Gravity Map of Earth

dr3vil writes "Interesting results have been published by the GRACE (Gravity Recovery and Climate Experiment) project, of the various gravity anomolies that exist at various places on Earth. The BBC report gives a good overview. Fascinating for me, a resident of California, to see us apparently sandwiched between a high and a low spot. Maybe that helps aids the tectonic flows around here?"

Comments

Previous Map

[crow]

BBC News also had a story on the previous map. This was prepared in anticipation of the current mission--apparently they needed a rough idea of what to expect.

Interesting trivia--you weigh 1% less in India than average.

Re:Previous Map

[Anonymous Coward]

Because variations in gravity are very small, units for gravity surveys are generally in milligals (mgal) where 1 mgal is one thousandth of 1cm/s^2
Unless India is really off-the-scale, I think the BBC made a math error with their 1% figure.
If the avg force of g is 9.81m/s^2
9.81m/s^2 * 1000 * 100 = 981,000 mGAL
60 / 981,000 = 0.006%

Re:Sub dectection

[whoda]

Well, inertially navigated submarines DO have a gravity map of the area they are operating in. At least the one's I was on.

The gravity map values are applied as correction factors to the inertial navigators.

The gyro's are attracted towards denser areas, which causes precession, which is picked up as an incorrect acceleration, and this throws the position of the inertial navigator off.

So we basically aplied a correction signal to keep the gyro's orientated to the correct reference planes in the math model so the 'real' accelerations could be correctly calculated.

Re:Seems to me ...

[stanwirth]

Maybe that helps aids the tectonic flows around here? Seems to me that the techtonic flows cause, rather than are caused by gravitational differences. Less mass in one area == less gravity, and so forth.

Tectonic movement is caused by density variations associated with the earth's being heated from within (decay of radioactive elements) and cooled from without. This drives convection currents (think chicken soup). What we see on the surface is the horizontal component of those convective movements. The gravity anomalies associated with these density variations are on the 100km-1000km length scale.

OTOH you can get gravity anomalies due to plain old topography, changes in chemical composition of the crust (e.g. an iron ore body, or uranium deposit) which are associated with both mass and density variations, but have nothing to do with either tectonics. The gravity anomalies associated with these effects are generally of a much shorter wavelength than the anomalies associated with convective (tectonic) forces.

Re:Practical uses

[HotNeedleOfInquiry]

Sorry about the broken link. Take out the space between node and 32.

http://stommel.tamu.edu/~baum/reid/book1/book/node 32.html

Is HTML really that hard?

[p3d0]

The link from the parent: http://www.csr.utexas.edu/grace/gallery/gravity/03 _07_GRACE.html

Re:mGal

[jfengel]

Actually a Galileo is 1 cm/s^2. An mGal is a milliGalileo, or 10^-3 cm/s^2.

Earths gravity is roughly 980,000 mGals, so the entire range they're measuring is 120 parts in a million. It looks like their precision is maybe 1 part in a million. That's pretty impressive.

Re:Practical uses

[fluffy666]

To be fussy, the gravity anolalies are more to do with out-of-equlibrium crust than simple highs and lows. So the East pacific rise (a mid ocean ridge) barely shows up at all, but the ridge over a hotspot at Iceland shows up a mile. In a similar way, the Plateaus of Tibet and S. America, which are currently undergoing gravitational collapse, show up strongly.

On a larger scale (see the Indian ocean), the really large scale anomolies are hypothesised to be the result of deep mantle convection.

Re:Seems to me ...

[Zan Zu from Eridu]

This is concidered not entirely true today. Today it is thought that subduction is the major driving force behind plate tectonics, not convection.

quote:

Until the 1990s, prevailing explanations about what drives plate tectonics have emphasized mantle convection, and most earth scientists believed that seafloor spreading was the primary mechanism. Cold, denser material convects downward and hotter, lighter material rises because of gravity; this movement of material is an essential part of convection. In addition to the convective forces, some geologists argue that the intrusion of magma into the spreading ridge provides an additional force (called "ridge push") to propel and maintain plate movement. Thus, subduction processes are considered to be secondary, a logical but largely passive consequence of seafloor spreading. In recent years however, the tide has turned. Most scientists now favor the notion that forces associated with subduction are more important than seafloor spreading. Professor Seiya Uyeda (Tokai University, Japan), a world-renowned expert in plate tectonics, concluded in his keynote address at a major scientific conference on subduction processes in June 1994 that "subduction . . . plays a more fundamental role than seafloor spreading in shaping the earth's surface features" and "running the plate tectonic machinery." The gravity-controlled sinking of a cold, denser oceanic slab into the subduction zone (called "slab pull") -- dragging the rest of the plate along with it -- is now considered to be the driving force of plate tectonics.

Re:Other Applications

[GeoGreg]

In fact, gravimetric data has been collected from Mars orbiters, although the precision is nothing like what the Texas researchers are doing.

I doubt that gravimetric data will be of much use in high-end physics research, unless it's somehow used to support experiments for the detection of gravitational waves, for instance. The data is very useful for putting better constraints on various models of geodynamics, though.

Re:Sub dectection

[Yet Another Smith]

Yeah, water compresses very very slightly. However, the gravity signature of the sub would be way below the noise level. It would be a signature probably much much smaller than a microGal - probably nanoGals (off the cuff estimate). MicroGals are very difficult to measure on land in perfect conditions. In the air over the ocean it would be impossible.

As for doing it with Grace, the data will be far too coarse to detect the sub's anomaly. The anomaly of a sub at periscope depth would be strongest, but the wavelength would only be about as long as the boat. To model it well enough to detect it, you'd have to sample several times across the wavelength. Grace is more likely to have a sample interval measured in kilometers than tens of meters. It would never detect a sub, even if it had a high-amplitude anomaly. It would be noise.