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geowizard
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Time Machine... ( 19:25:59 WedAug 23 2017 )

Hi All,

I have been building a "Time Machine". You've probably already seen efforts to make one of these by eccentric old scientists working out in their garage or in the basement.

This one is a little different;

Companies involved in GOLD Exploration are using a Time Machine that can find conductive mineralization including GOLD. The system measures the Electromagnetic response in the Time Domain. It's called TDEM which stands for Time Domain Electro-Magnetics.

Most readers are probably familiar with Pulse Induction metal detectors. They are very popular for finding GOLD and artifacts.

Companies like Zonge Engineering in Tucson market a large system that can be applied to the exploration for GOLD.

Discussion?

- Geowizard

  
geowizard
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Re: Time Machine... ( 21:29:13 WedAug 23 2017 )

The Time Domain;

To begin with, a switch is turned on that applies current to a loop of wire. The loop of wire generates a strong magnetic field.

After one second, the switch is opened.

The magnetic field collapses or 'decays' over a short period of time.

The time DEPENDS on the conductivity of the surrounding ground.

The changing magnetic field generates (Induces) a current in the ground. A secondary (opposing) magnetic field results from the induced current flow.

It is equated to an expanding "smoke ring";

The Expanding magnetic field goes deeper into the earth over a very short period of time.

A receiver coil is located on the ground to measure the secondary magnetic field and record the voltage during specified time periods.

- Geowizard

  
geowizard
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Re: Time Machine... ( 21:37:17 WedAug 23 2017 )

The Transmitter;

All that is needed is a 12 volt battery, a switch and 100 feet of 12 Gauge insulated wire.

I use a set of relays connected to a microcontroller. The relays can be accurately timed to switch the battery voltage. The relay switching produces an alternating square wave voltage with a one second "off" period in between.

- Geowizard

  
Jim_Alaska
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Re: Time Machine... ( 00:34:21 ThuAug 24 2017 )

Most people do not recognize the early symptoms of Alzheimer's, But most of us recognize the symptoms of full blown Alzheimer's. :devil:

But, continue on my friend. :eeekyellow:



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geowizard
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Re: Time Machine... ( 03:14:40 ThuAug 24 2017 )

The Time machine has excellent memory;

After the transmitter switches the current to "zero", the process of measuring the receiver voltage begins. The receiver coil voltage is measured at fifteen exact points in time during the decay of the magnetic field.

Referred to as channels, the times are as follows;

180 microseconds
220 microseconds
280 microseconds
350 microseconds
440 microseconds
560 microseconds
710 microseconds
1100 microseconds
1410 microseconds
1790 microseconds
2220 microseconds
2850 microseconds
3600 microseconds
4490 microseconds
6700 microseconds

Are we having fun yet?

- Geowizard
[2 edits; Last edit by geowizard at 03:18:09 Thu Aug 24 2017]

  
Fleng
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Re: Time Machine... ( 16:19:03 ThuAug 24 2017 )

Complicating factors can interfere with the explanation of electromagnetics. We use a constant for the magnetic permeability of a substance. We use a constant for the electrical permeability of a substance. The problem is that pressure and temperature can affect these constants and create artifacts in our samples. Most people think of water as either solid ice, fluid water, or gaseous steam for instance. Ice has at least 16 known phases that have unique characteristics. Steam takes up 1700 times more volume than room temperature water but is even more complicated to model than ice. Tables have to be used for high pressure and temperature because the density is highly nonlinear. Water, ice, and steam can coexist at a single pressure and temperature at a triple point.

TDEM systems depend upon a static value for permeability which is closely aligned with density. Things can get very complicated in a hurry where water is present. Did you know that that there is more water inside of the earth than in all of the oceans and lakes?
This is why the reports can get muddy and inconclusive.

Don't get me wrong about this. There is much that can be learned about locating minerals from EM systems. We just have to be careful not to oversimplify what can and cannot be concluded based upon a given device's report.

Some basic facts are helpful. In general the denser the media the slower the EM wave will propagate.
[1 edits; Last edit by Fleng at 16:35:00 Thu Aug 24 2017]



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geowizard
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Re: Time Machine... ( 16:47:13 ThuAug 24 2017 )

Fleng,

A good reference for technical information on TDEM systems, methods and theory can be found at http://www.geonics.com .

Given the porosity of Monzonite and Shale in the realm of less than 2 percent to zero, Water content isn't a factor. The relative permeabilities of water and conductive minerals are on opposite ends of the chart.

Airborne TDEM has to correct for propagation through air. So, I would note that ground applications have only the ground response.

- Geowizard
[1 edits; Last edit by geowizard at 17:10:08 Thu Aug 24 2017]

  
Fleng
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Re: Time Machine... ( 17:43:56 ThuAug 24 2017 )

The issue with water relates to conductivity tests. Pure water is an insulator but even minute dissolved mineral causes water to become a conductive medium. 2% water in a solution could mean a signal to noise reduction to as little as 17 db. If the method worked so good everyone would be using it! Refer back to my statement about the usefulness of TDEM devices.

From the technical library at the link you provided:

"Note that the time/frequency range used above was one-million to one in order to separate out the various stages of the time decay. To the best of the authorís knowledge there are no measurements covering such an extended time or frequency range and thus no guarantee that the amplitudes of the different time-constant components are all equal as was assumed above. "

Another of their technical excerpts:

" For this reason, although the EM31 and EM38 are widely used for metal detection, Geonics Limited did not pursue the metal detection application for these instruments since we knew that target interpretation (including even simple accurate location) would be very difficult."

This is an example of a very complicated theory of operation that isn't easily simplified.


  
geowizard
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Re: Time Machine... ( 19:27:55 ThuAug 24 2017 )

Fleng,

Good point.

The audience here on the Forum might appreciate an overview of TDEM for purposes of GOLD Exploration.

So, this isn't a PhD dissertation on the technical aspects - rather a basic down to earth (no pun intended) view of the method.

The TDEM method doesn't need to be proven at this point in history. The method has been used for 50 years or more and proven to be a valuable tool in the search for precious metals and base metals.

TDEM systems are available for rent or purchase. Companies that market the systems provide training. Also for those readers that have large GOLD mining projects and that have a Geologist or Geophysicist, they may broaden their understanding of the tools and exploration methods available.

- Geowizard

  
geowizard
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Re: Time Machine... ( 19:11:32 WedAug 30 2017 )

Is a DIY TDEM System possible?

We're about to find out! :smile:

Step 1;

The first step in making a DIY TDEM system is to construct the Transmitter. As I mentioned, the Transmitter switches current from a battery through a wire loop.

I chose to use a microcontroller to control the transmitter switching. The microcontroller also has an internal analog to digital converter. The analog to digital converter will be used to measure the receiver signal at the appropriate times listed above.

The microcontroller is one of my favorites, a PIC 18F4550. It is a 40 pin microcontroller that operates at 20 MHz in this application. I will discuss more later on how this microcontroller works in making an operational TDEM system.

The manufaturer of the PIC 18f4550 is a company called Microchip. Microchip provides a FREE software package called an "assembler". The assembler is needed to write the software in assembly language and assemble the "code" into binary form. A programmer that plus into the USB port of a laptop or desktop can be used to program the flash program memory in the microcontroller. The flash memory can be reprogrammed if we ever need to change something. The programmer i use is a "PICKIT(tm)3". It is a low cost programmer.

The microcontroller is a 40 pin chip. It has digital input and output pins. The digital input and output pins can be programmed to be used as output pins or input pins.

I incorporated an LCD Display. The LCD display can show four lines with twenty characters per line.

I also incorporated an 8 GB micro-flash memory. The memory card connects to the microcontroller serial port. Data that is acquired by the TDEM system can be stored as a text file on the flash memory. The flash memory can be removed and plugged into a flash memory port on a laptop or desktop pc.

One last feature is amplification. The signal that is received can be very small. This TDEM system incorporates a Programmable Gain Amplifier abbreviated PGA. More on that later.

- Geowizard

  
Fleng
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Re: Time Machine... ( 19:17:23 ThuAug 31 2017 )

Wiz-
Good luck on the DIY. I'm a big fan of home projects as well. My current project is a CNC milling machine for woodworking that can be upsized for aluminum and light steel.

One thing that you are going to need to consider for your TDEM is the switching system to drive your transmitter. As you probably know, the more power transmitted the deeper the penetration and more information. The microcontroller is fine for your signal source. Using one of the available DSP systems will allow you to compare the reflected signal to your source signal. This is the "easy" part.

The difficulty is going to be amplifying the 20MHz source signal from the microcontroller. Power amps in this frequency range can get very expensive. High power high frequency amps that radio stations use are often driven by tubes in the final stages. Tubes can be very tempermental after being subjected to vibration.

Another big issue with amplifying high frequency is the inductive load of your transmitting coil. High frequency doesn't like big inductors who look like a high impedance brick wall. Transmitted power =/= amplifier power.

Anyway good luck with this and I hope it works out for you.
If it doesn't, consider a pulsed system as this would be considerably cheaper.

  
geowizard
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Re: Time Machine... ( 19:21:09 ThuAug 31 2017 )

What TIME is it?

The "Time Machine" is able to measure TIME from the instant that a PULSE is turned off... at specified TIMES as listed above.

TIME is related to DEPTH. The PULSE is propagated into the earth at a predictable RATE.

The DEPTH of measurement is 50 to 60 Meters (164 to 196 feet) at 6000 microseconds.

- Geowizard

  
micropedes1
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Re: Time Machine... ( 00:38:25 FriSep 1 2017 )

I am curious. How much output power from amp? Resistive? Single transformer or staged with filters?

  
geowizard
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Re: Time Machine... ( 01:12:17 FriSep 1 2017 )

Transmitter Loops;

Transmitter Loops create a magnetic "Moment". The Moment is calculated as the product of the number of turns (N) x Current in Amperes (I) x Area in square meters (A). Moment = N x I x A.

Example: A Loop having one turn that is 10 meters x 10 meters with a current of 10 amperes... Moment = 1 x 100 x 10 = 1000.

The Loop generates a Magnetic Moment. The LOOP is an Inductor. When the Loop is energized - a Magnetic Field is created. Next, the current is turned OFF. The receiver is turned ON to measure the Decay of the Magnetic Field over a period of time. The Conductivity of the ground is related to the rate of Decay. A highly conductive earth causes secondary current flow and secondary magnetic fields - it takes longer to decay. A non Conductive earth has no response - no secondary current, no secondary magnetic field and rapid decay.

- Geowizard
[1 edits; Last edit by geowizard at 01:14:28 Fri Sep 1 2017]

  
micropedes1
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Re: Time Machine... ( 04:05:25 FriSep 1 2017 )

I realize that coil size and power requirements are inversely proportional (approximately). But for the sake of portability, I was considering transmit and receive coils on the order of 1 meter. But would require many more coils for signal resolution.. but greater current reserves. BTW, I am only wanting 10-15 meter depth.

My question was actually about the amplifier. Would you use a single transformer or multiple transformers with filters between to minimize s/n ratio?

  
geowizard
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Re: Time Machine... ( 13:12:37 FriSep 1 2017 )

Amplifier coupling - on the receiver?

Good question!

The signal is a DC time-decay voltage. We need to keep in mind - the very nature of the signal is the "response" to a magnetic pulse. The magnetic pulse changes rapidly at first. The changing magnetic field induces current in the earth. The earth is a conductor. The quality of the conductor depends on the conductive minerals in the ground. The response is detected with a coil that has many thousands of turns. The receiver coil may have 4,000 to 10,000 turns!

The receiver coil is DC coupled to the amplifier. Usually the first stage amplifier is contained in the receiver coil housing. It is powered by a couple of 9 volt batteries. All of this reduces noise from the amplifier and makes the signal larger.

The challenge is to detect a small signal in a NOISY world. The earth is noisy because of two things. there is variation in the magnetic field of the earth that comes from the magmatic core. The magnetic noise causes electric current to flow in the ground. At the receiver coil, the noise is detected and changes the received signal.

Solar noise (from the sun) reaches the earth and creates a second source of electrical and magnetic noise. A third source is cultural noise from power plants, power lines, etc. Cultural noise is less of a problem in remote areas.

Noise is reduced by AVERAGING;

The system remains in one place while repeated measurements are taken. It's easy to realize that if many voltage measurements are taken and all of the measurements are averaged, the noise which adds and subtracts from the REAL signal is averaged out.

All of the channels are measured once a second. In one minute, sixty measurements can be averaged. Noise is usually low frequency. The noise could vary slow enough that averaging for three to five minutes is sufficient.

The amplifier used in my system is a Programmable Gain Amplifier. (PGA). Gain can be bumped up as the signal decays and gets smaller. The software is designed to increase the gain of the receiver amplifier between each of the later channels.

- Geowizard
[1 edits; Last edit by geowizard at 13:16:06 Fri Sep 1 2017]

  
geowizard
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Re: Time Machine... ( 13:36:16 FriSep 1 2017 )

With reference to transport;

The transmitter loop is made from a 40 Meter length of single conductor wire. I plan on using 12 AWG stranded THHN T90 Nylon coated copper wire that you can get at The Home Depot. Coiled up for transport, it's about 16 inches in diameter and weighs about 5 pounds.

The TDEM Transmitter and Receiver fits nicely on a 12" x 18" piece of 1/4" perfboard. Two 12 volt 7AH batteries included.

- Geowizard

  
Fleng
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Re: Time Machine... ( 16:45:22 FriSep 1 2017 )

For useful probing of subsurface structures 10-20 meters down it is going to take far more than what has been discussed. A time domain probe that senses soil moisture is a mature technology. This is why I brought up moisture. Here is an example of a system that uses time domain reflectometry:

Sensor

This system buries a probe in the ground to sense the water content at depth. Not exactly what you want but it gets around a very big problem.

To penetrate the ground takes power and lots of it. To get coherent signals back from 10-20 meters down takes even more. The signal that comes back from the reflection is going to be measured in picoamps not milliamps or microamps or even nanoamps. That part can be done, but the signal will be so muddy by that time due to the moisture, ice, irregular gravel and the like that it won't be discernable. What is needed is a signal to noise ratio below 120 db (or in simple math 1 part in a trillion).

Simply penetrating the skin of the soil will reflect most of your transmitted power. Here is an MIT EM lab note that give you the real physics.
ONE physics lecture

Simply put the change in density between the ground and your transmitting coil is going to cause most of your power to be reflected back to your receiver. What little is left to penetrate the ground will be subject to destructive interference. Once you consider things like permafrost your reflected power from depth is covered in the noise of millions of smaller reflections.

The things that hurt a radio signal in high density like soil can actually help when you talk about acoustics. ELF radio is used for submarine communication because the losses are so low. If you want to talk about pure acoustics, the seismograph is a proven system for locating subsurface structures. Probably not what you want to hear but it is cheaper than spinning your wheels.

  
geowizard
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Re: Time Machine... ( 19:36:46 FriSep 1 2017 )

Fleng,

The Geonics website covers the theory. It WORKS! :smile:

Soil conductivity meters that measure dirt conductivity is an entirely different subject. I covered the concept and instrumentation last winter for Resistivity and IP systems that can measure as deep as you want to go.

The physics is well established. Doesn't require much discussion.

- Geowizard

  
micropedes1
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Re: Time Machine... ( 21:04:26 SunSep 3 2017 )

I see 2 different designs: one active transmit on 3 different axes, each in sequence, and the other active only in horizontal. Seems that the 3 axis system gives better resolution for ore bodies at the margins of signal propagation. I understand that this is to better define suspected metallic signals defined by aerial mapping. But if I am only interested in 10 meter depths, then my on-the-ground survey lines would necessarily be spaced something like 8 meters apart (with an overlapping margin). Might be difficult to accomplish in mountainous terrain, huh?

I am still sourcing parts for a 3 axis system. But I have wrapped three one-meter square transmit coils of heavy insulated wire on to a balsa/fiberglass frame (no metal connectors) and soldered gold-plated coaxial connectors to each. I have a National acquisition computer on order with a couple of multi channel A/D converters.

Still trying to find a variable frequency switching transmit and a receiving PXI module for that acquisition computer.

Will wrap receiving coils this week. Any suggestions on wrapping sequence, number of turns, orientation of wrapped coils. My assumption is that receiver coils should be in the same orientation as the transmit coils. Correct me if I am wrong. Will also incorporate a small preamp board into each receiver coil to help reduce s/n ratio between coil and main amplifier.

That acquisition computer greatly simplifies signal interpretation and storage options. Military grade is heavy duty stuff!

  
micropedes1
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Re: Time Machine... ( 21:27:54 SunSep 3 2017 )

After re-reading this post one more time, I realized that you will accomplish your averaging by operating in a static mode, taking multiple readings in a stationary position. And then moving a set distance and repeating the operation again.

It seems to me that you would be better served to operate in a dynamic mode by taking multiple readings by a bank of evenly spaced receivers and averaging the results, while eliminating spikes that lie outside a predetermined range. But to be fair, it would definitely take a much higher grade of processing and storage and that can get expensive. Fast!

Any thoughts about orientation sensors of receive coil(s) relative to vertical? Gotta have a GPS and antenna on this coil array to map position, too.

  
geowizard
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Re: Time Machine... ( 12:28:51 TueSep 12 2017 )

Processing power;

Averaging is simple. Add A plus B and divide by two.

The natural ambient noise is random and it adds to and subtracts from a signal. Averaging the receiver signal improves the quality of the measurement.

Binary data averaging in a digital system is very simple. Add two measured values and rotate the result RIGHT.

The process actually improves the resolution of the measurement because the result of averaging produces a fractional remainder.

In a typical TDEM system, the ground is pulsed once each second. Letting the system sit in the same place and measure over one minute, gives sixty measurements. I prefer multiples of two. Two exponent six = 64. Take 64 measurements - adding the result and rotating the total RIGHT six binary digits.

- Geowizard

  
geowizard
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Re: Time Machine... ( 12:36:24 TueSep 12 2017 )

Internal noise;

TDEM systems use sensitive amplifiers to measure the very small signal. One of the problems in all electronic amplifiers is noise that results from heat. This noise is referred to as THERMAL noise. As the gain is increased, the noise is amplified as well as the signal. Averaging the combined signal and noise reduces the effect of the noise and improves the signal.

- Geowizard

  

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