Editing Seismometer (section)

== Modern instruments ==
[[File:LaCoste suspension seismometer principle.svg|thumb|left|120px|Simplified LaCoste suspension using a zero-length spring]]
[[File:CMG-40T Triaxial Broadband Seismometer.JPG|thumb|CMG-40T triaxial broadband seismometer]]
[[File:Seismometer awi hg.jpg|thumb|Seismometer without housing; presented during a demonstration for children about earthquakes at Alfred Wegener Institute.]]

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&nbsp;Hz to 0.00118&nbsp;Hz (1/500 = 0.002 seconds per cycle, to 1/0.00118 = 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 [[Lucien LaCoste|LaCoste]] suspension. The LaCoste suspension uses a [[Spring (device)#Zero-length springs|zero-length spring]] to provide a long period (high sensitivity).<ref>{{cite web |url=http://physics.mercer.edu/earthwaves/zero.html |title=Physics of the Zero-Length Spring of Geoscience |website=physics.mercer.edu |access-date=28 March 2018}}</ref><ref>{{Cite web |url=http://www.mssu.edu/seg-vm/bio_lucien_lacoste.html |archive-url=https://web.archive.org/web/20070320042235/http://www.mssu.edu/seg-vm/bio_lucien_lacoste.html |url-status=dead |title=A Biography of Lucien LaCoste, inventor of the zero-length spring |archive-date=March 20, 2007}}</ref> Some modern instruments use a [[Galperin configuration|"triaxial" or "Galperin" 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: [[geophone]]s, 50 to 750 [[volt|V]]/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 instruments 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 seismograph can be simply input to a computer.  It presents the data in a standard digital format (often "SE2" over [[Ethernet]]).

=== Teleseismometers ===
[[File:Seismometer kum hg.jpg|thumb|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 [http://www.mbari.org/mars/default.html Monterey Accelerated Research System].]]

The modern broadband seismograph can record a very broad range of [[frequency|frequencies]].  It consists of a small "proof mass", confined by electrical forces, driven by sophisticated [[electronics]].  As the earth moves, the electronics attempt to hold the mass steady through a [[feedback]] 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 [[peak ground acceleration|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]]. Some instruments use a [[variable capacitor|linear variable differential capacitor]].

That measurement is then amplified by [[electronic amplifier]]s attached to parts of an electronic [[PID loop|negative feedback loop]].  One of the amplified currents from the negative feedback loop drives a coil very like a [[loudspeaker]]. 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 Streckeisen 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]].

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]] 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 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 man-made structures, through [[earthquake engineering]]. The recordings of such instruments are crucial for the assessment of [[seismic hazard]], through [[Engineering Seismology|engineering seismology]].

A strong-motion seismometer measures acceleration. This can be mathematically [[Integral|integrated]] 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.

Strong motion sensors are used for intensity meter applications.

=== Other forms ===
[[File:Kinemetrics seismograph.jpg|thumb|A Kinemetrics seismograph, formerly used by the [[United States Department of the Interior]].]]

[[File:Seismograph measuring visitors stomping their feet (often deliberately) at the Thomas A. Jaggar Museum, Hawaiian Volcano Observatory.webm|thumb|Seismometer in operation recording a seismogram.]]

Accelerographs and [[geophone]]s are often heavy cylindrical magnets with a spring-mounted coil inside.  As the 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 1&nbsp;Hz.  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]] sponge or a conductive fluid through a [[magnetic field]].

=== Interconnected seismometers ===
Seismometers spaced in a [[seismic array]] can also be used to precisely locate, in three dimensions, the source of an earthquake, using the time it takes for [[seismic wave]]s to propagate away from the [[hypocenter]], the initiating point of [[Fault (geology)|fault]] rupture (See also [[Earthquake location]]).  Interconnected seismometers are also used, as part of the [[International Monitoring System]] to detect underground [[nuclear test]] explosions, as well as for [[Earthquake early warning]] systems. These seismometers are often used as part of a large-scale 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 [[Tomographic reconstruction|tomography]].  The data reduction methods resemble those of computer-aided tomographic medical imaging X-ray machines (CAT-scans), or imaging [[sonar]]s.

A worldwide 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 event]]s or [[nuclear explosion]]s 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".<ref>{{cite web |url=http://psn.quake.net/ |title=Redwood City Public Seismic Network |website=psn.quake.net |access-date=28 March 2018 |archive-date=26 March 2018 |archive-url=https://web.archive.org/web/20180326175018/http://psn.quake.net/ |url-status=dead}}</ref>

Seismographic systems used for petroleum or other mineral exploration historically used an explosive and a wireline of [[geophone]]s 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.

=== Fiber optic cables as seismometers ===
A new technique for detecting earthquakes has been found, using [[fiber optic]] cables.<ref><!-- {{Harvnb|Marra|Clivati|Luckett|Tampellini|2018 -->
{{Citation |first1=Giuseppe |last1=Marra |first2=Cecilia |last2=Clivati |first3=Richard |last3=Luckett |first4=Anna |last4=Tampellini |first5=Jochen |last5=Kronjäger |first6=Louise |last6=Wright |first7=Alberto |last7=Mura |first8=Filippo |last8=Levi |first9=Stephen |last9=Robinson |first10=André |last10=Xuereb |first11=Brian |last11=Baptie |first12=Davide |last12=Calonico |date=3 August 2016 |title=Ultrastable laser interferometry for earthquake detection with terrestrial and submarine cables |journal=Science |volume=361 |issue=6401 |pages=486–490 |doi=10.1126/science.aat4458 |pmid=29903881 |doi-access=free |hdl=11696/59747 |hdl-access=free}}.</ref>
In 2016 a team of metrologists running frequency [[metrology]] experiments in England observed noise with a wave-form resembling the seismic waves generated by earthquakes. This was found to match seismological observations of an {{m|w|6.0|link=y}} earthquake in Italy, ~1400&nbsp;km away. Further experiments in England, Italy, and with a submarine fiber optic cable to [[Malta]] detected additional earthquakes, including one 4,100&nbsp;km away, and an {{m|l|3.4|link=y}} earthquake 89&nbsp;km away from the cable.

Seismic waves are detectable because they cause [[micrometre|micrometer]]-scale changes in the length of the cable. As the length changes so does the time it takes a packet of light to traverse to the far end of the cable and back (using a second fiber). Using ultra-stable metrology-grade lasers, these extremely minute shifts of timing (on the order of [[femtosecond]]s) appear as phase-changes.

The point of the cable first disturbed by an earthquake's [[p wave]] (essentially a sound wave in rock) can be determined by sending packets in both directions in the looped pair of optical fibers; the difference in the arrival times of the first pair of perturbed packets indicates the distance along the cable. This point is also the point closest to the earthquake's epicenter, which should be on a plane perpendicular to the cable. The difference between the P wave/S wave arrival times provides a distance (under ideal conditions), constraining the epicenter to a circle. A second detection on a non-parallel cable is needed to resolve the ambiguity of the resulting solution. Additional observations constrain the location of the earthquake's epicenter, and may resolve the depth.

This technique is expected to be a boon in observing earthquakes, especially the smaller ones, in vast portions of the global ocean where there are no seismometers, and at much lower cost than ocean-bottom seismometers.

=== Deep-Learning ===
Researchers at Stanford University created a [[Deep learning|deep-learning]] algorithm called UrbanDenoiser which can detect earthquakes, particularly in urban cities.<ref name="deep1">{{Cite journal |last1=Yang |first1=Lei |last2=Liu |first2=Xin |last3=Zhu |first3=Weiqiang |last4=Zhao |first4=Liang |last5=Beroza |first5=Gregory C. |date=2022-04-15 |title=Toward improved urban earthquake monitoring through deep-learning-based noise suppression |journal=Science Advances |language=en |volume=8 |issue=15 |pages=eabl3564 |doi=10.1126/sciadv.abl3564 |pmid=35417238 |pmc=9007499 |bibcode=2022SciA....8L3564Y |issn=2375-2548}}</ref> The algorithm filters out the background noise from the seismic noise gathered from busy cities in urban areas to detect earthquakes.<ref name="deep1" /><ref>{{Cite web |title=A deep-learning algorithm could detect earthquakes by filtering out city noise |url=https://www.technologyreview.com/2022/04/13/1049763/a-deep-learning-algorithm-could-detect-earthquakes-by-filtering-out-city-noise/ |access-date=2022-04-17 |website=MIT Technology Review |language=en}}</ref>