We move on to the IGY Bulletin issue #8 from February, 1958. The download of this 20-page issue from the AGU website can be found here. The articles in this issue are:
- First sea surface gravimeter
- A report on the United States Program
- Satellite telemetry
- IGY satellite program notes
- Thule neutron monitor station
Here I will summarize the first article. From an earlier post, gravity objectives of the IGY included:
- augmenting the global network of gravity measurements, especially in the Southern Hemisphere and polar regions
- better determination of the solid Earth's response to the tides
- enhanced measurement of gravity at sea
This article first reports on the first successful surface measurement of gravity in the open sea on Nov. 22, 1957, by J. Lamar Worzel of Lamont Geological Observatory, Columbia University. He used a Graf-Askania gravimeter developed at the Technical University of Munich by Anton Graf. The USS Compass Island provided a gyro-stabilized platform for this work.
In a basic physics course, we'd learn that the acceleration due to gravity at the Earth's surface is 9.8 m/s2. However, because of the shape of the Earth, gravity varies on a longitudinally-averaged Earth from 9.7803267714 m/s2 at the equator to 9.8321863685 m/s2 at the poles, 0.5% higher, because you are closer to the Earth's center at the poles of our squashed (oblate spheroid) Earth. Gravity is also affected by elevation, local terrain, tides, moving measurement platforms (such as ships), and mass anomalies in the subsurface. Gravity surveys in geophysics are often used, after correcting for the other factors that affect gravity, to infer the nature of anomalous masses in the subsurface. Signals as small as 0.00001 m/s2 (or about 1 part in 1,000,000 of the Earth's gravity field) are measurable with sensitive gravity meters (gravimeters) and can indicate significant "gravity anomalies."
Early gravimeters used on land were precision pendulum systems, since the period of a pendulum swing is related to the acceleration of gravity. Measurements at sea present a special challenge due to lack of stability and the addition of non-gravitational accelerations caused by the motions of the ship. In the 1920s, the Dutch geophysicist Vening Meinesz developed a system of two and then three balanced pendulums that were able to measure gravity underneath the sea surface by nulling out movement in the relatively quiet environment of a submarine. The later Graf instrument was basically a very sensitive spring-type balance that was able to measure gravity on the noisier platform of a ship at the surface. A measurement with the Graf could be taken in 9 hours and the data reduced in half a day, compared to two days and two weeks, respectively, for a measurement with the old pendulum systems. Today, measurements with modern gravimeters take a matter of minutes.
Seaborne gravity measurements are important since the oceans cover about 3/4 of the Earth's surface. The measurements to be made during the IGY were to fill major gaps in the extant world gravity database, especially in the polar regions and at sea, and then to meld the different national surveys together into a global grid.
As mentioned in an earlier post, the former German Democratic Republic (East Germany) issued a set of four se-tenant geophysics (but not IGY) stamps in 1980. The 20-pfennig stamp showed a modern gravimeter and how it might be used in delineating coal-bearing sedimentary basins. I just bought the block of stamps on eBay, from where the image below comes.
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| GDR block Scott #2146a, including the gravity stamp (Scott #2143), upper left |
On my trip to Sarasota two weeks ago, I visited parks, the beach, played golf, ate good food, went to botanical gardens. I also perused the John and Mable Ringling Museum of Art (yes, the circus Ringling); I was not so keen on the old Italian art, but I liked the modern art, including this 1988 piece entitled Gravity, by Yuriko Yamaguchi:
What goes up, must come down, according to gravity!
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