Gravity - measurement and applications
« on: September 09, 2020, 07:11:38 PM »
The gravity vs. Upward acceleration/ equivalency principle arguments are an interesting set of discussions. They commonly quote the constant value of 9.8 m/s2 for g. This is true enough for our every day lives.

The problem is that Earth's gravity is nowhere near that uniform once to start using more sensitive instruments in different areas - i.e. the significant digits after 9.8 become significant to the discussion. Gravitational strength varies based on a number of regional factors, like your latitude (because of the earth's rotation, you weigh very slightly less at the equator than you do at the poles, even though you're at a greater distance to the center of the earth).

Ignoring large regional effects, local variations in earth's gravity occur over as little as tens of meters! And in mapping out these changes, weve been able to discover geologic features like buried mineral deposits, oil and gas reservoirs, and buried bedrock valleys that may host large aquifers capable of supplying groundwater for large municipalities. (e.g. Greenhouse and Williams, 1986. A gravity survey of the Dundas buried valley west of Copetown, Ontario. Canadian Journal of Earth Sciences, v.23: 110-114 available free online)

Aside from the multi-billion dollar applications of gravity for exploration, understanding temporal variations in earth's gravity is becoming increasingly effective. The GRACE satellite system can now detect tiny changes in gravitational strength that relate to changes in water and ice storage on land on seasonal and multi-year timescales. These help measure climate change impacts and long-term over use of major aquifer systems that are causing subsidence problems in many cities (examples in California and Arizona are widespread in google searches https://lmgtfy.com/?q=land+subsidence+aquifers )

How do these measured changes in local acceleration due to gravity fit within a FE framework? The UA would induce an apparent acceleration of 9.8 m/s2 uniformly across earth's plane, and the equivalency principle is really only valid for local reference frames and cannot account for these local variations.
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Offline Tom Bishop

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Re: Gravity - measurement and applications
« Reply #1 on: September 09, 2020, 07:45:14 PM »
Review these articles:

https://wiki.tfes.org/Weight_Variation_by_Latitude

The classic scale and weight diminution experiments show variations by latitude, but were not done in a vacuum chamber.

https://wiki.tfes.org/Gravimetry

The gravimeter is not measuring gravity directly, and is based on an indirect theory of underground density variations, which is why the anomalies are associated with seismic zones and have a negative association with mountains and continents. Latitude variations are corrections to the data, even for absolute gravimeters. The corrections are based on those original 300 year old weight diminution experiments from the first article which were not done in a vacuum chamber.
"The biggest problem in astronomy is that when we look at something in the sky, we don’t know how far away it is" — Pauline Barmby, Ph.D., Professor of Astronomy

Re: Gravity - measurement and applications
« Reply #2 on: September 09, 2020, 07:46:24 PM »
Tom, thanks for providing the link to the wiki. It was an interesting read. My questions stemmed from that, and I will try to rephrase.

My issue is that the gravimeter readings of local changes in relative pull, as the wiki puts it, are supported by the subsequent drilling of boreholes and measurement of the properties from the subsurface materials. In mining camp settings, there are hundreds of boreholes drilled to provide the necessary ground-truthing.

I would argue that the wiki article misrepresents the density differences that are interpreted from the changing reading on gravimeters. It's really the total amount of mass beneath the meter (the relative densities of subsurface materials just influence the total mass because denser materials have more mass per unit volume).

The other issue is the seismometer-gravimeter comparison. The wiki article seems to argue that the fact that the two are variations of the same setup invalidates either of them. That's just not true because both are devices that measure acceleration. Why would they need to be different? The fact that they differ in precision and frequency is a function of the nature of the acceleration they are developed to measure at-large amplitude high frequency seismic waves vs. Low amplitude low frequency gravitational variations.

The wiki article does no seem to provide any explanation as to why the relative pull measured by a gravimeter would change over small areas, nor why temporal changes in gravitational strength are observed across large areas.

If you could provide further clarification to those last points that would be great!
Cheers

Edit, these variations are often in the interior portions of tectonic plates and far from mountains or other features
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Offline Tom Bishop

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Re: Gravity - measurement and applications
« Reply #3 on: September 09, 2020, 08:06:52 PM »
Quote
The wiki article does no seem to provide any explanation as to why the relative pull measured by a gravimeter would change over small areas, nor why temporal changes in gravitational strength are observed across large areas.

We are told to correct the data as we move.

http://www.geol-amu.org/notes/m10-1-4.htm

    "Latitude correction: The earth's poles are closer to the centre of the equator than is the equator. However, there is more mass under the equator and there is an opposing centrifugal acceleration at the equator. The net effect is that gravity is greater at the poles than the equator.

    For values relative to a base station, gravity increases as you move north, so subtract 0.811 sin(2a) mGal/km as you move north from the base station. (where a is latitude)."

The anomalies are otherwise associated with the seismic zones. More seismic noise = greater gravity anomaly. There is always background seismic noise in the Earth. Some places stronger than others.

Quote
I would argue that the wiki article misrepresents the density differences that are interpreted from the changing reading on gravimeters. It's really the total amount of mass beneath the meter (the relative densities of subsurface materials just influence the total mass because denser materials have more mass per unit volume).

The other issue is the seismometer-gravimeter comparison. The wiki article seems to argue that the fact that the two are variations of the same setup invalidates either of them. That's just not true because both are devices that measure acceleration. Why would they need to be different? The fact that they differ in precision and frequency is a function of the nature of the acceleration they are developed to measure at-large amplitude high frequency seismic waves vs. Low amplitude low frequency gravitational variations.

The device isn't measuring gravity directly, and relies on the theory of underground density variations. A seismometer can also function as a gravimeter. The 'gravity tides' are found in the subseismic band.

Looking for the tides in the subseismic band with data analysis is a bit different than a direct experiment where a test mass is being pulled towards the Moon as it passes by overhead. If this is an indirect test of gravity, as you acknowledge, then the results may well be caused by something else which is not gravity.
"The biggest problem in astronomy is that when we look at something in the sky, we don’t know how far away it is" — Pauline Barmby, Ph.D., Professor of Astronomy

Re: Gravity - measurement and applications
« Reply #4 on: September 09, 2020, 08:23:55 PM »
East-west profiles over short distances (1-3km) still register the local changes in gravimeter readings I'm talking about. This isnt just theory about density changes though. The subsurface properties are investigated by drilling and measurement of density/specific gravity etc.

You're 100% right that gravimeters dont measure gravity directly. They measure a pull on a mass.

BUT. Thousands of measurements spanning all the continents over many decades document changes in the pull over local scales. These are correlated with the observed properties of subsurface media (rocks, sediments, ice, water) in exploration settings, or with sun-moon positions, as you alluded to. Collectively, the local variations of the pull measured from all these gravimeters point to spatial and temporal variations in that pull. This collection of measurements and supporting ground truthing data is the evidence that gravity is the force exerting the pull, rather than a uniform upward acceleration.
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Re: Gravity - measurement and applications
« Reply #5 on: September 10, 2020, 12:19:28 AM »
Quote
the equivalency principle is really only valid for local reference frames and cannot account for these local variations.

Its also only valid for objects in free fall.  Not everything that falls is free fall.

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Offline Tom Bishop

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Re: Gravity - measurement and applications
« Reply #6 on: September 10, 2020, 01:46:44 AM »
Quote
BUT. Thousands of measurements spanning all the continents over many decades document changes in the pull over local scales. These are correlated with the observed properties of subsurface media (rocks, sediments, ice, water) in exploration settings, or with sun-moon positions, as you alluded to. Collectively, the local variations of the pull measured from all these gravimeters point to spatial and temporal variations in that pull. This collection of measurements and supporting ground truthing data is the evidence that gravity is the force exerting the pull, rather than a uniform upward acceleration.

Are you sure that the weight in a seismometer/gravimeter device isn't returning to its zero state in the raw readings of these gravity signals?

Look at the first graph in black called "SG Raw Gravity" here, from a superconducting gravimeter:

https://www.researchgate.net/figure/Raw-gravity-black-contains-all-signals-Residual-gravity-with-the-step-function-blue_fig2_264122117



The first graph in black, labled "Raw Gravity" seems to be returning to a zero state. If the mass in the superconducting gravimeter device (suspended by magnetism) was being pulled upwards or downwards - and stayed that way - we should see something more like the step functions and residual graphs, constantly rising or lowering, rather than something that looks like a seismograph that returns to its zero state.

Here is a live reading from a gravimeter in Sweden:

http://holt.oso.chalmers.se/hgs/SCG/monitor-plot.html



The Observed Gravity (middle black) shows that there is a bunch of noise, labeled with a 'Typical noise level' range, and the thinner yellow line is the gravity signal.

So it looks like:

- There is a bunch of noise which dwarfs the gravity signal.
- The signal in the raw results returns to a zero or common state and the signal does not stay suspended or suppressed, except in step sizing algorithms and filters

The Sweden Gravimeter site also has a 24 hour gravity plot:

http://holt.oso.chalmers.se/hgs/SCG/daily-residual-plots.html



When gravity gets stronger, the yellow gravity signal grows thicker.



The black is presumably still the noise like in the previous graph.

It gets vertically thicker because the earth is moving more violently up and down, like in a vertical component seismometer.



Of course, this 'violent' movement is very small.

It is telling that we have to look into noise to get the gravity signal. If the gravity signal is dwarfed by noise then the gravity signal could be a component of whatever else is causing that noise, such as seismic activity, which would explain why those gravity anomaly maps on the gravimetery page are all associated with seismic zones.
« Last Edit: September 10, 2020, 01:14:55 PM by Tom Bishop »
"The biggest problem in astronomy is that when we look at something in the sky, we don’t know how far away it is" — Pauline Barmby, Ph.D., Professor of Astronomy

Re: Gravity - measurement and applications
« Reply #7 on: September 10, 2020, 02:35:24 AM »
Hey Tom, thanks for the continued input. Interesting study you linked about the gravimeter calibration and apparent drift.

I would point out the scale on the y axis, and that the 'typical noise' range is 20 nm/s2... converted into standard g values, that's 2.039x10^-9 g in noise.... that's a lot of significant figures to carry around.

The bouguer gravity anomalies that define buried valleys and mineral deposits are about three orders of magnitude stronger (lower value of~1.5 mGal from the paper I cited earlier).

Again I emphasize that the readings are then ground truthed in exploration studies, so we can then verify whether the observed differences in pull match with the density contrasts of the rock/sediment beneath.
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Re: Gravity - measurement and applications
« Reply #8 on: September 10, 2020, 06:32:51 AM »
So the level of noise Tom’s talking about, compared to your survey data, is like the person behind you coughing in the middle of a ZZ Top concert. Or sniffling during Tchaikovsky’s 1812 overture as the cannons are fired?
« Last Edit: September 10, 2020, 07:08:51 AM by Longtitube »

Re: Gravity - measurement and applications
« Reply #9 on: September 10, 2020, 12:14:14 PM »
To be fair, gravimeter measurements are noisy, and if you tried to take measurements during a seismic event, the additional accelerations would make it impossible. The instruments are so sensitive that we had a had time completing a survey along a busy road because any large trucks driving by created enough vibrations that we had to wait until they had passed.

But there are still large, pronounced responses that are observed in exploration surveys (whether that's for mineral deposits or groundwater incestigations) that we know with a very high degree of certainty are caused by varying properties of material beneath our feet, namely the different local distribution of mass.

 in the examples provided above, the noise level would be like repeatedly using one of those rolling distance measuring wheels to measure the distance around the curved part of a regulation Olympic sized track then plotting the difference in measurements you get in millimeters. You'll never get exactly 100 m, but you can get really close every time.

This just wasnt the discussion I wanted to have, but not that weve gone through the measurement and precision side of things, maybe we'll be able to move toward the issue at hand of the observed changes in force at different locations and at different times. I would argue that these data support the idea that the force is gravity and is problematic for a constant upward acceleration concept, but would happily consider other options to explain the phenomena that account for the observations
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Offline Tom Bishop

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Re: Gravity - measurement and applications
« Reply #10 on: September 11, 2020, 03:26:09 AM »
If the gravimeter is really detecting seismic waves, then when you move to a different area the waves need to transmit through different materials to get to the device in a new area. Seeing different readings would tell you that the material beneath you is different, as different materials might block or allow waves differently.
"The biggest problem in astronomy is that when we look at something in the sky, we don’t know how far away it is" — Pauline Barmby, Ph.D., Professor of Astronomy

Re: Gravity - measurement and applications
« Reply #11 on: September 11, 2020, 12:31:41 PM »
If there were active seismic waves (I.e. from a major event that causes notable vibrations of the the earth) passing by, we wouldn't be able to get our readings. As I mentioned earlier, the passing of large trucks forced us to pause because the vibrations made it impossible to take a reading.

We actually do multiple types of seismic investigations as well. Seismic reflection surveys involve a seismic source (vibrator or shockwave) and a series of receivers (think micro phones). As you said, different media transmits seismic waves differently (denser media transmit the waves the fastest, and the energy of the wave is both reflected and refracted at interfaces between layers of different properties. By measuring the time and amplitude of the seismic energy reflected back upward to the receivers, we essentially get a picture of the geometry and layering of different materials beneath the surface.

 In exploration contexts, the gravity survey will tell us whether there is a valley beneath your feet, the seismic survey will gives you clues as to what's actually in filling the valley, and then drilling will allow you to measure the properties, test the chemistry, measure permeability etc..

Gravimeters are absolutely able to detect ground vibrations from seismic waves. The problem is that gravimeters are so sensitive that they cant actually read them.

Since they both do they same thing- measure acceleration  they are similar instruments, but the nature of the accelerations they measure  (the enormous differences in their amplitudes and frequencies) requires them to be built very differently.  I would say you can think of them like boats. Depending on where you want to go, you would want a different type of boat. A 12-foot aluminum boat is great for getting you up and down a flat stretch of river, but I wouldn't want to be caught in one in the middle of a big lake on a windy day!
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"Earth isnt round or flat. It's fucked."
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Re: Gravity - measurement and applications
« Reply #12 on: September 11, 2020, 06:44:25 PM »
It would help if we had some survey data of some sort to see what Iceman2020 is describing, whether a gravimeter can detect different densities of below-ground deposits. Happily, Tom has supplied a link to some: if you follow his link to the "SG Raw Gravity graph" etc, you can scroll down a little to a download of the full scientific paper – https://www.researchgate.net/publication/264122117_Detecting_small_gravity_change_in_field_measurement_Simulations_and_experiments_of_the_superconducting_gravimeter_-_IGrav/download

It's a very interesting read, investigating whether a superconducting gravimeter could be used to monitor in-ground storage of carbon dioxide for reducing global warming. The calibrating experiments include one where a heavy weight is placed directly above the gravimeter to see if any change in the downward, Planet Earth-induced force of gravity is found, plus another to measure any variation in gravity when the instrument is simply raised a short distance above the ground, but I won't spoil your enjoyment by posting spoilers. Suffice to say, the idea of Universal Acceleration struggles to explain the results found.
« Last Edit: September 11, 2020, 06:45:58 PM by Longtitube »

Re: Gravity - measurement and applications
« Reply #13 on: September 11, 2020, 07:48:34 PM »
Yeah fair point!

Here's another example that is available free:

conservancy.umn.edu/bitstream/handle/11299/60798/1/mgs-264.pdf
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Re: Gravity - measurement and applications
« Reply #14 on: September 11, 2020, 08:01:17 PM »
Paper from Canada:
nrcresearchpress.com/doi/pdfplus/10.1139/cjes-2016-0224

Others:
Gabriel et al. 2003 - geophysical investination of buried Pleistocene subglacial valleys in northern Germany

Moller et al 2007 - gravity field separation and mapping of buried quaternary valleys in Lolland Denmark. Journal of geo dynamics 43(2), 330-337.
The Scintrex operator manual from 1995 has some good examples of applications

« Last Edit: September 11, 2020, 08:23:01 PM by Iceman2020 »
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Offline Tom Bishop

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Re: Gravity - measurement and applications
« Reply #15 on: September 11, 2020, 09:05:34 PM »
Quote
It would help if we had some survey data of some sort to see what Iceman2020 is describing, whether a gravimeter can detect different densities of below-ground deposits. Happily, Tom has supplied a link to some: if you follow his link to the "SG Raw Gravity graph" etc, you can scroll down a little to a download of the full scientific paper – https://www.researchgate.net/publication/264122117_Detecting_small_gravity_change_in_field_measurement_Simulations_and_experiments_of_the_superconducting_gravimeter_-_IGrav/download

It's a very interesting read, investigating whether a superconducting gravimeter could be used to monitor in-ground storage of carbon dioxide for reducing global warming. The calibrating experiments include one where a heavy weight is placed directly above the gravimeter to see if any change in the downward, Planet Earth-induced force of gravity is found, plus another to measure any variation in gravity when the instrument is simply raised a short distance above the ground, but I won't spoil your enjoyment by posting spoilers. Suffice to say, the idea of Universal Acceleration struggles to explain the results found.



This is a poor setup. The gravimeter is very sensitive to noise. Placing a mass on top of the gravimeter can change the dampening.

Per the complex model they claim it matches, any phenomena can be made to match a model under any imagined assumption. Such a model could really be describing how mass stabilizes the readings of gravimeters, with gravity as the assumption, as an example.

This experiment should  be done with the mass suspended over the device without touching it.

Of course when you touch a device sensitive to noise, things are going to change. Ideally there would be multiple experiments conducted on the gravimeter in different ways to demonstrate the matter indisputably. Not a single experiment conducted on a device not designed for that procedure, with a claim that it matches a complex model and a conclusion. This is quite unsatisfactory.
"The biggest problem in astronomy is that when we look at something in the sky, we don’t know how far away it is" — Pauline Barmby, Ph.D., Professor of Astronomy

Re: Gravity - measurement and applications
« Reply #16 on: September 11, 2020, 09:13:35 PM »
Tom, this is just one single paper that you dug up. These types of tests and calibrations have been done thousands of times in hundreds of settings, on dozens of different instrumental designs.

I would reiterate that this method is part of billions of dollars in exploration studies around the world, which in turn lead to discovery of mineral bodies, oil and gas reservoirs, and help us identify and understand groundwater systems. The signals they measure provide only insight into what is happening in the subsurface. These are clues as to areas that need to be followed up with other types of geophysical and geological investigations in order to fully understand what is happening beneath our feet. The resounding evidence that changes in the gravitational strength observed at a given location result from the changing  properties of the materials.

I realize that I probably wont be able to change your mind on this issue. And that's OK.  I can only hope that others reading this will use the resources provided to decide for themselves.
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Offline Tom Bishop

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Re: Gravity - measurement and applications
« Reply #17 on: September 11, 2020, 09:42:34 PM »
Quote
I realize that I probably wont be able to change your mind on this issue. And that's OK.

There is a way to show that gravity exists. Unfortunately these papers aren't it. These authors are not trying to prove that gravity exists. They already assume that it exists and make complex models with an assumption of gravity's involvement for any particular observation. The goal is not to prove gravity. The goal is to describe with a set of assumptions, which makes most of these papers generally meaningless for the topic of FE vs RE.
"The biggest problem in astronomy is that when we look at something in the sky, we don’t know how far away it is" — Pauline Barmby, Ph.D., Professor of Astronomy

Re: Gravity - measurement and applications
« Reply #18 on: September 11, 2020, 09:46:37 PM »
I already conceded that you dont need to believe a gravimeter directly measures gravity. It measures the magnitude of downward pull experienced at a given location.

Collectively, the detection of changes in that measured pull, and the strong correlation between those changes and the physical properties of the rocks which underlie them, providence evidence that the measured pull results from differences in mass at different locations locally. Those point to gravity as the best possible explanation.
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"Earth isnt round or flat. It's fucked."
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Re: Gravity - measurement and applications
« Reply #19 on: September 11, 2020, 09:58:20 PM »
This is a poor setup. The gravimeter is very sensitive to noise. Placing a mass on top of the gravimeter can change the dampening. .... This experiment should  be done with the mass suspended over the device without touching it.

Reading a little more carefully, you'll see two cabinets were placed either side of it to make a platform to support the weight above the SG and left for some hours to stabilise. The reduction in gravitational reading is, like the reduction by raising the SG on a lifting platform, difficult to square with Universal Acceleration. Indeed, if the methodology is so poor, why is it cited by a number of other scientific papers on applications of gravimeters? Scroll down your original link and you'll find them.