Gravimetry

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. The 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. The theory behind gravimetry is that it is detecting tiny density variations in the noise which are called "gravity waves" and "infragravity waves" 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. There are also several items which do not make sense with the gravity anomalies if they were actually a measurement of mass. Mountains and continents are seen to dampen gravity readings rather than to increase them.

Further, the differences by latitude and altitude in the gravimetric end product data are seen to be artificial corrections that are added or subtracted to the data and reference model, rather than information that is from the measurements. Operators are told to make a correction for latitude, essentially inputting the variations. Even absolute gravimeters determine local gravity from an equation which uses the gravitational accelerations of the equator and poles with the device latitude.

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 (Archive)

=What is Gravimetry?=

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

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

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

The Global CCS Institute (Archive) says:

A New Mexico Tech Exploration Geophysics II course says (Archive):

=Seismometers are Gravimeters=

Comparative 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 Gravimeters to the Seismometers, the gravity spectra is nearly identical:

Tidal Detection
Further, the above paper states that seismometers are also able to detect the tides -- p.204, second paragraph:

The STS-1 is a seismometer (see caption text in last diagram).

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

A definition of "subseismic band" (Archive):

Is a study of subseismic activity a study of gravity?

Gravimeter Mode
From https://en.wikipedia.org/wiki/Gravimeter (Archive) 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/ (Archive)

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

On p.4 of The Gravity Method (Archive) by Dr. Nicolas O. Mariita we read another reference:

Again, we see that the gravimeter is actually a seismometer, and will be easily disrupted by seismic activity and other noise.

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

This agrees with the 'low-frequency' statement given by Dr. Marrita above. The gravimeter is a low-frequency seismometer, taking data out of those low-frequency bands.

On p.14 from a University of Hawaii lecture (Archive) we read:

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 gravimeters drop bodies, others do not. Consider how the following gravimeter works:

https://schmidtocean.org/rv-falkor/operations-and-science-systems/gravimeter/ (Archive)

The above device does not contain a falling object. The magnetism suspends the cylinder. The system then measures changes to that cylinder. The article continues:

As directly 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. The interest is in the tiny noises that affect the mirror in the device while the body is in free fall and disconnected from the Earth:

Monitoring earthquakes with gravity meters
From the abstract of a paper titled Monitoring earthquakes with gravity meters (Archive) 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 (gPhone) and seismometers (STS-2):





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.

Onsala Space Observatory / Gravity Laboratory
Look at these gravimeter readings from the Onsala Space Observatory's Superconducting Gravimeter (Archive) and decide if the readings looks like the output of a seismometer with values above and below zero. Determine which values are the raw readings before a noise reducing filter is applied.

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

24 hours of gravity residual, 1-s sampling



=Gravity Wave Theory=

A study titled Seafloor Compliance Observed by Long-Period Pressure and Displacement Measurement (Archive) uses gravimeters to study the gravity of the ocean. On p.2, para.4 its authors call the gravimeter a long-period seismometer.

On the same page we read about the theory behind the measurements:



We see that the theory behind the measurements involve the theories of "Gravity Waves" and "Infragravity Waves". Wikipedia describes them as:

https://en.wikipedia.org/wiki/Gravity_wave (Archive)

https://en.wikipedia.org/wiki/Infragravity_wave (Archive)

Essentially, the "Gravity Waves" are slight motions and vibrations picked up by the gravimeter (seismometer). A chart is provided, showing the frequencies that the winds and tides appear in:



In line with the previous seismometer section, the tides appear on the low frequency bands.

=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 may be fallacious.

Note: The reference 'for values relative to a base station' may imply that this is referring to a relative gravimeter.

United Nations University
On p.9 of Seismic Activity, Gravity, and Magnetic Measurements (Archive) by LaGeo as part of the United Nations University Geothermal Training Program we read:

The section goes on to list a number of corrections, including corrections for latitude and elevation, which is not data contained in the measurement readings:

These are artificial corrections which are added or subtracted to the data and reference model. If the earth were really elliptical or rotating, and if the devices were really measuring gravity in full, then these artificial corrections would not be necessary. It is seen that the devices are seismometers and that these corrections are artificially added into the data as modifications.

Absolute Gravimeter Corrections
A common response to some of the references above is to declare that even thought he sources do not specify, the sources must solely be talking about relative gravimeters, and that absolute gravimeters are completely different devices which measures gravity in full. Yet, despite this argument we see that even absolute gravimeters determine local gravity through a model involving the gravitational acceleration of the equator and poles.

Terrain-aided navigation with an atomic gravimeter (Archive)

Introduction

"The purpose of the paper is to provide a solution for surface or sub-surface navigation by Terrain Matching using an absolute gravimeter."

On the third page:

''III. A METHOD TO MAP THE GRAVITY ANOMALY WITH THE ATOMIC GRAVIMETER''



Elsewhere it describes that "Φ is the longitude and λ the latitude. g(Φ, λ) is the modulus of the local gravity acceleration vector"

To determine the local gravity acceleration the device invokes a model involving an equation using gravitational acceleration at the equator and poles with the latitude, and the results are then added to the gravity anomaly (Last line: g(Φ, λ) [Local gravity] = g0(λ)[standard gravity accounting for latitude gravity gradient] + ga(Φ, λ) [gravity anomaly]) We see similar equations (sin 2 lat) as in the previous latitude corrections. Why should this be necessary to involve the gravitational accelerations of the equator and poles to determine the local gravity? If an absolute gravimeter is measuring gravity in full then it should measure gravity in full.

Mobile Atom Interferometer
Similarly, we read the following about latitude corrections for a precision free-fall device:

Gravity surveys using a mobile atom interferometer (Archive)

Introduction

~

Latitude and terrain correction

~

References and notes

We again see a precision free-fall gravimeter which is corrected for latitude.

WGS84 Ellipsoidal Gravity Formula
On p.13 of a paper titled Invited Review Article: Measurements of the Newtonian constant of gravitation, G (Archive) we see a summary of the WGS84 ellipsoidal gravity formula:

This is a very similar equation to the absolute gravimeter local gravity equation given in a section previous to this (sin 2 lat).

The text around this p.13 quote also strongly indicates that the WGS84 equation for the gravity variations was determined based on the weight change experiments conducted at different latitudes and which affects pendulums and scales. From the sentence immediately prior to the above quote:

We hence see that the gravimeters, including absolute gravimeters, are adjusting the output for local gravity based on a latitudinal formula that was determined by a different experiment (Note: This is a determination which may be flawed in interpretation; see Weight Variation by Latitude). Once again, if the absolute gravimeter is measuring gravity in full, why should equations involving gravity's latitudinal differences of the equator and poles be necessary to determine local gravity?

=Seismic Map Similarities=

Convergent Plate and Earthquake Map
Compare the gravity anomalies (first image) to a map of the plate boundaries and the earthquake zones (second image):




 * Gravity Anomalies Source (Archive)
 * Convergent Plates Map Source (Archive)

World Volcano Map
Compare the gravity anomalies (first image) to a world volcano map (second image):




 * World Volcano Map Source p.10 (Archive)

Seismic Hazard Maps
Compare the gravity anomalies to seismic hazard maps of various locations.

South America



 * South America Seismic Hazard Map Source (Archive)

Alaska



 * Alaska Seismic Hazard Map Source (Archive)

Earthquake Map
Compare the gravity anomalies to a map of earthquakes with magnitudes of 6.5 and above.




 * Earthquake Map Source

=Perplexing Anomalies=

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

Gravity Anomalies Contrary To Theory
Bouguer Anomalies Over The Continents and Oceans (Archive) in the Journal of the Geological Society of India tells us that the anomalies are greater over the ocean than over the land, which is contrary to gravity 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.

On discrepancies, one writer states:

Bouguer Anomalies - Australia
We find the following depiction of Australia's Complete Bouguer Anomalies and Free Air Anomalies on a University of California Berkeley lecture on gravimetry (Archive) p.3, showing that the unfiltered 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 (Archive)

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

Inverse anomalies
It is theorized by isostasy that there are inverse anomalous structures beneath the mountain ranges, which mirrors the topography, giving the low gravity readings:

https://web.archive.org/web/20130529124307/http://earthsci.org/education/teacher/basicgeol/earthq/earthq.html



Seismic Interpretation
The negative gravity readings at the mountains are conventionally attributed to Isostasy. However, if gravimeters are really seismometers which pick up low frequency seismic waves originating from within the earth then we should expect the bulk mass of a mountain to dilute a seismic wave more so than other areas, creating a negative anomaly.

=Seismic Wave Propagation=

Background Seismic Noise
It should be noted that there is constant background seismic noise, which emanate from the earth. From http://microglacoste.com/gPhoneNoise/gPhoneSeismicNoise.pdf (Archive) we read:

Airborne Seismic Waves
It should also be noted that seismic waves can become airborne, which could explain the ability of gravimeter devices on airplanes to register the anomalies.

https://en.wikipedia.org/wiki/Seismic_wave (Archive)

Underground Detection
Q. If the gravimeters are picking up background seismic activity, how is it that gravimeters can detect underground oil deposits and other structures? A. If seismic vibrations are originating from deep within the earth and are passing though various substances as they reach the detector on the surface, and those signals change slightly when the detector passes over the underground body, it may be possible to subtract some elements to see others through data analysis, just as how seismographers try to use seismic waves to detect tunnels. See: Phys.org - Detecting tunnels using seismic waves not as simple as it sounds (Archive)

=Noise Analysis=

On analysis of the gravimeter we find its primary purpose is to measure noise and use many software algorithms to interpret and filter the noise. Every part of the device's components—the lasers which measure the motion, the mirrors, the springs which launch the mass up and down the small tube, and all component parts—are all connected in some manner to the earth, and are subject to noise and vibrations.

https://link.springer.com/chapter/10.1007/978-3-662-03482-8_9 (Archive p.1 p.2)

Marine Gravimeters
Below we see an example of gravimetry devices and methods for marine gravimetric surveying:

http://www.iitk.ac.in/nicee/wcee/article/WCEE2012_1237.pdf (Archive)

An example is given of outputs from VSE, Accelerometer, and Gradiometer devices on a mid-size ship navigating Toyama Bay, Japan:



Following the illustration we find the following:

From the above we see that a need to use algorithms and filters to 'find the gravity' from noise. The levels of g are not readily apparent, and must be constructed by subtracting from the noise to 'find gravity'. The above passage states that "We can expect to find the variations of gravity consists of components with long period, however the vibration of the carrier with short periods."

The reader may decide whether the process of subtracting vibrations with one characteristic to reveal other vibrations is truly measuring gravity. Why does "gravity" need to be found?

Galathea-3: A global marine gravity profile
The Danish vessel Galathea-3 describes its methods for gravimetric surveying:

Galathea-3: A Global Marine Gravity Profile (Archive)

We see that the process involves extensive data analysis, filtering, and clean-up from noise — to 'find gravity'.

Airborne Gravimetry
Airborne gravimetery methods reveal the same:

Improving the Accuracy and Resolution of SINS/DGPS Airborne Gravimetry (Archive)

Again, we see that filtering is necessary to clean up the noise.

General/Land Based Absolute Gravimetry
Per normal land-based gravimeters, the following paper describes the process of turning a laser signal from an absolute gravimeter into a "variation of gravity":

Ultra-high Precision, Absolute, Earth Gravity Measurements (Archive)

The paper describes that the fringe signal from the laser beam in an absolute gravimeter is digitized and processed:

Processing Steps



The fringe signal from the laser must be filtered with software:



The remainder of the document describes how the filtering occurs. All sources, to the best of the author's ability, are given an "uncertainty budget," to which is subtracted from the noise. The uncertainty budgets are estimated ranges to which a phenomenon may be contributing to the noise. All possible phenomena in nature must be considered and precisely defined. Everything from air drag, electrostatic fields, pressure, seismic vibrations, &c.

In the Table of Contents we find a list of items, showing the various elements which are subtracted:


 * 4.6 Uncertainty budget of the measured imbalance – Method II. . . . 84
 * 4.7 Uncertainty budget: COM and OC adjusted – Method II. . . . . . 84
 * 5.1 Uncertainty budget for air drag . . . . . . . . . . . . . . . . . . . 89
 * 5.2 Uncertainty budget for outgassing . . . . . . . . . . . . . . . . . . 90
 * 5.3 Uncertainty budget for eddy currents . . . . . . . . . . . . . . . . 91
 * 5.4 Uncertainty budget for electrostatic field . . . . . . . . . . . . . . 91
 * 5.5 Uncertainty budget for instrumental masses . . . . . . . . . . . . 94
 * 5.6 Uncertainty budget for laser verticality . . . . . . . . . . . . . . . 96
 * 5.7 Length standard specifications. . . . . . . . . . . . . . . . . . . . 96
 * 5.8 Uncertainty budget for laser stability . . . . . . . . . . . . . . . . 97
 * 5.9 Frequency standard specifications. . . . . . . . . . . . . . . . . . . 98
 * 5.10 Uncertainty budget for clock stability . . . . . . . . . . . . . . . . 98
 * 5.11 Uncertainty budget for corner cube rotation . . . . . . . . . . . . 98
 * 5.12 Uncertainty budget for radiation pressure . . . . . . . . . . . . . . 99
 * 5.13 Uncertainty budget for beam divergence . . . . . . . . . . . . . . 100
 * 5.14 Uncertainty budget for temperature gradient . . . . . . . . . . . . 101
 * 5.15 Uncertainty budget for seismic noise . . . . . . . . . . . . . . . . 107
 * 5.16 Uncertainty budget for speed of light . . . . . . . . . . . . . . . . 108
 * 5.17 Uncertainty budget for effective height . . . . . . . . . . . . . . . 109
 * 5.18 Specifications of Photoreceiver. . . . . . . . . . . . . . . . . . . . 110
 * 5.19 Uncertainty budget for amplifier . . . . . . . . . . . . . . . . . . . 111
 * 5.20 Uncertainty budget for solid Earth tides . . . . . . . . . . . . . . 113
 * 5.21 Uncertainty budget for ocean loading . . . . . . . . . . . . . . . . 113
 * 5.22 Uncertainty budget for polar motion . . . . . . . . . . . . . . . . 114
 * 5.23 Uncertainty budget for environmental pressure . . . . . . . . . . . 115
 * 5.24 Uncertainty budget for Coriolis force . . . . . . . . . . . . . . . . 116
 * 5.25 Uncertainty budget for MPG-1. . . . . . . . . . . . . . . . . . . . 117
 * 5.26 Uncertainty budget for MPG-2. . . . . . . . . . . . . . . . . . . . 118

Once all of these items and their theoretical uncertainties are subtracted, we are left with "gravity".

Gravimeter Tides
The above pdf mentions that long-term analysis of the gravimeter noise can detect the tides:



The main conclusion from this is that the tides may be related in some manner to vibration or noise that is being processed. No mechanism is presented, or is identifiable, from a long term analysis of noise trends.

Absolute Gravimeter Fringe Noise
Still on the topic of general/land-based gravimetry, we look at the following paper discussing absolute gravimeters:

Comparison of three digital fringe signal processing methods in a ballistic free-fall absolute gravimeter

From the introduction the author states that the input is a noisy fringe signal.

The g value must be extracted with various methods. It is not direct and apparent:

Multiple methods, functions, to "extract" the gravity value from the "noisy fringe signal."

Not an obvious and apparent method; multiple models are involved:

Filtering agreement can be made with other fringe signal processing methods:

These processes are filtering out the noise to find trends. From the conclusion we read:

This document was written in 2010. If Gravimetry were really "simple direct measurement of the acceleration," as some have claimed, then the reader might ask, why was it limited by pre-2010 computing capabilities? The answer is that it is not direct at all. It is the many algorithms necessary to interpret the "noisy" fringe signal (even for land-based absolute gravimeters) which needs large amounts of computing power. The process is interpretation of a noisy fringe signal from what we learned earlier was a seismometer.

From the paper:

Uncertainty budgets, just as we saw in the previous paper, are subtracted from the noise in the effort to find gravity.

=Underground Target Detection=

While gravimetry results have a relationship with geographical areas relating to positive anomalies at the seismic zones and negative anomalies on the continents and mountains, the use of gravimetry for reliable detection of underground targets is murkier. Gravimetry must be used alongside other techniques, as gravimetry alone provides poor understanding of the earth.

From a Los Alamos National Laboratory associated geophysics program - https://permalink.lanl.gov/object/tr?what=info:lanl-repo/lareport/LA-UR-17-30673 (Archive)


 * Underground-gravity-targets.png
 * Underground-gravity-targets.png
 * Underground-gravity-targets.png

From An Introduction to Geophysical Exploration - http://www.science.earthjay.com/instruction/HSU/2016_spring/GEOL_460/lectures/lecture_07/geophysical_expoloration_gravity.pdf (Archive)



From a Doctoral Thesis, Gravimetry for Geothermal Exploration - https://doc.rero.ch/record/255651/files/00002456.pdf (Archive)


 * Gravity anomaly interpretation



From a course, Physics in Proportion - https://www.mtholyoke.edu/courses/mpeterso/phys103/PhysicsInProportionI.pdf (Archive)