Metal Detecting in an A-Star?

[Geowizard]

A good tip and good advice!

The question for our purposes here is; "HOW?"

The answer to the question involves increasing levels of understanding to get there from here! To begin with, it should be safe to assume that anyone that is reading this thread has a basic understanding of WHAT a metal detector is and how to operate a metal detector. From a purely recreational point of view, a hand-held metal detector is fine for walking about looking for nuggets.

Large Deposits;

If a person has an interest in finding a large deposit of GOLD or other valuable minerals, a LARGER system is needed. A hand-held metal detector is too limited. Depth of penetration is limited to a foot or two at the very best. Alaska is covered with Tundra that is several feet deep. Overburden including dirt, sand, and gravel may be tens of feet deep. The physics that allow a metal detector to detect metals is directly related to the size of the coil, the energy produced by the coil and the sensitivity of the detector.

The Response;

Metal detectors measure and respond to objects within the limits of detection. A nugget must have "contrast" compared to the surrounding ground. Usually a nugget is more responsive and therefore, it has contrast. A larger nugget represents a larger mass and provides a larger response. There are certain physical characteristics that improve the response. The response is said to be "quantitative" because the response is proportional to the "quantity" of mineralization. The response is controlled by "depth" of the mineralization. Deeper mineralization is more difficult to detect. The response is reduced by 1/4 as the depth doubles.

The response and frequency;

Metal detectors are designed to operate at specific frequencies. A conventional TR metal detector Transmits a waveform. The Receiver is designed to receive the transmitted wave. Most of us probably are already aware of that. The frequency of the TR metal detector is important. Low frequency can penetrate deeper than high frequency.

Two coil TR systems;

The Whites TM-800 Treasure Master is a good example of a metal detector. The advantage of separation of the transmit coil and receive coil is that the area between the coils increases. The area (volume) of earth that contributes to the response is greatly increased.

Without duplicating 100 years of literature on the subject, suffice it to say, "It works!"

Next, the scaled up version.

Stay tuned...

- Geowizard

[Micropedes1]

I first started my detecting with a GB 2, which is about as good a surface VLF detector as you can get (in my opinion). But it is limited to surface, or near surface, depths of a few inches, which is fine. I bring a rake and a garden hoe, ‘cause I am not going deep.
Bigger targets and locations where I know that will be doing some shallow digging, I will bring the Whites GMT or MXT. Both are fine machines (different frequencies for different size gold). With them I bring a trowel or small digging tool and a bucket to dump some of the material from the bottom of the hold (found lots of fine gold that way)
Cool weather, high mineralization, hot rocks, and hard hit areas, I bring out the Minelab 5000. With it I also bring a short shovel and also something cold to drink because I am going to be digging some pits, knee deep or better. That can be hot work; sometimes rewarding. But still lots of conductive trash in the upper strata that must be sorted thru and carried out. Think about metal detecting where a city kid has been shooting rocks with a 22 and you get a strong desire for solitude in hard to reach places!
But stuck back in the corner is a seldom used Garrett GTI 2500 with a 2-coil Treasure Hound set up. VLF technology. Good for finding pennies at the park. I don’t use it much anymore. And that’s a shame. Because it is really good at finding conductive metal objects at depth. When I bring that thing on a prospecting trip, I DRIVE up close to where I plan to work. When I locate a suitable target, I plant an umbrella shade and drag up a cooler of chilled beverages and plant the wife in a lawn chair. The coil spacing on the 2-coil set up is about 1 meter, which is about how deep that thing reads, unless you start playing with the frequency. Have you ever dug a 1 meter hole? In rocks and cobbles? I’m good for one, maybe 2 in a day’s time. With lots of pick-action and cussing (usually) before me and that #2 shovel part company. Somewhere In there the wife gets tired of watching and starts throwing rocks at me and back into the hole, generally telling me that its time to pack it in. Come back tomorrow and finish (without her)! Found some good gold. Some larger gold with that thing. But at depths that none of the other detectors can reach. Virgin ground at its finest!

[Micropedes1]

I kinda hate to admit it, but Chuck lit a fire under my a$$ with the discovery of those area-wide EM surveys in Alaska. It took a lot of studying to finally understand them enough to utilize another engineer’s interpretation of the data. And even more to go back to the raw data and my own analysis (without all the averaging of data). For all of that, you have my thanks. There is a lot of barren ground in my prospecting area that I no longer need to sample.

But now that I know approximately where to look along those ¼ mile spaced flight lines, how do I go about finding and recognizing the mineral outcropping that may be as much as 30 feet deep? You see, I am only interested in those higher frequency, shallow deposits that I can mine economically as a surface deposit. That shallow EM strata is something like 30 feet of depth. Try reading that with a hand-held metal detector! Or digging the hole with a #2 shovel!

[Geowizard]

An electronic hole;

Excellent lead-in question - Thanks, Glen!

At least two possibilities exist. Let's get Geophysical...

If you can shoot a squirrel with a 22... then you can shoot a deer with a 7mm mag... It's all about target size and selecting the proper tool for the job.

We can do this.

The "Electrical Method";

I mentioned on the prior Alaska Gold Forum that there are methods of measuring the reciprocal, using a DIY system that YOU can build at home. You need four stakes, and a voltmeter. One other component for the purist is a "switch".

Drive the stakes in the ground along a line with equal distance between the stakes. The distance controls the depth of penetration. Greater distance = greater depth. IF the stakes remain centered on the SAME point and the separation is incrementally increased, it is possible to do a Sounding.

Connect the the two outside stakes while measuring. Measure between the two inner stakes. Calculate by using the formula. "G" is called the Geometric Constant. The Geometric Constant depends on the "Geometry" of the stakes.

Next... "Geometric Constant"

Stay tuned...

-Geowizard

[Geowizard]

Wenner spread;

There are different geometries for placement of the stakes.

The configuration is one possible geometry:

If the distance measurements used in the formula are in feet, the apparent is stated in ohm-feet.

If the distance measurements used in the formula are in meters, the apparent is stated in ohm-meters.

Other configurations are possible.

Stay tuned...

- Geowizard

[Jim_Alaska]

many years ago I discovered EM imagery quite by accident. I used to use false color imagery for looking at remote places I might like to run a trap line, or to find cabins. These maps we available through the University of Alaska-Fairbanks.

Quite by accident I found that they had EM imagery also. These were single images on a single page and they were expensive. Now, with the computer age in full swing it makes scanning these images affordable and easy, right from our desk top.

[Micropedes1]

A uniform symmetrical spacing is all well and good on flat terrain, but not always possible in the real world of mountains, rivers, and gullies. So, a slight modification is in order:
a………………..b……..c………….....……………d
voltage across [bc] and current across [ad] as per Geowizard’s drawing.

The constant (K) for your formula is (distance [ab] times distance [ac]) then divided by the distance [bc]. This pretty much approximates your cross sectional area.

Then your resistivity is calculated: R = K * V[bc]/I[ad] If your spacing is laid out in meters, then units will be in ohm-meters. Or if you wish to measure conductivity, simply take the reciprocal (1/R) for an answer measured in siemens per meter (S/m).

Please bear in mind that conductivity and resistivity surveys are influenced by soil permeability, salinity of the water table, and permafrost to name just a few that you will need to consider when dealing with Alaska terrains. I thought that I had struck it rich with one of these surveys only to discover a bitsy coal seam snaking thru the permafrost. Darn!

[Micropedes1]

Jim, do you mind divulging the location of those UA maps that you were referring to earlier?

[Geowizard]

The other options;

There are systems that fall under two headings.

I bought a gently used system. It can operate and the spacing is variable. Systems are available (as rentals) from companies. Their system is for prospecting.

- Geowizard

[Geowizard]

Here's How?

Navigate to the website...

Select the survey desired.

Now we're ready to get started.

- Geowizard