Microgravimetry is one of the best suited geophysics methods for non-invasive search of underground density anomalies, such as cavities, galleries, subsidence, voids…
It is based on the application of Newton’s law that states that the intensity f of distant of d, is where G is the universal gravitational constant. Gravity is the addition of the gravitational force of the Earth and of the Universe (virtually only the effects of the Moon and the Sun are noticeable), of inertia forces caused by the rotation of Earth, and of relief, altitude and latitude effects. Variations in gravitational acceleration are linked to variations in subsoil density. These variations are measured with a microgravimeter at junctions of a meshing of stations, evenly spaced out over the whole studied area. The mesh width depends on the dimensions and depth of the anomalies searched. The bigger their dimensions and depths are, the looser the mesh will be. On the contrary, the smaller their dimensions and depth, the tighter the mesh. Mesh are usually between 2 and 10 m wide. The rate of measures is approximately 100 stations a day.
In the end, the data is gathered in a microgravimetric anomaly map, highlighting negative anomaly areas (in red, below) that can correspond to density anomalies in the subsoil.
These anomalies then have to be the object of mechanical boreholes. We sometimes associate them to a video examination of the cavities found to assess their size. Microgravimetry not only allows us to detect density anomalies in the subsoil, but also prevents us from making boreholes blindly, and consequently decreasing their number, costs, and the duration of work.
INNOGEO takes more than a thousand microgravimetry measures each year as part of projects on building construction sites, engineering structures, roads or railways…