Local surface gravity varies by up to a ten-thousandth, about .001 m/s: https://en.wikipedia.org/wiki/Gal_(unit) https://en.wikipedia.org/wiki/Gal_(unit) Sensi

Local surface gravity varies by up to a ten-thousandth, about .001 m/s: https://en.wikipedia.org/wiki/Gal_(unit) https://en.wikipedia.org/wiki/Gal_(unit) Sensitive detectors can measure differences in hundreds of millionths (10^-8 parts) or around .1 um/s^2. A 1 million tonne mass on the other side of the earth will create a TOTAL acceleration of 4*10^-16. Changes in that will not be detectable.