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Black Sands Again...


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I just read "phrunt's" post.:  December 23, 2018 in Minelab Equinox Fan Club

...about black sands.  I did post on another forum about this and I hope I am not violating an etiquette but the topic is recent here and comments helpful.

I am newly using the Anfibio Multi but beginning to realize perhaps this detector was not the best choice and perhaps a PI better.  I liveaboard my boat on Vancouver Island pretty much year round voyaging the entire coast Vancouver to Alaska including islands. Today, first time using on the beach the AM could not settle down over regular looking sand interspersed with layers of black sand. Since most all of my detecting will be coastal coves, harbours and their beaches most all with black sands I am wondering what detector would be more suited. 

This was said by poster "phrunt" which was interesting with regard to making the "right" choice for my hunting grounds.

"I find the fine coatings of black sand to be the most difficult no matter if I use a multi frequency or a PI."

 

Thank you
 

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Use the detector to find good deposits of black sand .......

Then set up a high banker sluice and run that sand with fine gold recovery mats.....

You'll come out ahead with the gold vs most trinkets you may find which will likely be junk i the areas you speak of.

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I have also heard there are some gold rich beaches on Vancouver Island...

I don't know what the regulations are that you might have to deal with though but might be worth looking into.

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On 1/25/2019 at 6:09 PM, SeaScene said:

I am newly using the Anfibio Multi but beginning to realize perhaps this detector was not the best choice and perhaps a PI better.

As much gold as johndoe gets working black sands for fine gold maybe we should all get shovels! :smile:

Beach detecting is challenging, and a main difference is that between Florida style white sand beaches and Oregon style black sand laden beaches. Black sand does not refer to color actually. It refers to magnetic minerals like magnetite that are black in color. If a beach is black in color and nothing happens when you drop a magnet it - this is not black sand. On a true black sand beach if you drop a magnet on the beach, a huge load of sand comes up with it. Magnetic sand, laden with magnetite.

Here is a picture showing magnetite layers on a beach. The sand is mostly quartz and feldspars, non-magnetic minerals. Magnetite as an iron ore is considerably heavier, and so wave action concentrates it into layers. See johndoes thread above for more examples of layering.

magnetic-black-sand-beach-magnetite.jpg
Black sand (magnetite) layers exposed on beach

magnetite-black-sands-on-beach.jpg
Closeup of magnetite black sand. Note dark gun metal granular nature of the sand. Highly magnetic.

I currently face a similar dilemma. I beach detect and I prefer a VLF for the discrimination. However, on the west coast you can lose half your depth or even more due to black sand concentrations. Single frequency detectors cannot ground balance to both saltwater and black sand at the same time. You can balance to the sand, or the water, but not both. Multifrequency fixes this problem, but severe black sands still affect multifrequency machine adversely. In fact severe magnetite will even cause a hit in pulse induction performance.... just not as bad as the other options. Pulse induction detectors lose less depth in extreme mineralization.

There are many dry land prospecting machines that might do the trick, like the Minelab PI detectors. However, for salt water beach use I personally insist on waterproof detectors and so I leave those dry land units for others to consider.

The waterproof pulse induction field is very limited at this time. It divides into two classes. Pulse induction metal detectors that ground balance, and those that do not.

A pulse induction (PI) detector by its nature tends to ignore mineralization, so much so that in milder conditions a PI works fine without a ground balance circuit. As I noted above however a PI is not immune to mineralization. A non-ground balancing PI detector will sound off when raised and lowered over true black sands. The more concentrated the magnetite, the more intense these signals will be. The bottom line is that on real bad black sand beaches even a basic pulse induction will sound off if the coil height is varied too rapidly over the beach. In the water with troughs and depressions false signals are all but impossible to avoid.

The most extreme situations require a ground balancing pulse induction (GBPI) metal detector.

To sum up:

  1. Pure white non-magnetic coral beaches - most any detector will work well
  2. Even a hint of mineralization - a multifrequency detector has an edge over single frequency VLF where there is both saltwater and magnetic minerals.
  3. Moderate mineralization - you want multifrequency or basic pulse induction (non-ground balancing PI)
  4. Severe mineralization - at some point a ground balancing PI (GBPI) is required.

The above conditions grade from one into the other seamlessly.

fully-submersible-waterproof-underwater-metal-detectors-pulse-induction-pi.jpg
Fully submersible pulse induction metal detectors

Here are the current mainline waterproof PI detector offerings:

Bounty Hunter - no PI
Garrett - Sea Hunter Mark II (PI) and ATX (GBPI)
Fisher - no PI (one on the way?)
Minelab - SDC 2300 (GBPI)
Nokta/Makro - no PI
Teknetics - no PI
Tesoro - Sand Shark (PI)
White's - Surfmaster Dual Field (PI) and TDI BeachHunter (GBPI)
XP - No PI

Finally, here are the key specifications for comparison:

fully-submersible-waterproof-underwater-metal-detectors-pulse-induction-pi-comparison.jpg
Fully submersible pulse induction metal detectors

Waterproof VLF Metal Detectors Compared

 

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I’m 3 hours from a beach and have zero plans on buying a waterproof ground balancing PI.  And yet, due to Steve’s thoroughly informative, detailed, linked up, charted up response I find myself being wiser than I was 10 minutes ago. 

You do an amazing job Steve, thanks  ?

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