Two experts brought a high accuracy absolute gravity meter to the St. John’s campus recently.
Jason Silliker and Rachel van Herpt, of the Canadian Geodetic Survey of Natural Resources Canada, brought their FG5-236, a.k.a. “Gertrude”, to Station 991399 for a 24-hour-long set of measurements in May.
Below the Science building
Station 991399 is one of only three absolute gravity reference stations on the island of Newfoundland. It is located in a cool, quiet concrete bunker under the Science building, off the Munnels.
The acceleration of gravity on the surface of the Earth is g = 9.8 metres per second squared (m/s²), varying subtly from point to point on Earth’s surface. Mr. Silliker and Ms. van Herpt found the value at Memorial’s reference station to be g991399 = 9.808195878 plus or minus 0.00000002 (m/s²).
‘St. John’s is sinking’
In obtaining this level of accuracy and precision, the researchers had to allow for the gravitational attraction of the sun and moon and the ocean tides, atmospheric pressure, sinking of the sea bed due to the weight of tidal water and the wobble of the Earth’s axis.
“This year’s measurement is slightly — one part per 65 million! — lower than when we last measured it in 2010, a difference probably explained by a difference in the amount of water in the soil above the station: the gravity value there is known to vary depending on the levels of precipitation at various times of the year,” said Mr. Silliker.
“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 millimetres per 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, which is particularly significant in this time of global climate change.
As well, gravity changes due to tectonic movements aid in our understanding of the structure and dynamics of our planet. They are also reference points for all other types of gravity surveys, such as those used in natural resource exploration.
Two types of meters
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 used in exploration surveys and various other studies.
Memorial’s 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.
“The good news is Memorial’s gravity meter is operating at the top of its game.”
It is periodically calibrated at station 991399.
“We used Memorial’s 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,” said Mr. Silliker.
“From these results, we determined how well the CG-5 is performing. The good news is Memorial’s gravity meter is operating at the top of its game.”