Cave Detection using Quantum Gravity ?

ChrisJC

Active member
Probably not on eBay just yet.

I do have an idea to make a cheap gravimeter, but I've never found the full equations for the frequency of a crystal (oscillator) to enable me to calculate if it's genius or idiocy.

Chris.
 

Cantclimbtom

Active member
We'll the underlying technology sounds pretty straightforward really
Could you knock something up in your shed with a large metal coffee tin and a couple of powerful LEDs? (plus judicious use of gaffer tape)


"industry-grade gravity gradiometer that is able to measure simultaneously the gravity acceleration and its gradient. This quantum gravity gradiometer relies on atom interferometry with cold 87Rb atoms: it measures the vertical gradient of g with a single vertical laser beam interrogating simultaneously two sets of laser-cooled atoms at different height [REF]. Such device relies on a physical principle and a set of technologies that have already been validated for absolute quantum gravimeters [2]. We present the performance of the device that has been integrated and discuss its stationary measurement capability, as a sensitivity of 60 E/?Hz (1E = 10-9 s-2 = 0.1 ?Gal/m) and a resolution of 1E are expected. We will also review the operational features of this new device including its transportability and ease of use. This new type of gravity gradiometer is to our knowledge the only demonstrated technology that allows for an absolute continuous drift-free monitoring of both gravity and gravity gradient over timescales from a few minutes to several months.
This work demonstrates the feasibility to operate a free-falling atom gravity gradiometer as a turn-key device and paves the way to practical investigation of both spatial and temporal gravity gradient variations at a level of 1 E in both laboratory and field conditions"

See what I mean, pretty simple
 

Dave254

New member
There is a diy project for a Gravimeter in the wilds of the internet if anyone has to knowledge to program the IC https://www.nutsvolts.com/magazine/article/january2013_Newton
 
Programming a PIC is no problem.  PM me if you are stuck.  The PIC in question is still in production, widely available, and costs a couple of quid.  What's novel in the article though is the "el cheapo" method of measuring gravity.

It seems that gravity variation really is measurable for near-surface voids, i.e. variations exceeding the noise level of the instrument.  The eventual practical problem is that after finding a big void 10m below the surface ? are you then going to engineer a 10m deep shaft to get into it?

The circuit presented is a kind of resistance meter.  On the face of it, a near 1% change in gravity, as shown on the LCD display in the article, would imply a void of perhaps 1/100th of a planet in proportion?  I?m sure Dave Gibson can put me right on the maths, and provide an interesting paper in the next BCRA CREG journal about gravity and caves.  Indeed, a cave which is 1% of the planet in volume would be ?very interesting? to visit.

Using the microcontroller?s hardware clock as a yardstick rather than some internal or external A/D module is a much more accurate way to measure things, which is a hopeful feature of this design.  The PIC chosen could only resolve voltage to 1 in 1024 parts using its internal A/D unit.  As this chip?s max clock rate is 2MHz, this implies that ?the thing measured? can be resolved to half a microsecond at best, so call that 1 in a million parts, which is about 1000 times better than if its A/D unit had been used for that purpose.

An exceedingly stable power supply is also of the essence for measuring voltages to incredibly fine resolution, and that?s definitely not going to be optimal with just a 7805 and just a 100nF capacitor across its output, as shown in the circuit.

But the ideas are interesting in principle and it?s a use for all those bits of anti-static foam occuping my desk drawers rather than opt for some sort of strain-guage as used in digital bathroom scales.

Anyone taken bathroom scales to bits to find out how they really work?  They're incredibly cheap.  I have this vision of a caver walking across an upland landscape, with bathroom scales in hand, weighing themself evey few steps to determine if there is an unknown void below...
 

ChrisJC

Active member
Dave254 said:
There is a diy project for a Gravimeter in the wilds of the internet if anyone has to knowledge to program the IC https://www.nutsvolts.com/magazine/article/january2013_Newton

I can't believe that works to any sort of useful level.

Chris.
 

ChrisJC

Active member
Stuart France said:
Anyone taken bathroom scales to bits to find out how they really work?

Yes, and we copied the design for this:
https://www.surepetcare.com/en-gb/pet-feeder/microchip-pet-feeder-connect
Assume a load cell with a maximum load of 1kg. The converter IC might claim a 24bit result (unlikely in practice!). But that equates to 1/16777216 of a kg, which is 59microgrammes.

My feeling is that is nowhere near fine enough to be detecting voids.

Chris.
 

Cantclimbtom

Active member
From what I can glean the big deal about this new fangled gizmo is not that it can measure gravity at one point, but rather it can measure gravity at two different points very very sensitively to give both the gravity and also a gradient (difference between the two points).  The sensitivity + gradient measurement, is the big deal about it. From my limited understanding anyway
 
How are you with carrying a beer tray and six pints, I mean before you've actually had anything to drink and you can still think what you're doing?

If "the thing being weighed" and "the weighing machine" are not horizontal then you'll get different answers depending on how far away from horizontal the stuff is... all that sine and cos business.

The lead weight sat on the anti-static-foam-sandwich will compress it the most when the foam thingy is horizontal, and it won't compress the foam at all when the sandwich is vertical when only the PCB "bread part of the sandwich" is in contact with the weight and some hard surface below.  Intermediate angles produce intermediate answers.

The reproducibility of your weight readings will depend on, amongst other things such as temperature and humidity, how standardized the sensor rig can be.

There's another problem.  A bigger void at greater depth surely will produce the same gravity variation (the same number on the meter) as a smaller void at shallower depth, all other things being equal.  It's like five apples and three oranges costs ?3.  So how much is an apple?




 
ChrisJC said:
Stuart France said:
Anyone taken bathroom scales to bits to find out how they really work?

Yes, and we copied the design for this:
https://www.surepetcare.com/en-gb/pet-feeder/microchip-pet-feeder-connect
Assume a load cell with a maximum load of 1kg. The converter IC might claim a 24bit result (unlikely in practice!). But that equates to 1/16777216 of a kg, which is 59microgrammes.

My feeling is that is nowhere near fine enough to be detecting voids.

1/16777216 kg-force is the modification to the gravitational force produced by about 875 missing tons of rock at a depth of 10 metres on a 1kg weight - equivalent to a sphere with a radius of about 4.5 metres in limestone. That's quite a big void quite close to the surface, but not rediculously so. At 100 metres depth, then it's 87,500 missing tons - more like a 20 metre radius void. You might be able to detect something like Titan.

But you have so many other factors that would mask the change - climb the height of one single step on a standard staircase in altitude and you'd get a similar change in gravitational force. You might have other large density changes in the rocks and various different depths. Atmospheric pressure changes + wind could cause a fair change depending on what instrument you are using..
 

mrodoc

Well-known member
A practical example of a gravity survey was displayed at the annual Jrat awards. It related to the resistivity survey done by Palmer many years ago that purported to show a large chamber near the big Lamb Leer main chamber.  A gravity survey in the area failed to show up 'Palmer's Chamber' but did show the main chamber and some smaller voids (so far unentered but nearby). There is  a long term dig in the area that might, one day, lead to confirmation of the presence of these voids.
 
I think it is the measurement of the field strength gradient that is the key.
From what I read on this forum the measurement is of the free fall acceleration of 2 groups of atoms separated by a vertical distance. The difference will directly give the difference in gravitational field strength and hence the average gradient over this distance.
The gradient will normally be negative, the field strength decreasing as height increases.
I find it easier to imagine something like a 10m diameter lead ball with it's centre 10m below the surface.
On the surface directly above the field strength will be made up of 2 components, the larger being the contribution from the Earth, with it's C of M about 6400km below and the other being the lead, with it's C of M 10m down.
Moving upwards, the contribution from the lead will fall off very quickly (to 1% over a distance of 90m) but that of the Earth will be largly unaffected, so the gradient above the lead will be more negative than nearby.
With very accurate instruments I imagine it would be possible to determine the size and depth of the lead ball.
With a void the effect will be reversed, the gradient being less negative (even positive over a short distance?)
 

ZombieCake

Active member
Soooo... if I just connect the Flux Capacitor to the Ghostbusters Proton Back Pack with a few jump leads and read the results on the Sonic Screwdriver, correlating the vibrations to the Jurassic park glass of water dinosaur detector, and chuck in a few 18650s to amplify the power I'll be OK. Cool.
 
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