What wavelength can penetrate soil to 15 feet?

[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.

 

[Gold Dredger]

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?

Most PI detectors have a depth of less than 30 inches. I am wanting a larger 5-15 feet depth capability. Current detectors are good at finding surface pay streaks of modern flood gold. The deposits I am after are the ancient deposits which will be larger pieces and much more concentrated. Try to imagine the epic floods that resulted as the glaciers retreated and you will understand why. My overhead is minimal as I only run a 5 inch dredge (10hp) motor and a BobCat. I am already profitable.

 

[Drakkith]

@Gold Dredger Well, you seem to be more knowledgeable than most who come here asking for help. Since I know virtually nothing about detectors of this type or of gold mining in general I think I'll bow out of this thread. Best of luck to you.