Time Domain/Transient Electro-magnetic (TDEM)

geowizard · Oct 5, 2026

In the world of mining geophysics is a method called Time Domain/Transient electro-magnetics (TDEM).

It is a pulse system similar to pulse induction (PI) metal detectors. This might be a subject of interest to Treasure Hunters!

This thread offers a glimpse into this option in geophysical surveys. :thumbsup:

- Geowizard

geowizard · Oct 5, 2026

Transient EM systems measure the Magnetic pulse response of a mineral or metal object or deposit as the case may be. Depth of the response is measured in Time. The measurement is called tau. Tau is conductivity * size. Time after the transmitter pulse edge is measured in milliseconds. Time represents depth to the conductive mineral or metal. The response is a time dependent decay curve measured in "windows" of time.

VTEM is a system used in surveys marketed by Geotech. The specification is attached.
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- Geowizard

k0bgalan · Oct 6, 2026

This is another case of a specialized type of detection. We all know the bigger the coil, the larger the 'find' has to be in order to be detected. For potential ore deposits, I see the possibilities, regardless of the cost involved. On a similar note, I'd love to buy a decent GPR, but a 'decent' one overages $50K or so. Makes me wonder how much the above setup costs?

Carl-NC · Oct 7, 2026

Time represents the decay response of the target, not its depth.

geowizard · Oct 8, 2026

It costs zillions! plus a helicopter, a crew of pilots, a room or three full of goggle-eyed geophysical scientists. Oh... and an office building.

Or you can use the services of a goggle eyed geophysical analyst. Or... Stick around! :thumbsup:

- Geowizard

geowizard · Oct 8, 2026

Here's an example of what one of the "deliverables" is:

- Geowizard

geowizard · Oct 8, 2026

In reply to any comment to the contrary on tau and sample time;

Tau is the response of a TDEM system measured in the form of conductivity * size at "depth".
Depth is a function of "Time". Time is measured in milli-second "windows" referred to as "channels".
The example shown above shows horizontal fiducials -69 to 6964.
Each fiducial is a physical sample location.
There are about 7000 sample locations along this single flight line.
The TDEM response (tau) is shown on the vertical scale.
Time channels are shown in color lines giving a 3D representation of sample location, depth (as time) and tau, the response! This is one flight line of many flight lines surveyed in a single survey.

geowizard · Oct 8, 2026

To bad actors;

I spend time and effort to compose informative and sometimes educational content to share with readers of the TN Forum. In most cases, I invest additional time and effort to provide references to sources to support the information.

There are a small number of comments made by what I refer to as "bad actors" that present a false narrative that is both misleading and confusing. Bad actors are not contributing to subject matter and as a matter of course are reported. Moderators rightfully judge reported posts as acceptable or unacceptable under Forum policies. I ultimately exercise my discretion to "Ignore" members that fall into the "nuisance" category.

I am an adult. I respect others and encourage the same respect in return.

- Geowizard

geowizard · Oct 8, 2026

In the spirit of sharing, I would like to add a reference on the electromagnetic system used in TDEM.

This is the transmitter waveform and it shows the reference for time (t0).

(From a published report in the public domain from Geotech.)

Geowizard

geowizard · Oct 8, 2026

The windows in time;

Time is divided into channels. The channels represent tau at depth. Greater time channels reflect the response of TDEM at increasing depth.

Please see attached pdf.

Ed note: revised attachment

Geowizard

geowizard · Oct 8, 2026

What is "tau"?

A short lesson... (warning math is used!)

- Geowizard

pepperj · Oct 8, 2026

geowizard said:
To bad actors;

I spend time and effort to compose informative and sometimes educational content to share with readers of the TN Forum. In most cases, I invest additional time and effort to provide references to sources to support the information.

There are a small number of comments made by what I refer to as "bad actors" that present a false narrative that is both misleading and confusing. Bad actors are not contributing to subject matter and as a matter of course are reported. Moderators rightfully judge reported posts as acceptable or unacceptable under Forum policies. I ultimately exercise my discretion to "Ignore" members that fall into the "nuisance" category.

I am an adult. I respect others and encourage the same respect in return.

- Geowizard

"It sounds like you are confusing a strong, objective disagreement with 'bad actor' behavior. Just because someone has the intellect to successfully challenge your viewpoint or point out flaws in your references doesn't mean they are trying to mislead people. If you truly respect others, you should be able to handle intelligent pushback without hitting the ignore button."

geowizard · Oct 8, 2026

pepperj,

You are following the path of a bad actor in not providing a reference to a single fact and challenging my comment about the same. Pushback without a reference to facts is a bad act. A public forum becomes a trial of public opinion rather than a discussion among gentlemen with a reference to facts.

Pushback without a basis in facts is an unsupported argument for the sake of argument. Argument for the sake of argument in any form is not an acceptable form of discussion. It's a bad actor acting with a pretense of superiority without supporting evidence. That's a bad act!

- Geowizard

geowizard · Oct 8, 2026

Carl-NC said:
Time represents the decay response of the target, not its depth.

This is a case in point. No reference is given. This piece of "misinformation" contributes nothing. It is a contradiction of the fact( s) and creates confusion in an otherwise technical discussion that should not be confusing. Technical discussion requires foundation for statements shared in a public forum.

In light of the facts, a bad actor attempting to assert superiority is exposed for having inferior understanding and discredits himself in the process.

I have dedicated pages of supporting foundation on this one liner to dispute blatant misinformation for the benefit of unseen, unknown readers in providing clarity and understanding of the fact(s).

- Geowizard

robertk · Oct 8, 2026

geowizard said:
Pushback without a basis in facts is an unsupported argument for the sake of argument. Argument for the sake of argument in any form is not an acceptable form of discussion. It's a bad actor acting with a pretense of superiority without supporting evidence. That's a bad act!

Unless I missed something, threads posted on TreasureNet belong to TreasureNet, not to the poster. And they are open to replies from anyone with any viewpoint they want, as long as it doesn't violate TreasureNet's rules. If you don't want people posting comments on your posts, then start a blog and don't allow comments. But if you post here, people can and will say whatever they like, whether it agrees with you or not.

(Yeah, like you'll see this. I'm already on your ignore list.)

geowizard · Oct 8, 2026

robertk said,

"Unless I missed something, threads posted on TreasureNet belong to TreasureNet, not to the poster. And they are open to replies from anyone with any viewpoint they want, as long as it doesn't violate TreasureNet's rules. If you don't want people posting comments on your posts, then start a blog and don't allow comments. But if you post here, people can and will say whatever they like, whether it agrees with you or not.

(Yeah, like you'll see this. I'm already on your ignore list.)"

My reply:

Thanks for chiming in! No thanks for the continued bad actor mentality that got you where you are!

It is destructive behavior to enter into a discussion with contradicting misinformation that attempts to discredit others involved in a good faith discussion. It is unacceptable malice on your part. Malicious behavior seeks only one thing. That is to take value from others. No value is added in your comment(s).

Information has value. The very existence of a Forum is based on added value in the form of contributions made by members. I am sharing information in good faith. It's a gift of sorts.

Behavior of actors in a discussion is important to constructive understanding of concepts under discussion. A positive takeaway by readers is not possible when a bad actor jumps in with an assault on the facts without support and with a motive to take away value, in essence, robbing others of the value in added knowledge and understanding. The bad actor has the means, motive and access to rob good faith victims of value!

- Geowizard

geowizard · Oct 8, 2026

Time is depth;

In Time-Domain Electromagnetic (TDEM or TEM) data, depth of investigation is determined by how long secondary eddy currents take to diffuse downward and outward into the subsurface after the transmitter current is switched off.

How Depth Works

• Time-Delay and Depth: Measurements are taken during the "time-off" period. Early time measurements capture signals from shallow depths, while progressively later time-gate measurements sense eddy currents at greater depths as the "smoke ring" of current propagates deeper.

NCDEQ-DWR :: Time Domain Electromagnetic Geophysics

- Geowizard

geowizard · Oct 8, 2026

Looking at an example:

The solid red line is the reading from a magnetometer called Residual Magnetic Intensity (RMI).

The TDEM response (tau) is given for each sample location over a flight line of about 7000 samples. Looking at the peak between 4800 and about 5400, the peak is maximum around 5150. Each color represents a "time" channel (the response) from early time (green) to late time (red). The higher the peak, the greater the conductivity * size product aka "tau".

- Geowizard