Time Domain/Transient Electro-magnetic (TDEM)

[pepperj]

Reading this would be above my pay grade.

Electromagnetic Methods in Applied Geophysics

Volume 2 covers, in depth, the physical basis of EM methods of exploration magnetometric resistivity method, profiling methods using small sources, large-layout harmonic field systems, EM soundings, time-domain EM prospecting methods, VLF, MT, CSAMT, airborne EM methods, borehole EM techniques...

books.google.ca

 

[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.

Understood.
The question is, "Where does science become art?" I can share examples where SkyTEM did inversions that took two weeks and crashed. Inversions that showed excellent conductors that had no correlation with physical drill holes. The drill holes were consistently "no shows". To be fair, there is the possibility that the holes missed their intended targets. The client company walked away.

The challenge for the analyst is that TEM anomalies don't provide qualitative information in terms of I-Q and phase. Phase has been shown by example in FDEM to provide discrimination - adding a qualitative dimension to targets. This dimension reduces risk in moving forward in exploration of precious metals.

- Geowizard

 

[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.

I understand.

Fundamentally, TDEM is a function of dv/dt and dB/dt which by definition include time. We know the functions dB/dt and dv/dt (decay time) are the response relative to T0. The electromagnetic "response" in the medium is in the Time domain. You are correct.

Travel Time for penetration through the medium is not instantaneous but trivial.

ed: added "trivial"

- Geowizard

 

[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.

My comment on Inversion;

There is a point where science becomes art. Raw data in terms of a real TDEM response is as valid as the data can get. Filtering, smoothing and inversions and doing a best fit perturb the data into art - not science.

The UBC.edu document summarizes six methods of obtaining tau.

SkyTEM openly admits inversions that require over 100 hours of number crunching and then crash.

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

Show me a response. I will do the rest!

- Geowizard

 

[k0bgalan]

Lots of instruments use time domain measurements. The one I'm most knowledgeable about is the TDR—Time-Domain Reflectometer. The one I have is used to detect anomalies (breaks, opens, shorts, etc.) in telephone cabling. It can even tell you where, if any, the loads are. Not cheap, but a lot cheaper than digging a huge trench!

And... All of this makes me wonder about Garrett's VX (Vortex) series. It is, according to Garrett, the only fully-digital hobby metal detector they make. I've also been told, the only time the depth measurement is accurate (no + or - percentage given) is when the built-in pin pointer is used—a fact I can attest to. I would tend to believe it uses a form of time domain measuring for this function?

 

[99thpercentile]

Understood.
The question is, "Where does science become art?" I can share examples where SkyTEM did inversions that took two weeks and crashed. Inversions that showed excellent conductors that had no correlation with physical drill holes. The drill holes were consistently "no shows". To be fair, there is the possibility that the holes missed their intended targets. The client company walked away.

The challenge for the analyst is that TEM anomalies don't provide qualitative information in terms of I-Q and phase. Phase has been shown by example in FDEM to provide discrimination - adding a qualitative dimension to targets. This dimension reduces risk in moving forward in exploration of precious metals.

- Geowizard

There doesn't need to be any infatuation with phase. Geophysicists don't really care about some of these details as much as the hobbyist does. We have had the education to understand that you can make a measurement in either the time domain or the frequency domain, and then convert from one to the other using either a Fourier transform or an inverse Fourier transform. For electrical or electromagnetic measurement we care about storage and loss. We care about electrical conductivity or resistivity, magnetic susceptibility or permeability, and at higher frequencies the permittivity. We know that all physical properties are complex (meaning real/imaginary, storage/loss, inphase/quadrature) and that unfortunately nature is not isotropic and homogensous. Which was we choose to measure a physical property is typically driven by the electrical engineering aspects of reliability or cost, not of an idealized measurement method. Once we complete a measurement, we have a number with an approximate spatial location. The goal of the geophysicist is to convert that set of numbers into physical properties at locations. After using geophysical inverse theory to accomplish this task, we may end up with a profile that shows electrical conductivity versus depth. Unfortunately, there is no guarantee that a location with an electrical conductivity anomaly that sometimes indicates gold has gold now. That is why we combine geology, geochemistry, and direct measurements through borings to try to increase the probability of success. Geophysics is all about balancing errors in data, errors in models, and reality. Geophysics isn't perfect, but it is the only science based approach to determining what is in the subsurface while measuring from the surface.

 

[geowizard]

The greater value of experience;

I can cite thousands of references where in-situ FDEM measurements made by DIGHEM over thousands of conductive mineral deposits that show unambiguous correlation of conductivity and phase relation of I-Q. There are literally thousands of examples that I have documented. The documented responses are consistent, unanimous and self evident. Any reasonable person can look at the raw, unfiltered data presented in graphic form and see conformity that is consistent with naturally existing conductive ore-bodies. As would be expected, every ore-body is different yet consistent in conforming to the rational response of infinitely conductive and infinitely varied combinations of elemental composition in ore-body form.

Observation of the response many times each day, day-after-day over a period of more than ten years (3650 days) over thousands of deposits establishes clear and convincing "empirical" evidence without exception.

Furthermore;

DIGHEM is multi-frequency. The response is not a single response! It is a multiplicity of responses at five or more frequencies of infinite quantitative and qualitative content measured 10 times per second at 90 miles per hour over survey courses of many thousands of line miles. Giga samples form a body of experiential foundation that is without possible measure!

- Geowizard