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Science

Gravity Map of Earth 72

Posted by michael from the light-reading dept.
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?"
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Gravity Map of Earth More

Gravity Map of Earth

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  • This will be most useful! (Score:3, Funny)

    by HTH NE1 ( 675604 ) on Thursday July 24, 2003 @05:41PM (#6526503)
    Now I know where the best places would be to search for Upsidasium mines!

  • mGal (Score:5, Interesting)

    by molo ( 94384 ) on Thursday July 24, 2003 @05:41PM (#6526508) Journal
    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
  • Is it just me or does this ship [utexas.edu] looks so much like someting out of frontier (Elite). Even the jet of flames from behind it.

  • Seems to me ... (Score:4, Interesting)

    by Quixotic Raindrop ( 443129 ) on Thursday July 24, 2003 @05:55PM (#6526617) Journal
    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 ... (Score:5, Informative)

      by stanwirth ( 621074 ) on Thursday July 24, 2003 @07:21PM (#6527432)

      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 ... (Score:4, Informative)

        by Zan Zu from Eridu ( 165657 ) on Friday July 25, 2003 @06:37AM (#6530438) Journal
        This is concidered not entirely true today. Today it is thought that subduction is the major driving force behind plate tectonics, not convection.

        quote: [usgs.gov]

        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 ... (Score:1, Insightful)

          by 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.
          • 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 conve

            • 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 cou

  • Previous Map (Score:4, Informative)

    by crow ( 16139 ) on Thursday July 24, 2003 @05:57PM (#6526649) Homepage Journal
    BBC News also had a story on the previous map [bbc.co.uk]. 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 (Score:1, Informative)

      by 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%
  • "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 (Score:3, Interesting)

      by Syncdata ( 596941 )
      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. [bbc.co.uk]
      • 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 (Score:4, Interesting)

      by MerlynEmrys67 ( 583469 ) on Thursday July 24, 2003 @06:29PM (#6526992)
      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 (Score:5, Interesting)

        by BigBir3d ( 454486 ) on Thursday July 24, 2003 @06:47PM (#6527154) Journal
        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...
        • "
          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 (Score:3, Interesting)

            by BigBir3d ( 454486 )
            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.
        • 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.
          • False. You can compress a liquid, just not much. Think physics... there is space between the molecules (how else would they move).

            link [physlink.com]
            • 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
            • Just saw, it does appear that I am wrong and that water can be compressed - just with great difficulty. I stand corrected.
      • 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

    • Re:Sub dectection (Score:5, Informative)

      by whoda ( 569082 ) on Thursday July 24, 2003 @07:19PM (#6527416) Homepage
      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.

      • 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.
        • 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."
        • 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 c

  • Places that really, really suck (Score:5, Funny)

    by RobertB-DC ( 622190 ) * on Thursday July 24, 2003 @06:01PM (#6526676) Homepage Journal
    Looking at the detailed map [nasa.gov], 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.

  • Practical uses (Score:4, Insightful)

    by HotNeedleOfInquiry ( 598897 ) on Thursday July 24, 2003 @06:33PM (#6527036)
    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.

    • 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 (Score:3, Insightful)

      by pyr0 ( 120990 )
      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 (Score:4, Informative)

        by fluffy666 ( 582573 ) on Friday July 25, 2003 @04:11AM (#6530101)

        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.

        • 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).
    • 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.

  • [subject] (Score:3, Funny)

    by charlie763 ( 529636 ) on Thursday July 24, 2003 @09:55PM (#6528387)
    [Insert comment about the gravity of the situation here]
    • 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 (Score:4, Funny)

    by kaltkalt ( 620110 ) on Friday July 25, 2003 @05:02AM (#6530215)
    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....]
  • 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?
  • 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

    • Re:Other Applications (Score:2, Informative)

      by GeoGreg ( 631708 )

      In fact, gravimetric data has been collected [wustl.edu] 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.

  • looks like an elevation map
  • Buy gold in the north east of the North American continent. Sell it in the northern regions of the Rockies.

  • Uh... Great reporting. (Score:3, Interesting)

    by pla ( 258480 ) on Friday July 25, 2003 @03:45PM (#6534723) Journal
    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. ;-)
  • Note in the image [nasa.gov] 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.

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