Seafloor Features Are Revealed by the Gravity Field
earthobservatory.nasa.gov
earthobservatory.nasa.gov
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/ [2] http://www.tested.com/science/space/558108-return-rock-still...
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.
I think it's better to say that we know it from Newton's laws of motion and gravity - they are older, much easier to understand, and can still explain the principles of the technique.
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.
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.
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.
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.
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.)
If the submarine were a perfect spherical shell of denser and lighter sub-shells, everything would cancel out. However, they're not. They're essentially really dense blobs attached to much less dense blobs.
You get a gravity anomaly due to the lack of spherical symmetry.
This is very cool! I never thought about the ocean as being bumpy.