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In article <11960@sun.uucp> chuq%plaid@Sun.COM (Chuq Von Rospach) writes:
>This is something that has been in the back of my mind for a while.  I know
>you can get good quality hobby (and professional) weather gear, but I've
>never heard of seismographic material that would be reasonable for home
>use.

Sure, you should be able to put together a home station for not too much
money.  The Geophysics Dept. at UW Madison puts portable seismic station
all over the world.  They are quite expensive and highly accurate, but
you don't need all the professional features in order to get pretty nice
seismic data for home use.  What you need for a home digital seismic
recording station is:


_______                      ----------          -----------
|     |                      |        |          |         |
|     |----------------------|        |----------|         |
|     |                      |        |          |         |
_______                      ----------          -----------
Geophone      Cable          Amplifier             Computer

1) Geophone
   Geophones are widely available with lots of different price/performance
   characteristics.  They come in 2 basic types: accelerometers and velocity
   transducer.  The accelerometers are used mainly for large motion.  The
   velocity transducers are used for recording smaller motion.  For home
   use, I think velocity transducers would be more fun.  In fact, both types
   are pretty much the same, they differ only in their frequency response,
   and in the size of the force for which they operate well.

     Geophones which have good low frequency response are, in general,
   larger, heavier, and much more costly than higher frequency phones.
   (However, there is a relatively recent technique called "force-balance",
   which has a lot of promise for reducing the cost of low frequency
   phones).  Since you are in an earthquake active area, you don't need
   low frequency phones.  A 8hz phone will probably do just fine.  I don't
   remember how much these phones cost, but I think that they are considerably
   under $100.  With the slowdown in the oil exploration industry, I expect
   that the cost of geophones has gone down considerably lately.

2) The purist would have coupling transformers on both ends of the cable,
   but you probably don't need them for home use.  Use insulated twisted
   pair wire for the cable.
   
3) The amplifiers are the hard part.  They should have diferential input
   and lots of gain.  The best way to choose the gain for home use, is
   to choose enough gain that the cultural background noise level is one
   or two counts high on your computer's A/D converter.  How much gain this
   is depends on how much cultural noise there is, (busy streets, pumps,
   etc., are really bad), and how large the motor constant on your
   geophone is.  (The motor constant is the number of volts per velocity).
   You are in a pretty good situation if you can feel the earthquakes.
   You will be able to get by with much less exotic amplifiers than
   if you were in a less seismicly active region.

   Consider some sort of lightning protection in your amp design.
   Consider some scheme to keep the amp's dc offset low.  (I.e., < 1
   a/d convertor count).  Capacitive coupling works.  Alternatively,
   you can remove the dc offset in software on your computer.  Most
   triggering algorithms require low offset voltages to work well.

4) The computer should contain an a/d convertor and graphics.  Sample
   the signal at around 100 to 200 samples per second.  A good triggering
   algorithm is, for each sample, take the ratio of the sum of the absolute
   value of the previous 100 samples to the sum of the absolute values of
   the following 100 samples.  If the ratio exceeds 4, you have a genuine
   seismic event.  (Or a truck, or a pump, or ...etc).

Disclamer:  I hope this information helps.  I am not really an expert
   in seismic data acquisition devices.  I spent a few years making
   seismic processing software.  There are people on the net who
   know a lot more about seismic data acquisition devices than I do.
   Hopefully, they will correct the mistakes in this posting and
   offer some more suggestions.  (Lee, Ben, are you listening?)

I think that a home seismic recording station could be a lot of fun.
With the hardware outlined above, you would have a better seismic
recording station than many 3rd world governments have.  (And they
paid a lot more for theirs than you would for yours!).
-- 

Bruce Karsh
U. Wisc. Dept. Geology and Geophysics
1215 W Dayton, Madison, WI 53706
(608) 262-1697
{ihnp4,seismo}!uwvax!geowhiz!karsh