A gravity meter, also known as a gravimeter, is used to measure gravitational fields on land and in seas and space. Gravity meters actually are accelerometers, measuring the varying downward acceleration of gravity’s pull. As such, they are similar in construction and design to other accelerometers, but they are capable of much more sensitive measurements. The differences they must measure across the Earth’s and seas’ surfaces are fractional in most instances. Being so sensitive, they are susceptible to vibrations such as noise; however, signal processing and vibration isolation capabilities attempt to counterbalance these interferences.
Relative gravity meters are one of two main categories of gravity meters in use that makes comparisons between gravity levels at multiple locations. Taking a calibration at a location already accurately measured and then moving the gravity meter to a second location for another reading gives a ratio of the gravity at both points. The most sensitive and reliably accurate of relative gravity meters, which are usually spring-based, are called super-conducting gravity meters, which feature a floating super-conducting niobium ball in a stable magnetic field. One such meter in Metsahovi, Finland, was so sensitive as to have measured a gradually increasing change in surface gravity as some workers were on the laboratory roof clearing off snow. After noticing that for a period of time there was no change, the scientists were informed that the snow removal workers were taking a tea break.
Absolute gravimeters have a method for measuring gravity that involves charting acceleration, while steadily attached to the ground, of a freefalling mass in a vacuum to measure the velocity of the mass. Air drag on the mass is reduced by setting air pressure in the chamber of the gravity meter to one billionth of the atmosphere. A portable absolute gravity meter can measure to a precision level of one part in 1 billion over a 24-hour period. The pull of tides, the compression of rocks and the wobble of Earth on its axis all have a bearing on the fact that Earth’s gravity changes many times over a day’s time. Such measurements discovered that the Earth’s surface is ascending in northern Manitoba, Canada, in a delayed response to the removal of a huge ice sheet.
Some specific gravity meters can determine the density of soils at depths to determine how much strengthening of soils is needed as a bed for laying asphalt roads. Not only soils, but also water and air samples can be measured using a digital specific gravity meter to arrive at weights for water and air and volume changes in both. Gravity measurements can be taken of seafloors using towed deep ocean gravity meters and produce data back to shipboard with ocean bottom measurements.
