Editing Seismometer (section)

=== 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.