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VLF Detectors And Depth


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Interesting, do you believe depth with VLF`s is frequency dependant ie  lower frequency = more depth and less sensitivity, higher frequency = less depth more sensitivity. Sensitivity being to smaller targets. I am finding with the Deus and its switchable frequencies the above is very true especially for coin sized targets. ( The Deus being my first switchable frequency detector since the 1700, thus please forgive me for not mentioning the others) Or is this lower frequency thing just because it isn`t targeting the smaller stuff it just appears to have more depth?

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Frequency does a number of things. George Payne was one of the engineers who patented many of the basic concepts used in VLF detectors to this day. Here is an excerpt from his article at http://jb-ms.com/Baron/payne.htm (2002):
 
"The r component acts differently. It is maximum at one particular frequency and decreases if you go up or down in frequency. We call the special frequency at which the r signal is maximum, the target’s “-3db” frequency. It also turns out that at the -3db frequency the x signal is one-half of its maximum value. This special frequency is unique to each target and is different for different target.

The higher the conductivity of the target the higher will be the targets -3db frequency. Conversely, the lower the conductivity the lower the -3db frequency. The -3db frequency of the high conductivity target will also make the r signal peak at a high frequency, normally well above the operating frequency of the VLF detector. This will make the high conductivity target have lower sensitivity on the VLF detector because the r signal amplitude drops if we are significantly below the -3db frequency. Simply put, maximum sensitivity on a VLF detector would be if we position the operating frequency directly at the target’s -3db frequency. For example, a dime and penny have a -3db frequency of about 2.7KHz. This is where their r signal peaks and would be the best frequency for picking them up using a VLF detector. However, a silver dollar has a -3db frequency of 800Hz. Nickels, on the other hand, have a -3db frequency, where its r peaks, at about 17KHz. Targets like thin rings and fine gold are higher still. Clearly there is no one frequency that is best for all these targets. The best you can do is have an operating frequency that is a compromise."

So that sets up the basics for air tests. The problem is we have to deal with the ground. Lower frequencies tend to be better at reducing ground issues, while higher frequencies light up both the ground and hot rocks lower frequencies might ignore. You are looking for a frequency which best lights up a target while minimizing ground effects. All in all mid frequency machines in the 10-20 kHz range offer good compromise solutions, above 20 tends to be the realm of specialized prospecting detectors, and under ten the realm of the "coin detector".

Minelab messes with peoples heads because their multifrequency FBS detectors are billed as operating at 1.5 kHz all the way up to 100 khz simultaneously. Sounds like they should simply do it all, right? Super deep on coins, super hot on tiny gold. Yet this really is not the case in practice. What they do is get very accurate discrimination on coin type targets at very good depths, but compared to single frequency prospecting machines they are no depth demons. I have seen all kinds of technical arguments and explanations surrounding this which all completely ignore practical reality. Reality is these machines were designed first and foremost to deliver exceptional accuracy for coin hunters looking for silver coins in park settings, and they excel at this task. Accurate discrimination as far as it can be pushed. They despite all the marketing fluff act like really good low to moderate frequency detectors. Possibly the best silver hunters made. Focus on what they do, not what they are marketed as.

These days EMI can be a big issue, and certain frequencies that in theory may work well may not in practice due to interference issues. The Fisher F75 looked great on paper but at 13 kHz fought EMI issues forever. Machines running at 19 khz seem nearly immune to urban EMI and so have an edge in that regard.

Finally, frequency is just a small part of it all these days. The efficiency of the ground balance method, voltage applied to the coil (transmit gain) and receiver gain, recovery speed (reactivity) etc all play into it, so frequency alone is only a bare indicator of basic operating parameters. Still useful though and operating frequency is certainly something I still always look at when choosing metal detectors. Units like the White's V3i or XP DEUS are fascinating to experiment with because you can switch frequencies on the fly and observe the effect on found targets.

One thing you will discover air testing a DEUS is that in air tests depths do not change all that much with the frequency on silver coins, but increase dramatically on low conductors like U.S. nickels or gold rings as frequency increases. Remember that aluminum is a low conductor, so using low frequencies in a park setting will do well on most coins while reducing signals on common trash.

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Back to the basic depth problem with VLF detectors.

Metal detectors have a basic limitation in how far they can detect gold items. From http://www.talkingelectronics.com/projects/200TrCcts/MetalDetectors/MetalDetectors-1.html  “the sensitivity is roughly proportional to the cube of the object diameter (as expressed as a function of the search coil diameter). Sensitivity is also inversely proportional to the sixth power of the distance between the coil and the object. All this means is that if the object size is halved the sensitivity is reduced to one-eighth. Also, if the depth is doubled the sensitivity is reduced to one sixty-fourth. It’s easy to see why all metal detectors which are designed to pick up small objects use small coils, (150 to 300 mm diameter) and really only skim the soil surface. If the search coil is doubled in diameter for greater penetration the sensitivity to small objects falls to one-eighth. You rapidly encounter the law of diminishing returns.”

Famed metal detector engineer Dave Johnson reiterates this in a different way at http://www.fisherlab.com/hobby/davejohnson/davejohnsonjohngardinerinterview.htm  “Getting extra depth out of a VLF, multifrequency, or PI machine is very difficult, because these machines follow an inverse 6th power law relationship between signal voltage and depth. If everything else is maintained equal, doubling the depth requires 64 times as much signal. If this is done by increasing transmitter power, doubling depth requires 4,096 times as much battery drain. That’s the basic reason why depth increases come so slowly in this industry.”

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Wow, that`s in Depth. No pun intended. Never bothered with air depth tests, only air test for IDs, which is not 100% either. In ground undug targets can only give a true indication. There on the Deus as I guess with all changeable frequency detectors a indication of the frequency change can make to detection of a target, have noticed a difference that confirms the lower frequency depth capabilities as at a higher frequency the sensitivity to small nuggets. More so on small gold, not a peep at 4Khz but a clear signal at 19Khz.

 

Certainly from coin sites had flogged with the A2B, back in 80s to going over those sites with the Deus now, VLFs haven`t gone far, even the discrimination features seem of not much value on such sites.

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Yes, and not all places have coins at depth. Most forums I read make a great deal of various "sink rates" regarding coins, and that no doubt holds true in moist climates with deep turf accumulations. But a lot of places with drier climates the ground surface changes but little over time and coins stay shallow even over hundreds of years. The first people in with detectors quickly got the bulk of them, no different than many nugget patches. Unfortunately, more depth, even when we can get it, just does not result in more finds. A lot of places got cleaned out of the really good stuff back in the late 60's and early 70's before we got this huge proliferation of aluminum trash. Parks really were just a lot cleaner and even the simple BFO and TR detectors of the day had no problem pulling out what would now be huge numbers of old coins.

I used to subscribe to all sorts of treasure magazines back in the 70's before I ever got into nugget detecting. The magazine covers had pictures like this one of Kay Modgling on a regular basis - people finding silver by the sackful and rings by the pound. With basic non-discriminating non-ground balancing detectors! Kay was just amazing. And it may be that the very lack of discrimination played a big part in it. When I got started there was no such thing, you just dug it all. No worries about target masking!

kay-modgling.jpg

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Kay was apparently one of the early stars of detecting. Pals with folks like the legendary (and happily still,with us) Jim Straight.

They Had the pick of the 19th and early 20th century deposits.

The Europeans have been waking slowly up to this for the last decades and are now on board, with their own detector manufacturers following.

Guess where the next great historical rush will be.

https://en.m.wikipedia.org/wiki/History_of_China

1500 BC!

Guess who will sell 10 million high quality metal detectors there?

SOMEBODY will - and if Apple's success in China is any guide, it might not be a low ball outfit - whether Chinese or otherwise.

The last big prize for detector sellers - who will win?

P.S. Forgot India, add 20 million more!

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My first detector was a Garret deepseeker vlf/tr. No vlf I have had since then went deeper. using reverse discrimination it was 100% accurate according to the discrimination setting. Of course thing got easier with the next model but the finds got leaner...

 

Steve; excellent primer or refresher on VLF's in the current ICMJ...

 

fred

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"Guess who will sell 10 million high quality metal detectors there?

SOMEBODY will - and if Apple's success in China is any guide, it might not be a low ball outfit - whether Chinese or otherwise.

The last big prize for detector sellers - who will win?"

 

funny, i was talking to my wife a couple of days ago about an idea i had of having our friend with all the mountain property inviting a group of hunters over to China to do some detecting on his property and having another friend who manages a cctv station in Shanghai film the whole operation for Chinese TV. Putting together something that sounded like a fun adventure and getting some publicity for jade mountain and the old temples and such that are on the mountain land, I didn't really think about the possibility of generating interest in metal detecting "IN" China, i just kind of assumed they already were? 

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