The short answers are:

There are other filters working besides your 8-pole Bessel, like the
earth, which is a pretty good low-pass filter.  All you really need to
filter is the electrical noise and some local cultural noise.

I believe your analyis is correct, but assumes that the noise is full
scale.

A perfect anti-alias filter is just too complex and has too many parts
for either amateurs or professionals to build. Compromises happen.

To read further, here's a link with a discussion of anti-alias filters
in the real world.
http://www.chipcenter.com/eexpert/shendrix/shendrix005-2.html

Doug Crice


David Saum wrote:
> 
> I am confused about the relationships between
> seismic LP filtering, A/D and SPS.
> 
> Suppose I have an 8 pole bessel filter
> with a -3 db point of 10 Hz and a 16 bit A/D.
> 
> Log10(2^16)=4.81, so in power that is 96 db
> dynamic range.
> 
> My 8 pole bessel response is down 96 db
> at about 80 Hz.
> 
> So I should sample at a minimum of 160 Hz  to
> avoid aliasing that will screw up my lowest bit ?
> Similarly, for a LP of 1 Hz  I should sample at
> a minimum of 16 SPS?
> 
> This seems to be a lot higher SPS than most folks
> are using.  Can we assume the seismic spectra
> naturally falls off at higher frequencies so that we
> do not have to depend on our LP filter
> at higher frequencies?
> 
> Or is my math screwy?
> 
> Confused,
> 
> Dave Saum
> 
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-- 
Doug Crice			 http://www.georadar.com
19623 Via Escuela Drive		      phone 408-867-3792
Saratoga, California  95070  USA	fax 408-867-4900
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