Dear Tom Schmitt,

    You wrote :-

>  In exploration seismic work geophone arrays were used to filter local 
noise.
>  The signals from several geophones would be summed.  The noise would be out
>  of phase at the geophones but the signal would be in phase. In reflection
>  work that assumption is more justified than in earthquake or refraction
>  seismology as in reflection the signal is coming more or less straight up.
>  Has this ever been used in earthquake seismology? 

    Your letter raises several questions. How many and how widely spaced 
locations were you proposing for your recording stations? The wavelength of 
earthquake waves is likely to be VERY long when compared to explosive 
sounding signals / environmental noise and what you actually record at a 
particular site is effected by the wave path, which may have different 
properties from different directions.

>  The usual answer to noise is a quiet location but with 
> educational seismometers rather than research
>  seismometers, location near the classroom is important.

    If you can use your many observers to identify the various man made noise 
sources, there is no reason in principle why you should not place an 
auxiliary sensor(s) closer to sources and feed the relatively strong 
interfering signals back to provide compensation, but don't expect a perfect 
match. It is easier to do this using digital delays, rather than analogue 
delay lines. Can you record an environmental noise channel to help in 
identification and the seismograph channel? Could the identification / 
characterisation / avoidance of local sources be a worthwhile project? If the 
local noise is terrible, could you get on line data from some quiet location 
for your 'real earthquakes' and use your local set-up as a teaching / backup 
aid?
  
>  Second.  There is a new chip that is up to a 6 pole analogue filter.   The
>  configuration of the filter is set in a device similar  to an EPROM burner.
>  The software that comes with it has full design features such as simulation
>  of amplitude and phase. 

    Filters of this type are produced for use in hearing aids, to vary the 
audio characteristics over a wide band. In your application, it is probably 
better to use four pole Bessel filters, which are relatively easy and cheap 
to make with a couple of OPAs. When you go to six pole filters, the delay can 
get rather long. You may find that shifting the corner frequency downwards 
just a bit is actually preferable to adding more filter stages. If you have a 
two channel recording system, could you use it to compare the simultaneous 
results of various filter choices? If you initially use 'plug in' breadboard 
(with silver plated contacts), you can easily change the filter 
characteristics.

    Regards,

    Chris Chapman
__________________________________________________________

Public Seismic Network Mailing List (PSN-L)