Announcement

**Reg** · 10-12-2006, 05:58 PM

Hi Gef,

I wouldn't get too excited about the claims of others as to how deep they can detect a nugget. There are simply too many variables to say just what is what when it comes to the detection of natural gold.

Now, as for some of the claims, there was just a post on the Prospecting In OZ forum about this subject and here is what was basically said as part of the answer.

"And then we have
another poster having trouble finding .5 gram nuggets at 20cm=8" with
a std 11" DD coil! Over a lot of our ground you would struggle to find
a .5 gram nugget at 10 cm let alone 20 cm!"

He is right about a realistic depth of detection of a .5 gram nugget. realistically, a 4" depth of detection is a good depth of detection for a nugget of this size. In fact, it is an extremely good depth.

Ok, so what is a realistic depth of detection of your .9 gram nugget? Well, once again, a big part of that answer lies in the nugget itself. Some will be detected deeper than others simply because of its shape, thickness, purity, etc. In the case of a .9 grain nugget, the actual depth of detection could vary a couple of inches or possibly more.

Now, as a general rule, if you are actually burying your .9 gram nugget to an actual depth of 4" in the ground and detecting it ok, then your detector is working fine. In fact, it is doing quite well.

One other test you can do is to bury the nugget 1" deep and then pass over the nugget and raise the coil on each multiple pass to see just how far you can raise the coil and still detect the nugget. If you do this, make sure to measure the final height above the ground you can detect the same piece of gold. Now, if you add the height of the coil plus the 1" of actual depth, don't be surprised if the combination exceeds the actual depth the same nugget can really be buried and still be detected.

Unfortunately, many people run similar tests and assume that if they can raise the coil "x" inches, then they could detect this same nugget if it were bured "X" inches plus the real depth. As such, they use this mechanism as a means to determine the depth of detection in their claims.

One should realize that such an assumption is not valid, but most people do not know that so they do use this technique as the means of determining the depth a nugget "could" be detected but state it as a fact.

This is why I really do not worry about what others are claiming to do and worry more about what I see as an actual increase.

Finally, there is the "ground" factor. The more you test, the more you will find the ground mineralization can have a very strong influence on the actual depth of detection.

Now, if you are determined to try to obtain more depth from your detector, then use the same nugget as your reference in your tests when you conduct your experimentsm, and always use the same ground when possible. So, what should be done is to bury that nugget at a depth where the detection is marginal at best and leave it there Now, if you actually do have any improvement, it will show much easier.

Now, the one big flaw in even this test is the possibility of external influences, especially if you bury the nugget and leave it for an extended period of time. By this I mean, one has to be careful that small pieces of tinfoil or other trash haven't contaminated the test location over time, thus giving a false indication on the test. Realistically, this can happen quite easily, especially if you are testing in an area where you can't completely control the test site.

The reason I mention this is because I have had this happen several times. So, if by chance you do see a sudden increase, then make sure to verify nothing else is actually being detected.

Because such contaminations can occur much easier than one might think, I recommend as a backup, that one does conduct an air test to make sure such depth increases also show up in the air test also.

Finally, there is the simple test of laying a nickel on the ground and raising the coil to see just how high you can raise the coil and still detect the thing. This is a really good test in some respects because it will show, over time, just how easily testing can be influenced. To illustrate just what I mean, I have found I could use this test and have a variation of as much as about 5" or so simply because of the external and internal noise factor.

On typical days, a realistic distance of 12" to 13" is practical on the nickel test. However, I have run this same test on a few days where for some reason, there was little or no noise and on those days, I could detect this same nickel at a depth approaching 18". Now, I do have to admit, such days are extremely rare but I have encountered them.

So, if you see a sudden jump either way in the depth of detection, make sure to run follow up tests before getting too excited.

One final note, I have found the HH does better with a DD coil than a mono in most locations, simply because of the ground factor. Now, to add to the complexity, I have found that round DD coils seem to do better than elliptical ones also. So, I have found that my 11" round DD coil appears to be the best combination.

I hope this helps.

Reg

**gef12** · 10-14-2006, 05:43 AM

thanks

Hi Reg
Thanks for that deep insight into detection..
I was kinda thinking that alot of persons out there use a magical rule or something. But they were saying in the ground .3 grams at 6inch and that to me is quite small for that depth .. to good to be true me thinks.....
Could you post some details of your dd 11inch (i think u said) coil on here ..
windings , shield used etc . I used my coiltek 10 inch mono yesterday and was picking quite a lot of noise in my back yard .. plus a bit of rubbish to ..
And if u would the mosfet ur using ..
hope im not asking too much ..
Myself am using a IRFI740G and will be trying others soon that i have purchased ..
anyhow thanks again.
gef

**Reg** · 10-14-2006, 04:59 PM

Hi Gef,

First, I use coil housings I purchase from Bill Hays for my DD coils, so the general dimensions of my 11" coil are for that form.

Now, I am not sure my design is the best either. I need to try several things regarding the actual shape of the final windings to see just how much difference can occur. Generally, I will intentionally use too much overlap in the center zone, rather than try to null the signal. I suspect nulling the signal will generate less ground noise, so you might keep that in mind.

Ok, the windings are about 8" in diameter. I have used between 8" and 8.4", though. The number of turns will depend upon the type of wire used since the insulation thickness will alter the inductance value. Generally, I will try for 300 uh, which will be somewhere around 22 to 24 turns, depending upon the wire insulation. Being a little high doesn't hurt since DD coils are more forgiving when it comes to reducing the delay.

I have made coils where both the transmit winding and the receive winding are the same inductance and I have increased the inductance of the receive windings to about 450 uh or so. Generally, I have found the coil with the matching 300 uh coils to be quieter and have less ground noise. So, I recommend one experiment with different windings to see what works best for them. Fortunately, trying different receive windings is not that difficult before the coil is finally sealed.

I also shield each winding using a technique similar to that used by Eric Foster. By this I mean, the windings are wound on a form. I will then wrap them or better stated, lace them with beeswax coated string to keep the windings tight. This keeps the windings tight so they don't bunch up when installing the spiral wrap. I then wrap the windings with spiral wrap, to provide a spacer between the shielding and the wire. I will use a 26 awg size teflon coated wire since it works well size wise and the final bundle diameter is such that the spiral wrap is snug and not too large. Finally, I will wrap the windings with a special conductive tape. I use a special copper or silver plated polyester tape for my shielding. Now, this is not a typical foil tape and is hard to buy in single rolls.

I have used several other things for shielding including something as simple as basic silver colored ribbon I purchased at Wal Mart. Some of the ribbon that is silver in color is conductive on one side. Now, not all is, so it has to be checked.

Eric uses lead foil tape on his coils. Unfortunately, I found it difficult to obtain so I have struggled to find something else to use.

Getting back to the wire type, I purchased some Teflon coated wire off EBay so I got it cheap and that is why I use it. I have used a litz wire that is about a 26 awg size I purchased from Surplus Sales of Nebraska. They have, or at least did have a fair amount of this wire at a very reasonable price. Again, the price was right, and was much cheaper than if I were to try to buy this same wire that was new from a major supplier.

I will be trying something as simple as 300V pvc hookup wire shortly. I do expect the bundle size to increase though. I will also be trying to build some coils using the flat wound method as mentioned by Woody or used in the design of coils on Gary's PI.

Reg

Announcement

Detection depths

Detection depths

Comment

Comment

Comment