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Microgravity Surveys
- Thread starter ChrisJC
- Start date
rhychydwr1
Active member
Perhaps someone can explain to me the difference between Microgravity and Resistivity.
sinker
New member
ChrisJC said:
Small world. I spent a few years carrying out gravity, seismic and borehole surveys for NIREX around Windscale/Sellafield in the mid / late '80s.
https://en.wikipedia.org/wiki/Nirex
The gravitational acceleration changes according to the density of the ground. Sensitive gravimeters, used over large areas can plot underground variations in density down thousands of metres.
https://en.wikipedia.org/wiki/Gravimeter
Finding voids (caves/mines) is very easy as the results are very obvious.
Our survey programme consisted of gravi survey, seismic survey and finally boreholes to 1500m and more. The object, although denied at the time but later admitted, was to find suitably dense, sound bodies of rock in which to mine out caverns to form repositories for underground disposal of nuclear waste.
https://www.researchgate.net/publication/249557436_Lithostratigraphy_of_a_concealed_caldera-related_ignimbrite_sequence_within_the_Borrowdale_Volcanic_Group_of_west_Cumbria#pf2
https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=&ved=2ahUKEwjqsI6yv4fvAhVTaRUIHXRIATIQFjADegQICxAD&url=https%3A%2F%2Frwm.nda.gov.uk%2Fpublication%2F023-sellafield-geological-and-hydrogeological-investigations-the-geological-structure-of-the-sellafield-site-1997%2F%3Fdownload&usg=AOvVaw0_f8q1Q-N86oMs2AhhmvWh
I was at Parys Mountain in 89/91 sinking the new Morris Shaft; the story at the time being that it was for the same purpose.
You decide.....
RobinGriffiths
Well-known member
Interesting, it appears that there are even MEMS gravitymeters these days http://eprints.gla.ac.uk/203113/, though I suppose that shouldn't be a suprise given the plethora of MEMS sensors already in our phones.
I wonder hnow feasible, accurate and cheap a diy ARM based gravitymeter would be. Could be a lockdown project..
I wonder hnow feasible, accurate and cheap a diy ARM based gravitymeter would be. Could be a lockdown project..
ChrisJC
Well-known member
Sadly (?) we are coming out of lockdown and I haven't finished all of my lockdown projects yet!!
But I agree, a low cost Arduino type project might be quite straightforward. I have an idea for a super cheap way of doing it without even a MEMS but I need the oscillating frequency of a quartz crystal formula with all of the various factors included.
The stuff on Esclusham Mountain seems like a good place to test it.
Chris.
But I agree, a low cost Arduino type project might be quite straightforward. I have an idea for a super cheap way of doing it without even a MEMS but I need the oscillating frequency of a quartz crystal formula with all of the various factors included.
The stuff on Esclusham Mountain seems like a good place to test it.
Chris.
RobinGriffiths
Well-known member
Raspberry Pi pico, gravitymeter, GPS module and an SD card module - Bob's your uncle.
AR
Well-known member
Sinker's already described what microgravity surveying is, resistivity surveying (affectionately known as "the electric zimmer frame" by archaeologists) picks up subtle differences in the resistance to an electrical current between the two prongs. Damp or porous ground carries current easier than dry or solid ground, hence how you can find buried walls (high resistance) and infilled ditches (low resistance) amongst other things.rhychydwr1 said:
Sounds very interesting! I would love to use one of these to see where the continuation of our passage is in Water Icicle. We've dug 20m(ish - can't recall exact distance) though a choke under a surface depression (30m depth). Would be interesting if something like this would confirm if we're going in the right direction or not!RobinGriffiths said:
ChrisJC said:
Yeah, definitely. It is something we've discussed a few times in the past, but it hasn't progressed any further than that. I think when I looked into it last, the depth of the passage below was right on the boundary of what was feasible - but in truth I don't really know that much about the capabilities and how the science/technology has progressed. It's certainly a project I'd love to see carried out. The end of Volcanic Bug Pusher is ideal in my mind - we've got big known passage that could be scanned, then try to see if we can identify the continuation beyond Oh No! Choke, and then compare the results.
ChrisJC
Well-known member
rhychydwr1 said:
Microgravity measures the earths gravitational strength at that point. If there is less gravity in a certain spot, it indicates less rock between you and the centre of the earth. This could be due to you being lower down, or there being a gap!
But you do need to measure pretty accurately...
Resistivity is just stuffing a couple of probes into the soil and measuring the electrical resistance. When you plot it out, you can see features buried in the soil like drains, wall foundations, Roman temples, etc. But it only works at shallow depths.
Chris.
Bob Mehew
Well-known member
Er No.RobinGriffiths said:
But as it happens I have been working on an accelerometer as described, namely the SparkFun LSM9DS1. Left on a surface for 100s without any movement gave an average value for g of 9.812 m/s^2 with a standard deviation of 0.028. The span of value recorded ranged from a maximum of 10.113 to a minimum of 9.703. So I fear this may require a far more sensitive instrument as well as stopping all local movement.
ChrisJC
Well-known member
Not surprised with that one Bob!
We need the sort of thing alluded to here:
http://theses.gla.ac.uk/7788/
and here:
https://www.nature.com/articles/s41378-019-0089-7
But I've not seen one off the shelf yet...
My thinking is that the frequency of a crystal oscillator has a dependency on G. Therefore you can measure the change in F as a function of G. But I've never seen an equation confirming that.
Chris.
We need the sort of thing alluded to here:
http://theses.gla.ac.uk/7788/
and here:
https://www.nature.com/articles/s41378-019-0089-7
But I've not seen one off the shelf yet...
My thinking is that the frequency of a crystal oscillator has a dependency on G. Therefore you can measure the change in F as a function of G. But I've never seen an equation confirming that.
Chris.
aardgoose
Member
Yes, the mems devices the Glasgow papers describe are fabricated in their own labs, so not yet at the commercial stage sadly. And it seems unlikely that there will be the scale of application such as phones driving their production.
The other problem is the work to get rid of all the other effects to isolate small scale features, which look non trivial in the original article above, although obviously routinely carried out (correcting for local topology and geology, tides etc).
The other problem is the work to get rid of all the other effects to isolate small scale features, which look non trivial in the original article above, although obviously routinely carried out (correcting for local topology and geology, tides etc).
sinker
New member
ChrisJC said:
Don't you just double it, divide by your house number and then subtract the number you first thought of....?
In reply to someone else's thought further up the thread, the sensitivity required is mind-blowing.
Also there are lots of local corrections that need applying to the results.
The point where the reading is taken has to be surveyed to sub-5mm accuracy and all other survey stations needs to be surveyed to the same accuracy to allow meaningful correlation between adjacent points; then local 'terrain corrections' need to be applied to allow for things like side slopes, adjacent rock outcrops, nearby lakes etc; anything which would have an effect on the reading.
We'd all sit in the bar in the evenings and work through the survey data, all logged on Husky Hunter personal lap-top computers, the first of their kind anywhere in the world I think? Anyone remember them??! The original "tough-book". 32k ROM, 208k RAM, proper keyboard, originally designed for rapid, real-time 'in-the-field' guided missile trajectory calculations!
Wow, its all coming back to me now. Wildly off-topic now I'm afraid!!
<< We'd all sit in the bar in the evenings and work through the survey data, all logged on Husky Hunter personal lap-top computers, the first of their kind anywhere in the world I think? Anyone remember them??! >>
Yes, The Husky hunter was my choice of first computer for outdoor and underground surveying. Water, shock and fireproof, but being intended for the armed forces, no software available - "Write your own ! "
A certain Dr Philips did a talk on resistivity surveying at a caving conference way back in the sixties.... It was tried out in Devon and successfully predicted caves later explored at Buckfastleigh.....
But that takes all the suspense out of exploration ! Especially if any details are posssible. Back to the Hazel Twig !
D Send.
Yes, The Husky hunter was my choice of first computer for outdoor and underground surveying. Water, shock and fireproof, but being intended for the armed forces, no software available - "Write your own ! "
A certain Dr Philips did a talk on resistivity surveying at a caving conference way back in the sixties.... It was tried out in Devon and successfully predicted caves later explored at Buckfastleigh.....
But that takes all the suspense out of exploration ! Especially if any details are posssible. Back to the Hazel Twig !
D Send.
pwhole
Well-known member
We got some reasonable results doing resistivity surveying of the Speedwell Vent in Castleton a few years ago, and did detect a possible void about 4m below surface in Middle Bank Gully during a follow-up session. But that was specifically to better interpret the agglomerate structure, rather than looking for cave passage - the void was just an accidental extra. Not that they'd ever let us dig it up to check.