[Max]

Originally Posted by Aurificus

The following mechanism that may enable (some) buried metal objects to
produce an electromagnetic signal that can be detected passively.

Sunlight striking the earth warms the ground

There is a gradient of heat (energy) from surface to depth (ignore geothermal)

Heat transfer is in the direction from hottest to coolest

The energy transfer through the soil is due to molecular vibration (phonons) and is relatively slow,

Thermal conductivity for soils around 1 - 2 W/(m.K)

For our preferred target metals: Gold 318 W/(m.K) Silver 429 W/(m.K)
Copper 380 W/(m.K)

Metals transfer heat by movement of free electrons, therefore rapid heat transfer.

Metal in the soil will transfer heat from top of object to bottom faster than surrounding soil.

Metal is an excellent electrical conductor .

Soil is a poor electrical conductor, but not an insulator.

Maximum thermal transfer will occur near top of target (from soil to target)

As target warms, electrons will migrate to bottom of object.

Accumulation of electrons at bottom will repel electrons in soil, leaving “holes”

The close contact of the metal object to the soil is analogous to a “Schottky” barrier.

Once the “forward voltage” potential is achieved the electrons will cross the junction and fill the holes.

Current flow (metal to semiconductor) is very fast and high.

And it will stop very quickly as it has next to nowhere else to go.

As the electrons are returning to a lower state they must release their energy.

This will manifest as a burst of electromagnetic radiation.

The metal target will also have lost its stored energy (ie “cooled)

Enhanced energy transfer from soil at top (phonons & free electrons) to cool target

Cycle repeats,,,,,,,Cool

Aurificus

Hi,
still the thermal gradient here...

Ok... but Seebeck's effect you describe works good with metallic junctions... more than between a metal and soil.

This is first problem.

Second problem is that you're talking about large (?) current...
What ? Seebeck effect between metals generates very small currents...and voltages.
As an example common themocouple alloys gives you a maximum of around 70uV/°K = 70uV/°C

So... if gradient is just 1°C you get maximum 70uV.
In the metal-soil interface you'll maybe get a voltage of some uV/°C at maximum.

About current... they are known to be really small... and usually thermocouple devices need proper preamplifier design with hi-impedance to get useful readings.

Indeed the power you could get from a single junction is really small... as another example: old radioactive generators used many thousands of them both for increasing voltage (in series) and for increase output current (parallel of them).
Such systems are widely inefficient... think that you need maybe 2KW thermal power (e.g. Plutonium-238 bars) to get maybe 100W electrical , like used in old spacecrafts (and also today's made are not much different about that). We are talking of something 5-10% power efficiency... you put in 2000 and get 100 at output!

About EM pulse... (?)
the thermal gradient you talk is there while things are going on... the heath moves from hot to cold and this will create small current... with no sharp cutoff... so not really an EM pulse... but maybe a small current which vary as function of time... then a magnetic field is surely present but of which amplitude ?

Maybe can e.g. deflect the needle of a compass if strong enough (maybe) but sure is not an EM pulse, and of course we are talking of very weak magnetic field.

Kind regards,
Max