TDR Logic, mental lapse

Geologists use TDR to study soil, rock, and underground moisture. It's interesting.

google geology tdr

--

John Larkin         Highland Technology, Inc 

Science teaches us to doubt. 

  Claude Bernard

It's what a fast ohmmeter would measure for a short piece of the line, short enough to ignore ohmic losses.

Or an equivalent fast measurement of a long line. Same thing.

--

John Larkin         Highland Technology, Inc 

Science teaches us to doubt. 

  Claude Bernard

Several years ago we leased our gas rights for a few years to a company tracing the Marsellis shale up into NY. They placed microphones (geo- phones?) up and down the roads around here. And then I think had some big thumper truck (or explosives?) To map out the geology. The shale is ~two miles down near my place.

It'd be fun to know more of the details.

George H.

Yes, I just cut it in the middle and attached my antenna analyzer. No grounding. just connected the antenna analyzer. Equal distance wire in both directions on the ground. It is a dipole antenna laying on the ground.(earth, dirt) I dipole in the air of the same length (260ft) would resonate at about

1.8MHz, but because this is on the ground and the VF is changed it is resonant at 1.19MHz. Mikek

Ya, that confused me for a while also, it is just the impedance, I was told it was an old term that is used less now.

Mikek

I need help on calculating Velocity factor.

300,000,000 / length(in M) / 2 = Frequency, divide by 2, because it a 1/2 wavelength dipole. (in Free space). This is how I'm calculating Velocity Factor. A 79.25 length dipole 300,000,000 / 79.25 / 2 = 1,892,744 Hz My Dipole on the ground has a resonant frequency of 1,139,000 Hz. (I moved the wire so it changed a little from before) VF = 1,139,000 / 1,892,744 = 60.2% VF Does that work to calculate VF?

Now just for fun, I added 10 ferrite sleeves, 5 on each side, spaced about 20 ft apart. The sleeves measure, R = 3.1? and X = 12.1? @ 1 MHz. This reduced the resonant frequency 1.139MHz to 1.08 MHz, If my first version of calculating VF is correct, then, VF = 1,080,000 / 1,892,744 = 57.0% VF Not as much change as I was expecting, but some. Just a note: The gurus say if you ferrite load it past 50% VF, you get a pattern reversal on the antenna. (the main lobe switches direction)

Mikek

60th HIGHWAY GEOLOGY SYMPOSIUM. (tdr) to monitor an active landslide - Terracon

Fig 4, Pg 433 An unterminated 50 ohm coax cable is buried in a vertical bore hole and connected to a TDR.

Graph 5 through 7, Pg 434 - 436 Shows the current and cumulative return waveforms caused by any horizontal displacements. There is some analysis on Pg 436 explaining the possible causes of reflectionsL:

The reflection coefficient (cable impedance) changes when there are changes in the cable dimensions, material property changes along the cable system, or changes in material properties surrounding the cable.

Photos of the hardware on Pg 431

50 ohm Foam Coaxial Cable, Heliax Coaxial Cable (FSJ4-50B), manufactured by Andrew Inc.

I think this is the pulser:

Please note that this earth movment detection system does NOT push low frequency pulses through the ground but instead relies on the reflections produced by the deformation of buried a coax cable, which is very different from what MikeK is attempting.

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Jeff Liebermann     jeffl@cruzio.com 
150 Felker St #D    http://www.LearnByDestroying.com 
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Skype: JeffLiebermann     AE6KS    831-336-2558

Anyone have any input, How do I calculate VF when I can calculate free space wire length with Resonant frequency and I know resonant frequency of my reduced VF wire? Or if I ask that better above... Mikek

Huh, and when I heard tdr and geology, I immediately thought of sending sound waves (vibration) into the ground and looking at reflections. (vs time)

George H.

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Mikek

ncy

Hi Mike, I'm not sure what velocity factor is. I've got a table that lists propagation velocity as, c/sqrt(epsilon) where c is the speed of light and epsilon is the dielectric constant. That might be for coax?

If there's some resonance nearby then I'm not sure you can use the transmission line equations...

When you ping on it with the tdr is there any ringing?

George H.

I suspect you do know. Here's an explanation shortened considerably from Wiki, "The velocity factor (VF) is the ratio of the speed at which a wavefront (of an electromagnetic signal, a radio signal, or a change of the electrical voltage on a copper wire) passes through the medium"

As I understand it, my wire and the earth are two plates of a capacitor, the wire is an inductor, together they make a transmission line, L-C-L-C-L-C, as in the coax. The wavefront velocity is slowed, resulting in, it has a ratio to the speed of light. (VF) I was confused because I got a couple different answers, I did get an explanation. Hams use a formula for a dipole, it is slightly altered, I think because of an end effect of a wire antenna. I used the formula to do my calculation, that is wrong, I need to use the speed of light. My wire is 79.25 Meters. Calculating the Resonant frequency of a wire in free space,

300,000,000 / 79.25 / 2 = 1,892,744 Hz My 79.25 meter wire is resonant at 1,190,000. Therefore, the ratio or VF is 1,190,000 / 1,892.744 = 62.9 %

I have not pinged it with my TDR, I may do that, but I'm working on my ground system and waiting for Miss Dig, one of my grounds is in the right of way, and there are several utilities there. I'd prefer to miss them all when I install my 10ft ground rod or more. It's a bit of a chore, I need to drag my scope out into the field along with the extension cords, Hide the screen from the sun so I can see it. It may not tell me anything usable, but just for the experience.

Mikek

That impedance is suspiciously close to 600 ohms, the old wireline impedance of telephone land lines.

Where did you get this TDR unit? Make and Model? Probably intended for diagnosing telephone wiring.

Joe Gwinn

In the early telegraph days, before the invention of AC, they measured the DC impedance of a component with a Whetstone Bridge powered by a battery, usinga galvanometer to detect balance.

If one interposed a telegraph key between battery and bridge, and balanced the bridge so there was no initial galvanometer deflection on keying, one had measured the surge impedance.

Joe Gwinn

I modified it from a 50 ohm out put to 575 ohms. Easy to change it back to 50 ohms.