From the European Space Agency: CryoSat goes to sea
CryoSat was launched in 2010 to measure sea-ice thickness in the Arctic, but data from the Earth-observing satellite have also been exploited for other studies. High-resolution mapping of the topography of the ocean floor is now being added to the ice mission’s repertoire.
The main objective of the polar-orbiting CryoSat is to measure the thickness of polar sea ice and monitor changes in the ice sheets that blanket Greenland and Antarctica.
But the satellite’s radar altimeter is not only able to detect tiny variations in the height of the ice but it can also measure sea level.
The topography of the ocean surface mimics the rises and dips of the ocean floor due to the gravitational pull. Areas of greater mass, such as underwater mountains, have a stronger pull, attracting more water and producing a minor increase in ocean-surface height.
Therefore, instruments that measure sea-surface height incidentally map the ocean floor in previously uncharted areas.
There have been several recent global gravity missions, such as ESA’s GOCE satellite, that provide extraordinarily accurate measurements of gravity at the spatial resolution of hundreds of kilometres.
But CryoSat’s radar altimeter can sense the gravity field at the ocean surface, so that seafloor characteristics at scales of 5–10 km are revealed. This is the first altimeter in 15 years to map the global marine gravity field at such a high spatial resolution.
Recent studies at the Scripps Institution of Oceanography in San Diego, USA, found that the range precision of CryoSat is at least 1.4 times better than the US’s Geosat or ESA’s ERS-1.
They estimate that this improved range precision combined with three or more years of ocean mapping will result in global seafloor topography – bathymetry – that is 2–4 times more accurate than measurements currently available.
“We know more about the surfaces of Venus and Mars than we do about the bathymetry of deep oceans,” said David Sandwell from the Scripps Institution of Oceanography in the US.
“This new mapping from CryoSat will revolutionise our understanding of ocean floor tectonics and reveal, perhaps, 10 000 previously uncharted undersea volcanoes.”
Most satellite radar altimeters such as the one on the joint CNES/NASA/Eumetsat/NOAA Jason-2 follow repeated ground-tracks every 10 days to monitor the changes in ocean topography associated with ocean currents and tides.
CryoSat’s 369-day repeat cycle provides a dense mapping of the global ocean surface at a track spacing of over 4 km. Three to four years of data from CryoSat can be averaged to reduce the ‘noise’ due to currents and tides and better chart the permanent topography related to marine gravity.
I would have thought that there were many factors influencing sea height. Shallow water/ deep water, density of the surface floor, planetary attraction, density of the mantle, effect of latitude on the sea level (thinking here of the Earth’s rotation), varying density of sea water itself (would dense sea water provide a gravitational attraction similar to the upper mantle and reduce the contour relationship ?).
Talk about proxies for proxies. This is an example of experimental scientists doing what they do best, using ingenuity to extract some measure of data from a previously hidden haystack. If they can just report the wonders of their discoveries, and not try to attach it to any political agenda, then they deserve the kudos they will get.
The Apollo13 and Hubble repair missions are examples of what good experimental science can achieve. Can this thing tell if I need to clean the bottom of my swimming pool ?
This is very interesting as well as remarkable.
I hope data is not abused.
That’s interesting. Yes, the geoid. Uhmm….. 2-4 times accurate? I’ll have to check, but I hope the ice thickness tech isn’t the same which went into the estimation of the Himalayan glacial measurements. -4 +/-20gt.
As to moving it to sea-level measurements….. I’d keep a wary eye. Envisat “died” right after they “corrected” their data, even data they never presented before! Now both Jason’s are seemingly stuck in stupid. I’ve chronicled the slow “adjustments” to Jason II here….. https://suyts.wordpress.com/2012/04/18/the-quickening-jason-ii-getting-in-the-act/
But anyone can go here, http://www.aviso.oceanobs.com/en/news/ocean-indicators/mean-sea-level/products-images/ and see some very strange things occurring, or rather, not occurring. Just knock off any adjustments (click on any radio button with the word “not” in its description) to see how strange this all is. Did they all collide up there?
From above – “But CryoSat’s radar altimeter can sense the gravity field at the ocean surface,”.
I did not know we understood enough about what gravity is to be able to detect in this fashion. I presume there is some proxy method of measurement being used here. If there is anyone that can shed some more light on the subject?
Thanks.
Excellent! A better understanding of the ocean floor topography and tectonics is highly desirable. “….10,000 previously uncharted undersea volcanoes”??? Oh, Baby!!!!!!!
So they are going to use the data from the parts of the orbit they previously didn’t use to do other science. That is ok. I only object when they start removing sensors from their primary mission to do some tertiary junk projects then miss the primary data.
……wind strength, direction, SOI changes.
I came across some impressive logic here as NASA tries to explain the 2010 drop in sea level.Check the top right diagram, water fell in Brazil and Australia reducing sea height……mmmm… where did it go after that ?
It would be really nice, to get a better picture, of the total volcanic activity happening and where. A few spot checks, should confirm the results definitively, I would think. GK
It’s ESA; situated in hyper-PC-warmist EU. They have rewritten the entirety of the Envisat record because the satellite wouldn’t play ball. They will do what it takes to prove what needs to be proven.
I have to read the apropriate links, but this smells. Somehow, the satellite designed to measure ice is now going to be reasigned to measure water? How can this be? Perhaps the ice measurements were not only wrong, but unadjustable?
As McIntyre would say, watch the pea. But that is wrong advice. Yes, it is a shell game the Warmistas are playing. But, the key to the shell game is that the pea is under none of the cups. No point in watching the pea; that is the whole scam – the audience watches the hand that supposedly hols the pea while it is in fact not in play, in the other hand.
@ Truthseeker: What the satellite is doing is like what a gravimeter does when measuring the gravity field on the surface of the earth. Gravity anomaly maps are used in oil/gas exploration and in mining to identify possible places for exploration/exploitation. Since it would be cost prohibitive to cover the oceans with the detail and speed the satellites can do, this is a good idea and will provide much needed information and understanding of the true contours of the ocean bottom.
DirkH,
Correct.
Here is Envisat data following “adjustment”:
http://wattsupwiththat.files.wordpress.com/2012/04/envisat2bhide2bthe2bdecline.png
Here is Envisat’s unadjusted data:
http://wattsupwiththat.files.wordpress.com/2012/04/msl_serie_en_global_ib_rwt_nogia_adjust.png
Isn’t this the same thing the GRACE satellite pair is supposed to be doing, topography (even underwater) by detecting gravity? Isn’t GRACE doing a good enough job? Do they need confirmation?
Although I wonder about how this is supposed to work. GRACE measures gravity by precisely measuring the distance between the satellites, when traveling over a larger mass the larger gravity force moves the satellites slightly closer together. By physics the satellites should also move slightly closer to the ground.
It says “…CryoSat’s radar altimeter can sense the gravity field at the ocean surface…” which it seemingly does by measuring the water height, higher water indicates greater mass under the surface. Some modeling with several parameters later, the sea floor map is generated. But CryoSat should also move slightly closer to the ground over a larger mass, and CryoSat will only be measuring height.
Oh well, crank it through a supercomputer, use calculated parameters (aka “educated guesses”) as needed, call it science. Don’t forget to publish the error margins.
I’ve looked at a web site of the ocean floor recently. Supposedly the best detail ever.
Without having to go look for an answer, can someone tell me why there are many straight lines and 90 degree angles etc on the ocean floor. Some of the features look impossible to be natural.
The ocean levels are primarily rising due to the production of water from fossil aquifers plus water formed when fossil fuels are combusted. The rise rate is 2.4 mm per year from the no or slow to recharge aquifers and 0.2 from combustion water formation for a total rise of 2.6 mm per year (year 2000 datum point). Due to water tables declining the production rate of the fossil water has been declining a bit.
TheEnvisat “adjustments” need to critically reviewed.
JFD
The problem with this technology is that it doesn’t seem to account for the changes on the ocean floor. The assumed geoid (flood contours on the surface) must change. And if people look at where the sea levels are rising, (according to the satellites) it is in the exact spots where we’d expect the geoid to change. Now, this is a crude discussion, but just look at the graphics if nothing else. http://suyts.wordpress.com/2011/06/25/discussion-so-far/ Clearly, there’s something here for more than one or two people. I quit bothering with it because as Dirk and Smokey have affirmed, it doesn’t really matter, they’ll lie anyway.
But, there’s your gravitational pull, there’s your geoid….. exactly where all the underwater volcanoes are. There’s your sea level rise.
greymouser70 says:
May 30, 2012 at 8:38 pm
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Thank you for attempting to answer my question, but looking at how a gravimeter operates (http://en.wikipedia.org/wiki/Gravimeter), there is no way that such a device orbitting many, many kilometers above the planet can accurately get readings from either the surface of the ocean or the ocean floor. It must be doing something else and that means it is using a proxy measurement of some kind. My question still stands, and by the look of some of the comments here, I am not the only one.
It actually looks like Cryosat is not going to be able to measure the sea ice thickness accurately enough, one of its primary missions. I’ve been looking at some of its raw data and it changes so much between the tiny swaths available in each orbit (waves, tides, etc) that it looks like it will only be able to provide some indication of seasonal averages. Maybe they are trying to find new purposes to justify the budget required.
kadaka (KD Knoebel) says:
May 30, 2012 at 8:43 pm
Isn’t this the same thing the GRACE satellite pair is supposed to be doing, topography (even underwater) by detecting gravity? Isn’t GRACE doing a good enough job? Do they need confirmation?
Although I wonder about how this is supposed to work. GRACE measures gravity by precisely measuring the distance between the satellites, when traveling over a larger mass the larger gravity force moves the satellites slightly closer together. By physics the satellites should also move slightly closer to the ground.
It says “…CryoSat’s radar altimeter can sense the gravity field at the ocean surface…” which it seemingly does by measuring the water height, higher water indicates greater mass under the surface. Some modeling with several parameters later, the sea floor map is generated. But CryoSat should also move slightly closer to the ground over a larger mass, and CryoSat will only be measuring height.
Oh well, crank it through a supercomputer, use calculated parameters (aka “educated guesses”) as needed, call it science. Don’t forget to publish the error margins.
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1. A radar altimeter does not measure gravity, it measures distance between the satellite and the surface of the ocean.
2. Grace measures the gravity field.
3. It seems counter intuitive that higher water height means greater mass underneath. If the water is being attracted to the greater mass then there should be a depression in the overlying water column.
4. Radar altimeter data actually penetrates the surface of the ocean as old rad alt generated images show sea floor expressions and underground tunnels in shallow water environments.
5. Seems like the scientists have dumbed this down to the point where rational people can’t make sense of their comments.
Robert of Ottawa says:
May 30, 2012 at 8:18 pm
I have to read the apropriate links, but this smells. Somehow, the satellite designed to measure ice is now going to be reasigned to measure water? How can this be? Perhaps the ice measurements were not only wrong, but unadjustable?
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Robert,
If you have an optical measurement to reflect off the ice (laser) and a radar altimeter to reflect off the conductive ocean’s surface you can measure ice thickness. Radar does not “see” ice because ice is not conductive.
If you decide to use a radar altimeter to measure the surface of the ocean then there is no difference between radar and laser. So mapping very small variations in ocean height is better accomplished with a laser-based device (shorter wavelength, higher accuracy).
http://www.esa.int/esaLP/ESAQ5JPV16D_LPcryosat_0.html
Underwater volcanoes are not simple things. Everyone always assumed the Emperor Seamounts were a geostationary blowtorch from the mantle that recorded the movement of the Pacific Ocean floor. The graphic above makes it look more like a midocean ridge than the series of islands in the current Google Ocean. I wonder which is right. I wonder how much a gravity line dance along this linear feature biases measurements that can only see the surface.
“This new mapping from CryoSat will revolutionise our understanding of ocean floor tectonics and reveal, perhaps, 10 000 previously uncharted undersea volcanoes.”
Surely not!
And after Bob Ward and his Thermogeddonists chums “proved” that Ian Plimer doesn’t know what he’s talking about?
\sarc
Robert of Ottawa says:
May 30, 2012 at 8:18 pm
I have to read the apropriate links, but this smells. Somehow, the satellite designed to measure ice is now going to be reasigned to measure water? How can this be? Perhaps the ice measurements were not only wrong, but unadjustable?
============
Really, both ice elevation and sea level are the same measurement — the distance between the satellite’s antennae and the reflecting boundary between the earth’s surface and the atmosphere. The big difference between Cryosat and, for example, Jason would presumably be orbital inclination which controls how close the satellite flies to the poles. If you are going to measure Arctic sea ice elevation, you want a very high inclination orbit that actually flies over the polar ice rather than a lower inclination orbit that only skims the Arctic/Antarctic.
That said, this article is a press release and was very likely written by someone with only the haziest idea of what the satellite is actually doing — after consulting with one or more people who hopefully do know something. It’s not clear to me that modestly better vertical resolution and substantially lower pass to pass horizontal intervals than Jason (design goal I believe of 1 cm vertical resolution) is somehow going to open up new vistas (vistae?) of scientific knowledge. But what do I know?
It seems quite incredible to me that this sort of measurement, with all of its indirect imprecision, confounding factors, and unknowns could add to the accuracy of undersea topographical mapping.
But I have no doubt that with application of enough modelling, adjustment, and esoteric algorithms, plus a little imagination, such a map of that topography will be produced.
Aah! The caption under the illustration may explain it better:
“When interesting features are discovered in satellite measurements, they can later be surveyed in fine detail by ships.”
That makes a little more sense – they really don’t intend to do finely detailed mapping, really they are just seeking interesting, otherwise unexplained ‘bumps’ to investigate.
Now THAT sounds sensible.
I suspect gravity measurements may vary with time, either drift in radial or longitudinal direction. Large percentage of gravity pull is due to the inner core, which is asymmetric, has degree of a wobble and possibly rotates at somewhat different rate to the mantle.
I hope Steve from Rockwood may elaborate further.
A satellite launched to measure sea ice thickness is remissioned without any results to date being published. Weird.
I looked at the Cryosat website.
http://www.esa.int/SPECIALS/Cryosat/SEMQK4908BE_0.html
It talks about how important it is to measure sea ice thickness, then switches topic to talk about evidence for land ice thinning, then concludes with a statement that would give the causal reader the impression Cryosat found sea ice is in fact thinning, but careful reading shows it says no such thing.
I can only conclude Cryosat can’t measure sea ice thickness because of some technical issue, or the data it was producing would cause too many problems, ie it shows thickening sea ice.
A satellite specifically designed to measure sea ice thickness gets remissioned after 2 years operation without any data being published. I can only conclude it either didn’t work as designed or it found alarming rates of increasing ice thickness.
The Cryosat website doesn’t give any clues.
JFD says: May 30, 2012 at 9:15 pm Water released from fossil fuel combustion adds to sea level rise.
Probably it does, but how do you judge if it goes into the sea or into aquifer recharge? Reference?
It all starts with G = m1*m2/d^2, m=mass, d=distance.
It all comes down to collapsing of a broad series of signals into a higher resolution model.
Gravity seen by a satellite is not confined to a single, narrow column between the satellite and the centre of the earth, like a laser beam through transparent media can be tuned. The gravity measured by a satellite, be it GRACE or another, is the complex total of all that is close enough for d to give a signal, plus the variation of m2 integrated over that wide field of view (assuming the satellite mass m1 is constant for this discussion). This set of equations cannot be solved without an a priori density for water and rock and probably water depth and salinity within the broad field of view. The density of all rock changes from place to place, so there is no solution without an assumption of density distribution, not just in underwater rocks from the sea floor, but also from deeper ones within the range of influence r.
It is somewhat akin to a computerised tomography scan, where a series of planes is X-rayed for density by a system moving around the perimeter of each slice in turn. The method can be tuned so show a contrast between materials of different X-ray absorption, but it has the ability to see “behind” dense objects as the X-ray tube-detector coupling is rotated around the subject. If the subject can be dissected and actual X-ray absorption properties measured on pure components, then a somewhat more calibrated CT scan can be obtained.
However, we seldom have the required knowledge of the change in rock density AND DISTRIBUTION below the ocean floor, so an absolute calibration is not possible. Lacks degrees of freedom.
It’s a nice try, but it relies on a trick like a movie relies on 30 frames per second to give the eye an illusion of flicker-free motion. That’s why many passes over the same path are needed. In theory, the size of blocks of rock of near-constant density that can be resolved from above decreases with increasing passes, but it does a law of decreasing returns exercise so that only so much can be gleaned – mainly because nobody knows the distribution of rock density on the sea floor and just below.
In evaluating performance, the critical parameter to check would be spatial resolution in 3D, in my book. Just like for long-established ground-based surveys on the land surface. How big is the circle of displacement of sea water surface that can be detected as coming from a point block of a high density or a low one? Theoretically it is spread over the whole geoid, but the displacement is greater the closer the apparatus is to it, in rough terms.
Steve from Rockwood says:May 30, 2012 at 10:01 pm
… 3. It seems counter intuitive that higher water height means greater mass underneath. If the water is being attracted to the greater mass then there should be a depression in the overlying water column..
You get higher levels because the stronger gravity is drawing water from surrounding areas with weaker gravity.
To demonstrate the drawing in effect, consider the moon and tides. When the moon is overhead, its gravity is opposite the earth’s, and it seems to pull the water level higher. Yet we also get high tides at the antipodes, where the moon’s gravity should by this reasoning be pulling the level lower.
High tides along the earth-moon axis on both sides of the planet indicate that it’s the sideways drawing of water into to the region of increased gravity responsible, rather than the up-down pull.
From http://www.esa.int/esaLP/SEM037ZWD2H_LPcryosat_0.html
“The topography of the ocean surface mimics the rises and dips of the ocean floor due to the gravitational pull. Areas of greater mass, such as underwater mountains, have a stronger pull, attracting more water and producing a minor increase in ocean-surface height.”
They are saying that underwater mountains attract more water and therefore the water column over these mountains is higher. The satellite does measure the height of the ocean’s surface.
“Therefore, instruments that measure sea-surface height incidentally map the ocean floor in previously uncharted areas.”
These instruments also measure sea-surface height in charted areas. I suspect they were trying to measure ice thickness and saw a lot of variation in sea-height. When they looked into it these variations mimicked the topography of the known ocean floor. Someone had the good sense to suggest that once calibrated to the known ocean floor these same measurements could map areas unknown.
These measurements would not be time-varying in the sense of the signal – the signal (rise of water column around underwater mountains) would be constant but other effects (tides etc) would be time varying.
I also suspect that such a signal would be spread out laterally to the extent that it would be unlikely they can identify individual volcanoes but they could certainly outline mountain ranges. They suugest the resolution is 5-10 km (not very detailed). But while the resolution doesn’t sound great imagine if we had temperature measurements spaced every 10 km in the oceans!
Their description of the satellite measuring the gravity field is misleading. It does not measure the gravity field directly but instead measures the distortion of the ocean’s surface, of which topography is one effect. They would also have to make some assumptions such as constant density of the underlying topography. We do this in surface gravity measurements in areas with large topographic changes. Google Parker Method for terrain corrections, for example.
SO after reading the Cryosat web-page again I believe they can do this. I’m not sure how much of the earth’s oceans are unmapped but they could discover a new under-water mountain chain – but not volcanoes directly.
From Steve from Rockwood on May 30, 2012 at 10:01 pm:
As noted.
Not exactly. GRACE itself is just a precise distance measuring device. Had to refresh myself on the details.
The twin satellites are separated by about 220 km. When the leading one passes over an area of higher gravity it “zooms” into the higher gravity field. This is an increase in separation, crank the distance numbers with data from an accurate time signal source to determine the leading one has sped up. The lagging one will subsequently zoom in also, shortening the gap. The leading one will be reluctant to leave the higher gravity field, so it will slow down on departure, gap decreases. Then the lagging one subsequently slows down on departing. In theory (might only happen for an instant) when both are over a gravity field of the same magnitude then the distance doesn’t change, their speeds match.
So when you break it down, GRACE is just two blocks flying through space with a tape measure strung between them. The time is noted when the distance measurement is taken, computers take the figuring from there.
It’s basically like gas molecules in space preferentially gathering around objects of greater mass. The possibly-confusing part is the “leveling off” effect. If there’s an area of less mass thus less gravity, I would think offhand there wouldn’t be that much of a depression as the water would “settle out” and “fill in” the space. This would come from the pressure of the water itself, pushing water into the low spots.
But there’s the power issue. Can you get a strong enough radar “beam” from orbit to map the bottom? Ships can do better, they’re in direct contact with the water also.
Actually, I wonder if they could do “Argo II” and equip the next generation of floats with some form of bottom-mapping radar, even a mere “depth to the bottom” measurement would be useful. The floats drift, over time they could yield a good map. Would be comparatively cheap to add in that capability to floats they’ll deploy anyway.
I’ll restrain my criticism to the press release writers. Especially considering how many have journalism or literature or “communications” degrees, expected to get jobs in print or perhaps TV media, thus never bothered learning that geeky math and science stuff anyway. Why would they ever need that egghead nonsense?
As many above have pointed out, there are a number of issues with inferring sea floor topography from satellite altimetry. The first assumption is that changes in the surface topography represent changes in gravity. To first order this is true, however ocean currents are associated with dynamic topography changes as well. To the extent that an ocean current does not change position or strength, there will be no way (from an altimetry signal alone) to separate topography due to gravity from topography due to ocean currents. Second, the extent to which sea floor topography is represented in the gravity field depends on where the topography was created. The crust under the topography is not uniform in thickness or strength. For example, when a seamount is created over new crust near a spreading center, the underlying crust is thin, weak, and cannot support the weight. Thus you get deformation of the crust which largely cancels out the “gravity signature” of the seamount. This will be true even when the seamount has drifted to a location far from the spreading center and is embedded in what is now thick, older crust. On the other hand, a seamount (or island) that forms in the middle of a plate will form on older stronger crust and have a strong gravity signal.
Small scale gravity anomalies decay rapidly with altitude. Thus grace cannot measure gravity well on the scale of a seamount. This is also the reasoning behind attributing small scale gravity anomalies to sea floor topography. Any gravity signature from deep small scale anomalies will have decayed before the surface of the ocean. Therefore anything you see at small scales must be shallow.
To summarize, small scale anomalies in altimetry data are related to sea floor topography. However, there is not a one-to-one relationship and it is possible to have significant topography with little sea surface signal. Such gravity anomalies will have largely decayed by the altitude of the grace satellites.
gymnosperm says:
May 30, 2012 at 11:13 pm
Underwater volcanoes are not simple things. Everyone always assumed the Emperor Seamounts were a geostationary blowtorch from the mantle that recorded the movement of the Pacific Ocean floor. The graphic above makes it look more like a midocean ridge than the series of islands in the current Google Ocean. I wonder which is right.
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If you want to look at a hotspot volcano chain in detail, there are, I believe, several of them that cross continental areas where you’d have both topographic and geologic mapping available. One worth looking at would be the New England seamounts that extend from the Great Meteor seamount South of the Azores North and West across the Atlantic, through the White Mountains and at least as far as Montreal
kadaka (KD Knoebel) says:
May 31, 2012 at 5:45 am
But there’s the power issue. Can you get a strong enough radar “beam” from orbit to map the bottom? Ships can do better, they’re in direct contact with the water also.
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I’m pretty sure that ships use sonar for mapping/depth determination. Sound doesn’t travel so well in space. And I believe that electromagnetic radiation at wavelengths a satellite might use doesn’t propagate that well through water. Doesn’t mean that there isn’t some clever way to map the ocean bottoms from space at high resolution. But I can’t think what it might be.
I strongly suspect that both the US and Russian navys have significant classified information on the ocean bottom topology. It’s nice that there will be an unclassified resource for researchers.
Mike McMillan says:
May 31, 2012 at 4:46 am
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I see it as a simple function, but a little more complicated than your description. If you consider a mountain ridge (an elongated prism say), one one side the ocean is drawn toward the mountain raising the apparent sea-level. If you go to the other side of the mountain, same thing. But directly over the mountain the water is drawn vertically downward to its top. So the transfer function would be a symmetric positive peak with a smaller negative peak in the center. This is what confused me a first. The water is drawn to the mountain and directly over the peak the attraction is downward, not upward.
kadaka (KD Knoebel) says:
May 31, 2012 at 5:45 am
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Grace measures gravity just as a gravity meter measures gravity. The only difference is the satellites themselves are the mass of attraction. The separation between the two satellites allows for the removal of effects that are common in time but not common in position (because one satellite follows the other). Normall gravity meters have a mass on a spring and the mass is attracted to the earth. The Grace satellites are the mass and their deviations in position are equivalent to the movement (and measurement) of the spring tension.
Don K says:
May 31, 2012 at 6:06 am
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Ships use sonar (acoustic) which easily penetrates water and planes / satellites use radar (electromagnetic) which easily penetrates air and fresh water but not conductive salt water. Satellite radar actually penetrates deeper into the ocean than people would believe. A Scottish geophysicist showed me an early radar image from space that clearly mapped the ocean bottom and revealed some pipelines lying on the ocean floor. This penetration is likely several tens of meters to perhaps a hundred meters – clearly not powerful enough to map the ocean floor other than along coast lines.
MikeP says:
May 31, 2012 at 5:48 am
“To summarize, small scale anomalies in altimetry data are related to sea floor topography. However, there is not a one-to-one relationship and it is possible to have significant topography with little sea surface signal. Such gravity anomalies will have largely decayed by the altitude of the grace satellites.”
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Great post Mike. Let’s separate Grace from Cryosat. The Cryosat data will miss topographic effects that are substantial where those features occur very deep in the oceans. The Grace data will see topographic effects as well distortions of the ocean’s surface from those same topographic effects. Plus ocean currents, plus tides etc. If scientists can map variations in acquifer levels using Grace, they can map topography based on ocean level variation. I’ve always been suspicious of the interpretation of Grace data. It seems to find problems in India but never problems in the USA where certain acquifers are being drained at alarming rates.
I am convinced that what the Cryosat scientists are doing is valid. They must have seen topographic signatures in their data where the ocean floor topography is known and someone went from the know to the unknown.
From Don K on May 31, 2012 at 6:06 am:
But it does travel very well in water. Except we’re in the age of “stealth mode” military, and sound travels too well. Thus I surmise an alternative method has been developed at least for military use, using a tight “beam” that doesn’t scatter much, that’s not sound based.
If we civilians ever hear about it, that’s another issue.
Sounds like trying to map the ground from the snow lying on top of it. Interesting that Google are involved also. The seabed is a new frontier and perhaps this survey will be a preliminary prior to more detailed surveys for deep sea mining of polymetallic nodules and minerals.
http://www.isa.org.jm/files/documents/EN/Brochures/ENG7.pdf
Multibeam echosounders attached to ships are the usual method of hydrographic surveying. Using GPS and other criteria such as the pitching and rolling of the ship the results are very accurate and the instruments quite small. But readers working in oceanographic research, cable laying and the oil and gas industries will know far more than I do.
Another method is Laser Detection and Ranging or LIDAR as used by the military for airborne mine and submarine detection and other other secretive activities, and for coastal and seabed analysis, the data being collected from an aircraft.
http://adsabs.harvard.edu/abs/1997SPIE.3107..288H
wsbriggs says:
May 31, 2012 at 6:20 am
I strongly suspect that both the US and Russian navys have significant classified information on the ocean bottom topology. It’s nice that there will be an unclassified resource for researchers.
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If I’m not mistaken, Bell Geospace developed a submarine gravity meter to map the ocean floor to compensate for variations in launch accuracy for intercontinental ballistic missles launched from submarines. The introduction of real time telemetry and satellite control made this technology obsolete and the technology was released into the mineral exploration and oil & gas industries originally as the “Falcon” system owned by BHP Mining. Those ocean floor maps should exist somewhere and were likely digitized into an amazingly accurate sea-floor map that we will never see. The Cryosat maps will have a lateral resolution of 5-10 km. The classified maps could have ten times more resolution.
Philip Bradley says – “A satellite specifically designed to measure sea ice thickness gets remissioned after 2 years operation without any data being published.”
Ding, Ding, Ding! We have a Winner! [i]Another wheel falls off the AGW band wagon.[/i]
Say anybody know how many wheels the Scam has left?
Pardon my ignorance but doesn’t that first picture have it backwards? I would expect that sea levels would be lower over mountains because the gravity is higher there and shallower over the deep trenches.
Do I have it backwards?
Geoff Sherrington says:
May 31, 2012 at 4:02 am
It all starts with G = m1*m2/d^2, m=mass, d=distance.
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It is somewhat akin to a computerised tomography scan, where a series of planes is X-rayed for density by a system moving around the perimeter of each slice in turn. The method can be tuned so show a contrast between materials of different X-ray absorption, but it has the ability to see “behind” dense objects as the X-ray tube-detector coupling is rotated around the subject. If the subject can be dissected and actual X-ray absorption properties measured on pure components, then a somewhat more calibrated CT scan can be obtained.
A problem with a CT analogy is that for the minimum tomography acquisition you require a projection angle of 180 degrees PLUS the cone angle of your imaging set-up.Try it with a smaller angle (as they sometimes do trying to make TEMs into nano-CT scanners) and serious geometric artefacts set in. Of course this is using the traditional Feldkamp cone bean reconstruction – if one moves to newer methods such as discreet tomography or maximum likelihood, it may be possible to trim down the angular requirements – and also the sampling angular frequency.
I have sometimes speculated as to whether, if you installed on the moon a large enough grid of neutrino detectors, and the sun was an intense enough source of neutrinos, then you could perform neutrino tomography of the earth. This would have the advantage of being spherical rather than the traditional cylindrical tomography, eliminating the DeFrise type high cone angle artefacts. But this would depend on there being differential absorption in different phases. This is likely between seawater and rock at least. However the “scan time” would be an uncompetitive several human generations. At least it would give time for development of the spherical reconstruction algorithms that dont exist AFAIK yet.
Thomas W. McCord says:
May 31, 2012 at 8:22 am
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Off to the side of a topographic high the water is drawn toward the feature and ocean level rises in this direction (this is what they are showing). But directly over the high the water is drawn downward over the feature. The Cryosat people probably didn’t want to confuse people with a complex drawing. The gravity field is almost always vertical. But when you are beside something with a lot of mass there is a horizontal component as well. The water is drawn in the direction of the horizontal component. Directly over the topo high the horizontal component is zero and the vertical component is increased (due to the excess mass below you). The transfer function would look like a pair of rabbit ears – very fat and odd-looking rabbit ears. Or for those who recall TTY (rotate CCW):
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Interesting comments–gotta add my two cents. I’ll be the last to know if anything changes, but last I heard the subs had to get an ELF signal instructing them to surface or near surface for more data–radar will not penetrate water. That’s how the ice thickness is measured, and that’s why you can’t heat the deep ocean from above. How much burned fossil fuel H2O goes to the ocean? All of it. What little sinks into the ground will be quickly pumped back out.
Another way to look at the “counter intuitive” water column is the reductio ad absurdum case: a gravity singularity or a thimble full of neutrons from a neutron star will certainly move the center of gravity and distort the field, raising sea level. Also counter intuitively, the earliest sat telemetry correctly showed high gravity over the Marianas Trench and low gravity over the Himalayas: resolution matters. So being so much closer to the mass distortion sea level resolution is a whole lot better. The ocean floor is more uniform in density than the continents, which float in the crust and get turned inside out over the eons, but rarely get sucked under with plate subduction. So it’s a reasonable assumption to neglect density problems for starters–except for the continental shelves sea bottom is igneous. –AGF
From Steve from Rockwood on May 31, 2012 at 7:30 am:
Steve, while the apparatus you’re describing is a useful historical instrument, it takes a vertical measurement. GRACE, as I’m reading about it and has been discussed on WUWT before over many months, relies on a precise horizontal measurement of the satellites’ separation. How can they be the same?
If I was measuring gravity, especially when movement is involved, I’d use deflection. For example, have a 1 tonne ball suspended from a cable. Move a 1 tonne cylinder around the suspended mass. Wherever the cylinder is, the ball will move towards it from the gravitational attraction. It will deflect towards the cylinder. Measure the position of the ball over time and you can calculate where the cylinder was, in direction and distance from the ball. Change the frame of reference and have the suspended ball move, measure position relative to the suspending framework, and you can determine what masses it passes by and the strength of the gravitational field. Pull some tricks to estimate the distance to a certain body, you could calculate its mass as well.
And if the ball was suspended in all three dimensions by springs, you can detect those other masses in all three dimensions.
After reviewing modern gravity sensing techniques, such as with Gravity gradiometry, I see that’s basically what they’re doing these days.
kadaka (KD Knoebel) says:
May 31, 2012 at 11:08 am
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Interesting post. As the first satellite approaches a greater mass (400 km below the satellite but say several hundred km away) the first satellite will accelerate toward the mass. Initially this acceleration will be almost horizontal so the satellite speeds up toward the mass and the distance between the two satellites increases. As the first satellite approaches directly over the greater mass the acceleration will essentially be vertical, tugging the satellite down toward the earth. This could also be measured as a change in the satellite altitude (although I suspect that algortihms exist that allow the horizontal acceleration (increase or decrease in distance) to be converted into equivalent vertical acceleration and then appropriate mass necessary to enact that acceleration. As the first satellite moves beyond the mass it slows down, but the second satellite is already over the greater mass and has accelerated toward it. Only after both satellites have crossed over the greater mass do they return to their original separation and altitude.
This is a gravimeter. The satellites are the mass being attracted increase in mass on earth. Rather than measure spring torsion they measure distance. But it’s still a direct measurement of gravity. If the satellite had no mass it would not undergo these accelerations. I wonder if they added weight on purpose to increase this effect.
A pressure ridge is a narrow feature. A large one may jut upwards twenty or thirty feet (and have roots going down one-hundred-eighty or two-hundred-seventy feet,) yet only be fifty or a hundred feet across. On either side there may lie miles of flat ice, only nine feet thick. How good is this satellite at picking up such narrow features? Would it have ther resolution to pick up a pressure ridge, or merely average everything out?
Steve from Rockwood says:
May 31, 2012 at 1:21 pm
I wonder if they added weight on purpose to increase this effect.
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Like a bag of golf clubs? You’re supposed to add something like sarc/on/off (I’m not sure) when you say something like that. –AGF
Would a difference in atmospheric pressure over various areas acting on the water surface give the illusion of differences in underwater topography?
Just think, subsea mountains could come and go with the wind.
Eyesonu, You’d need persistent deviations in atmospheric pressure. Otherwise, like ocean eddies, the signal would average out over time.
They certainly wouldn’t want Cryosat to expose PIOMAS for what it is……..