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

This will be most useful!

[HTH NE1]

Now I know where the best places would be to search for Upsidasium mines!

mGal

[molo]

BTW, if you want to know how much gravity differs (and how damn sensitive these sats are), look at this chart:

http://www.csr.utexas.edu/grace/gallery/gravity/ 03 _07_GRACE.html

And note the range of the legend -60 to +60 mGal.

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

Thats damn sensitive.

-molo

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:mGal

[Gyl]

I know others have also made conversions, but for my own sake, and possibly others:

Normal gravity is about 9.8 m/s^2, 1 mGal = 1 x 10^-3 cm/s^2 = 1 x 10^-5 m/s^2. 60 mGal = 6 x 10^-4 m/s^2 = 0.0006 m/s^2

so the measurment range, (assuming 9.8 is perfectly accurate) is 9.8006 to 9.7994 m/s^2.

Seems to me ...

[Quixotic Raindrop]

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.

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: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.

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.

Places that really, really suck

[RobertB-DC]

Looking at the detailed map, it's fair to say that, in the Western Hemisphere, Alaska sucks. Also, Montana seems to suck, and southern Mexico sucks, too. Colombia and Chile, though, really suck.

On the other hand, the Bermuda Triangle totally blows.

Interesting how the map relegates Europe to the fringes... I'm suuuuuuuure it's just because the Prime Meridian happened to cut France in half.

Re:Sub dectection

[Syncdata]

From what I understand (very little), we already have very sensitive equiptment detecting submarine presence.
Again, I'm trying in vein to remember the source, but I recall listening to a Radio program on how satellites are used to note peculiar wakes in the ocean, and can even differentiate between a submarine and a large whale.
I can't find anything technical on google, but apparently The chinese government was interested enough to steal the info.

Re:Sub dectection

[MerlynEmrys67]

I wonder if this technology is going to also be used to locate subs?

Can't work. Basically you are measuring the denisty/mass differences in large areas (lets even say you can do it down to the centimeter though). So an area that has high mountains, highly dense subsurface is going to have a higher pull from gravity.
A submarine with neutral boyancy (not going up or down, just level) would displace an equivalent volume of water, therefor not change the gravity field around it.
That said, there is no reason why we can't us other things like detecting the change in magnetic flows because a large nuclear reactor just went underneath, and things like that

Practical uses

[HotNeedleOfInquiry]

Anyone installing a highly precise weight scale uses this data. Also note that the apparent gravity changes from a high level at the poles to a low level at the equator due to the surface rotational speed of the earth.

I'll leave it as an exercise to the reader to determine how much.

Re:Practical uses

[pyr0]

Actually if you knew a little bit of geology and some geography to go with it, you would see that the gravity highs exist around areas of *very* thick crust. In other words, areas of mountain building due to either collision of plates or from volcanism (including the mid-ocean ridges).

Re:Practical uses

[HotNeedleOfInquiry]

No, you are wrong, sir, and I am right.

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

I guess you missed that simple physics class

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

Re:Practical uses

[A Bugg]

I apologize i remembered incorrectly.
A Bugg

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:Sub dectection

[BigBir3d]

If the sub is moving, their is a layer of compressed water on one side, and a layer of slightly less compressed water on the other side (kind of like how radar sees a moving volume). Not to mention any bubbles generated by the spinning prop(s) out back decreasing the density of the water significantly. Question is; can we detect it fast enough to matter? Knowing where a sub was 30min ago is useless.

IIRC, magnetic harmony has already been reached between sub and ocean. I think that was something they did back in the 50's or 60's. Not 100% on that though...

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:Sub dectection

[BigBir3d]

Sub launches are detected by traditional means; optical satellites, or men/women on the ground in the area.

2 days means nothing in a world full of ICBM's tipped with multiple nuclear warheads.

Is HTML really that hard?

[p3d0]

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

[subject]

[charlie763]

[Insert comment about the gravity of the situation here]

So, I weigh less on the West coast than the East

[kaltkalt]

Seems one weighs more in california than in new york. Am I the only one who has a great idea for a new 'guaranteed' weight-loss plan to sell on ebay? [running to the US patent office....]

GPS, GRACE and Gravity Question

[mrthoughtful]

Grace uses GPS for it's findings.
GPS receivers use a gravity map to aid in their calculations.
Does this mean that Grace can improve it's GPS signals, and iteratively improve upon it's measurements?

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.

Uh... Great reporting.

[pla]

If you were to fly over the red areas, you would be tugged ever so slightly downwards;

As opposed to? Yes, in our normal experience, gravity acts as an attractive force.


the blues mark regions where the planet's attraction is much weaker.

"Much weaker"? The entire range corresponds to about 0.1% of Earth's mean gravitational attraction. For comparison, the apparent decrease in local gravity at the equator due to centrifugal force FAR exceeds the differences shown in the linked article (by a factor of about 5, in fact). And even that only changes the local apparent force to 9.78m/s/s.


Then again, I suppose we have separate careers called "journalist" and "scientist" for a reason. ;-)

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.

Re:Sub dectection

[SEWilco]

I'm trying in vein

That's "trying in vain", unless the submarine which you're dealing with is a shrunken one inside a human and Raquel Welch is in it.