PSN-L Email List Message

Subject: Re: LP filters, A/D bits and SPS
From: Karl Cunningham karlc@..........
Date: Fri, 12 Jan 2001 11:33:58 -0800

Hi Dave --

Your math sounds fine to me.  But unless you are very close to an
earthquake, there isn't going to be much energy at 80Hz.  

The large dynamic range needed for seismic recording is primarily to
accommodate the wide variation in peak amplitudes from seismic signals.
16-bit accuracy is not needed, and really 8 bits of accuracy is more than
enough for most purposes.

The fact that the lower bits are being affected by aliasing isn't much of a
concern if the higher bits are recording good data.

Karl Cunningham

At 12:06 PM 1/12/2001 -0500, you 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?
>Dave Saum  
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Larry Cochrane <cochrane@..............>