Halo Effect and Related Ground Oddities

[Jim Hemmingway]

Dave Johnson has produced another very interesting read entitled "Halo Effect and Related Ground Oddities". Wondered if you guys might enjoy this... Jim.

http://www.fisherlab.com/hobby/davejohnson/Halo Effect.pdf

 

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[TerryC]

Dave Johnson has produced another very interesting read entitled "Halo Effect and Related Ground Oddities". Wondered if you guys might enjoy this... Jim.

http://www.fisherlab.com/hobby/davejohnson/Halo%20Effect.pdf

Nice read, Jim. I am a proponent of the "halo" theory but can we say it is still a theory? Tnx for the link. Dave (and you) is always a good read. TTC

 

[Jim Hemmingway]

Terry… me too… at least with respect to subsurface corroded iron. Rust is maghemite… it plunges my F75’s ground balance way down from GB85 into the GB 40s here. As a result it acts similarly to a positive non-conductive hotrock, and doubtless helps to boost a metallic iron signal as DJ notes.

But I would point out that in a prospecting context, as one example… that acanthite… a silver sulfide chemically similar or identical to tarnish on ‘dug’ silver coins or household silverware… will issue a “soft” signal in a bench test to a prospecting-capable unit that is adjusted to high sensitivity in an all-metal stat mode.

But otherwise I don’t subscribe to the halo theory with conductive metals. Other than perhaps very pure gold, precious metals and/or alloyed constituents do react with soil chemicals resulting in molecular leaching / deposition of metal ions into the soil. However, I doubt the amount involved can possibly sustain electromagnetic energy in the form of eddy currents sufficient to return a measurable signal. It’s tough enough for a sensitive metal detector to give a useful response to disseminated native metals in rocks.

I’m more inclined to a view that metal detector performance over disturbed ground struggles either from the loss of electrical alignment / continuity within the disturbed soil, perhaps further aggravated by the disruption of the magnetic (field) fraction of a soil’s composition. That’s just guesswork. What does matter is that we can identify the effect of disturbed ground on metal detectors over freshly buried targets as contrasted to targets that have naturally settled into undisturbed ground. So at the end of the day I don’t think it’s important whether we call it “halo effect” or whatever. The phenomenon is readily acknowledged by manufacturers and is nothing new to experienced coin hunters.

A personal note. Following your posts I see that you have been doing some commendable work with newcomers to nuggethuntng at your claims and at some personal expense. You’ve taken these people under your wing, and under your roof to help them. I think that is a really fine thing for you to have done… deeds speak.

Jim.

 

[TerryC]

Jim, In my "old age" I have pretty much done all the big adventures I set out to do. It is now time to "give it all back". So, I am having fun spending my money on others than myself. I have some more ideas in the works..... stay tuned! TTC

 

[mjarvis]

Jim, In my "old age" I have pretty much done all the big adventures I set out to do. It is now time to "give it all back". So, I am having fun spending my money on others than myself. I have some more ideas in the works..... stay tuned! TTC

That's great terry!! We need more people like you, make this world a better place!! Happy hunting!

 

[TerryC]

That's great terry!! We need more people like you, make this world a better place!! Happy hunting!

Yes, mjarvis, there are others out there like me. I just make more noise than most. It feels good to know others appreaciate (tnx) my efforts. The reason I make noise is I hope it spurs others on to sharing that extra buck or two. I have my charities and ..... one of them is www.woundedwarriorproject.org Tnx. Take care. Terry

 

[McKinney_5900]

Thanks,,,this was a great read. I definitely concur that Texas ground and moisture, or lack of, effects depth. martin

 

[TerryC]

Thanks,,,this was a great read. I definitely concur that Texas ground and moisture, or lack of, effects depth. martin

Martin, Your statement has aroused my interest in knowing (or asking, I guess) just HOW water changes the conductivity of the ground. There is no question that seawater and "dry" lakes in the great basin (Nevada) dramatically affects the flux lines under a vlf detector coil. If you have ever detected at the seashore, or dry lakes, with a vlf, then you have probably screamed at that confounded machine! I was wondering if there is any paper that determines if distilled water affects the detector's depth. To what extent does rainwater affect it? Maybe Dave J can tell us. I sure would like to know what the detector techs can tell us. TTC

 

[Jim Hemmingway]

Martin… yes DJ makes a special point of mentioning the feedback across the country about dry soils generally decreasing VLF detection depth. But then too, there are exceptions to the rule.

Some soils become more reactive as soil moisture increases. This phenomenon can reduce EMF penetration and play havoc with target ID. Some experts attribute this condition to enhanced soil mineral (non-conductive iron) ionization that results directly from increased soil moisture. Let’s look at a documented Alabama ‘red’ soil sample lab report from one of our leading detector manufacturers.

The lab report describes the soil in a dry state as “bad ground” but well within the ground rejection range of their current detector models. It recommends that soil type should only be searched when in a dry state. Below is a direct quote from the report about that same red soil in a wet state.

“When wet the sample becomes very had ground, difficult to reject and or penetrate. This type of iron oxide, because of its consistency, achieves a solidity or connection between iron particles making it much harder to reject when wet.”

Terry… it sounds like Dave doesn’t have a satisfactory explanation, although he seems to feel that there is something to it over predominately non-conductive soils. In my ground dry to compactive soil moisture content gives the best detection depth. When wet, those deeper targets in my test plot are harder to acquire. How much that has to do with our semi-annual fertilizing is an open question. Quote from DJ below…

"Most reports of improved depth after a good rain come from wetter climates. I used to attribute this to easier digging, not to any electrical phenomenon, since soils of wetter climates usually have moderate to low conductivity whether wet or dry. However, the great drought of 2011 changed my mind. Just too many reports from detectorists from Texas to the Atlantic who have planted test gardens, and they noticed that the drier things got, the weaker the target signals got, and when it finally rained a good one the signals came back. Digging didn’t have anything to do with it. So, now I believe the effect is real. I’m aware of several theories which attempt to explain it, and I don’t like any of ‘em. So for now I just accept that it really does happen and I don’t know why."

Now as to salt / alkali question, I don’t hunt such ground, never been at a seashore either, so anything I might say would be talking through my hat. No direct experience. While I think most of us know what the general suggestions are… it would be nice if a few of the guys here who do hunt such areas would comment.

I know you’ve got a copy of Jim’s “Advanced Prospecting & Detecting for Hardrock Gold”. He discusses the wherefores in considerable detail starting on pp.19 and 20… then again on pp.25 and 26, and more extensively on pp.31 to 36 with a few suggestions included to deal with those conditions when wet. It sure makes for an interesting read.

There’s no question you’ve hit on the key point Terry. Dave has referred to widespread information to suggest that some amount of rainfall (excepting alkali areas) does improve detection depth. Of course it’s a given there will always be anomalies / exceptions as described in the first example above.

Jim.

 

[scott565]

Thank you for posting the link Jim.
I took a bit of interest in the "do targets move" section.
Here in northern Ohio we of course have freeze -thaw, but quite mild soil with respect to minerals.
What I have observed is that areas where the land gets very wet, targets go deep.
For like items, way deeper than you would otherwise expect.
Around here, you can find coins 100 year old around 5 inches down where the soil is not subject to much hydration.
But a couple of places I detect - an old ballfield adjacent to a river bottom, and a rural bit of yard(not been turned in 100 years) I find much more recent coins at similar depths.
The only real difference in the two kinds of areas is the amount of water in the soil (it is saturated for weeks in a typical year) over a longer period, compared to drier soils where the coins appear at shallower depths.
It's only my observation, but I don't have any other explanation why the more moist soils would present at such different depths than drier ones.

-Scott

 

[Jim Hemmingway]

Good point Scott. We have a park in town that has a low flat area where we used to play summer football as kids. All the ground around it is more elevated, so there is no drainage… surface water must percolate down into the soil. Most of the spring and early summer that area is wet. I’ve experienced the same thing… modern coins found at depths comparable to much older coins at higher elevations.

The other aspect to your comment is that when we do find silver in wet areas, it is always more blackened with silver sulfide… similar to silver coins found in bottom sediments between boulders at the Lake Simcoe. Stir up those sediments and a hydrogen sulfide (marsh gas) odor is produced. I believe similar anaerobic conditions persist in the wet park area for at least part of the year, such that it supports chemical reduction of silver to silver sulfide. Some silver found in lake bottom sediments is so reduced there is very little actual silver remaining… especially with coins that date back to the Victorian era.

Jim.

 

[mjarvis]

Great post Jim!! Lots of info I didn't know, I'll keep looking for your posts!!

 

[Jim Hemmingway]

Thanks Matt… that’s most kind. I feel the same about many of the guys who post to this forum, we’re very lucky to have them contributing here.

An issue for many of us is that we cannot justify buying the “latest and greatest” just for the sake of doing a personal assessment. These forums give us access to others who do use a variety of equipment over variable geological settings and are willing to share their experience. Forums are not ideal at conveying nuances of communication, and sometimes this factor alone can precipitate disputes, but overall they’re a very good format to exchange information and ideas.

As you read posts about metal detectors, keep in mind that results are dependent on the application and search conditions. What works well in one area or set of circumstances does not necessarily confer a similar advantage elsewhere. I tend to evaluate detector suitability for a task in terms of ground mineral magnetic susceptibility, hotrocks, ferrous trash density, and of course... predominant nugget / ore size one is likely to encounter.

Jim.

 

[mjarvis]

Wow Jim your right on the money with that!! So many factors and time needed to apply
that knowledge to get the results. Keep swinging it!! Good luck and thxs again for the information!!