The geophone is basically an accelerometer, it looks like. That's why I commented that bumping it might cause the output to go pretty high. On the other hand, the bandwidth is pretty low, so that may not be a problem.
OP has a 24-bit ADC. Lets say the input was 30V scaled to 2.5 to protect the ADC. That still gives the equivalent of 1.7 µV resolution. Consider a 2.5V reference and only 30mV full scale input to a 24-bit ADC that still gives you 19-bit resolution. I think the external ADC will be plenty for the job.