VLF Detectors And Depth

[green]

On 12/8/2015 at 7:11 PM, Steve Herschbach said:

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.”

Doubling target diameter should increase signal about eight times. Wire ring targets, 1, 2, 4, 8, 16, 32, 64, 128, 256, 512 and 1024 inches in diameter. 8inch diameter coil. Wire ring targets should have similar time constants(frequency). Would signal increase 8 times each time target diameter is doubled or is there a target diameter/coil diameter where signal increase is less than 8 times? Wondering if there is a maximum target size for a coil (how big is the ocean?).

[GB_Amateur]

32 minutes ago, green said:

Doubling target diameter should increase signal about eight times. Wire ring targets, 1, 2, 4, 8, 16, 32, 64, 128, 256, 512 and 1024 inches in diameter.

This doesn't pass the sniff test.  Yes, within assumptions what was said is true.  IMO you are going way outside of the assumptions that were in place when this statement was made.  "The devil is in the details."

[kac]

There is a general fall off on the coils detection ie can only pick up a sewer cover just so far as the side of a car or truck would be similar. There are 2 box systems that do go deeper but even those have limitations.

[EL NINO77]

I think there is a certain optimal range of coil size 11 "-13" .... to the size of the target .... / for example a small coin / - where increasing the size of the coil really increases practically the range of the detector to targets of this size ..

maybe even a 15 "coil can provide some advantage in depth ..

on the contrary ... the 4-5 "coils that could not optimally increase its range on larger targets ... ,, it sets the range limit here..for any large coin target ...

On the other hand, too large a coil .. size 18 "scatters the TX /Vlf/ detector more into the space than necessary .....
and this can also cause losses on the receiving part of the detector coil - so the detector becomes less sensitive and has a smaller impact on smaller coin targets

...according to me tx must have .. for a certain coil size you have a certain minimum density of the electromagnetic field ,, so that it can well induce small ,, but deep targets...

....so the size of the 18 "coil no longer has to show increased range.-on normal TX.

..of course in the case of a VLF detector in addition to the size of the coil ... the frequency of the detector and the type of conductivity of the target also play a big role ... where some type of conductor will have the best range ..

on the other hand, when I want to find out how a given detector is naturally sensitive to a type of conductors .... I do comparison tests on coins of the same diameter ... but different conductivity ... to exclude the target size factor on the detector range ... and found out on what conductor the detector will have the highest range ...