Time Domain EM!

[Geowizard]

Hi Guys and Gals,

There has been a movement lately toward Time Domain EM also known as TDEM.

This method is also known as Pulse Induction. That is because the principle that makes it work is based on an electromagnetic pulse that is "induced" into the subsurface and into conductive mineralization. There is much that can be learned and appreciated about this technique.

The principle;

When a pulse of electric current flows in a coil of wire, a magnetic field is also generated. Large coils and large currents can produce large magnetic fields that penetrate deep into the earth. After the pulse is turned off, a receiver is turned on that measures the secondary magnetic field. In barren ground, there is little or no secondary field. In conductive, mineralized ground, there is a measured response!

It's a fascinating method of metal detecting and has many uses from recreational metal detecting to large scale surveys. Here's a place to discuss this interesting topic!

- Geowizard

[Geowizard]

What is Time Domain?

When an electromagnetic pulse is transmitted into the ground, the ground returns a signal that changes over a period of "time". The pulse is equated to a smoke ring that moves outward and downward. As the pulse interacts with different conductors, it causes a secondary signal to be returned to the receiver.

- Geowizard

[Micropedes1]

This unit has the ability to set the delay period after the power to the transmit inductor has been switched off to minimize noise from the transmitted signal. Actually, I think that the processor is receiving signals from that zero point, and at 15 microsecond intervals, but you can set the minimal starting point for data logging. That way, you can get more meaningful data to your graphing software. With a laptop tied to the data logger, it is a simple matter off turning a knob until the interference disappears. It simply means that fewer channels (fewer of those 15 microsecond intervals) are employed in your graph.

Wait, I may be getting ahead of myself. Perhaps a bit of simplified theory of operation may be in order.

[Micropedes1]

Lay out a big loop of wire over the area that you want to check for conductive ores.

Mine is limited to 50 feet of #1 welding cable because I wanted the heavy insulation for abrasion resistance. The length, or diameter of coil, limits the depth of penetration of primary field. Mine is relatively small because I am working in a mountainous region.

My receiver array is positioned roughly in the center of the loop.

[Micropedes1]

Another reason for the smaller loop is that I am using limited power with it and thereby am limiting myself to shallower depths of ore. I am trying to correlate data obtained from this unit to the Alaska DGGS 56K that I have been trying to analyze.

Wiz, help me out with a better explanation. Getting the data is the easy part. The proof is in graphing software.

[Micropedes1]

The best analogy that I can think of is this:

Walk into a dark room and flick the light switch on and off as quickly as possible. Your eyes likely did not have enough time to take in all the details of your surroundings. You merely got a snapshot of the room.

Now, flick the light switch on and off repeatedly. Each time you are able to visualize more and more of the room until you have a very complete picture of your surroundings.

That is essentially how TD works. It broadcasts an EM signal which permeates the ground beneath it. This causes magnetically susceptible materials in the ground to become inductively coupled to your transmit loop. When that EM signal is switched off, conductive ore, clays, etc will each gradually lose their magnetic responses. That term “gradually” is actually a misnomer, because it happens within very small increments of a second. But that rate of decay is instrumental in determining both the identity and extent of the ore. But you only got a “snapshot”. And your data logger recorded it all.

If you want a better representation of what lies beneath, your microprocessor must “flick the switch” on and off many times over several minutes until you are satisfied that you have a “clear picture” of what lies beneath.

Then graph the data. And advance your coil a few meters. And do it all over again.

[Micropedes1]

Seems like a whole lot of work. But when I start digging with my backhoe, or dozer, or whatever...I want to be darned sure that I am digging into the most conductive and thereby promising ore supplying the gold that I am finding on the surface.

[Geowizard]

Glen,

You're doing a good job explaining this!

Geonics is a company that makes and sells TDEM systems. They have a library of technical notes that explain the details of the theory.

http://www.geonics.com/pdfs/technicalnotes/tn7.pdf

It works;

Suffice it to say, "It works very good"... There's a lot of theory that requires a relatively high level of physics and math that many readers are simply not prepared to dive into. The information is there for the very inquisitive types that need to know more!

The objective for a GOLD prospector is to find GOLD. TDEM represents a tool that works very well to that end. A GOLD prospector at the end of the day simply needs to be able to look at information laid out in front of him or her and make a decision on where to dig!

- Geowizard

[Geowizard]

In "theory" everything works;

In theory everything works perfect! The question is, how can we be sure it really works!

As with every form of metal detecting, there is a learning curve. The end user has to learn how this new tool works from the end user point of view.

As is the case with most large systems, this isn't going to fit in a back pack along with your picnic lunch.

Geonics shows several TDEM systems that range from roll-around carts to larger systems that involve a small crew to set up.

In every case, the end user needs to have spent time in the field with the system, operating the computer and collecting data. This work should be done on barren ground as well as conductive/mineralized ground.

- Geowizard

[Geowizard]

The Geonics Technical Note 7;

This type of reading for many, represents a NEW language!

You might say... I don't understand... I don't get it... It's over my head!

I can only suggest... "Read it". It won't make any sense! Read it again... and again... Before you know it, you will find bits and pieces that begin to fit together and start making sense. Metal Detecting involves technology. The subject involves understanding a NEW language!

- Geowizard