EM Penetration Depth Into the Ground

[david_nelson]

TL;DR Summary

Wondering about best EM to scan the underground

Hello,
I'm wondering about the best EM to penetrate deep into ground and sense/detect what's there.

The authors here (https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2005JE002528) are recommending P-band with 430 MHz to probe few meters into the ground of Mars; that's about 70-cm wavelength. Another paper has claimed P-band can penetrate 5-meters into arid deserts on Earth.

Well; I want to detect/sense certain materials underground with a minimum of 5-meters depth (the deeper, the better) with 100% accuracy using each of those materials' unique spectral signature. So, what's the best EM penetration into ground that would enable me to do that?

I understand that short-wavelength high-energy X-Ray and Gamma are excellent in penetration and dangerous due to ionization; the danger part isn't a problem because I work in areas with no apparent living organisms (certainly no humans or animals; can't worry about a reptile underground that I can't see), but still: such equipment isn't easily accessible and isn't user-friendly, although I'm willing to buy one if user-friendly portable/handheld ones were available and accessible.

Which brings me back to: jump UV, Visible, and IR, as they all cannot penetrate ground. But Microwave and Radio are long enough not to help in sensing/detection, right? Or we can use them as carrier, and modulate some short wavelength onto them, so they just carry that short wavelength into ground, and using spectrometry we detect whatever we want to detect; make sense?
Thanks.

 

[Baluncore]

Welcome to PF.
The depth of penetration will be determined by ground conditions. What are the conditions in your search area? What rock type, below what soil depth, what water content?

Well; I want to detect/sense certain materials underground with a minimum of 5-meters depth (the deeper, the better) with 100% accuracy using each of those materials' unique spectral signature.

You will have a problem looking for spectral signatures underground. You will certainly be able to measure the electric and magnetic properties of the near surface. That is because you will only see what is reflected back to the surface by impedance discontinuities.
What in particular are you looking for? What are the spectral features that might be seen?

Or we can use them as carrier, and modulate some short wavelength onto them, so they just carry that short wavelength into ground, and using spectrometry we detect whatever we want to detect; make sense?

The problem with modulation is that the carrier needs to be at a frequency greater than the modulation. Deep penetration and reflection will attenuate the carrier frequency more than the modulation frequency.

There are ways of getting better images, but do you need images, or would a bulk spectral signature be sufficient?

 

[davenn]

WOW, someone with my own name welcome to PF

Summary:: Wondering about best EM to scan the underground

I'm wondering about the best EM to penetrate deep into ground and sense/detect what's there.

Summary:: Wondering about best EM to scan the underground

Well; I want to detect/sense certain materials underground with a minimum of 5-meters depth (the deeper, the better) with 100% accuracy using each of those materials' unique spectral signature.

100%, well I think that's being unrealistic and I suspect impossible to achieve with current technologyhave a look here for some background
https://www.sensoft.ca/range-of-gpr/

and there are plenty of other sites

regards
Dave

David Nelson

 

[sophiecentaur]

Summary:: Wondering about best EM to scan the underground

I'm wondering about the best EM to penetrate deep into ground and sense/detect what's there.

That all depends on what ground and what you're looking for. If you envisage just pointing an 'antenna' (Hollywood style) at a piece of ground and bouncing signals off the various layers and objects down there then you have to be very lucky. I found a very practical application of a method for finding underground water in this paper. It may not relate to what you're interested in but it will give you an idea of the sort of trouble you need to go to and a possible technique, even for large deposits of a suitable mineral.

 

[david_nelson]

Welcome to PF.
The depth of penetration will be determined by ground conditions. What are the conditions in your search area? What rock type, below what soil depth, what water content?

Thank you.
We are doing geophysical research in Jordan, so it's fair to assume dry arid hot environment, which is not the case during winter of course, but the majority of the country is so. We might find reddish, brownish, or white sands/rocks at various layers: clay, loam, phosphate...etc; rocks come in all forms; in the south we have Wadi Rum, which is the location of the movies "Martian" "Star Wars: Episode IX" "Red Planet"... water content might be few meters away, and might be very deep.. I would work in places where I want to detect something between ground surface and the water bed for sure, hence I don't need any penetration of the water... so, we are talking about highly variable soils and rocks across the country. I need the detection system to work regardless of all that; there should be something available!

What I am looking for is diverse, hence I want a technology that would depend on the unique reaction/resonance/natural frequency of each material to detect it; I want to map zinc, copper, phosphate, gold, silver... but I don't want some sort of generic metal detection, I want to be able to precisely determine the detected thing based on its unique characteristics, regardless of its shape.

Imaging is always good; I wish to have handheld X-ray device that would penetrate the ground for 5 meters; would be better that generic scanning for 100 meters; backscatter x-ray scanners are excellent for shallow detection, but poor with their depth, like https://www.heuresistech.com/hbi120 and https://www.rapiscansystems.com/en/products/ase-mini-z.

 

[david_nelson]

WOW, someone with my own name welcome to PF
100%, well I think that's being unrealistic and I suspect impossible to achieve with current technology
have a look here for some background
https://www.sensoft.ca/range-of-gpr/
and there are plenty of other sites
regards
Dave
David Nelson

Thank you, Dave... pleasure being here with all you guys.
GPR depends on radar technology; I talked during a conference in London, UK with the technical guys making some European GPR products, and they assured me that the wavelength of the GPR won't be helpful even at 5-meter depth; it will give you images, but no reliability below 3-meter... If I go with Figure 6 of the Sensoft website, that seems right. Also, their detection is very unfriendly: lines and parabolas all over the place, with no discretion at all; GPR is not good at all for my applications.

 

[david_nelson]

That all depends on what ground and what you're looking for. If you envisage just pointing an 'antenna' (Hollywood style) at a piece of ground and bouncing signals off the various layers and objects down there then you have to be very lucky. I found a very practical application of a method for finding underground water in this paper. It may not relate to what you're interested in but it will give you an idea of the sort of trouble you need to go to and a possible technique, even for large deposits of a suitable mineral.

Thank you.
The developments in THz technology are brilliant, and can come close to that kind of Hollywood style, but they are still facing problems with the energy. Backscatter X-Ray is great too, but again energy isn't helping. I'm in geophysics, hence I want to scan large areas, with 100m x 100m grid cell, for many materials; zinc, copper, silver, gold, phosphate, iron... to come up with high-res unique perspective about the 3D layering there underground; I don't need to detect all of them at once; one at a time, using its unique responses to the irradiation coming from ground... there must be something.

 

[Baluncore]

I want to map zinc, copper, phosphate, gold, silver... but I don't want some sort of generic metal detection, I want to be able to precisely determine the detected thing based on its unique characteristics, regardless of its shape.

Everyone wants that, and no one has it yet.

Are you looking for the metals as solid conductive metal, or as say oxides or sulphides?
Why do you insert phosphate in the middle of the list of conductive metals? That would suggest you are a prospector rather than a treasure hunter. Which is it ?

To find shallow deposits you might use down-hole XRF.

 

[david_nelson]

Everyone wants that, and no one has it yet.

Are you looking for the metals as solid conductive metal, or as say oxides or sulphides?
Why do you insert phosphate in the middle of the list of conductive metals? That would suggest you are a prospector rather than a treasure hunter. Which is it ?

To find shallow deposits you might use down-hole XRF.

The physics is there; just need someone resourceful to harvest it; maybe through Surface Plasmon Resonance or NMR combined with THz; I'm working on it for few months new, but as a geophysicist, I'm not really into signals and systems that much; but there are many kinds of waves that penetrate ground like a knife goes through butter; think about seismic waves: you don't hear anyone complaining about penetration of Earth with them; why not harvest them one way or another!
I don't need to list all the solid materials in the periodic table, do I? Just examples of what's there underground, conductive or not.
My path might cross with treasure hunters or prospectors; I don't really care: I'm a geophysicist.

 

[Baluncore]

There is another possibility.
We are usually handicapped by the presence of vegetation. When jungle covers the ground you can fly a gas analyser to sample the vapour produced by the vegetation. The rock type changes the plant chemistry so the plant activity can be used to identify the chemical geology.

It appears you do not have that advantage of jungle growing down into the weathered rock. Nor do you have grass or shrubs.

But soil contains bacteria that can concentrate various chemicals including heavy metals. So sampling and mapping the soil bacteria from shallow pits or holes may well give you the information you require. There might well be a PCR test for zinc converting bacteria. Such a system would need to be calibrated by sampling bacteria from different known ore body sites in the region.

 

[david_nelson]

There is another possibility.
We are usually handicapped by the presence of vegetation. When jungle covers the ground you can fly a gas analyser to sample the vapour produced by the vegetation. The rock type changes the plant chemistry so the plant activity can be used to identify the chemical geology.

It appears you do not have that advantage of jungle growing down into the weathered rock. Nor do you have grass or shrubs.

But soil contains bacteria that can concentrate various chemicals including heavy metals. So sampling and mapping the soil bacteria from shallow pits or holes may well give you the information you require. There might well be a PCR test for zinc converting bacteria. Such a system would need to be calibrated by sampling bacteria from different known ore body sites in the region.

Thank you.

 

[sophiecentaur]

Backscatter X-Ray is great too,

OH boy - using a hand held device?? I don't know what thickness of lead your hand-held device would need to produce a well collimated beam of sufficient power. Health and safety would be a very significant issue, I think.

I would think that a large antenna, dragged over the ground would be the way to start - followed by a method of analysing the low frequency and (I'd bet) very broad band responses of the stuff you're looking for.

Someone has already made the point that you are only one a many people who would love this particular philosopher's stone. The answer, if it's every attainable, will have to be something incredibly complex. With all measurements, the bottom line is Signal to Noise Ratio. Given enough time and effort you can more or less measure anything - consider gravity waves.