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.

Modern instruments

Modern instruments use electronic sensors, amplifiers, and recording instruments. Most are broadband, operating on a wide range of frequencies. Some commercially-available research seismometers receive frequencies from 30Hz (0.03 seconds per cycle) to 1/850 Hz (850 seconds per cycle). Seismometers unavoidably introduce some distortion into the signals they measure, but professionally-designed systems have carefully-characterized frequency transforms. Sensitivities come in three broad ranges: geophones, 50 to 750V/meter; local geologic seismographs, about 1,500V/meter; and teleseismographs, used for world survey, about 20,000V/meter. Instruments come in three main varieties: short period, long period and broad-band. The short and long period measure velocity and are very sensitive, however they 'clip' 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 a part per million.

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Delivered seismographs come with two styles of output: analog and digital. Analog seismographs require analog recording equipment, possibly including an analog-to-digital converter. Digital seismographs simply plug in to computers. They present the data in standard digital forms (often "SE2" over ethernet).

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The modern broad-band seismometer (so called because of the capacity to record a very broad range of frequencies) 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.

Related Topics:
Frequencies - Electronic - Feedback

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Another type of seismometer is a digital strong-motion seismometer, or accelerograph. This data is essential to understand how an earthquake affects human structures.

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A strong-motion seismometer measures acceleration. This can be mathematically 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.

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Accelerographs and geophones 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 Hz down to 4.5 Hz (cheap) to as low as 1 Hz (pretty expensive). Some have electronic damping, a low-budget way to get some of the performance of the closed-loop wide-band geologic seismographs.

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

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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.

Related Topics:
Electret - Magnetic field

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Today, the most common recorder is a computer with an analog-to-digital converter, a disk drive and an internet connection. Many observatories now use computers. For amateurs, a PC with a sound card and software is adequate, and saves a lot of paper.

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An algorithm often used to eliminate insignificant observations uses a short-term average and a long term average. When the short term average is statistically significant compared to the long term average, the event is worth recording.

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