Gravimetry

This is a Work in Progress

Gravimetry and Gravity gradiometry is a field of science which supposedly measures the strength of the earth's gravitational field. In these discussions Gravimetry is often used as evidence that the gravitational field of the earth varies with latitude and by location.

It has been found that Gravimetry is not directly measuring gravity at all. Gravimeter devices have been described by professionals in that field as long-period seismometers that are measuring small "jerks" in the background seismic noise and interpreting it as variations in gravity. Gravimeters are, in truth, seismometers. Seismometer devices have been described as having a "gravimeter mode." Seismometers can double-purpose as gravimeters, and can detect the "gravity tides". Gravimeters are often similarly double-purposed as seismometers to detect earthquakes thousands of miles away. Perhaps most illustrative, gravitational anomalies on gravity maps are indistinguishable from the seismic zones. Finally, there are several items which do not make sense with the gravity anomalies, if they were actually a measurement of mass.

The theory behind the field of Gravimetry is that the masses in the subsurface are creating tiny variations or jerks, presumed to be due to "gravity", that are measured by the devices in a unit of measurement called ugal or mgal. We read a description of Gravity Gradiometry on Wikipedia:

https://en.wikipedia.org/wiki/Gravity_gradiometry

=What is Gravimetry?=

A quote from the Enhanced Geothermal Innovative Network for Europe (Arch) explains:

From Gravity surveying: a brief introduction (Arch) we read:

In The Gravity Method (Arch), its author Dr. Nicolas O. Mariita tells us:

The Global CCS Institute says:

=Seismometers are Gravimeters=

Comparitive Study
Comparative study of superconducting gravimeters and broadband seismometers STS-1/Z in seismic and subseismic frequency bands (Archive)

Diagram from p. 212:



The paper says that when comparing with Gravimeters to the Seismometers, the gravity spectra is nearly identical:

Further, seismometers are also able to detect the tides -- p.204, second paragraph:

It is mentioned that the "gravity tides" are found in the subseismic band:



A definition of "subseismic band":

Is a study of subseismic activity a study of gravity?

Gravimeter Mode
From https://en.wikipedia.org/wiki/Gravimeter we read:

The reader might ask, if gravimeters are entirely different devices than seismometers, how could seismometers have a "gravimeter mode"?

=Gravimeters are Seismometers=

This inventor describes gravimeters as follows:

http://www.njsas.org/projects/tidal_forces/magnetic_gravimeter/baker/

Another gravimeter = seismometer reference is found in The Gravity Method by Dr. Nicolas O. Mariita. On p.4 we read:

Again, we see that the gravimeter is actually a seismometer.

Recall from the above seismometer section that the seismometer was detecting gravity tides on subseismic bands, which was described as:

Hence, the gravimeter is a low-frequency seismometer, like the seismometer above, taking data out of those low-frequency bands.

From p.2 of a paper on seafloor measurements (Archive) from the Journal of Geophysical Research we read another reference:

Function of a Gravimeter
It is often argued that a gravimeter works by dropping a body and measuring the full acceleration. While some types of graviters drop bodies, others do not. Consider how the following gravimeter works:

https://schmidtocean.org/rv-falkor/operations-and-science-systems/gravimeter/#images

With the above device there is no falling object. The magnetism suspends the cylinder. The system then measures changes to that cylinder.

As stated, the interest isn't in the total pull. The designers mainly care about using the device as a seismometer.

Absolute Gravimeter Description
From Geophysics From Terrestrial Time‐Variable Gravity Measurements we read about a device that does involve a falling object:

Gravity anomalies observed before earthquakes
Pre-seismic gravity anomalies before Linkou Ms 6.4 earthquake by continuous gravity observation of Crustal Movement Observation Network of China (Archive)

The reader may ponder why the gravity of the earth would change before an earthquake.

Monitoring earthquakes with gravity meters
From the abstract of a paper titled Monitoring earthquakes with gravity meters we read:

The ending two sentences of that abstract even imply that gravity meters may be superior for measuring seismic elements.

Earthquake Comparison Readings
From the study we see several seismic reading comparisons between gravimeters and seismometers:





=Corrections for Latitude=

It is asserted that gravimetry has shown trends at different latitudes, and so this is validation of the idea that it is really measuring "gravity". We find that this assertion is unfounded.

From a university course on gravity surveying we read:

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

If the objective of gravity surveys is merely to look for deviations from a round earth reference model with the vibrating gravity theory, then the final computed number in meters per second squared would becomes meaningless for the purpose of discussion. Any modifications to the reference values are constructed on an entirely theoretical basis.

The above page tells us that there is a theoretical model and that the goal of gravity surveys is to modify that model. Further down we see, among the list of corrections to be made, the latitude correction:

We read that we are subtracting or adding values to the reference model and the data to make the corrections for latitude, which is very different than using the data to determine the latitude. The claim that the final number is meaningful as evidence to showcase any particular point is shown fallacious.

=Seismic Map Similarities=

Convergent Plate and Earthquake Map
Compare the gravity anomalies to a map of the plate boundaries and the earthquake zones:



Seismic Hazard Maps






=Perplexing Anomalies=

The anomalous deviations of gravity, as detected by the gravmieter, are called Bouguer Anomalies.

https://en.wikipedia.org/wiki/Bouguer_anomaly explains:

Note that a correction for attitude needs to be made, and that the data is not in the gravimeter readings itself.

Gravity Anomalies Counter To Theory
Bouguer Anomalies Over The Continents and Oceans tells us that the anomalies are often grater over the ocean than over the land, which seems to be contrary to theory:

The anomalies are negative in continental areas and positive in oceanic areas. The anomalies are also negative in the mountains. These anomalies appear to go against the theory that the anomalies are due to the attraction of mass.

Bouguer Anomalies - Australia
We find the following depiction of Australia's Bouguer Anomalies and Free Air Anomalies on a University of California Berkeley lecture on gravimetry (p.3), showing that the anomalies are negative over continental areas and positive over oceanic areas:



Bouguer Anomalies - Alps of Germany
https://www.leibniz-liag.de/en/research/methods/gravimetry-magnetics/bouguer-anomalies.html

The above map and description shows that the anomalies are negative in the Alps of Germany.

=Temp=

The goal of gravity surveys is to modify this model"... Lower down it says that the corrections for height and latitude are made to that model.

- Seismometer and Gravimeter Results

Technical Assessment

-Noise

Latitude Corrections

Map Examples

Volcanoes

Fault Lines

Seismic Zones

Complete Bouguer Anomalies

https://www.leibniz-liag.de/en/research/methods/gravimetry-magnetics/bouguer-anomalies.html

Quote

This map shows the Bouguer anomalies over the whole of Germany and surrounding areas, in a detailed but still clear way.

...

The resulting gravity anomalies vary across the mapped area from -170 mGal in the Alps to +40 mGal around the gravity low in the Magdeburg area.

Low in the Alps of Germany.

He tells us about the Bouguer anomalies previously mentioned, and the volcanoes:

Quote

The most commonly used processed data are known as Bouguer gravity anomalies, measured in  mGal. The interpretation  of Bouguer gravity anomalies ranges from  just manually inspecting the grid or profiles for variations in the gravitational field to more complex methods that involves separating the gravity anomaly due to an  object  of  interest from some sort of regional gravity field. From this, bodies and structures can be inferred which may be of geothermal interest.

Volcanic centres, where geothermal activity is found, are indicators of cooling magma or hot rock beneath  these  areas as shown by the recent volcanic  flows, ashes, volcanic domes and abundant hydrothermal activities in the form fumaroles and  hot  springs. Gravity studies in volcanic areas have effectively demonstrated that this method provides good evidence of shallow subsurface density variations, associated  with the structural and magmatic  history of a volcano. There is a correlation between gravity highs with centres of recent volcanism, intensive faulting and geothermal activity. For example, in the Kenya rift, Olkaria, Domes and Suswa geothermal centres are located on the crest of a gravity high.

https://en.wikipedia.org/wiki/Seismic_wave

Quote

Primary waves

Primary waves (P-waves) are compressional waves that are longitudinal in nature. P waves are pressure waves that travel faster than other waves through the earth to arrive at seismograph stations first, hence the name "Primary". These waves can travel through any type of material, including fluids, and can travel nearly 1.7 times faster than the S waves. In air, they take the form of sound waves, hence they travel at the speed of sound. Typical speeds are 330 m/s in air, 1450 m/s in water and about 5000 m/s in granite.

The multiple levels of filtering, trending, analysis, interpreting, obfuscates the real mechanism.

Background Noise:

From http://microglacoste.com/gPhoneNoise/gPhoneSeismicNoise.pdf we read:

Quote

It is interesting to speculate on the precise origin of the background seismic noise. Haubrich et al ii for example, open their article with the following description of the seismic noise background and the large interest it has generated over the years as well as the intractability of its investigation:

" The low‐level background unrest of the earth, called microseisms or earth noise, has puzzled seismologists and other scientists for nearly a century. The problem of its nature and causes has proved particularly unyielding, not, however, for lack  of  investigation.  A  bibliography  covering  work up  to 1955  [Gutenberg  and  Andrews,  1956] iii lists  over  600  articles on  the  subject;  one  covering  the  years  from  1955  to  1964 [Hjortenberg,  1967] iv lists  566.  Unfortunately,  much  of  this work has advanced the subject but slightly. "