Thanks to both Chris and Brett for their thoughtful replies.  I better 
understand that an upgrade to 24 bits is not a "no brainer"  upgrade, 
but part of design strategy.   All the parts must fit together to get 
superior results and fulfill site goals.

There are new devices that may hold promise for future amateur use.  One 
such device is the Cirrus Logic CS5530 24 bit ADC with integrated 
amplifier.  I don't know if this could stand alone with a dedicated 
computer (or maybe not dedicated) using a serial port, or if it must be 
combined with a microcontroller.  I also don't know if the input must be 
pre-filtered to prevent aliasing, or if the integrated amplifier is 
sufficient  for amateur seismic work.

Thanks for your comments

Roger
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