Konrad Lehnert @Berkekey about MEMS gravimeters and other very sensitive accelerometers focused on ‘gravitational’ sources

Physics @Berkekey: Oct 13 Colloquium: Konrad Lehnert at https://www.youtube.com/watch?v=qbqTWe1qBbY

Konrad Lehnert, Three axis MEMs gravimeters can be made sensitive enough to measure precisely the vector tidal sun moon signal that is the main part (about 98.5%) of the signal at a superconducting gravimeters station. Using the JPL ephemeris this is just the sun’s acceleration at the station minus the suns’ acceleration at the center of the earth, plus moon at the station minus the moon at the center of the earth, plus the centrifugal acceleration of the station due to earth rotation. Pu that in station North East Vertical and it only needs a linear regression (offset and multiplier to fit. It is so sensitive that small changes in position and orientation are measurable. Gravity is not affected by the ionosphere or oceans, or underground or shielding.  A great way to calibrate global networks. The broadband seismometers reporting velocity can be differentiated to get the acceleration, and for quiet stations all three vector signals are there – linear regressions.

The residual after subtracting this Newtonian signal is mostly microseisms and atmospheric density changes. There are atom interferometer gravimeters, tensor gravimeters that can fly on drones. You can use atomic force microscope sensors as gravimeters, you can monitor any movement precisely take the first and second derivatives and have a “gravimeter”. I set the hurdle for the term “gravimeter” at “can measure and stay locked to the sun moon tidal signal”. But the next stage is “measure the speed of gravity”, that requires precision timing, but now that is mostly available in lots of ways.

There are “Bose Einstein gravimeters”, “electrochemical gravimeters”, and several what I call “direct potential sensors” like Mossbauer, gravitational time dilation, atomic clock and some of photon interferometer detectors. If you can monitor at Mega Samples Per Second you can use arrays to image inside the earth, in the atmosphere, in the oceans, in side the moon, inside the sun. It is very similar to “passive time of flight correlation imaging”.

It does help to use vibration isolation, but better is to have stages for measurements with smaller and more sensitive detectors stacked on less sensitive ones. If all your detectors are “null” (no movement), then the “holding constant” signals simply have to be recorded precisely. If you are not already part of SEMI-MEMS, you might want to look at those groups. I did work with someone a few years ago to adapt the atom force noncontact cantilevers as gravimeters (sensitive accelerometers). The higher modes are more sensitive, you show you can drive and excite and measure those.

GravityNotes Org has some links.  There is a spreadsheet at the top with a sample. That month long data stream only requires TWO numbers and the JPL ephemeris, station location, WGS84 and earth rotation.

Richard Collins, The Internet Foundation