Gravimetry measures the Earth’s gravitational acceleration (absolute gravimetry) or variations of its value in a reference point (relative gravimetry). We work with relative gravimetry.
A relative measurement indicates a spatial variation of the value of g (terrestrial gravitational acceleration). The aim is to attribute this change to a corresponding variation in mass (parts) underground. To this we must first apply several delicate corrections that eliminate the effect of other variables.
There is another way forward, sometimes called gravimetry differential over time. It is taking the measure at two different times, see if it has changed and, if so, to attribute this variation to the modification of the subsoil (eg resizing of a cavity) in that time interval.
We have a digital gravimeter Scintrex CG-5. It is accurate field (optimal conditions) about 5 microgal. Therefore allows handled properly perceive tiny variations in gravity associated with changes in mass produced not only in mining (traditional use) but civil works, archaeological or hydrogeological engineering.
Our experience with the CG-5 work extends hydrogeology, mining cavities detection and jobs in underground coal mines, where strong logistics have required adaptation – to detect changes associated with gas injection.