  The measurement of tiny variations in local gravity enables the observation
of subterranean features. Gravimeters have historically been extremely
expensive instruments, but usable gravity measurements have recently been
conducted using MEMS (microelectromechanical systems) sensors. Such sensors are
cheap to produce, since they rely on the same fabrication techniques used to
produce mobile phone accelerometers. A significant challenge in the development
of MEMS gravimeters is maintaining stability over long time periods, which is
essential for long term monitoring applications. A standard way to demonstrate
gravimeter stability and sensitivity is to measure the periodic elastic
distortion of the Earth due to tidal forces - the Earth tides. Here we present
a nineteen day measurement of the Earth tides, with a correlation coefficient
to the theoretical signal of 0.979. The estimated bias instability of the
proposed gravimeter is 8.18 microGal for an averaging time of ~400 s when
considering the raw, uncompensated data. The bias instability extracted from
the sensor electronic noise sits just under 2 mircoGal for an averaging time of
~200 s. After removing the long-term temperature and control electronics
effects from the raw data, a linear drift of 268 microGal/day is observed in
the data, which is among one of the best reported for a MEMS device. These
results demonstrate that this MEMS gravimeter is capable of conducting
long-therm time-lapse gravimetry, a functionality essential for applications
such as volcanology.
