
Seafloor Features Are Revealed by the Gravity Field - Mz
http://earthobservatory.nasa.gov/IOTD/view.php?id=87189
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stoey
This reminded me of NASA's GRACE mission [1] that I recently heard about. It
can track the depletion of California's aquifers (for example) based on the
tiny changes in orbit of the two tandem satellites.

Was mentioned on a recent Still Untitled podcast episode by NASA CTO David
Miller [2], the whole thing is worth a listen if you want to hear him eagerly
talking about all the cool things NASA is up to.

[1]
[http://www.nasa.gov/mission_pages/Grace/](http://www.nasa.gov/mission_pages/Grace/)
[2] [http://www.tested.com/science/space/558108-return-rock-
still...](http://www.tested.com/science/space/558108-return-rock-still-
untitled-adam-savage-project-122215/)

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snowwrestler
These maps were made by detecting small differences in sea surface altitude,
and inferring the gravitation field. But changes in gravity can also be
measured directly, using a gravimeter.

However, they tend to be finicky to work with--difficult to calibrate and to
eliminate sources of error. After all, we know from Einstein that there is no
measurable difference between gravitational acceleration and inertial
acceleration (like, say, bumping a sensor).

Still, gravity seems like an area of immense promise for sensors of the
future. We've spent immense effort improving and miniaturizing sensors for
things like magnetic fields, electrons, and photons. But we've recently
discovered--using gravity--that these things might represent only 5% of the
total mass of the universe. We haven't even detected a gravity wave yet. We're
just barely getting started at using gravity to observe and understand the
universe.

~~~
wycx
_However, they tend to be finicky to work with--difficult to calibrate and to
eliminate sources of error. After all, we know from Einstein that there is no
measurable difference between gravitational acceleration and inertial
acceleration (like, say, bumping a sensor)._

Gravity gradiometry is used in exploration geophysics. Typically this is done
as a ground survey, taking measurements on a grid. I don't know how they do
the error correction, but aerial gravity surveys can also be done. Obviously,
the more stable the platform the higher quality the data, which is by De Beers
used a zeppelin for surveys in Botswana looking for kimberlite pipes. This
company [1] uses a BT-67 (an updated DC-3) as their fixed wing platform. The
company I used to work for had a small gravity survey flown in the mid 2000s.
When chatting with the technician from Bell, I facetiously asked whether they
had considered an Antonov An-2 [2], and was surprised to hear that it had been
(briefly) considered it as a platform.

[1] [http://bellgeo.com](http://bellgeo.com)

[2]
[https://en.wikipedia.org/wiki/Antonov_An-2](https://en.wikipedia.org/wiki/Antonov_An-2)

~~~
snowwrestler
When I studied geology we did a gravity survey along a road. The gravimeter
was a big metal can; inside there was a weighted arm suspended by a spring.
Measuring the differences in the static deflection of the arm gave us a
reading of local gravity at each point along the survey.

The can had to be placed perfectly level, and sit for a couple minutes (to
allow any vibration in the sensor suspension to dissipate) before taking the
reading. We also couldn't take any readings when a vehicle was going past.
Luckily this was along a fire road in the mountains, so there was very little
traffic.

I've heard of both aerial and underwater (submarine or towed sensor) gravity
surveys, and it's so impressive that they get usable data from a moving
platform. I think one trick is to do multiple passes so that chaotic sources
of error (like turbulence) average out at any given point--while real
differences in gravitation would persist.

Edit to add: Going back to my comment above, I can't imagine how a truly
portable (like, arbitrarily hand-held) gravity sensor could be developed, the
way we have portable sensors for light, radio, sound, ambient pressure,
magnetic field, etc.

~~~
jofer
Most of the moving measurements aren't gravimeters. They're gradiometers.

You don't actually get the same data out, and you can't use it in the same
way.

The key part isn't just the acceleration due to motion. It's that you have to
know your absolute elevation very precisely if you're using a gravimeter.
Otherwise, the data you get can't be corrected relative to your other
measurements and is more or less useless. (The method mentioned in the article
actually measures the geoid directly by measuring the sea surface, which is an
equi-potential surface.)

However, if we don't worry about the absolute acceleration due to gravity, and
instead measure the local rate of change in gravity, we don't need to know
elevation precisely.

That's referred to as gradiometry. You can't use the data in the same way, but
it's still very useful.

In a nutshell, most of the movable gradiometry sensors work by using multiple
accelerometers. Acceleration due to motion affects both equally (with some
caveats when rotation comes into play). The differences in acceleration
between the two accelerometers is therefore purely a result of the "tilt" of
the geoid locally.

It's difficult to integrate this back into an accurate picture of what the
free air or Bouger anomaly would look like, but it's still useful information.
We can't necessarily calculate the same things from it, but it's a great edge
detector.

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ajmurmann
Applying the reverse of this is actually really fascinating as well. A friend
of mine used to work on "navigation systems for submarines". According to him
some submarines measure the pull of gravity to help keep track of their
location.

~~~
breatheoften
Some of these shipboard gravimeters originally developed for subs during the
cold war ended up on research ships -- and are still on them.

In my old life I would tend to these instruments (still in operation).
Finicky, old, entirely analogue systems ... but they keep "working" 35+ years
later.

~~~
ajmurmann
And here I thought this was cutting edge technology...

~~~
jofer
We've been doing gravity surveys for over a century. All you need is to 1)
know the elevation precisely, and 2) have a very well calibrated spring.

In fact, the really old Lacoste & Rhomberg style gravimeters are often thought
to be more accurate than their modern counterparts because the springs used to
be very meticulously hand-crafted instead of mass-produced.

Even Sandwell & Smith's method for inverting for bathymetry from the Free Air
Anomaly (i.e. local sea level) has been around for a long time at this point.
(Still damned neat, though.)

The cutting edge stuff these days is in satellite methods that can measure
gravity anomalies over land (e.g. GRACE and GOCE). They're still much lower
spatial resolution than the sea-surface based methods, but you can use them
over the entire globe.

GRACE, for example, measures the distance between two satellites ~100km apart
to an accuracy of ~1 micrometer. Pretty impressive! GOCE has multiple very
precise accelerometers at different ends of the satellite. Because
acceleration of the craft will affect accelerometers equally, the differences
in acceleration must be due to local changes in the geoid. (In other words,
GOCE is gradiometry based.)

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mlpinit
paraphrasing: Underwater mountains exert a gravitational pull on the water
above and around them causing small but measurable bumps.

This is very cool! I never thought about the ocean as being bumpy.

~~~
FreeFull
And a similar thing happens with large rifts, except the bump goes the other
way.

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lmm
These maps use data from JASON-1; JASON-3 is launching soon, possibly in mid-
January.

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dang
Url changed from [http://gizmodo.com/heres-the-most-complete-ocean-floor-
map-e...](http://gizmodo.com/heres-the-most-complete-ocean-floor-map-ever-
made-1750109568), which points to this.

