This semester at the Colorado School of Mines all 350 freshmen will be=20
working on the
design of an inexpensive seismic system.  They will be divided into 70=20
groups of 5 for
this effort.  Because the goal is to design an inexpensive instrument (less=
=20
than $150)
some things may have to be sacrificed,  including linearity and single-axis=
=20
sensitivity.
The $150 limit (exclusive of a PC) would make this instrument a candidate=20
for the
GLOBE program, an educational program with international scope and
many collaborators that is currently focused on weather, atmosphere, and=20
biology
issues.

I will encourage the students to join mailing lists,  such as this one, to=
=20
get information
and advice for their projects.  The PSN group collectively has a wealth of=
=20
experience that
they could benefit from.

A letter with added details follows.

Cheers,
John


>August 20, 2002
>Robert D. Knecht
>Colorado School of Mines
>
>Dear Dr. Knecht,
>
>The IRIS Consortium Education and Outreach Committee is pleased that the=20
>EPICS Project for this
>year will be the design of an inexpensive sensor for seismic=20
>waves.  Computers, which are widely
>available in K-16 classrooms, have replaced the paper-drum and pen=20
>recorders of earlier years.
>IRIS has sponsored the development of AmaSeis, a free, PC-based program=20
>that monitors the real-
>time stream of data from a seismometer so that near and distant=20
>earthquakes can be easily recorded
>and viewed graphically.
>
>The principal remaining barrier to more widespread, school-based=20
>earthquake recording systems is
>the availability of an inexpensive, yet sensitive, long-period seismic=20
>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=
=20
>about $500, which
>precludes purchase by very many teachers or schools.  If this cost could=20
>be reduced to $150 or less,
>it would be within the range of many more budgets and would also be a good=
=20
>candidate to become
>an instrument recommended for use by the Globe=20
>program.  
>
>Design Goals and Options
>
>Sensitivity
>
>For the system to be useful in any location on the Earth, it must be able=
=20
>to record large earthquakes
>at any distance -- teleseisms in the terminology of seismologists.  This=20
>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=20
>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 [  5.61E+01 =B5m]  [  1.76E+01=
 =B5m/s]
>Expected 1s period body wave amplitude     [  7.93E-01 =B5m]  [  4.98E+00=
 =B5m/s]
>
>Magnitude 7 at 180 degrees distance:
>Expected 20s period surface wave amplitude [  1.81E+01 =B5m]  [  5.68E+00=
 =B5m/s]
>(No direct P-phase at this distance.)
>
>Construction Strategy
>
>One strategy might be to look at commercially available recording systems,=
=20
>such as the AS1
> on display at CSM, the Sprengnether
>, and the designs of=20
>the Public Seismic
>Network  and
>. Explore ways in which the=20
>cost of manufacture, and
>hence, the cost of purchase of these instruments could be reduced by=20
>eliminating expensive or
>carefully machined components.
>
>Either a high-school teacher with little or no mechanical and fabrication=
=20
>skill should be able to
>build one from scratch for about $150, or, if you choose to reduce costs=20
>by distributing instruments
>in kit form, the cost to the fabricator must be such that an=20
>easily-assembled kit will sell for less than
>$150.
>
>Sensor
>
>The sensor could be based on the voltage generated by relative motion=20
>between a coil and magnet,
>on the proximity of a magnet to a Hall-effect sensor, on the amount of=20
>light reaching a photo diode
>that is partially blocked by a flag, on the strain of a piezoelectric=20
>crystal, or on the output of a micro
>machined accelerometer similar to those used to trigger the release of an=
=20
>automobile's air bag.
>Given that new, high-density, low-cost disk drives are continually coming=
=20
>to market, there may
>well be some sophisticated, mass-produced, and yet inexpensive components=
=20
>that could also be
>used for this application.
>
>Recording Options
>
>The completed system must be able to record data continuously onto a PC=20
>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)=
=20
>an inexpensive, 12-
>bit, analog to digital converter that could be used for this system.  It=20
>is model number DI-154.  It
>has terminals for analog input and digital-serial output and is powered by=
=20
>the serial port of a PC.
>They also sell a 10-bit unit for $25 (DI-194RS).   
>
>The free AmaSeis software for seismic recording is available from Alan=20
>Jones' web site for the DI-
>154RS AD unit.   
>
>To use the either Dataq AD unit, one would need to construct a sensor that=
=20
>would connect to the
>AD via an amplifier and filter circuit.  Because of the cost of the AD,=20
>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=20
>amplifier/filter circuit to allow  a
>direct connection to the serial port of a PC.  That would cut down on the=
=20
>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=
=20
>designed to sense the
>resistance of the joy stick.  A field effect transistor (FET) can be used=
=20
>to simulate a variable resistor
>and the FET "resistance" can be made to vary with the voltage generated by=
=20
>the amplifier/filter
>circuit.
>
>
>Sincerely,
>
>
>John C. Lahr
>Geophysicist
>US Geological Survey
>National Earthquake Information Center
>(303) 273-8596 (tel)
>(303) 273-8600 (fax)
>
>
>
>Thomas Boyd
>Associate Professor
>Department of Geophysics
>Colorado School of Mines
>  (303) 273-3522 (tel)
>(303) 273-3478 (fax)

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