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Overview of the hardware components
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- EAI S-102 Seismometer: Seismometers turn the tiny ground motions into voltage between two wires. In the past, the only cheap off-the-shelf seismometers were "short period" instruments - good for detecting and locating nearby events. Research-quality broad-band instruments can record both nearby and teleseismic events, but were quite expensive. The EAI S-102 is a new generation of inexpensive feed-back broad-band seismometers. The S-102 produces a voltage proportional to ground velocity over the period range of 1 to 20 seconds, and it easily resolves earth noise in this band, while also recording large teleseismic earthquakes.
- A-to-D board: Seismometer output is analog voltage as a function of time. The A-to-D board takes this analog voltage and "digitizes" it by rounding off the voltage to the nearest discrete value. A 12-bit A-to-D board divides up the input voltage range into 2^12=4096 discrete steps, while a 16-bit board divides up the input voltage range into 2^16=65,536 discrete steps. Furthermore, the A-to-D board samples the voltage at discrete values of time (for examples, 10 samples per second). In this fashion, the input seismometer signal is turned into an on-going sequence of integers.
- Time: All seismographs in the world need to use the same clock. OhioSeis use a 24 hour clock, the hour is set to Coordinated Universal Time (UTC). For example, UT time is advanced by 5 hours compared to Eastern Standard Time in the USA. Furthermore, the clocks have to be accurate to a fraction of a second. Thus, even if the built-in computer clock could be set to UT time exactly, it would eventually drift off from this time standard. Thus, some external reference is required to keep "absolute time". While there are possible solutions to this problem via the internet, the SeismoGraf system uses the widely available Garmin GPS II+ hand-held GPS receiver to get absolute time from the GPS satellites.
- A Macintosh computer to run the software and provide file storage and internet communication.
Last update March 10, 2003
Ohio Seismic Network http://www.dnr.state.oh.us/OhioSeis
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