Proactive disclosure Print version ![Print version Print version](/web/20061103053902im_/http://gsc.nrcan.gc.ca/esst_images/_printversion2.gif) ![ÿ](/web/20061103053902im_/http://gsc.nrcan.gc.ca/esst_images/_spacer.gif) | ![ÿ](/web/20061103053902im_/http://gsc.nrcan.gc.ca/esst_images/_spacer.gif) | ![Strong and safe communities Strong and safe communities](/web/20061103053902im_/http://gsc.nrcan.gc.ca/esst_images/2002iscom_e.jpeg) Natural Resources Canada > Earth Sciences Sector > Priorities > Strong and safe communities > Geodynamics
Geodynamics Absolute gravity
![Absolute gravity meter Absolute gravity meter](/web/20061103053902im_/http://gsc.nrcan.gc.ca/geodyn/images/absgrav_.jpg) Absolute gravity meter
|
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).
|