Gravimetry
Work in Progress
Gravimetry
What is Gravimetry?
A quote from the Enhanced Geothermal Innovative Network for Europe (Archive):
“ Gravimetry
As the gravitational field of the earth depends on the density of the rocks, variations of the gravitational field (Bouguer anomalies) observed at the surface or in a borehole are due to density changes in the subsurface, which can be interpreted in terms of changes in the composition and/or geometry of the geological layers. ”
From an article called Gravity surveying: a brief introduction (Archive) we read:
“ Introduction
Everything is gravitation ally attracted to everything else. And the gravitational attraction of an object is proportional to its mass. So if the rocks below you at a given place are denser, then the gravity there will be slightly larger. The changes in gravity from place to place are small: gravity, g, at the Earth’s surface is about 9.81ms^−2, but the local variations are a tiny fraction of this; often we are measuring differences of 10−6 ms−2. If you can measure how g changes from place to place, you can learn something about how the density of the rocks below you varies ”
From The Gravity Method Dr. Nicolas O. Mariita tells us about what gravimetry depends on:
“ The success of the gravity method depends on the different earth materials having different bulk densities (mass) that produce variations in the measured gravitational field. These variations can then be interpreted by a variety of analytical and computers methods to determine the depth, geometry and density that causes the gravity field variations. ”
Seismometers are Gravimeters
Comparative study of superconducting gravimeters and broadband seismometers STS-1/Z in seismic and subseismic frequency bands (Archive)
“ Superconducting gravimeters and broadband seismometers (vertial component) both measure gravity, but whereas the former are most sensitive to very long period signals (gravity tides with periods longer than 6h), the latter are designed for recording the seismic band (elastic normal modes with periods shorter than 1h) ”
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:
“ A first attempt to use broadband seismometers outside their traditional spectral range was made by Pillet et al. (1994), and they showed that the STS-1 is able to receive strong tidal signals around diurnal frequencies. ”
It is mentioned that the "gravity tides" are found in the subseismic band:
A definition:
“ "subseismic band" (i.e. frequency lower than 0.03mHz) that has very strong background noise; ”
Is studying of subseismic activity a study of gravity?
Gravimeters are Seismometers
This inventor describes gravimeters as follows:
http://www.njsas.org/projects/tidal_forces/magnetic_gravimeter/baker/
“ A seismometer usually looks for the smallest possible acceleration changes. Since gravity is physically the same as acceleration, gravimeters are merely versions of seismometers with an infinitely long period response. ”
Another gravimeter = seismometer reference:
https://orkustofnun.is/gogn/unu-gtp-sc/UNU-GTP-SC-10-0405a.pdf on p.4
“ An important factor in obtaining useful gravity values in detailed surveys is determining the earth tide effect as their gravitational effects may be greater than the gravity field variations due to the anomalous features being sought. The final aspect of reading a gravity meter concerns seismic activity or cultural movement such as those of vehicles or people. These will disrupt the readings (the meter is actually a low-frequency seismometer) and even though the Scintrex meter has an anti-seismic filter (the La Coste-Romberg meters are also mechanically damped to lessen the effects of earthquakes), readings will still be disrupted. ”
The author is Dr. Nicolas O. Mariita. Again, we see that the gravimeter is actually a seismometer.
Recall from above that the seismometer was detecting gravity tides on subseismic bands, which was described as:
“ 'subseismic band' (i.e. frequency lower than 0.03mHz) that has very strong background noise ”
Hence, the gravimeter is a low-frequency seismometer, like the seismometer above, taking data out of those low-frequencies.
From https://agupubs.onlinelibrary.wiley.com/doi/full/10.1002/2017RG000566 we read:
Quote
In an absolute gravimeter, a test laser beam bounces off the free‐falling body before being reflected back to the interferometer, where the test beam interferes with a reference one. While the dropped mass is completely isolated from the Earth's vibrations during its fall, anthropogenic and natural microseismic noises continuously modify the position of the reference mirror of the interferometer. Even in the absence of an earthquake, the displacements of the Earth's surface are persistent and location and season dependent, reaching up to a few micrometers close to the coast (Kedar et al., 2008), while one should measure the free‐fall distance at the 1 nm precision level in order to achieve a precision on gravity of 10 nm/s2. In the first white‐light gravimeter, the measurements of gravity were corrected by using the records from a 1 s period seismometer. Early in the 1980s, Rinker (1983) developed the so‐called Super Spring, that is, a modified seismometer providing an inertial reference system at periods shorter than about 1 min—the suspended mass of a seismometer provides an inertial reference frame, independent from the motions of the Earth, at periods shorter than the resonance frequency (Aki & Richards, 2002). The challenge consisted in producing a suspension device of which the free period is about 1 min, that is, longer than the periods ranging 5–20 s, where microseism is the strongest.
Monitoring earthquakes with gravity meters
https://www.sciencedirect.com/science/article/pii/S1674984715301920
Quote
Abstract: Seismic waves from a magnitude 8.3 earthquake in Japan were consistently recorded by five nearly identical gPhone gravity meters in Colorado. Good correlation was also found in the response of two different types of gravity meters and a standard seismometer in Walferdange, Luxembourg to an earthquake of magnitude 8.2 in Japan, indicating that all of them were capable of measuring the surface waves reliably. The gravity meters, however, recorded 11 separate arrivals of Raleigh waves, while the seismometer only one. Thus the gravity meters may be useful for obtaining new information in the study of seismic velocities, attenuation and dispersion.
The end two sentences of that abstract even implies that gravity meters may be better for measuring seismic elements.
"Gravity Anomalies" observed before earthquakes
https://www.sciencedirect.com/science/article/pii/S1674984717300034
The mass of the earth changes before an earthquake? Quite the mystery, indeed.
Monitoring earthquakes with gravity meters
A comparison of Gravity Meters and Seismometers.
https://www.sciencedirect.com/science/article/pii/S1674984715301920
https://ars.els-cdn.com/content/image/1-s2.0-S1674984715301920-gr9.jpg
Figure 9. Seismic records by a gPhone (blue) and a STS-2 seismometer
https://ars.els-cdn.com/content/image/1-s2.0-S1674984715301920-gr10.jpg
Figure 10. A set of five-minute S-wave records with an STS-2 seismometer and a gPhone
https://ars.els-cdn.com/content/image/1-s2.0-S1674984715301920-gr11.jpg
Figure 11. A set of five-minute records of background variation with an STS-2 seismometer and a gPhone
Funny how Gravity Meters and Seismometers agree like that.
- 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. "