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!

Re:This will be most useful!

[Anonymous Coward]

Hey Moose! This teenage crowd is much too young to appreciate your comment.

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

[Xenophon Fenderson,]

Sorry, but I feel like putting on my "physics pedant" hat on. Gravity is measured in units of distance per units of time squared (i.e. acceleration). 1 mGal = 1 cm / s^2.

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

[Anonymous Coward]

If time and space are "really" the same, is acceleration really 1/time i.e. a frequency?

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.

Something from frontier

[brejc8]

Is it just me or does this ship looks so much like someting out of frontier (Elite). Even the jet of flames from behind it.

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.

Re:Seems to me ...

[Anonymous Coward]

yes, but you must remember that both ridge processes (e.g. "ridge push") and subduction processes (e.g. "slab pull") are the crustal manifestations of convective processes in the mantle.

Re:Seems to me ...

[Zan Zu from Eridu]

I think you missed the point. Conventional theory holds the plates are floating freely and convection makes them move by pushing them apart at the oceanic ridges. When two plates collide head on one slides over the other, pushing the bottom plate down into the subduction zone. Now it's thought that the cooler oceanic slabs of the plates sink into the subduction zone by their own weight, even without another plate pushing them down. This subduction by own weight contributes more to plate tectonics than convection effects.

Re:Seems to me ...

[stanwirth]

Now it's thought that the cooler oceanic slabs sink into the subduction zone by their own weight...

The buoyancy contrast as driving force you describe -- between cold (dense) slab and warm (less dense) mantle -- is by definition thermal convection.

The fact that some Earth Scientists actually argue over "whether it's slab pull, ridge push, or thermal convection," is just proof that they went into Earth Science because they failed their undergraduate classical mechanics and hydrostatics courses.

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%

Sub dectection

[satanami69]

"This is the only satellite technology that can actually see through the surface of the ocean; everything else gives you a measure of what's going on in the top metres.

"But with this, you actually see straight to the bottom."

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

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

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

[mstorer3772]

"
Question is; can we detect it fast enough to matter? Knowing where a sub was 30min ago is useless.
"

I disagree. While 30-minute-old information might not be of much use in a sub-battle, it could be quite useful to know that "them dang ians just launched 4 subs towards 's coast... and we can expect them to arrive in less than 2 days" on a strategic or political level.

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.

Re:Sub dectection

[bill_mcgonigle]

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.

Just curious - how do you know where you are on the map? I'm assuming you can't get GPS at depth.

Re:Sub dectection

[Anonymous Coward]

The US has a VLF transmission system - very low frequency electromagnetic waves are transmitted *through* the Earth and Oceans (also the basis for Tesla's power transmission scheme, which was unfortunately squashed)

I would guess triangulation from a few VLF sources would pretty accurately determine the location of the sub.

Re:Sub dectection

[bill_mcgonigle]

I would guess triangulation from a few VLF sources would pretty accurately determine the location of the sub.

Yeah, that seems like a really good way to do it. What confused me though was the poster talked about 'inertially navigated submarines'. I'll presume they're more accurate than previous systems, maybe for navigating canyons and such - if they're using gravity maps they need a high degree of precision for something.
The only way I could think of to do it was to get a known good reading (e.g. surface for GPS), then take continuous readings from accelerometers until the next surface. However, the readings would have to be exceedingly accurate and any errors would forward propogate until recalibrated. I considered this unlikely and error-prone, but, hey, maybe we can make some really good gear these days.

Re:Sub dectection

[Wyatt Earp]

They know where they were when they left harbor. They have advanced inertial navigation systems and they have guys with maps and grease pens.

http://www.raytheonmarine.de/highseas/products/s ub /navigation/index.html

When they stream a radio antenna to get messages from command, they have a gps antenna on that as well.

"Give me a stopwatch and a map, and I'll fly the Alps in a plane with no windows."

Re:Sub dectection

[Anonymous Coward]

Actually 30 min old info could be quite good given that you had a steady stream of that info. It could be used to track general location and a reasonable assumption of bearing based on passed data. At some point the data would be to old to have any value but a steady stream of slightly dated data could be highly useful.

Re:Sub dectection

[onyxruby]

No! Compressed air for a plane, ship or vehicle, sure. Compressed water? Not gonna happen. You can't compress water, it simply not possible. Think about it, with all the weight of the worlds oceans, water still hasn't compressed. As for water moving faster at different points, sure, but getting compressed, not going to happen.

Re:Sub dectection

[BigBir3d]

False. You can compress a liquid, just not much. Think physics... there is space between the molecules (how else would they move).

link

Re:Sub dectection

[whoda]

We navigate by knowing our exact starting position.
From that position, any acceleration in any direction is detected.

The accelerations in all dimensions are integrated over time, which, after lots of math, gives you a velocity vector.
Add this vector to the last point, and now you know where you are.

Obviously there are many outside influences that can cause errors, heat and vibration being 2 obvious choices. (Friction, not enough decimal points, etc...)

We can control heat and vibration to an extent, and correct these and the rest of the errors with physical or mathematical corrections.

We cannot control gravity, but we can measure it accurately enough to correct the effects of it.

Periodically, we would do as other posters have mentioned, and we re-align the gyro's using GPS fixes, or in the old days, LORAN.

Don't be fooled though, we could make a whole patrol underwater, with no corrections from a GPS fix, and still be close enough to our computed position that the missles would land on the right city in Russia. ;)

Re:Sub dectection

[Yet Another Smith]

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

You're right. Magnetic Anomaly Detection (MAD) is a common method for detecting submarines, although it's generally only used to pinpoint the location of a sub that's already been detected by passive sonar. Of course, it's the iron in the hull you're detecting, not the reactor (or atleast not the nuclear parts of the reactor, but the steel parts). It works just as well for Diesel/Electric submarines as it does for nuke boats.

Sub-hunter helicopters have magnetometers in a 'bird' that they lower on a cable (to get it away from the airship's magnetic signature). Fixed wing planes like the P-3 Orion have their magnetometers in odd-looking 'booms' sticking out the back of the plane. Look a bit like the plane's got a stinger.

Interestingly, there were rumors floating about that the Russians built their Alpha class subs (predecessor to the Sierra and Akula classes) with titanium hulls, which, not being ferromagnetic, doesn't have nearly as big a MAD signature. The titanium was for a stronger hull, rather than the magnetic properties, though. Dunno if that was true or not, but sounded like a cool idea.

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.

Re:Sub dectection

[Anonymous Coward]

"there", not "their"

The shape of a submarine is designed to reduce water compression, as any kind of pressure change could be detected by a nearby enemy.

And if you really knew anything about submarine technologies you would know that "bubbles generated by the spinning props" happen due to "cavitation". Cavitation is carefully avoided, as it causes a lot of noise, so you're not going to detect a submarine that way unless the sub either knows that it is safe or knows that it is in big trouble. And cavitation is a very local effect and will not alter water density significantly.

Rent "Red October" a second time and pay more attention this time.

Re:Sub dectection

[Anonymous Coward]

Think about it, with all the weight of the worlds oceans, water still hasn't compressed.

Your thought doesn't alter whether it compressed or not. Provide information which supports your baseless claim that water at the bottom of the ocean is not compressed. (Yes, I know the characteristics of water, but you seem to think that you are providing an example of some sort)

Re:Sub dectection

[Anonymous Coward]

I would guess triangulation from a few VLF sources would pretty accurately determine the location of the sub.

A guess does not indicate any relationship to reality.

The VLF transmission system penetrates water better than higher frequencies. Because it can not send signals quickly, it only sends short codes with specific meanings (similar to telling a sub that it has email but not delivering the mail in that signal).

VLF also requires a very long antenna. To do triangulation they'd have to do something like trail their antenna across one area, then cross through that area at another angle so they can get another signal reading. Even if they could do direction finding with such awkward tools, the long wavelength of a VLF signal would restrict the precision so the result would be a very fuzzy location estimate.

Summary: You are wrong.

The other people are right. The whole purpose of an inertial navigation system is that it can know its position without external information. An ILS is now quite good, and additional information is merely nice to have and not required except when there is a malfunction.

Re:Sub dectection

[onyxruby]

Just saw, it does appear that I am wrong and that water can be compressed - just with great difficulty. I stand corrected.

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:Places that really, really suck

[Anonymous Coward]

Well you know, France is just _old_ europe. :)

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

[A Bugg]

You are wrong sir. The fact that earth rotates means that it bulges out around the equator which means theoretically there is more gravity. Since there is a bulge around the equator there is less mass around the poles therefore less gravity. The reason it appears the way it is has more to do with concentrations of mass in the continents and plates. A simple physics class will tell you this.
A Bugg

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:Practical uses

[pyr0]

Yeah, I hear ya. My point was that from simple observation of those pictures, 2 and 2 could have been put together to come up with a more meaningful explanation than the change in rotational velocity at the surface. I find it hard to get technical about geologic information with people who aren't geologists, meaning most of the slashdot readers. That plus I'm a geochemist and not a geophysicist, so I'm talking a out of my area of expertise here (been a while since I had my global plate tectonics class).

Re:Practical uses

[anubi]

I don't wanna poke too much into this, but I question the phrase,

"Anyone installing a highly precise weight scale uses this data."

It was my understanding that this is one of the reasons for using a counterbalance-style scale in critical situations.

Variances in gravitational attraction affect both the object being weighed, and the counterbalance, in identical proportions, thus assuring the accuracy of the balance irregardless of gravitational force.

Re:Practical uses

[thelandp]

Or... er... just click on this...

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:Is HTML really that hard?

[Anonymous Coward]

wow, that's the worst karma whore that I've ever seen

[subject]

[charlie763]

[Insert comment about the gravity of the situation here]

Re:[subject]

[SEWilco]

Slashdot needs a location-adjusted Funny moderation system to compensate for differences between the gravity of the article at the locations of the poster, moderator, and reader.

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:GPS, GRACE and Gravity Question

[Anonymous Coward]

I think it is more interesting than that.

The time on the GPS satellites is affected by the gravity which they pass through. Knowing gravitational variations better will allow better compensation for variations, leading to better GPS, leading to better gravitation measurements, leading to better GPS...but who needs to know their location to a nanometer?

Re:So, I weigh less on the West coast than the Eas

[mess31173]

Come to think of it, I do too.... stop eating.

It's a joke, laugh.

Other Applications

[mizidymizark]

While it is all well and good that we can use these maps to show that more water is close to the Equator, but I think the real usefulness of these satellites will come into play when this research can be applied to understanding gravity itself. It is nice to see where it is strongest on Earth, but we know so little about gravity and how it works, it would be nice to see this used to work towards the Unified Field Theory.

I think these satellites could also be used on other heavenly bodies, such as the Sun and Mars. Imagine placing these over Mars, it could potentially be used to see if there are fluctuations in the gravity field which could indicate if Mars has any movement under it's surface.

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.

Now..

[kmak]

Chick can lose weight the easy way! (Without moving to, say the moon)

looks like an elevation map

[extrarice]

looks like an elevation map

Earthquakes?

[saskwach]

Maybe I'm just advertising my ignorance, but it seems to me that stress at a fault line should cause an increase in ground density, what with the squishing and whatnot. Could this be used to predict earthquakes?

Trade in precious metals?

[Jerk City Troll]

Buy gold in the north east of the North American continent. Sell it in the northern regions of the Rockies.

Re:Trade in precious metals?

[SEWilco]

Buy gold in the north east of the North American continent. Sell it in the northern regions of the Rockies.

Steal gold between those two regions.

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. ;-)

Obvious Use of Gravity Map

[Anonymous Coward]

Won't someone mention perpetual motion machines?

Re:Obvious Use of Gravity Map

[Anonymous Coward]

Perpetual motion?

What do you intend, to move a weight on a spring from a low gravity area to a high gravity area? It won't move enough to create enough power to move itself back to the low gravity area.

Going Down!

[Anonymous Coward]

Ha.

Trying to measure ocean levels, some people think that they measured rising ocean levels in the Pacific islands between SE Asia and Australia.

Geologists, however, know that the land in his area is sinking. Of course water is higher on the land.

Notice on the map the high gravity under that area. That's the denser, cooler material in the downward convection flow which happens to be under that area.

Yucatan half-circle

[Mazzaroth]

Note in the image just North of the Yucatan peninsula (South-East of Florida), the strong half-circle anomaly. I've read before that this particular anomaly is probably the signature of the asteroid that hit the Earth 60 millions years ago and caused the extinction of the dinosaurs.