Comment on Earthquake Seismograph Using Bismuth Magnetic Levitator – about state of the art in low cost gravitational detector arrays

Earthquake seismograph using bismuth magnetic levitator as sensor at https://www.youtube.com/watch?v=aIANNzpec7Y

Tomislav Stimac,

The “standard” still for accelerometers is the small but global network of super conducting gravimeters (SGs). They use liquid helium cooled niobium magnetically levitated spheres as the test mass. The sensitivity is about 0.1 nanometers per second squared (nm/s2) at 1 sample per second. That is sensitive enough that the main part of the signal is the vector tidal gravity signal due to the sun and moon then the atmospheric and ocean signals, then magnetic and seismic and electromagnetic signals.

The problem with the SGs is the only have single vertical axis. Since the sun moon signal is a three axis signal, only one of the axes is recorded. So it cannot be used for solving precisely for the position and orientation of the instrument.

You are right that room temperature magnetically levitated diamagnetic bismuth seismometers/gravimeters is not an organized field on the Internet. But there are many examples of people trying it. Albeit most never refine it to the point of a working and accurate device.

“bismuth” “seismometer” gives 557,000 entry points (3 Nov 2021, Google, GMT 1:37 PM)

The more usual material for magnetic levitation is “pyrolytic graphite” because it is much more diamagnetic than bismuth.

“magnetic levitation” “graphite” has 7.-3 Million entry points

“magnetic levitation” (“graphite” OR “pyrolytic”) (“seismometer” OR “accelerometer” OR “gravimeter”) has 1.04 Million entry points.

“magnetic levitation” “diamagnetic” (“seismometer” OR “accelerometer” OR “gravimeter”) is more narrow. but much more specific. It has 67,300 entry points. Among them “Optimization of a 3D micro-accelerometer based on diamagnetic levitation” at https://www.researchgate.net/publication/29608633_Optimization_of_a_3D_micro-accelerometer_based_on_diamagnetic_levitation

I don’t know what your goal might be. For gravimeters and gravitational imaging arrays the most useful devices are atom, ion,. electron interferometer devices. But there are about 40 basic types of approaches. Most everyone working alone, all trying to patent their tiny piece of what ought to be a global industry.

The broadband, long period seismometers in the IRIS.edu global network of seismometers, in quiet locations, can be used as three axis gravimeters. There are several groups who have made variations on the MEMS accelerometers to make three axis accelerometers sensitive enough to detect the sun moon signal. The sun moon signal varies at most locations by about +/- 1000 nm/s2. So it can be used to calibrate the MEMS devices continuously to remove long period drift, and variations due to temperature and many other reasons.

If a seismometer (which measures and reports velocity usually) signal is differentiated, it operates as an accelerometer. Most devices on IRIS.edu have “impulse response functions” so they can operate both as seismometers and accelerometers. The difficulty is separating the signals that get injected from many sources. So the “gold standard” is a three axis, time of flight accelerometer in global arrays where the magnetic, electromagnetic, and gravitational signals can be separated clearly by what are basic synthetic aperture array methods. But few have gotten all the pieces together to run accelerometer at Giga Samples per Second rates needed (Gsps). Many groups and companies are aiming at distributed sensing arrays now, but mostly for traditional acoustic and electromagnetic array imaging and communication.

One of the most exciting things I found is that a three axis gravimeter can be used with stationary devices to calibrate against the sun and moon so precisely it is possible to determine position and orientation as accurately as VLBI (very long baseline interferometry). I call it “gravitational GPS”.The advantage is that it works underground, and is less bothered by magnetic variations. I had to use that because the seismic “transportable array” had seismometers whose orientation was not known or precise. But you can minimize the sum of squares of residuals for three axis measurements against the sun moon signal, and get a stable correlation. Using arrays, and time of flight, the whole network can operate at VLBI precision but locking down the gravitational part.

I have been following these technologies for 20 years now. I calibrated the superconducting gravimeter array back about 1981 to measure the speed of gravity. Then the seismometer arrays. In recent years I am encouraging LIGO groups to use atom interferometer, Bose Einstein condensate, “quantum” and other desktop or hand held devices to work together globally. But it is worse than herding cats.

You might take a look at GravityNotes.Org and look at an example spreadsheet at the top. Or look at my notes on Hackaday.IO

Low Cost, Time-of-Flight Gravimeter Arrays – Gravimeter array imaging requires building low cost, high sensitivity, time-of-flight (aka high sampling rate) sensors. at https://hackaday.io/project/164550-low-cost-time-of-flight-gravimeter-arrays

Again, I don’t see what you are aiming to do. And the field is so huge. I actually started back in 1975 at the University of Maryland at College Park where Charles Misner, Kip Thorne and John Wheeler, Joe Weber and Robert Forward were laying the foundations of gravitational wave detection. Misner was my academic advisor, but Joe Weber convinced me to follow what Robert Forward recommended. Robert Forward had an expression for the gravitational energy density, which I was able to show is in equilibrium with the magnetic energy density in certain situations. The earth’s gravitational field is equivalent to a magnetic field of about 379 Tesla and the random fluctuations show up in every sensitive experiment as various types of noise. But with time of flight methods those can be uniquely separated.

https://en.wikipedia.org/wiki/Gravitation_(book)
https://en.wikipedia.org/wiki/Robert_L._Forward
https://en.wikipedia.org/wiki/Weber_bar
https://en.wikipedia.org/wiki/Joseph_Weber

Robert moved to California and was part of creating LIGO. Nowadays, he would be recommending everyone use some type of desktop system, so that every country and university and research group could afford to be part of global arrays. In Roberts dissertation and elsewhere he recommended combining gravitation and electromagnetism on a practical basis. That is what my research was focused on. The Japan earthquake created a gravitational signal strong enough to registered on both the SG network and the broadband seismometer networks as a signal from the changing density of voxels due to the spreading seismic wave. The seismic wave spreads out at the speed of waves in the earth and the speed of light and gravity signal from the changing gravitational potential registers on sensitive detector that can be used for imaging the atmosphere, the oceans, and the interior of the earth. The data rates are high and the computer processing is stiff, if you try to work in real time. But much can be done with data over long periods.

It has been a bit frustrating having to wait 45 years for technology to advance far enough to achieve the vision that Joe Weber and Robert Forward started with – that gravitational fields can be used for imaging, precise location and orientation, communications, and levitation. I recommended using a simple rule – if you can use a computer to use fields to move an object in 3D on a precise path, it is “gravity”. It turns out that the field at the surface of the earth is not that deep. It is effectively equivalent to a soft xray field with energy density comparable to a magnetic field of 379 Tesla. Something easily achievable with lasers and beams of many kinds. The story and implications is very long, and I am a bit busy now.

There are lots of ways to monitor the position and orientation of your test mass over time. I have followed all the “super resolution” methods that allow using statistical methods to resolve and image down to nanometer and picometer resolutions and track that over time. My favorite is electron methods using noise in cameras sensors and memory devices. But electrochemical seismometers, Mossbauer, atom interferometer (chips now), atomic force methods (hundreds of variations), and many simply labeled “quantum” all can pick up variations in the gravitational potential at all frequencies accessible to electromagnetism.

In August 2017, there was a merger of two neutron stars that generated both gravitational and electromagnetic signals that arrived at detectors near earth at precisely the same time. That showed that the speed of gravity and the speed of electromagnetism are identical. Not just close, but identical. To me that meant gravity and electromagnetism must share the same underlying potential. That greatly simplified the whole thing. When I was going to UMD, I worked with Steve Klosko at Wolfe Research on a NASA contract to take many kinds of data to solve for more and more precise models of the earth’s gravitational potential field. I follow those groups, and the GRACE follow-on is now imaging field changes inside the ocean and over the whole earth. I keep hoping they will try these faster sensors. But there are groups using the small detector methods for lunar and Venus field measurements. It is a lot, and it is messy. Part of the reason I spent the last 24 years following global groups on the Internet, is to find ways to help coordinate global research with groups made of of many individuals all working alone or in small groups.

Richard Collins, Director, The Internet Foundation

​ @Tomislav Stimac I was just trying to say that magnetic levitation for seismometers is not that unusual. If you are interested in the field and want to do more, experiment and create new sensors and devices, that is good news. Lots of people and groups and ideas around. It is not clear if you want to improve what you started, or just display it as an art piece. I am not being mean, just joking. But a one time creation, however well done, is different from something that builds.

I like the light that you used, It could probably be used with high resolution, high frame rate cameras to measure more precisely and at higher frequencies. At higher frequencies is is called an acoustic or vibration sensor. All these technologies are position sensors. First time derivative gives velocity and you call it a seismometer. Second time derivative “accelerometer”. Second time derivative and very sensitive a “gravimeter”.

I like what you did. Just saying hello, and wondering what you might do next, or if you wanted to improve and use what you learned from this. When you talk about things and make improvements, your popularity and interest on YouTube grows. You meet more people, you learn more.

And bismuth is diamagnetic, but just happens to be one of the first to be tried. The most diamagnetic substance is the electron itself.

Keep up the great work! Hope you will do more.

Richard Collins, Director, The Internet Foundation