public domain designs use thin foil hinges in a clamp.

Another issue is that the transfer function A transfer function is a mathematical representation, in terms of spatial or temporal frequency, of the relation between the input and output of a (linear time-invariant) system. With optical imaging devices, for example, it is the Fourier transform of the point spread function (hence a function of spatial frequency) i.e. the intensity of a seismograph must be accurately characterized, so that its frequency response is known. This is often the crucial difference between professional and amateur instruments. Most instruments are characterized on a variable frequency shaking table.

Strong-motion seismometers

Another type of seismometer is a digital strong-motion seismometer, or accelerograph. The data from such an instrument is essential to understand how an earthquake affects manmade structures.

A strong-motion seismometer measures acceleration. This can be mathematically integrated Integration is an important concept in mathematics and, together with differentiation, is one of the two main operations in calculus. Given a function ƒ of a real variable x and an interval [a, b] of the real line, the definite integral later to give velocity and position. Strong-motion seismometers are not as sensitive to ground motions as teleseismic instruments but they stay on scale during the strongest seismic shaking.

Other forms

A Kinemetrics seismograph, formerly used by the United States Department of the Interior The United States Department of the Interior is the United States federal executive department of the U.S. government responsible for the management and conservation of most federal land and the administration of programs relating to Native Americans, Alaska Natives, and Native Hawaiians, and to insular areas of the United States.

Accelerographs and geophones A geophone is a device which converts ground movement 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 the earth are often heavy cylindrical magnets with a spring-mounted coil inside. As case moves, the coil tends to stay stationary, so the magnetic field cuts the wires, inducing current in the output wires. They receive frequencies from several hundred hertz down to 4.5 Hz or even as low as 1 Hz with higher quality models. Some have electronic damping, a low-budget way to get some of the performance of the closed-loop wide-band geologic seismographs.

Strain-beam accelerometers constructed as integrated circuits are too insensitive for geologic seismographs (2002), but are widely used in geophones.

Some other sensitive designs measure the current generated by the flow of a non-corrosive ionic fluid through an electret Electret is a dielectric material that has a quasi-permanent electric charge or dipole polarisation. An electret generates internal and external electric fields, and is the electrostatic equivalent of a permanent magnet. Oliver Heaviside coined this term in 1885. Materials with electret properties were, however, already studied since the early 18 sponge or a conductive fluid through a magnetic field Magnetic fields surround magnetic materials and electric currents and are detected by the force they exert on other magnetic materials and moving electric charges. The magnetic field at any given point is specified by both a direction and a magnitude ; as such it is a vector field.

Modern recording

Today, the most common recorder is a computer with an analog-to-digital converter, a disk drive and an internet connection; for amateurs, a PC with a sound card and associated software is adequate. Most systems record continuously, but some record only when a signal is detected, as shown by a short-term increase in the variation of the signal, compared to its long-term average (which can vary slowly because of changes in seismic noise).

Interconnected seismometers

Seismometers spaced in an array can also be used to precisely locate, in three dimensions, the source of an earthquake, using the time it takes for seismic waves Seismic waves are waves of force that travel through the Earth or other elastic bodies, for example as a result of an earthquake, explosion, or some other process that imparts forces. Seismic waves are studied by seismologists, and measured by a seismograph, which records the output of a seismometer, or geophone. For seismic studies of oil to propagate away from the hypocenter The hypocenter or hypocentre , refers to the site of an earthquake or a nuclear explosion. In the former, it is a synonym of the focus; in the latter, of ground zero, the initiating point of fault In geology, a fault is a planar fracture or discontinuity in a volume of rock, across which there has been significant displacement. Large faults within the Earth's crust result from the action of tectonic forces. Energy release associated with rapid movement on active faults is the cause of most earthquakes rupture (See also Earthquake location The primary purpose of a seismometer is to locate the initiating points of earthquake epicenters. The secondary purpose, of determining the 'size' or Moment magnitude scale must be calculated after the precise location is known). Interconnected seismometers are also used to detect underground nuclear A nuclear weapon is an explosive device that derives its destructive force from nuclear reactions, either fission or a combination of fission and fusion. Both reactions release vast quantities of energy from relatively small amounts of matter; a modern thermonuclear weapon weighing little more than a thousand kilograms can produce an explosion test explosions. These seismometer are often used as part of a large scale, multi-million dollar governmental or scientific project, but some organizations, such as the Quake-Catcher Network, can use residential size detectors built into computers to detect earthquakes as well.

In reflection seismology, an array of seismometers image sub-surface features. The data are reduced to images using algorithms similar to tomography. The data reduction methods resemble those of computer-aided tomographic medical imaging X-ray machines (CAT-scans), or imaging sonars.

A world-wide array of seismometers can actually image the interior of the Earth in wave-speed and transmissivity. This type of system uses events such as earthquakes, impact events or nuclear explosions as wave sources. The first efforts at this method used manual data reduction from paper seismograph charts. Modern digital seismograph records are better adapted to direct computer use. With inexpensive seismometer designs and internet access, amateurs and small institutions have even formed a "public seismograph network."[7]

Seismographic systems used for petroleum or other mineral exploration historically used an explosive and a wireline of geophones unrolled behind a truck. Now most short-range systems use "thumpers" that hit the ground, and some small commercial systems have such good digital signal processing that a few sledgehammer strikes provide enough signal for short-distance refractive surveys. Exotic cross or two-dimensional arrays of geophones are sometimes used to perform three-dimensional reflective imaging of subsurface features. Basic linear refractive geomapping software (once a black art) is available off-the-shelf, running on laptop computers, using strings as small as three geophones. Some systems now come in an 18" (0.5 m) plastic field case with a computer, display and printer in the cover.

Small seismic imaging systems are now sufficiently inexpensive to be used by civil engineers to survey foundation sites, locate bedrock, and find subsurface water.

See also

References

  1. ^ Richter, C.F. (1958). Elementary Seismology. San Francisco: W.H. Freeman.
  2. ^ Erhard Wielandt's 'Seismic Sensors and their Calibration'- Current (2002) reference by a widely consulted expert.
  3. ^ Sleeswyk AW, Sivin N (1983). "Dragons and toads: the Chinese seismoscope of BC. 132". Chinese Science 6: 1–19.
  4. ^ Needham, Joseph (1959). Science and Civilization in China, Volume 3: Mathematics and the Sciences of the Heavens and the Earth. Cambridge: Cambridge University Press. pp. 626–635.
  5. ^ Zero Length Springs in Seismographs
  6. ^ A Biography of Lucien LaCoste, inventor of the zero-length spring
  7. ^ Public seismograph network- many resources for amateurs and underfunded institutions

External links

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