Absolute gravimeters measure the acceleration of a mass during free fall in a vacuum chamber. Several of these are available commercially with a range of accuracies from 2 – 10 microGal absolute accuracy.
Most relative gravimeters determine gravity by the stretch of a spring supporting a weight or the electrostatic force required to keep the spring length constant. Transportable instruments have been used for many decades on land for oil, gas and mineral resource exploration, and for multiple other research purposes. For regional mapping they obtain accuracies of some tens of microGal. For detailed mapping better accuracy can be obtained.
The core of a Quad relative gravimeter (shown at right) is used in its remotely operated (ROVDOG) instrument. The sensor has 1 microGal reading resolution. Quad has adopted the dedicated control and recording electronics, remotely operated gimbals and deep-water pressure cases initially developed at Scripps Institution of Oceanography.
Superconducting gravimeters are the most precise relative instruments, with sensitivity as low as 1 nanoGal. However, they lack portability and cost is an issue.
Heikki Virtanen. Studies of Earth Dynamics with the Superconducting Gravity Meter. 2006. Academic dissertation in Geophysics presented to the Faculty of Science at the University of Helskinki