detection depth

[bbsailor]

Target, 20 inches deep. Best coil size, 40 inch diameter. Target just detectable with the 40 inch coil. How to approximate detection depth with a 20 inch diameter coil?

Keep inductance and current profile the same.

If you specify the time constant (TC) of the target, you can calculate the best PI parameters to optimize the potential detection of that target. Here is an example.
Let us assume that the target has a 5uS TC. We would need a TX pulse at least 5 times longer than the target TC or 25 uS. The full stimulation of that target would require a coil with a coil discharge TC of one fifth the target TC or 1 uS. That means that we need a coil of 300 uH that has a 300 ohms effective damping resistance value. This effective value includes the input resistor to the first op amp stage which clamps the input resistor in parallel with the damping resistor during the flyback pules down to about 0.6V. If you set the delay to 10 uS your maximum target signal will be 2 TCs or about 86.5 percent lower than the maximum target induced signal.

Knowing the target TC can help you reverse engineer a coil resistance, inductance, effective damping value along with the proper TX and RX parameters for that target. Since target TCs are unknown, we tend to cluster our coil sizes/types and PI parameters for certain classes of targets such as coins in a beach or small gold nuggets in a stream bed or small nuggets in mineralized soil.

There is a limit to how much power you can pump into a target area as the doubling of detection distance in theory needs 64 times more power. That is 2 to the third power to stimulate the target and 2 to the third power for the return trip of the target eddies back to the coil which adds up to 2 to the sixth power or 64. Then there is minimizing noise in the RX circuit when using the highest gain to maximize sensitivity. Here is where Eric Foster integrated many RX pulses to lower the noise level much like what is used in Lock-in amplifiers.

I hope this helps?

Joseph J. Rogowski

[Skippy]

I have no experience of nugget-hunting, so this is just a curiosity question:
What sort of size/weight nugget would you viably expect could be detected at 24" depth? 15mm diameter? Golf-ball size? Fist-size?

I'm aware nuggets aren't spherical, and have inclusions of rock and quartz etc, so they may be harder to find than their apparent size would suggest.

Have you (Green) done any buried or air tests with your available coils, with real or 'simulated' nuggets?

[Dave J.]

There is a limit to how much power you can pump into a target area as the doubling of detection distance in theory needs 64 times more power. That is 2 to the third power to stimulate the target and 2 to the third power for the return trip of the target eddies back to the coil which adds up to 2 to the sixth power or 64. Joseph J. Rogowski

Magnetic dipole reciprocal third power law both ways for magnetic field strength. Electrical power is proportional to the square of the field strength. All other things being equal (which they never are of course) to double detection distance you'd have to increase transmitter power by a factor of 4096.

Apart from basic theory, it's easy enough to discover the same thing the old fashioned way: put a metal detector on the oscilloscope and see how the returned signal voltage varies with the distance of a large coin along the axis of the searchcoil. That's the "rule of 64" in metal detectors. Voltage.

[green]

I have no experience of nugget-hunting, so this is just a curiosity question:
What sort of size/weight nugget would you viably expect could be detected at 24" depth? 15mm diameter? Golf-ball size? Fist-size?

[Good question. What size coil?]

I'm aware nuggets aren't spherical, and have inclusions of rock and quartz etc, so they may be harder to find than their apparent size would suggest.

Have you (Green) done any buried or air tests with your available coils, with real or 'simulated' nuggets?

[Most of my tests have been air tests with GEB on or off. I did measure a lead weight at 10 inches in an air test, integrator out with GEB on adjusted to cancel ground signal. Then buried the weight 10 inches in the ground that needed GEB and got the same answer. The nuggets I have(4, 10 and 18 grain) aren't close to large enough for 24 inch depth.]

Wound a 200mm diameter coil and one 200/1.5(133mm) and another 200*1.5(300mm). Charted signal strength vs distance for the three coils(same inductance)and got similar results as Eric's curves. Curves cross at diameter(coil A + coil B)/4. The chart suggests a 48 inch coil would give the highest signal strength. Maybe not the best signal to noise. Can't swing it if the ground isn't smooth. I would think a coil diameter equal to the depth would be a good place to start if terrain allows.

chart for the coils I posted awhile back.

[green]

Doubling the coil diameter reduces the target signal to 1/8. What does doubling coil diameter do to the ground signal?

[Ferric Toes]

What does doubling coil diameter do to the ground signal?

Everything else being equal. it stays the same. At least, that is what my tests in Australia indicated, a few year back.

Eric.

[green]

Everything else being equal. it stays the same. At least, that is what my tests in Australia indicated, a few year back.

Eric.

Thanks for the reply

Was wondering about noise with different size coils. Some tests with 133mm, 300mm diameter mono coils and Rx 2(200mm diameter figure eight). Scope pictures varied each scan, stored a typical scan. Moving the noise source effected the noise signal. The 300mm coil appears to be noisier than the 133mm coil, noise sources were fairly close to the coils, wondering if the recordings would be different if stronger noise sources were 100 meters or more away. Guessing the figure 8 coil wouldn't be effected as much by orientation if the noise source was a lot farther away, maybe not? Scope scales not the same for each recording, looking at the picture it would be easier the see the difference if they were the same scale.

Is the noise increase with the 300mm coil over the 133mm coil what would be expected or should it be higher or the same?

[green]

For a target of a given diameter, if you're thinking only of relative voltage induced in the receiver, it's a fairly straightforward (when you know how) axial air coupling calculation. However a target that's "just detectable" is most often limited by a number of factors other than mere voltage induced in the receiver. Also your question as you worded it doesn't make sense: "best coil size 40 inch diameter, target just detectable" means that any other size searchcoil won't detect that target.

So, in the real world your question has no straight answer. The non-straight answer is that if a searchcoil of a given radius has a weak response on a particular target buried at a depth equal to that of the coil radius, an electrically equivalent searchcoil half that radius will kinda sorta more or less usually give a better response. Your mileage may differ.

[EDIT] now the question about a cache 2 feet deep, what size searchcoil will be best. There is no straight answer especially since there is no "given" regarding the effective diameter of the cache or what size of searchcoil is known to detect it. You might do fine with an 8 inch searchcoil and not need "best".

Thanks for your replies. Didn't get a diameter number but a lot to think about. Your second paragraph says a coil diameter equal to target depth might be better than one with a diameter twice the depth from the graph. A place to start.

[green]

I charted some detection distances for a US nickel, dime and quarter from a different detector site, different threads. Didn't record which detector was used and the coil diameter wasn't listed. A large spread. I can't detect the distances listed in the third column with my detector with a 8 inch diameter coil. Thinking a larger coil might help. What size coil should I try to best detect a 19 inch US nickel or a 16.5 inch US dime? Should I be looking at something other than coil diameter first? Using a PI detector, would the answers be different if using a VLF detector?

[jladre]

I would also like to ask if which of this coil can offer more depth? A 13 inches round dd coil or the 10x14 inches elliptical coil?

[Dave J.]

assuming both are DD's and electrically identical, there would be a slight difference in coverage on small shallow targets but virtually no difference on targets deeper than about 4 or 5 inches.

[kt315]

i think you must build both coils and tell us your experiment about.

[green]

Re: Dave's "Understanding cache locating" comment. I get the impression no-one does it, or they don't know anything about it. I started a thread on Dankowski's Forum about it last year, as I've been trying my hand at it, and thought the experienced folk there would have some wisdom to pass on. Zip Nul Nada Sweet.Fanny.Adams. Not one reply...oh well, I tried.

Back to Erics graphs: Here's the graph, I can't find the original explanation.

This is for PI machines, for circular mono coils, not accounting for ground effects.
Basically each solid curve is for one particular size target. Bigger targets have a higher-up curve. So take the point where the 40" coil just detects the large target 20" away. Now move left down that line, and when the coil-size reads 20", you will see a target depth of 17.5", as I quoted earlier.

I have never tested out this data, I'm not a PI guy, and how circular mono-coils compare to elliptic DD's is not my specialised subject : )

http://www.geotech1.com/forums/attac...2&d=1501120250

Still thinking about making a bigger coil. Question. Using link above, if I can detect a US nickel at 15 inches with an eight inch diameter coil. What is the largest diameter coil I could use and still detect the nickel? Looks like over 40 inch diameter coil, but don't know if it makes sense that I could use a coil that big.

[green]

http://www.geotech1.com/forums/attac...2&d=1501120250

Still thinking about making a bigger coil. Question. Using link above, if I can detect a US nickel at 15 inches with an eight inch diameter coil. What is the largest diameter coil I could use and still detect the nickel? Looks like over 40 inch diameter coil, but don't know if it makes sense that I could use a coil that big.

Still wondering about best coil size. Example: an eight inch coil can detect a target at sixteen inches, what diameter coil would detect the target the greatest distance and what distance would that be? Everything the same(inductance, current amplitude and profile, PI timings, noise level, etc.)except coil diameter. I charting some data with a 200mm and 300mm coil vs output from Hyperphysics. Entered twice the distance in Hyperphysics to get it to make sense. Still not close to my measured data. Think I'm doing something wrong with Hyperphysics. Wanted to chart distance vs amplitude with Hyperphysics for a 200, 300, 400 and 500mm coil to see if that would help determine best coil size. Any ideas where I'm going wrong(maybe the wrong law in Hyperphysics) or how to calculate best coil size for the example?

[Skippy]

I have lots of target strength vs distance charts plotted out (VLF machine, 11" x 7" bi-axial DD coil) . So I laboriously plotted your 200mm Hyperphys data onto paper to compare. And you've definitely got something wrong. All my small targets dropped away much faster than your HP figures. The only target that resembled your HP data was a 3" long ferrite rod, end-on to the coil. It was actually a good match (when close to the coil) , but I don't think there's anything to learn from the match. The rod is not a small target - when it's at 0", some of it is 3" away.
It looks like your HP model isn't correct when targets are close to the coil - there just isn't the steep fall-off in signal strength as distance increases.

But seeing as I don't know how you've modelled your coil & target, I've no idea what's gone wrong.