Absolute Gravity

An absolute gravimeter measures gravity by timing the free fall of a test mass in an evacuated chamber (upper yellow cylinder). The air pressure in the cylinder has been reduced to one billionth of an atmosphere to reduce air drag on the mass. Natural vibrations of the floor are compensated for by a "super spring" (lower yellow cylinder). The electronics (left) control repeated drops of the test mass, and measure its acceleration using a laser interferometer and an atomic time standard. This is an FG5 absolute gravimeter manufactured by Micro-g Solutions Inc., Erie, Colorado.

Using a transportable absolute gravimeter the force of gravity can now be measured with a precision of one part in one billion. The instrument takes 2 or 3 hours to set up at a station and another 24 hours to collect enough data for a precise measurement. Gravity is continuously changing as a result of a number of phenomena affecting the whole Earth such as, earth tides, the wobble of the Earth on its axis, density variations in the atmosphere, etc. Gravity also varies as a result of very small changes in the elevation of the station and in the density of the material beneath the station. We measure these changes by repeating gravity measurements at regular time intervals. We expect to measure the changes due to the surface movement and compression of the rocks taking place where two tectonic plates meet. Gravity changes have already been measured in northern Manitoba where the Earth's surface is moving upward in delayed response to the removal of the 2 km-thick Laurentide ice sheet - (see Mid-continent Tilt Project).