Franek Hasiuk, Department of Geological and Atm Sciences

Absolute Gravity Measurement at Iowa State University

 

As part of the Geoid Slope Validation Survey (2014), the National Geodetic Survey will measure absolute gravity (that 9.81... m/s^2 number) on the campus of Iowa State University in Summer 2014. Dr. Franek Hasiuk is facillitating the measurement by acting as a focal point for communication between stakeholders at ISU and at the NGS.

 

Why measure absolute gravity at ISU?

The first reason is that the National Geodetic Survey is updating the North American Datum, which is the reference frame cartographers and geodesists use to measure elevation. Today, we typically hear elevations being given in reference to sea level ("The summit of Pike's Peak is 14,114 feet above sea level"). The National Geodetic Survey seeks to establish a fixed reference for measuring elevation that is not related to sea level because of projected sea level rise over the next century.

To ensure the accuracy of this new reference, there are a few locations that need some special attention because of particular anomalies. NGS's program to address these anomalies is called GRAV-D and the surveys are called "Geoid Slope Validation Surveys." One was completed in 2011 from the Gulf Coast of Texas to Austin. The survey line in Iowa runs along US Highway 30 from Denison in the West, to Cedar Rapids in the East .

While Ames is known for being surrounded by fertile—and flat—countryside, deep below Ames is a geological structure known as the Mid-Continent Rift. The Rift formed over a billion years ago when the paleo-continent Laurentia began to break apart. Scientists are still actively researching the cause of this rifting episode and why it ultimately failed. The Mid-Continent Rift is full of a large volume of dense igneous rocks. That large volume of dense material that fills the Rift results in one of the largest gravity anomalies on Earth existing right under Ames.

An old anecdote is that you weigh half an ounce more in Ames than you do in Cedar Rapids, which is far from the Rift's axis. To the best of our knowledge the US Olympic Team is not planning on locating training facilities on the Rift to take advantage of this gravity anomaly, like they take advantage high-altitude training with their Colorado Springs Training Center.

 

Here is a map of Gravity Anomaly associated with the Mid-Continent Rift.

 

A modern example of a rifting continent is Africa where is has successfully rifted from Arabia along the Red Sea and the Gulf of Aden, rifting processes are ongoing under East Africa, but it is unclear whether this rifting will fail or proceed to completion (the formation of an ocean basin like the Red Sea).

 

Timeline:

15 July 2013: a 1-inch diameter brass benchmark was installed in the floor of the Christian Peterson Art Museum. LINK to short story at Iowa State News Service.

June 2014: Absolute gravity measurement in Morril Hall, Iowa State University

Links:

NGS Grav-D Webpage

Geoid Slope Validation Survey 2011 (Texas Gulf Coast to Austin)

Geoid Slope Validation Survey 2014 (Iowa Mid-Continent Rift)

Iowa Magnetic and Gravity Maps and Data at the USGS

KML Geology at South Dakota State University