Time Domain EM!

[Geowizard]

An overview of TDEM;

Please note, The following information does not imply an endorsement of a product or attempt to advertise a product or service.

Zonge Research is a company in Tucson with offices in other locations in the world. Zonge has been in this business for decades. They design, manufacture , sell and lease equipment as well as offer TDEM survey services. Here is a link to their webpage on the subject. It provides an overview:

http://zonge.com/geophysical-methods/electrical-em/tem/

- Geowizard

[Geowizard]

An example involving UXO;

Most prospectors are not prospecting for Unexploded Ordinance (UXO) ...

This reference shows an application of TDEM and an example of what is involved and what the instrumentation provides to the end user...

http://zonge.com/wp-content/uploads/201 ... XOmap3.pdf

- Geowizard

[Geowizard]

Another example;

Because examples help give a better view of things that are not always easy to understand, I would like to share another good example:

http://zonge.com/wp-content/uploads/201 ... pMetal.pdf

- Geowizard

[Geowizard]

The magnetic field;

An electromagnetic pulse from a transmit coil generates a brief magnetic field...

When the magnetic field travels into the earth, it interacts with conductors...

(called the process of Induction)

The magnetic field induces a current called an "eddy current" in the conductor(s)...

That eddy current creates a "secondary" (equal and opposite) magnetic field that is dependent on the size and quality of the conductor.

A receiver coil is located in close proximity and it has a current induced in it from the changing secondary magnetic field.

A magnetic field is given the letter "H". The world represents a 3 dimensional space with three axes, X, Y, Z.

https://en.wikipedia.org/wiki/Magnetic_field

The resulting secondary field can be measured by three receiver coils arranged in three perpendicular directions, X, Y, and Z. Hence Hx, Hy, and Hz...

- Geowizard

[Geowizard]

The receiver coils;

Here's what the receiver coils look like:

http://www.zonge.com/legacy/PDF_Equipment/Tem-3.pdf

They have built in amplifiers.

- Geowizard

[Geowizard]

The data?

So, you send a pulse into the ground and you record DATA... Now what?

The "Father of TDEM" is Dr. Misac Nabighian. He is to TDEM what Thomas Edison is to the Light bulb.

He is explaining his role in reading the DATA early in the days of TDEM:

Note: His accent combined with poor quality audio make it moderately difficult to understand.

Please appreciate this type of discussion is rarely seen.

https://www.youtube.com/watch?v=725dVEYK3bk

- Geowizard

[Geowizard]

Time Domain Data;

After the big bang... The receiver "listens" to the resulting magnetic field collapse for a period of "time".

The signal is sampled on a logarithmic scale of time.

Here's a Log base 2 example;

Signal is sampled at 2 microseconds, 4 microseconds, 8 microseconds, 16 microseconds, 32 microseconds, 64 microseconds, 128 microseconds, 256 microseconds, 512 microseconds, 1024 microseconds, 2048 microseconds, 4096 microseconds, ... Each time is called a "channel".

A survey is conducted along a line called a traverse. Survey data is collected at fixed intervals. Over the course of many sample points called stations, the data may show peaks in the data that represent conductive deposits in the ground.

- Geowizard

[Micropedes1]

I am not sure that it is pertinent, but I had to move my battery/transmitter as far away from my transmitter loop as possible. It was interfering with the shallow response of my system. Transmit loop only 5 meters on each side. Repositioning a loop even this small takes forever. Were my terrain flatter, I would build a cart of some type to speed things up.

I recently built a pond and incorporated a section of PVC into the dam at an angle. I can fill the pipe with different ores and check the response. Results shortly. Not sure how crushed ore will appear on magnetic system. Hope it is not the feeble signal of standard metal detector.

[Geowizard]

Accuracy;

Measurement systems... Metal Detectors included, must provide accuracy!

If we are not certain that the measurement is accurate, then we are no better off than we would be with no measurement at all.

Calibration;

A metal detector can be calibrated. Calibration adds accuracy to the process of metal detecting. Most prospectors simply go out into the field with a metal detector and go through a process of ground balancing and adjusting the sensitivity and threshold of their detector.

For calibration, I have a "test nugget". I tape a 5 gram GOLD nugget on a card and place it on the ground where I plan on detecting. Different sizes of test nuggets can be used to establish the sensitivity required for smaller nuggets. Test nuggets can be buried in test holes having varied depth.

- Geowizard

[Geowizard]

Calibration coils;

Because a metal detector is responding to a secondary magnetic field, it is possible to take a short length of copper wire and form a small single turn loop. A fixed resistor can be placed in the loop to control the current in the loop. The loop can be taped to a card for rigidity.

The resistor is simulating the resistance (resistivity) of the ground or simulating a conductor in the ground having a given resistivity. Digital displays on metal detectors give a reading of the response. Knowing the reading of a 5 gram nugget, a simulated nugget can be made with a coil and a resistor. A technologically savvy prospector can make different calibration coils to simulate different sizes of nuggets.

- Geowizard