What wavelength can penetrate soil to 15 feet?

[Gold Dredger]

TL;DR Summary

My question is what wavelength would penetrate 15 feet of soil, sand, and clay but be stopped when it struck gold?

As my name suggests I am a gold prospector. My question is what wavelength would penetrate 15 feet of soil, sand, and clay but be stopped when it struck gold? The goal is to create a device that will allow for looking into the ground in such a fashion as to be able to create a visible display showing the presence of gold. Any thoughts on the subject are welcome. Clearly the potential of such a device would change gold prospecting and it's creators lives immeasurably.

 

[Baluncore]

Welcome to PF.

Summary:: My question is what wavelength would penetrate 15 feet of soil, sand, and clay but be stopped when it struck gold?

Clearly the potential of such a device would change gold prospecting and it's creators lives immeasurably.

If it was possible, gold would become almost worthless. I guess you have a case of the gold fever, with those crazy dreams of being able to see through the overburden.

It will depend on the soil. Gold is conductive so it will stop any EM radiation, while inducing a counter current in the metal. If there is a water table, or a salt layer present, it will make it more difficult.

You could use ground penetrating radar, but 15 feet is a long way down for VHF GPR. The problem with imaging is that the resolution required to see 1 oz nuggets is going to require short wavelengths, which will not penetrate so deep. The antenna array on the surface will need to be a similar size to the depth of penetration.

The usual imaging technique is to sweep the area with a metal detector, then scrape off the top 6” with a bulldozer and repeat. After 30 cycles you will be about 15' down and can make a sketch of where the gold was found.

1. What is the geology of the area you prospect ?
2. What climate and water table ?
3. Are there any salt horizons in the soil ?

 

[sysprog]

If it was possible, gold would become almost worthless.

Although supply versus demand is important in the markets, it's not the only factor $-$ wouldn't gold retain its inherent value?

I think that probably its market value would sharply decrease, at least temporarily, if large newfound deposits of it were to suddenly become available, but it wouldn't become almost worthless $-$ its abundance/scarcity as an elemental component of Earth would not thereby change, nor would its highly desirable properties $-$ we've seen platinum become cheaper than gold to purchase when more of it is mined and offered for sale, but it has never become almost worthless $-$ it remains more scarce than gold, and its inherent properties make it remain, e.g., the best catalyst material for a catalytic converter.

An influx to the gold market of greater apparent supply due to counterfeit ingots (made mostly from tungsten, the density of which is very close to that of gold) caused a drop in the price of real gold, but gold remained a precious metal $-$ based on its unchanging scarcity and inherent properties, I think that even if all the gold on Earth were available to the markets, it still wouldn't be almost worthless $\dots$

 

[berkeman]

Any thoughts on the subject are welcome. Clearly the potential of such a device would change gold prospecting and it's creators lives immeasurably.

So if we help your with this, what is our cut?

 

[Gold Dredger]

Welcome to PF.

If it was possible, gold would become almost worthless. I guess you have a case of the gold fever, with those crazy dreams of being able to see through the overburden.

It will depend on the soil. Gold is conductive so it will stop any EM radiation, while inducing a counter current in the metal. If there is a water table, or a salt layer present, it will make it more difficult.

You could use ground penetrating radar, but 15 feet is a long way down for VHF GPR. The problem with imaging is that the resolution required to see 1 oz nuggets is going to require short wavelengths, which will not penetrate so deep. The antenna array on the surface will need to be a similar size to the depth of penetration.

The usual imaging technique is to sweep the area with a metal detector, then scrape off the top 6” with a bulldozer and repeat. After 30 cycles you will be about 15' down and can make a sketch of where the gold was found.

1. What is the geology of the area you prospect ?
2. What climate and water table ?
3. Are there any salt horizons in the soil ?

I have been a gold dredger over 30 years. I have done pretty well for myself using conventional means such as a 5 inch gold dredge and a variable pulse induction metal detector. This isn't simply a case of "gold fever"... I make a nice living from it (in a state where that is rather hard to do Indiana) by training people on dredges, giving panning lessons, and selling my gold. I have tested GPR with I.U.'s (Indiana University) Geology Department (one of the best in the country). As you pointed out it's capabilities are limited by depth but also very poor discrimination for my target of gold and it requires fairly level clear ground. The geology is glacial till down to bedrock along the terminal moraine lines (often just 5-15 feet deep in my areas of interest.) The water table varies greatly from location to location some are creeks and rivers that require my dredge with very high water tables, others are bench deposits high and dry on bedrock shelves up the hill sides. Very little salt in most locations here in Indiana with a few exceptions such as "Salt Creek" In Brown County Indiana (so named due to it's natural deposits.) but that is the exception not the rule here. As for your assertion gold would become almost worthless that simply is wrong for two reasons first it is used in every electronic device made today and has industrial uses and second people love to wear it. If the price dropped much the demand would sky rocket and people would wear more of it especially in countries like India (a culture obsessed with gold) where brides are adorned with as much gold as their families can afford at their weddings. The fact silver is much more common but still not "worthless" also is an undeniable fact. Thanks for your reply.

 

[Gold Dredger]

Although supply versus demand is important in the markets, it's not the only factor $-$ wouldn't gold retain its inherent value?

I think that probably its market value would sharply decrease, at least temporarily, if large newfound deposits of it were to suddenly become available, but it wouldn't become almost worthless $-$ its abundance/scarcity as an elemental component of Earth would not thereby change, nor would its highly desirable properties $-$ we've seen platinum become cheaper than gold to purchase when more of it is mined and offered for sale, but it has never become almost worthless $-$ it remains more scarce than gold, and its inherent properties make it remain, e.g., the best catalyst material for a catalytic converter.

An influx to the gold market of greater apparent supply due to counterfeit ingots (made mostly from tungsten, the density of which is very close to that of gold) caused a drop in the price of real gold, but gold remained a precious metal $-$ based on its unchanging scarcity and inherent properties, I think that even if all the gold on Earth were available to the markets, it still wouldn't be almost worthless $\dots$

I agree with you. Gold is used in most electronics and has industrial uses, plus it is valued for it beauty and worn as jewelry. If the price suddenly dropped many more would buy it both as an investment and for it's properties. Supply and demand means it will never be "worthless".

 

[Gold Dredger]

So if we help your with this, what is our cut?

If you worked with me creating and testing said device and patents were applied for and granted I believe a 50/50 partnership would be fair. I would do the field work testing it and taking it to mine sites (for a commission of the gold found also to be split 50/50) leaving you to the lab work to make improvements and advance the technology as well as handle any patent issues that may arise (unless experts in that field were needed in which case costs would be shared equaly by the partnership). Sound fair to you?

 

[Vanadium 50]

So, this engineer interviews for a job. The interviewer says "We have this great product we're developing. You order a food item, and out it comes! Say 'apple pie', and out pops a slice of apple pie! Just like Mom used to make. Say 'steak and potatoes' and out pops steak and potatoes - the best you've ever tasted! Say 'spaghetti and meatballs' and out pops spaghetti and meatballs. What do you think?"

"That's amazing, the engineer says. How does it work?"

"You're the engineer - that's what we are hiring you to figure out!"

 

[Gold Dredger]

Engineer's have changed the world many times. The greatest thing about them is when people say "That is impossible" they take it as a challenge not a fact. The joke was funny and I apreciate a good sense of humor. Is your intellect as sharp as your wit? Let's solve this! And thanks for the laugh!

 

[DaveC426913]

Apologies Gold Dredger for the pile-on. The one I heard was for Authors: every author has a "buddy" who thinks he has a bestseller: "We can be 50/50 partners! I'll come up with the ideas, all you have to do is write the words!"

Seriously though, I think you can reasonably look to PF for suggestions about interesting avenues to pursue, but breakthroughs tend to be 1% idea and, like, 99% perspiration. That's got to be on you.

That said, I have nothing useful to contribute. Carry on.

 

[Baluncore]

The greatest thing about them is when people say "That is impossible" they take it as a challenge not a fact.

I guess I must declare a conflict of interest here. Your gold fever induced dream device is typical of gold prospectors, who often approach me with requests like this. My work in geophysics, electronics and computers suggests that if the problem was solvable, I should have done it by now. Finding landmines can be more dangerous than finding gold. The challenge of finding things hidden underground has been an active field of research for over 100 years. There must be some reason why it is not more advanced.

The problem here is that many good ideas can be proven mathematically impossible. No amount of engineering can make them work. What must be changed are the assumptions, the design constraints, or the prospect location.

At the moment pulse induction is the best technique for finding gold. But the best technique for making money is salting the creek with gold flakes, teaching tourists to pan for it, then buying the flakes back, and doing it all again this afternoon. Making and selling PI metal detectors to gold prospectors is also quite profitable.

Hand held detectors are small and portable, but can only detect shallow gold in the surface soil. The bigger nuggets will be found on or in weathered bedrock, below the clay layer in the soil. Deeper detection requires bigger coils. Maybe it is time to build some vehicle towed PI detectors, with many bigger diameter coils, that can sweep broad-acre farmland to bedrock depth, recording the GPS location of hot spots. Then you can return with an excavator later.

 

[Gold Dredger]

Apologies Gold Dredger for the pile-on. The one I heard was for Authors: every author has a "buddy" who thinks he has a bestseller: "We can be 50/50 partners! I'll come up with the ideas, all you have to do is write the words!"

Seriously though, I think you can reasonably look to PF for suggestions about interesting avenues to pursue, but breakthroughs tend to be 1% idea and, like, 99% perspiration. That's got to be on you.

That said, I have nothing useful to contribute. Carry on.

Dave,

No worries or need to apologize. I deal with gold miners regularly and some have a pretty twisted sense of humor. I have thick skin and can take it. I have some knowledge of variable pulse induction and its applications for finding shallow or near surface gold. I will agree breakthroughs are 99% perspiration! I just thought some here may have knowledge of wavelengths that I lack and therefore be a starting point. I don't exspect a breakthrough overnight.

People seem to forget just how far metal detecting technology has advance since just 1975.

The first generation of metal detectors (1975 to 1984) were capable of detecting large nuggets close ot the surface, but due to ground interference or “noise”, smaller pieces of gold could not be detected.The second generation of metal detectors (1985 to 1991) had improved technology that enabled the device to reduce ground noise and increase the penetration depth.The third generation of metal detectors began in 1995 after the invention of Multi-Period Sensing (MPS). These detectors were capable of finding gold nuggets as small as 0.3 grams, as well as locating larger nuggets at greater depths than ever before.The fourth generation of metal detectors began in 2006 via improvements in the ability of monocoils to reduce ground interference noise on highly mineralised terrain.The fifth generation of metal detectors began in 2010 with further developments on the ability of coils to reduce ground interference. The fifth generation uses Pulse Induction (PI) technology to further enhance the capability of metal detectors to reduce noise.The sixth generation of metal detectors began in 2015 with the development of Zero Voltage Transmission (ZVT) technology. ZVT has the advantage of transmitting a constant and stable magnetic field.With that said I know this is not an easy undertaking... nothing worth doing is easy or everyone would do it! If gold prospecting was easy no one would take a "real" job they would be out there like me dredging, detecting, and digging!

Thanks for your reply.

 

[Gold Dredger]

I guess I must declare a conflict of interest here. Your gold fever induced dream device is typical of gold prospectors, who often approach me with requests like this. My work in geophysics, electronics and computers suggests that if the problem was solvable, I should have done it by now. Finding landmines can be more dangerous than finding gold. The challenge of finding things hidden underground has been an active field of research for over 100 years. There must be some reason why it is not more advanced.

The problem here is that many good ideas can be proven mathematically impossible. No amount of engineering can make them work. What must be changed are the assumptions, the design constraints, or the prospect location.

At the moment pulse induction is the best technique for finding gold. But the best technique for making money is salting the creek with gold flakes, teaching tourists to pan for it, then buying the flakes back, and doing it all again this afternoon. Making and selling PI metal detectors to gold prospectors is also quite profitable.

Hand held detectors are small and portable, but can only detect shallow gold in the surface soil. The bigger nuggets will be found on or in weathered bedrock, below the clay layer in the soil. Deeper detection requires bigger coils. Maybe it is time to build some vehicle towed PI detectors, with many bigger diameter coils, that can sweep broad-acre farmland to bedrock depth, recording the GPS location of hot spots. Then you can return with an excavator later.

With all due respect metal detectors have advanced by leaps and bounds since 1975 with every few years some amazing breakthroughs have happened.

We agree the assumptions must be changed. That is why I joined this forum. Fresh ideas from young minds open to new ideas, untested methods, and applying existing knowledge in new ways. I don't think breakthroughs will be in an existing metal detecting company... they make small improvements to their products but seldom are the source of true game changing technology.

Before GPR no one thought there was any possible way to "look" under the ground. Now it is in regular use in the geophysics field.

Large detectors like the EM61 LX2 are capable of reaching over 30 inches in depth (unheard of just a few years ago)!

I am not looking for professors who say "if the problem was solvable, I should have done it by now". I want the fresh minds who say "lets figure it out"!

But thanks for the reply.

 

[Gold Dredger]

My Grandfather Charles Wayne Bensheimer was an Army engineer for over 38 years. He won many Gold plated sets of Craftsman tools for his designs. Many told him it would be impossible to create an off-road firefighting vehicle with a large water tank capacity and that a 4x4 truck was the best option. He refused to accept what the rest of the Army Core of Engineers and his Commanders told him was "possible or impossible" and went on to design a tracked firefighting vehicle that worked better than anyone thought possible... from a modified command APC that held 10 times the water capacity of the 4x4 trucks and could reach fires where they could not. When he told me the story he said something I will never forget... "If you think it is impossible it will be IMPOSSIBLE". Just thought this relevant given the tone of the conversation.

 

[Baluncore]

With all due respect metal detectors have advanced by leaps and bounds since 1975 with every few years some amazing breakthroughs have happened.

You are exagerating. We had PI back in the mid-70s for looking through salt pans. It just took a long time to reach the lower cost metal detector market. The advances in low cost metal detectors have been in very small steps, not leaps and bounds.

Pulse induction can be used to detect copper ore bodies 3000 feet below the surface. Variations in the Earth's magnetic field are used as the signal source. It takes a week to lay out a set of detector coils on the surface. Then you record data for a week. For deep imaging the frequencies analysed are below 1 Hz. The deeper the body, the lower the frequency.

The problem with GPR below VHF is that it interferes with too many radio services and propagates world wide. A license to operate the equipment will be refused unless the source of energy is radiated by natural lightning strikes. I could synthesise fake lightning strikes, but the lightning mapping networks would identify my position in less than an hour. I would be quickly charged by the authorities with transmitting in a non-permitted mode without a license.

I am not looking for professors who say "if the problem was solvable, I should have done it by now". I want the fresh minds who say "lets figure it out"!

If a particular method is impossible, then no amount of beating your head against it is going to make it possible. You must change the game.

Antenna arrays are well understood and can be designed for the job, but it is provably impossible the way you describe it. The dream of such an accurate image being generated by a small detector antenna is as illusory as a perpetual motion machine. You will require a much larger antenna array and you will need to mount it on a trailer of some sort. The first step would be to build a larger vehicle mounted PI system. That test bed can generate income while it is being upgraded to make an imaging system.

"If you think it is impossible it will be IMPOSSIBLE". Just thought this relevant given the tone of the conversation.

True. But I believe it is possible. The question becomes; how big must the antenna array be ? and how long must you wait before moving to the next sample point ? That determines how clear a picture you would get before excavating a deep target, and how much area you can survey to that depth in a week.

 

[Gold Dredger]

You are exagerating. We had PI back in the mid-70s for looking through salt pans. It just took a long time to reach the lower cost metal detector market. The advances in low cost metal detectors have been in very small steps, not leaps and bounds.

Pulse induction can be used to detect copper ore bodies 3000 feet below the surface. Variations in the Earth's magnetic field are used as the signal source. It takes a week to lay out a set of detector coils on the surface. Then you record data for a week. For deep imaging the frequencies analysed are below 1 Hz. The deeper the body, the lower the frequency.

The problem with GPR below VHF is that it interferes with too many radio services and propagates world wide. A license to operate the equipment will be refused unless the source of energy is radiated by natural lightning strikes. I could synthesise fake lightning strikes, but the lightning mapping networks would identify my position in less than an hour. I would be quickly charged by the authorities with transmitting in a non-permitted mode without a license. If a particular method is impossible, then no amount of beating your head against it is going to make it possible. You must change the game.

Antenna arrays are well understood and can be designed for the job, but it is provably impossible the way you describe it. The dream of such an accurate image being generated by a small detector antenna is as illusory as a perpetual motion machine. You will require a much larger antenna array and you will need to mount it on a trailer of some sort. The first step would be to build a larger vehicle mounted PI system. That test bed can generate income while it is being upgraded to make an imaging system. True. But I believe it is possible. The question becomes; how big must the antenna array be ? and how long must you wait before moving to the next sample point ? That determines how clear a picture you would get before excavating a deep target, and how much area you can survey to that depth in a week.

This is progress. I agree a larger array or coil will be required. Perhaps a device that is trailer mounted would be more practical or something that say 4 men could position and allow to remain stationary for the required length of time for the depth of the site (some areas I am looking at have as little as 5 feet to bedrock while others are 10-15 feet deep). Then move the array in a grid pattern based on the previous readings (to follow any pay-streak detected).

I was not aware of this fact "GPR below VHF is that it interferes with too many radio services and propagates world wide." This is the type of information I am seeking. As it is pointless to develop a device that would be illegal to operate. So thank you.

So what is the largest current PI system in use? Thank you again for the assistance.

 

[Vanadium 50]

If the price suddenly dropped many more would buy it both as an investment

I doubt that very much. "People have just lost 90% of their investment. Gonna get me some of that!"

Also, there is history. Aluminum was once a rare and exotic metal, but after the Hall process, prices dropped. To half a percent of what they were.

gold would become almost worthless

And we would then all use zinc. (Bonus points if people get the reference)

15 feet

There's gold at 15 feet, sure. But that's not where the gold is. The largest mine in North America and 2nd largest in the world went down to 8000 feet.

breakthroughs tend to be 1% idea and, like, 99% perspiration

It's not even an idea, There is no suggestion of how this thing works. Just, in the words of Brian Wilson, "Wouldn't it be nice?"

 

[DaveC426913]

It's not even an idea, There is no suggestion of how this thing works.

I meant the idea we come up with as to how it might work, that might - with 99% work - lead to a practical solution.

 

[Drakkith]

The general idea of using ground penetrating radar to find deposits and veins of minerals, even gold, is not new and has already successfully been done:

Application of ground-penetrating radar in placer mining: a case study from Guyana's Potaro region
https://www.researchgate.net/publication/4020525_Applications_of_GPR_for_surface_mining
An Evaluation of Placer Deposit Exploration Through GPR, DC Resistivity, Drone Imagery and Lidar

The goal is to create a device that will allow for looking into the ground in such a fashion as to be able to create a visible display showing the presence of gold.

Not going to happen. The absolute best you could hope for is something that works similar to an ultrasound. Ultrasound machines require training and experience to get useful information out of their images, so a device that just 'shows the presence of gold' is as much of a pipe dream as an ultrasound device that just 'shows the presence of cancer'. An ultrasound will show you that something is there, but it's up to the doctors to determine what it is using their expertise and other testing methods. Similarly, GPR can get you a 'map' of what's beneath you, but it's up to you to know how to use that map.

And keep in mind that GPR wavelength is several meters, so anything smaller than several feet across can't be made out since it is much smaller than the wavelength of the radar. And gold deposits are not made of lumps of gold several feet across, but in small, thin, disperse amounts mixed in with various minerals and other metals. We're talking amounts in the range of ounces of gold per ton of rock. Amounts as minute as this are not going to show up on any radar of any wavelength once you factor in their size and the reflections and interference from layer boundaries, impurities, and other metals and minerals that surround the gold.

 

[DaveC426913]

...a device that just 'shows the presence of gold' is as much of a pipe dream as an ultrasound device that just 'shows the presence of cancer'.

It would be cool if a device could detect the decay of some of gold's radioactive isotopes.
Then you wouldn't need to worry about interpreting artefacts in an image.
But that's still just sci-fi.

 

[russ_watters]

As an engineer I'm pretty sure I can figure out anything technical and as a fan of the TV show Gold Rush I'm relatively certain I understand gold mining well enough to be competent at it.

The problem is, in placer mining the gold concentrations are on the order of perhaps 0.05 oz per cubic yard, which means this near solid layer of gold you are looking for simply doesn't exist.

 

[Baluncore]

There's gold at 15 feet, sure. But that's not where the gold is. The largest mine in North America and 2nd largest in the world went down to 8000 feet.

Most gold is nearer the centre of the Earth.
The detection technology must be matched to your mining technology and the regional geology. There is no advantage detecting gold that you cannot reach with your excavation technology.

So what is the largest current PI system in use? Thank you again for the assistance.

Pulse Induction requires a circular eddy current be induced, that needs to be a significant loop area in the conductive material. Deeper gold nuggets need to be bigger to be detected from the surface. Disseminated gold dust will be invisible. Large PI loops are used to detect the regional volume of saline groundwater in aquifers.
It could be mobile, something like this.
https://www.usgs.gov/media/images/h...-loop-deployed-below-airborne-electromagnetic
https://www.csiro.au/-/media/LWF/Files/Water/FGARA/Fact-sheets/FGARA_factsheet_electromagnetic.pdf
Or it might be a 1 km square coil laid out on the surface as connected segments of multi-conductor cable with multi-pole plugs.

 

[Gold Dredger]

As an engineer I'm pretty sure I can figure out anything technical and as a fan of the TV show Gold Rush I'm relatively certain I understand gold mining well enough to be competent at it.

The problem is, in placer mining the gold concentrations are on the order of perhaps 0.05 oz per cubic yard, which means this near solid layer of gold you are looking for simply doesn't exist.

You are correct to some degree. It is not a "solid" layer, but it is concentrated greatly on the bedrock. Particularly here in Southern Indiana as we have had 3 major glacial events that bulldozed the till from Canada flattening much of the Northern part of the state and depositing that material in glacial termination lines on our southern bedrock. I have dredged here for over 30 years and found concentrations far greater than .05 oz per cubic yard. I have learned enough during that time to know where gold SHOULD be concentrated but I don't like making assumptions and want a device to confirm what I suspect before I begin. That said I have been correct far more than mistaken. I normally deal with areas that have overburden no more than 5-15 feet deep sitting on bedrock.

 

[Gold Dredger]

I doubt that very much. "People have just lost 90% of their investment. Gonna get me some of that!"

Also, there is history. Aluminum was once a rare and exotic metal, but after the Hall process, prices dropped. To half a percent of what they were.And we would then all use zinc. (Bonus points if people get the reference)

There's gold at 15 feet, sure. But that's not where the gold is. The largest mine in North America and 2nd largest in the world went down to 8000 feet.

It's not even an idea, There is no suggestion of how this thing works. Just, in the words of Brian Wilson, "Wouldn't it be nice?"

You brought up deep lode gold deposits. Indiana geology has no lode gold. Our gold is all placer gold on the surface of bedrock. I am not trying to start a lode mine down to 8000 feet but rather make surface operations more economical and profitable. Silver has gone from $4.30 an ounce in 2002 to a record of $48 an ounce in 2001... in spite of it being more common than ever in history. Prices dropped below $15 an ounce in 2018 and millions took notice and began "stacking" (buying rounds and bullion) and now it is near $25 an ounce due to higher demand from investors and industry. My point is if gold were to drop it would be widely invested in by both individuals and manufacturing (due to its superior qualities more components could affordably be made from it). There is already a discussion about large PI detectors possibly suited to my goals. Do you have anything to contribute to the conversation or are you just making background noise?

 

[Gold Dredger]

Most gold is nearer the centre of the Earth.
The detection technology must be matched to your mining technology and the regional geology. There is no advantage detecting gold that you cannot reach with your excavation technology.Pulse Induction requires a circular eddy current be induced, that needs to be a significant loop area in the conductive material. Deeper gold nuggets need to be bigger to be detected from the surface. Disseminated gold dust will be invisible. Large PI loops are used to detect the regional volume of saline groundwater in aquifers.
It could be mobile, something like this.
https://www.usgs.gov/media/images/h...-loop-deployed-below-airborne-electromagnetic
https://www.csiro.au/-/media/LWF/Files/Water/FGARA/Fact-sheets/FGARA_factsheet_electromagnetic.pdf
Or it might be a 1 km square coil laid out on the surface as connected segments of multi-conductor cable with multi-pole plugs.

The airborne electromagnetic mapping information was good and shows the possibilities of larger PI arrays. Thanks for sharing this. The gold I seek is normally found within 15 feet of the surface. As you said no point in "detecting gold that you cannot reach". Which is why I set a goal of 15 feet. I can reach 25 feet with my current equipment but costs go up in proportion to depth. I think a large PI loop is a perfect solution to my problem. Possibly a modular unit portable by 4 men that can remain stationary until the depth is reached then moved in a grid to follow the deposit. Thank you for your reply.

 

[Drakkith]

You are correct to some degree. It is not a "solid" layer, but it is concentrated greatly on the bedrock.

Any idea what concentration you need to be profitable? Or what concentration PI loops can detect?

Also, a bit of googling shows a bunch of PI detectors of various types are already available on the market. Are these not sufficient for you? Or are you looking at scaling up your operation?

 

[Vanadium 50]

It would be cool if a device could detect the decay of some of gold's radioactive isotopes.

Gold is isotopically pure. It has no long-lived radioactive isotopes.

 

[DaveC426913]

Gold is isotopically pure. It has no long-lived radioactive isotopes.

I know it has radioactive isotopes (Au198 has a half life of <3 days); but I don't know how long-lived they need to be to be useful (probably longer than 3 days).

 

[Vanadium 50]

If the gold was around since the formation of the earth, that's half a trillion half-lives. There is nothing left. You are quibbling. Natural gold is monoisotopic.

 

[DaveC426913]

If the gold was around since the formation of the earth,

That's a good piece of information to pin to the discussion, yes. Earth's gold was formed in supernovae.
But until that's provided here, I wouldn't call it "quibbling"; I'd call it "exploring". But YMMV.

And I did say a 3 day half life is "probably" useless.

 

[Baluncore]

Natural gold is monoisotopic.

Even if gold had a decay product, only gamma rays might escape through 15 feet of overburden.

There are a couple of techniques that could detect gold in the surface, but probably wouldn't work underwater or underground. One is XRF, the other is neutron activation followed by a scintillator and a pulse height analyser.

 

[Klystron]

If you are brainstorming for detection methods deep into soil (the thread title does not specify gold), I dimly recall reading about promising acoustic resonance methods for mapping underground structures that also detect underground water, old water channels, and certain crystals. IMS water and quartz crystals, aside from their intrinsic value, are associated with gold, silver and copper deposits.

Techniques to synthesize quartz crystals perfected when I was a child, likely reduced the hunt for natural crystal except for gem quality stones such as amethyst. I was curious about piezoelectric and anti-piezo effects to detect various crystals underground in response to pressure waves generated by natural events and acoustic location techniques. No idea if locating underground quartz applies to your search.

 

[Baluncore]

I was curious about piezoelectric and anti-piezo effects to detect various crystals underground in response to pressure waves generated by natural events and acoustic location techniques.

Most bedrock does produce a sort of bulk piezo voltage during a seismic event, but it also produces a granular noise like a carbon microphone as telluric currents are modulated by changes in rock conduction. Unfortunately, making a fine grid of electrodes suitable for imaging through glacial till will prove to be a serious challenge. Interpreting just what is meant by the signals is anyone's guess, unless you have a quartz crystal ball.
https://www.sciencedirect.com/science/article/abs/pii/S0031920120300212
https://en.wikipedia.org/wiki/Telluric_current

Mapping with Induced polarization, IP, using telluric currents is good for deep regional mapping and ore body location. It cannot really be adapted to fine detail mapping, where it is easier to reverse the IP and use PI.
https://en.wikipedia.org/wiki/Induced_polarization

 

[Klystron]

Most bedrock does produce a sort of bulk piezo voltage during a seismic event, but it also produces a granular noise like a carbon microphone as telluric currents are modulated by changes in rock conduction.

Excellent description. Yes, sound not unlike an overloaded mic felt viscerally as well as aurally.

Survivors of Loma Prieta and Northridge earthquakes in California describe the loud persistent sound.

Mapping with Induced polarization, IP, using telluric currents is good for deep regional mapping and ore body location. It cannot really be adapted to fine detail mapping, where it is easier to reverse the IP and use PI

Old books written prior to plate tectonic theories often refer to telluric currents as established science. I assumed either fantasy or they meant the Earth's magnetic field.

 

[Baluncore]

Old books written prior to plate tectonic theories often refer to telluric currents as established science. I assumed either fantasy or they meant the Earth's magnet field.

Any electrical current that flows in the Earth is a telluric current.
It is an old term derived from Latin "tellus" meaning Earth or soil.
Telluric currents arise from many different sources, and they are very real.