Absolute Gravity Readings at Memorial University
The St John’s Campus of Memorial University is the host of one of only three absolute gravity reference stations on the island of Newfoundland. Station 991399 is located in a cool, quiet concrete bunker off the Munnels under the Science Building. Last month, gravity experts Jason Silliker and Rachel van Herpt from the Canadian Geodetic Survey of Natural Resources Canada brought their high accuracy absolute gravity meter FG5-236 (aka “Gertrude”) for a 24-hour-long set of measurements at the station.
The acceleration of gravity on the surface of the Earth is g = 9.8 m/s2. More precisely, Jason and Rachel found the value at our reference station to be g991399 = 9.808195878 ± 0.00000002 m/s2. In obtaining this accuracy and precision, they had to allow for the gravitational attraction of the Sun and Moon and the ocean tides, atmospheric pressure, loading from the ocean and the wobble of the Earth’s axis.
The determined value is 0.00000015 m/s2 lower than when they last measured it in 2010, a difference Jason says is probably explained by a difference in the amount of water in the soil above the station: the gravity value here is known to vary depending on the time of year. As part of adjustments to surface elevation after the melting of a Canada-wide ice sheet from the last ice age, St John’s is sinking relative to the centre of the Earth at a rate of 1.09 mm/year. However, the resulting tiny increase in gravity is not detectable at this site.
Absolute gravity measurements have three important applications. First, they can be used to track changes in such quantities as the amount of groundwater at sites across Canada, particularly significant in this time of global climate change. Second, gravity changes due to tectonic movements aid in our understanding of the structure and dynamics of our planet. Third, they are reference points for all other types of gravity surveys, such as those used in natural resource exploration.
FG5 absolute gravity meters work by timing a mass dropping in a vacuum. Relative gravity meters work by measuring the extension of a spring: because the properties of the spring are constantly changing, they must be regularly calibrated at reference stations. These relative meters are use in exploration surveys and various other studies.
The Department of Earth Sciences owns a Scintrex CG-5 survey gravity meter. This meter (unlike Gertrude!) is easily portable and it is used in undergraduate and graduate courses in exploration geophysics and for graduate projects on oil and gas and mineral deposits. It is periodically calibrated at station 991399. Jason and Rachel used our CG-5 to measure how gravity changes with elevation (the gravity gradient) by placing it on platforms at three well-determined heights above station 991399. From the results they could determine how well the CG-5 is performing. The good news is that our gravity meter is operating at the top of its game.
Current and historical results of station 991399: https://webapp.geod.nrcan.gc.ca/geod/data-donnees/station/cgsn_graph_report.php?id=99131999
Surface elevation at the St John's active control site (ACS): https://webapp.geod.nrcan.gc.ca/geod/data-donnees/station/report-rapport.php?id=920000
Operation of FG5 absolute gravity meters: http://www.microglacoste.com/fg5x.php