Seismometer

Seismometer (in Greek seismos = earthquake and metero = measure) are used by seismologists to measure and record seismic waves. By studying seismic waves, geologists can map the interior of the Earth, and measure and locate earthquakes and other ground motions. The term seismograph is usually interchangeable, but seismometer seems to be a more common useage.

Locating an event

Pressure and shear waves

An earthquake induces two types of seismic waves in the ground - a pressure wave (called the P wave) and a number of shear waves (called S waves). The P wave can be likened to a sound wave, alternately compressing and rarifying (by a small amount) the density of rock, while the S wave is more akin to the ripples on a still pond when a stone is dropped into it. These two waves travel at different velocities (the P wave is much faster). The P wave can also travel directly, deep in the earth, while the S wave is a surface wave. With appropriate adjustments the difference in time of arrival of these two waves at a seismic observatory can be used to determine the distance to the event. Observations from at least three stations may be used to determine the event location.

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When an earthquake occurs, seismographs near the epicenter are able to record both S and P waves, but those on the other side of the Earth can only record P waves. S waves cannot pass through liquids, because liquids do not transmit shear. This is how Oldham proved that the Earth had a liquid core, and the moon and Mars were proven to have solid cores.

Related Topics:
Epicenter - Oldham

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Before the advent of precise electronic clocks and modern high-speed electronic communications this was the principal method of determining the location of distant events.

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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 to propagate away from the epicenter, the point of fault rupture. Interconnected seismometers are also used to detect underground nuclear test explosions.

Related Topics:
Seismic wave - Epicenter - Fault - Nuclear

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In seismography, an array of seismometers images 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.

Related Topics:
Seismography - Tomography - Sonar

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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 seimograph network." (See references).

Related Topics:
Impact event - Nuclear explosion

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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 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" plastic field case with a computer, display and printer in the cover!

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Small, inexpensive seismic imaging is now sufficiently inexpensive that it is used by civil engineers to survey foundation sites, locate bedrock, and find subsurface water.

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