Gravimetry is the study of the measurement of gravity ("g") and its variations in time and space.
g depends on :
- Mass repartition in the Earth;
- The Earth’s rotation (speed and position of the rotation axis);
- The seasons (groundwater mass variations);
- The relative position of the Moon and the Sun, which generate tidal forces.
Until recently, measuring vertical land movement could only be achieved using repeated levelling measurements. This was done using theodolites since the end of the XIXth century. However, it is hopeless to apply this technique to detect land movements smaller than a centimeter per year. Read
Usually, seasonal variations are visible in the gravity measurements. Those effects are caused by the recharge of the aquifers during the winter, causing an increase in gravity at spring time, and conversely, a decrease during fall. Read
The kilogram is the only remaining base unit of the International System of Units (SI) whose definition is based on a physical artefact rather than on fundamental properties of nature. To prepare the SI for the increasing needs of science and technology, a replacement of the kilogram based on fundamental constants is needed. Read