> Subject: RE: Honduras Quake 5/28/09
> From:    "Kareem Lanier" 
> Date:    Thu, 28 May 2009 14:48:59 -0700
>
> Question:
>
> Since Rayleigh waves are vertical but vertical geophones are more designed
> for vertical, higher frequency motions, would the geophone still pick up
> these Rayleigh waves very well?

It is a common misconception that 4.5 Hz geophones can not pick
up seismic signals below 0.1 Hz, or that they need special
post processing to do so.

I often pick up 0.05 Hz or lower signals from strong teleseismic events
with my horizontal 4.5 Hz geophone systems that do not have any
spectral emphasis processing on the lower frequencies

Here is one of my 4.5 Hz horizontal geophone
teleseismic detections:

2003 January 20 08:43:06.0 UTC
Magnitude 7.2 - SOLOMON ISLANDS: 13,500 km from Bailey's Crossroads, VA

spectra from horizontal 4.5 hz geophone with no filtering
http://www.infiltec.com/seismo/030120f.gif

Amaseis 24Hr Helicorder display with 0.1Hz low-pass
http://www.infiltec.com/seismo/030120h.gif

display of time data with 0.1 Hz low-pass
http://www.infiltec.com/seismo/030122w.gif

for more examples, see
http://www.infiltec.com/seismo/inf-qm45.htm

It helps to use a low-noise amplifier with auto-zero
to reduce 1/f noise, do 16 bits or better A/D to preserve
your dynamic range, to have a multi-pole low pass
Bessel filter for antialiasing, and to sample fast
enough to allow your antialiasing to keep your noise
spectra from folding back on itself. All of that will
lower your background noise in the lower frequency
bands of seismic interest.

Dave
http://www.infiltec.com/seismo
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