Time Domain/Transient Electro-magnetic (TDEM)

[robertk]

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.

Your response is based on false assumptions about my motivations. I have no malice toward you or the information you post. Any information I contribute is based on what I believe to be truth -- whether you see it as "misinformation" or not is entirely your perception, and has nothing to do with my intent.

No value is added in your comment(s).

No value is added in your rebuffing either. But then there's no requirement that there should be, is there?

 

[Carl-NC]

It is destructive behavior to enter into a discussion with contradicting misinformation that attempts to discredit others involved in a good faith discussion.

GW, you have often posted some good tech info. When you do, I read it and sometimes even learn from it. But occasionally you post stuff that is misleading or just wrong, and when I see wrong info being posted I tend to step in and add my 2c. It ain't just you, I do this with lots of people. Sometimes folks are appreciative (they learn something new!) and sometimes they debate my 2c, and either one of us eventually learns something new (maybe me!) or we agree to disagree. You seem to be especially thin-skinned and don't want anyone to ever disagree with you. I can't help ya with that, but I can help with understanding TDEM responses. If you want, you can even enter into a back-in-forth about it, or you can just ignore me.

TDEM responses do have an exponential curve but only in the late-steady response region. Here is the response of a US silver quarter (I personally took this data; it is published in ITMD3):

The slope of the dashed line represents the tau of the quarter (~150us) and is valid for the single-mode exponential part of the response. Notice that the early part of the response has increased curvature -- this is the diffusion region, where the eddies are still spreading outward and downward. In this region the response is not exponential, but rather power-law, or power law on top of exponential. In my experience, it takes about 1 tau for the diffusion to settle out and enter a single-mode exponential. Literature states this should happen much faster (0.1-0.2 tau) but that might be based on a spherical target, which is the norm for published papers. That's an issue I have not investigated. The very late part of the curve gets ratty-looking, and this is just the noise limits of the receiver.

There are 2 points of interest here. First, in neutral ground, you will get the same response no matter how deeply the quarter is buried. The only difference is that a deeper target will have a weaker (and noisier) response, but the curvature will be the same. Second, if you sample early and look at the slope hoping to identify the tau of the target, you will get a wrong answer. The target will appear to have a lower conductivity. The higher the conductivity of the target, the longer you have to wait to get a right answer. This depends on the diffusion depth of the eddies, not the buried depth of the target.

In non-neutral ground you also have to deal with the ground response itself, which gets mixed in with the target response(s). Some soils have mostly magnetite which has an extremely fast TDEM response that has little effect on the target response, but which can bend the TX field and reduce depth. Some soils have viscous minerals (maghemite) which store/release magnetic energy and have a power-law (not exponential) decay that can extend far into a target response:

These soils are worst in old weathered desert areas like Australia or the US Southwest. But even where I live (Pacific NW) the area is full of viscous volcanic hot rocks. And some soils have conductive minerals (mostly salts) and they tend to have an exponential response with very low conductivity; that is, they get out of the way fairly quickly.

The graph above shows that in the early time (<t1) the ground response can be significant, then falls away in the mid time, but in the late time the conductor exponentials catch up with the ground. None of the diffusion effects are shown. One difference between TDEM metal detecting and TDEM aerial surveys is that the latter operates on a much bigger scale, with decays typically ranging 10-100ms instead of 5-500us.

GW, one thing I should thank you for is the attachment in post #9, which shows the VTEM TX waveform:

Those jaggies are caused by shorting the coil during the TX pulse. Minelab uses the same method in the GPX 6000:

It's nice to find out that, if ML tries to patent this, there is prior art. See? I learned something new!

 

[geowizard]

Geophysical Mapping in Alaska: Unlocking Alaska's Critical Resources;

Alaska represents the "last frontier" for exploration and mining in the free world!

Here is a recent publication - fresh off the press on the subject... exploration and discovery of critical minerals resources in Alaska may be a solution to the demand for Critical minerals!

- Geowizard

 

[geowizard]

Geophysical methods are the solution;

Metal detection on steroids! TDEM methods using Geotech and SkyTEM are leading us into a new frontier in exploration

Here's part II of the publication...

- Geowizard

 

[geowizard]

Candle Greek, Alaska:

A report on Candle Creek Placer mine:


It is located on BLM domain. BLM Alaska has info on mining claims. There are no mining claims there!

I mention Candle Creek because there was a recent TDEM survey done that shows WHERE to dig!

Geowizard.

 

[geowizard]

Beginning with a topo map of Candle Creek:

The placer mine produced 6,130 ounces of Gold! If there was any gold left, they would still be there mining it!

Where did the Gold come FROM?

- Geowizard

 

[geowizard]

Time channels are shown as the large peaks decaying into smaller peaks over TIME with depth of the response. Peaks represent a unique response shape that is unique to the shape of a mineral deposit!

Geowizard

 

[geowizard]

How do we KNOW if the deposit is GOLD? We don't!

Time Domain provides a response in TIME channels. There is no phase information. This point becomes a matter of discussion!

- Geowizard

 

[geowizard]

How CAN we KNOW what the content of a deposit IS?

In Frequency Domain EM, phase of the response is measured. Phase is a shift in TIME of the secondary response returning from the deposit!

IF, in fact, TDEM is measuring the response in time dependent channels, the response is a time dependent decay of the transmitted pulse! We can calculate the change in time at each channel compared to a predicted Resistance - Inductance (RL) time- decay curve...

Another concept is Inductive Reactance (XL):

XL = 1/ (2 * pi *F * L) where L = inductance, F = Frequency.

Frequency is time dependent wave length (lambda) = 1 / Frequency (F).
XL depends on Frequency.
Wavelength and XL are time dependent.
Phase is measured in time as an angular displacement!

- Geowizard

 

[geowizard]

The foundation of metal detection is "Inductance"!

The deposit (a conductor) is subjected to a magnetic pulse! The deposit simultaneously conducts an electric current because of a sudden, increasing surrounding magnetic field!

Geowizard

 

[geowizard]

Almost like magic, when the magnetic field changes (disappears), at the end of the magnetic pulse, a very large Counter-electromagnetic current flows in the deposit! The very large counter electromotive current simultaneously creates a very large counter-magnetic field (Opposite). The very large counter electromotive magnetic field is referred to as the "secondary" magnetic field.

- Geowizard

 

[geowizard]

Fundamental "Foundational" electric concepts form the basis for our understanding of all things electric;

Ohms law is the most fundamental explanation of electric relationships. It forms a foundation for other electric concepts reaching into the use of capacitors, inductors, and relationships with frequency in reactance, impedance, resonance and further into the realm of "electronics", the control of electrons.

Amazingly, these concepts can "Get you on the Gold!"

- Geowizard

 

[geowizard]

It's called "Geophysics" It's all covered in detail on the "Geophysics Forum".

The link: https://www.treasurenet.com/forums/geophysics.42/

- Geowizard

 

[geowizard]

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

Correct. I am speaking with reference to "depth" as related to round-trip (a time, speed, distance calculation). Depth to the target.

Speaking to readers;

The response (amplitude) of the target is measured as a "changing rate of change" in pico volts (pV) as delta v/delta t and measurement of B field dB/dt. (1pico volt =.000000001 volt.)

Each channel is a slice of time and each channel has a different rate of change!

Where dv = change "delta" in voltage, and dt = change "delta" in time. Two variables voltage and time.

Think of it like a car decelerating at the "rate" of 2 mph per one second. = 2 (delta velocity / one second delta time). Then decelerating at the rate of 2 mph per two seconds = 1.

Ed note: Readers should appreciate the double delta division as simple division, however,

It's an introduction to complex math (calculus) . It is related to the changing slope of the voltage decay curve. It's a curve with a changing rate of change over time.

- Geowizard

 

[geowizard]

Where Time Domain takes a hard right turn;

In airborne time-domain electromagnetic (AEM) data, the time decay constant or t (tau) is not a fixed physical constant you look up in a table. Instead, it is a calculated parameter derived directly from the measured off-time voltage decay of the transient electromagnetic signal.

The purpose of measurement of anything is to obtain a definitive "quantitative" answer. In TDEM there is NO definitive answer! What is the answer? Is TDEM playing with us?

I refer readers to an earlier post with the playful tiger cubs...

Decay Characteristics of Airborne Time-Domain Electromagnetic Data:
Where to find the Constant (TAU)

https://www.tau-geophysics.ca/resources/Geosyntesis%202011_Decay%20Constants%20in%20AEM.pdf

Where and How t (tau) is Found

1. Geophysical Survey Reports and Databases

• Tau values are typically found in the processed datasets, geophysics logistics/assessment reports, and map products (such as a TAU map or B-field/dB/dt profiles) delivered by the airborne contractor (e.g., Geotech VTEM, Fugro, CGG).

• They are presented as grid files, channel-specific columns in database tables (like Geosoft .gdb files), or contour maps showing conductivity/conductance variations.

2. Calculated from Time-Window (Time-Gate) Tables

• Tau is computed by fitting the measured secondary magnetic field or induced voltage (V(t) decay curve to an exponential function:

F(t)=Ae^(-t/tau)

• Specifically, tau represents the time it takes for the transient voltage to decay to 1/e (about 36.8% or 37%) of its initial or reference value (V_37).

• Contractors calculate it by:

• Fitting selected early, middle, or late off-time receiver channels (time gates) to a single exponential curve in semi-log space, where the slope equals -1/tau.

• Using semi-heuristic or weighted-average algorithms across specific time windows surrounding the V_37 drop-off point.

3. Theoretical Relationship tau = L/R

• For a discrete subsurface target modeled as an equivalent resistor-inductor (RL) circuit, tau is physically bound by the target's inductance (L) and resistance (R):

tau = L/R

A slow decay rate (high tau) indicates low resistance (high conductance/conductivity), pointing to strong bedrock conductors like massive sulfides. A fast decay rate (low tau) indicates high resistance or poor conductors (e.g., overburden, clay, or weak fracture zones).

- Geowizard

 

[99thpercentile]

Correct. I am speaking with reference to "depth" as related to round-trip (a time, speed, distance calculation). Depth to the target.

Speaking to readers;

The response (amplitude) of the target is measured as a "changing rate of change" in pico volts (pV) as delta v/delta t and measurement of B field dB/dt. (1pico volt =.000000001 volt.)

Each channel is a slice of time and each channel has a different rate of change!

Where dv = change "delta" in voltage, and dt = change "delta" in time. Two variables voltage and time.

Think of it like a car decelerating at the "rate" of 2 mph per one second. = 2 (delta velocity / one second delta time). Then decelerating at the rate of 2 mph per two seconds = 1.

Ed note: Readers should appreciate the double delta division as simple division, however,

It's an introduction to complex math (calculus) . It is related to the changing slope of the voltage decay curve. It's a curve with a changing rate of change over time.

- Geowizard

I don't think you understand that electromagnetic induction is a diffusion equation governed system. At frequencies up to low or mid kHz there is not a time/speed/distance calculation. Only at higher frequencies when the electromagnetic systems transitions to wave equations do time/speed/distance matter. Ground penetrating radar (GPR) operates from mid kHz range to low GHz frequency ranges. At lower frequencies where metal detectors or TDEM systems operate it is a diffusion equation.

 

[99thpercentile]

Looking at an example:

View attachment 2267900

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

What you are showing here appears to be the raw TDEM decay data, not the inverted data which is what geophysicists would provide as a deliverable. We would apply a series of inverse models such as laterally constrained inversion (LCI) and then spatially constrained inversion (SCI) to the raw decay data to generate 2D profiles of electrical conductivity versus depth. The SCI data can then be interpolated into 3D conductivity volumes.

 

[99thpercentile]

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

One of my PhD committee members, Dr. Misac N. Nabighian who was the chief geophysicist at Newmont Mining, invented the concept of the smoke ring.

 

[99thpercentile]

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?

You don't need to spend $50K to get a decent GPR. My ImpulseRadar PinpointR is about $20K, my ImpulseRadar CrossOver CO1760 was about $30K, but my ImpulseRadar Raptor-45 18 channel array was $130K.

My Loupe TDEM instrument was about $75K, and I have used it to survey 600 line miles on a 20,000 acre site. My Loupe instrument gives pretty good data from ground level to a depth of investigation (DOI) of up to about 40 m (depending on the electrical conductivity of the subsurface). If you want to survey a larger area with a towed TDEM instrument, the TEMcompany tTEM is the best option right now as it can image to about 130 m wich collection speeds of up to 25 km/hr.

Airborne EM systems are rarely sold, they are custom built for the operators and customers pay for the data collection, processing, and analysis. The processing and analysis are sometimes performed by third parties. Airborne EM surveys are priced by the size of the survey area, the required spatial resolution (line spacing), and how far the survey area is from an airport. Airborne EM surveys start at around $350K as a minimum price and go up quickly from there. I budgeted a small one in Morocco ($1M) earlier this year and a giant one in Pakistan ($5M).

These instruments are all used for geologic mapping, not locating individual targets. A metal detector array like my Geonics EM61-FLEX array that is a TDEM instrument with one large transmitter coil and multiple receiver coils can be hitch mounted and towed behind a truck or UTV. I have four of these systems that I purchased from a government auction. They were custom built for the US Marines to locate UXO in Iraq and Afghanistan. I got a really good deal on them that was much less than the $100K each that the Marines paid.

 

[geowizard]

One of my PhD committee members, Dr. Misac N. Nabighian who was the chief geophysicist at Newmont Mining, invented the concept of the smoke ring.

That simple statement is epic. It will go down in history. You were in great company!

- Geowizard