F75 SE: Why did lower sensitivity to 41 reveal more non-ferrous targets?

The site I have been hunting/ training on is totally carpeted with square nails in various stages of decomposition. When ya dig a plug, you can run your garrett pinpointer through it and it will indicate the presence of metal. Even a random plug. I usually am patient enough to find what is making my pinpointer sound off. And it will be a half decayed square nail, or a whole square nail. In that plug I can see the orange evidence of rust. I have went over this site a hundred times. I gave up on it last summer. I told everyone that I had hunted it out, even though I hadn't found much. Coin-wise, just one old wprn out Indianhead that was so worn that it was "Index card paper" thin! I could barely tell that it was an I.H.!!! But finally made out the headress on the Indian's head. And the ONE CENT on the back. I had used a F75SE and a Garrett AT Pro and a Tek G2 on this site and officially called it hunted out. I had been detecting for about two months. I was a real pro. LOL
I sold all of those detctors over the course of time just so that I could buy and play with other detectors. Alas, I bought a new F75SE and a new Garrett AT Pro. I headed back to this site one day because I had learned some new unmasking tricks and wanted to try them, plus, it's really the only place I had to go that day. Since I have been re-hunting it, I have been carrying home relics and coins.
5 Wheat backs. A 1890 V Nickel. A No-date Standing Liberty quarter. A 58 cal Minie ball bullet. A 52 cal Sharps Carbine solid base bullet. Many many more 1919 cattle tags (1919 tick infestation tags). An old pocketknife with the loop on the end that swivels. An old spoon. A Civil War era suspender buckle. The list goes on and on and I will put all of these new finds together and take a pic when I really am done with this site. BUT I LEARNED A VALUABLE LESSON two days ago. I had been hunting this site with both of my detectors jaked up as high as they would go without interference. (electrical) The SITE CAME ALIVE AGAIN with new non-ferrous targets. For some odd reason and for lack of anything better to do I lowered my sensitivity on my F75 to 41. Using the 11" DD coil within the nail bed, I started hearing brand new non-ferrous signals. I thought that I had got them all!!!! Yesterday, two more 1919 cattle tags, several more period relics. And a couple of nickles dating in the 50's. This is over a site that I have crisscrossed a hundred times!!!!!! Was the high power of the high sensitivity making the square nails over bearing?????? It has puzzled me becasue I thought that the best unmasking came from the highest sensitivity possible. I am going back today with the 5inch coil again but with a lowered sensitivity. Now this site is right in the middle of town and electrical interference may very well have been my problem. I SHOULD start keeping notes on how my units are set up so that I can start making a comparartive analysis, but I'm too lazy and don't think of it until I am swinging the coil. Then it comes to me: I FORGOT MY NOTEPAD! I will report back tonight if I find anything. I really think that there is more silver under those nails. Along with Civil War relics as this place was under Union occupation and the place I am at was supposedly a hospital during the Shiloh battle by command of Grant. This stuff is just interesting as can be isn't it? Anyway, the reason I posted is to see if you guys and gals have noticed the benefits of a lowered sensitivity when hunting highly nail littered areas. Thanks for reading.

Headlights in the fog. Finding a way to tweak a detector to get the most out of it is what this hobby is all about. Congrats on your finds. You have two fast recovery detectors there. They should do the job.

I did some testing today on my Omega. Buried a clad time at 7 1/2 inches. I'm in Idaho. Soil is fairly mineralized. Three coils, 5" DD, 11" DD and stock 10" ellipitical concentric. Ran 50-60-70-80-90-99 sens. The hardest hits were at 90. 60-70-80 got a signal but it was progressively not as loud as the sens was dialed down. 99 was too erratic with falses. 70-80-90 produced same amount of falsing with coil in motion. 60 is where it stopped falsing. What really shocked me was that only the 5"DD & 10" concentric picked up a signal. The dime was dead to the 11" DD no matter the sens level. No other metal around the dime that I could pick up with any of the coils. Kinda flies in the face of all I have read about which type of coil is best in mineralized soil. The 11" has been bench tested umpteen times and works properly. Weird !!

Terradigger- Those are interesting results with your Omega. Strange that the 11''DD didn't get a response, where the other coils did.

Kevin B- You learned the most important lesson about the F75 in heavy trash-'' Keep er tame''. Too much sensitivity can make a mess of things.

Kevin......I would perform a head-to-head. When you find a non-ferrous target in this hunted-out site...by using a lower sensitivity.......... THEN switch over to increasingly higher levels of sensivity....... and see what happens.

NASA-Tom Wrote:
-------------------------------------------------------
> Kevin......I would perform a head-to-head. When
> you find a non-ferrous target in this hunted-out
> site...by using a lower sensitivity.......... THEN
> switch over to increasingly higher levels of
> sensivity....... and see what happens.


Tom, I did that yesterday. At 41 sensitivity, I would get a great signal, in the low 60's (it would turn out to be another of those round tags with Hardin Co. Tennessee 1919) and it was surrounded by the decaying nails. When I shot straight up to 99 sens. and start swinging over it, I would get strong iron signals and an occasional one way non ferrous signal. I was in 2F tone. But I only performed that one head to head test and will start performing more. I have the wonderful luck of having an old untouched, turn of the last century, cotton gin to go to this afternoon after I drop by the hospital for a visit with Momma. I am pumped about this new site. Also, an old store nearby from the same era. Structures are gone so nails will likely be present.
Back to the the 'hunted out' site. Alot of the old decaying nails are up close to the surface. They seem to go down to a depth of 5 inches roughly. It sounds like it's about time for another phone session with you because I can't type fast enough to convey what's in my head. I will email you this evening when I get home for a phone session this week. I will try to be more organized with my questions.
And to the other folks who replied: Thank you for your replies. Your feedback is valuable to me and I appreciate that you took the time to read my post. I will try to shorten my questions from now on, but dad gum, I love talking about these detectors. LOL Happy President's Day to everyone.

hardin county border the county i live in

*** Buried a clad dime at 7 1/2 inches. What really shocked me was that only the 5"DD & 10" concentric picked up a signal. The dime was dead to the 11" DD no matter the sens level. ***

According to Omega air tests done by Monte on this site:

11" DD, Sens 65, All-Metal, TID's dime to 7" and 5" DD, Sens 70, All-Metal, TID's dime to 6"
About the same results with Disc 20.

11" DD, Sens 65, All-Metal, Audio response to 9" and 5" DD, Sens 70, All-Metal, Audio to 8"
About the same results in audio with Disc 20.

11" DD, Sens 99 in Disc 1, TID's to 7"
11" DD, Sens 99 in Disc 1, Audio to 8.5"


If the 11" DD can have audio response to 8.5" in neutral soil then the contributing factor dropping audio depth to < 7.5" would be iron soil mineralization. The 5" DD gets audio to about 70% of the larger 11" DD under neutral soil. But the 5" has decreased response to the mineralization due to the decrease illumination volume.

So, the 11" DD drops from 8.5" audio to say 7" in mineralized soil, giving no response. Yet the 5" DD does not drop equivalently. It might be expected that the 5" DD would drop to say 5" due to an equivalent decrease in bad soil. However, because the smaller coil is less affected by soil mineralization it drops from ~8" to only 7.5", and picks up the dime.

The 10" elliptical concentric coil has a different E-M footprint. It is affected by mineralized soil too but air testing does not show any significant TID or audio depth difference over against the 11" DD. In this case, the 11" DD's E-M footprint was deflected whereas the 10" concentric managed to cut the coin to produce mutual induction, otherwise - no signal.

When iron oxide minerals in undisturbed ground lay in a horizontal layer they act as a kind of shielding. That is, the E-M flux passes more easily through the iron than the surrounding soil matrix, bending the field into the horizontal. The field lines always take the path of greatest permeability - and iron has very high permeability. The total field flux is a constant in the soil, and if some is diverted it necessarily means whatever is in the void will not be illuminated.

The reason for all this is that the soil is not homogeneous and has widely different affects on the lines of flux, which is rarely considered. But the effect is such that a broader footprint (concentric) can exceed the capabilities of a narrower DD footprint and the smaller soil volume illumination of a 5" DD can exceed the depth of the larger reactive-X producing signal of a larger DD coil.



Edited 1 time(s). Last edit at 02/20/2012 09:06PM by Johnnyanglo.

Thanks for the technical explanation JohnnyA. I was thinking that perhaps the volume of mineralization seen by the 11" coil might be a culprit in the coil's performance. Your explanation though is more illuminating than VoM. However, I was not aware of the reasons of why the other coils performed the way they did. Really useful info to me

p.s. maybe that's another reason why Monte uses the concentric for most Omega hunting and the 5" for trashy sites. He lives about 300 miles from me. Our soils have pretty much the same level of mineralization. (northwest U.S.)



Edited 3 time(s). Last edit at 02/21/2012 01:15AM by TerraDigger.