August 20, 2002
Robert D. Knecht
Colorado School of Mines
Dear Dr. Knecht,
The IRIS Consortium Education and Outreach Committee is pleased that the EPICS Project for this year will be the design of an inexpensive sensor for seismic waves. Computers, which are widely available in K-16 classrooms, have replaced the paper-drum and pen recorders of earlier years. IRIS has sponsored the development of AmaSeis, a free, PC-based program that monitors the real- time stream of data from a seismometer so that near and distant earthquakes can be easily recorded and viewed graphically.
The principal remaining barrier to more widespread, school-based earthquake recording systems is the availability of an inexpensive, yet sensitive, long-period seismic sensor that can attach to a PC's serial port via an analog-to-digital converter. The least expensive commercially available system (excluding the PC) costs about $500, which precludes purchase by very many teachers or schools. If this cost could be reduced to $150 or less, it would be within the range of many more budgets and would also be a good candidate to become an instrument recommended for use by the Globe program. <http://www.globe.gov/>
Design Goals and Options
For the system to be useful in any location on the Earth, it must be able to record large earthquakes at any distance -- teleseisms in the terminology of seismologists. This requires a good signal to noise ratio in the period range from 1 to 20 seconds. The following table gives a rough idea of the amplitude and velocity of ground motion that would be expected from a magnitude 7 earthquake at 90 and 180 degrees distance.
|Magnitude 7 at 90 degrees distance amplitude velocity|
|Expected 20s period surface wave amplitude||Amplitude = 5.61E+01 Ám||Velocity = 1.76E+01 Ám/s|
|Expected 1s period body wave amplitude||Amplitude =7.93E-01 Ám||Velocity = 4.98E+00 Ám/s|
|Magnitude 7 at 180 degrees distance|
|Expected 20s period surface wave amplitude||Amplitude = 1.81E+01 Ám||Velocty = 5.68E+00 Ám/s|
|(No direct P-phase at this distance.)|
One strategy might be to look at commercially available recording systems, such as the AS1 <http//jclahr.com/science/psn/as1/> on display at CSM, the Sprengnether <http://www.geocities.com/meredithlamb/page026.html>, and the designs of the Public Seismic Network <http://www.geocities.com/meredithlamb/> and <http://jclahr.com/science/psn/gldn_psn.html>. Explore ways in which the cost of manufacture, and hence, the cost of purchase of these instruments could be reduced by eliminating expensive or carefully machined components.
Either a high-school teacher with little or no mechanical and fabrication skill should be able to build one from scratch for about $150, or, if you choose to reduce costs by distributing instruments in kit form, the cost to the fabricator must be such that an easily-assembled kit will sell for less than $150.
The sensor could be based on the voltage generated by relative motion between a coil and magnet, on the proximity of a magnet to a Hall-effect sensor, on the amount of light reaching a photo diode that is partially blocked by a flag, on the strain of a piezoelectric crystal, or on the output of a micro machined accelerometer similar to those used to trigger the release of an automobile's air bag. Given that new, high-density, low-cost disk drives are continually coming to market, there may well be some sophisticated, mass-produced, and yet inexpensive components that could also be used for this application.
The completed system must be able to record data continuously onto a PC computer disk, and this will require the use of an analog-to-digital converter.
The Dataq Company sells the DI-154RS for $100 (with educational discount) an inexpensive, 12- bit, analog to digital converter that could be used for this system. It is model number DI-154. It has terminals for analog input and digital-serial output and is powered by the serial port of a PC. They also sell a 10-bit unit for $25 (DI-194RS). <http//www.dataq.com/>
The free AmaSeis software for seismic recording is available from Alan Jones' web site for the DI- 154RS AD unit. <http://www.geol.binghamton.edu/faculty/jones/as1.html>
To use the either Dataq AD unit, one would need to construct a sensor that would connect to the AD via an amplifier and filter circuit. Because of the cost of the AD, either $125 or $50 would be available for the sensor/amplifier/filter combination.
Another option would be to add an AD with serial output to the amplifier/filter circuit to allow a direct connection to the serial port of a PC. That would cut down on the cost, but AmaSeis might need to be modified to accept a new serial format.
Still another option would be to use the PC's game port. The game port is designed to sense the resistance of the joy stick. A field effect transistor (FET) can be used to simulate a variable resistor and the FET "resistance" can be made to vary with the voltage generated by the amplifier/filter circuit.
John C. Lahr
US Geological Survey
National Earthquake Information Center
(303) 273-8596 (tel)
(303) 273-8600 (fax)
Department of Geophysics
Colorado School of Mines
(303) 273-3522 (tel)
(303) 273-3478 (fax)