Analysing a Geophone Circuit
Try this, hopefully I haven't done my math wrong: -




If you imagine R11 is inside the op-amp, the feedback components R12 and R13 act to produce a gain of two for the cap C9. This keeps the sallen key filter stable but the bonus is a 10x higher gain where the actual op-amp output is (because R11 is approximately 10x R12+R13)
R7 sets the dc bias for the final stage but as the filter is extremely slow (0.0015Hz) you might not have noticed things sliding towards a power rail due to leakage currents.
The junction of the 2x 220k resistors is the analogue reference point for the circuit therefore to keep noise low the sensor has a return to that point.
Correct.