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Detector Magnetic Mineralization Measurements


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Occasionally in posts I see reference to mineralization and how it affects detector performance.  It is often vague with usage of words like 'mild, moderate, severe'.  In fact several detectors have the capability of measuring it.  Two of mine -- Fisher Gold Bug Pro and Fisher F75 -- have that capability.  Until now I've typically paid little attention to it.

(Magnetite) mineralization and its effects are clearly important in many detecting for native gold situations, but it's also meaningful for other forms of detecting, if nothing else when comparing notes on detector performance.  Extreme cases we've seen are Tom Dankowski's reports on the depth he gets in the sandy, non-mineralized soils of Florida vs. Steve Herschbach's difficult Reno parks.  An even more extreme example has been reported multiple times by Mark Gillispie in coal ash laden school yards.

Here are the pertinent sections from the F75 and F19 (Gold Bug family member) user manuals respectively regarding the meanings of their magetite measurements:

Fisher-F75_mineralization-msmt.png.f704bcefacb714b98e117c8a65cb1b9e.pngFisher-F19_mineralization-msmt.png.5ad90db7e55ea8d301ff03b975365cb9.png

Personally I prefer the F75 readout since it quantifies the meaning of the bar levels.  Not sure why the Gold Bug scale wasn't equivalently defined, maybe another example of "don't step on the toes of the more expensive model" syndrome....

Yesterday I made measurements in my back yard with both detectors, two coils each (stock Fisher 5" round DD and 7"x11" DD).  In all cases I ground balanced before taking the measurements.  For the F75 the ground phase values in my backyard as a whole were 61 with its 5" coil and 64 with the 7"x11" coil.  In those locations (still F75) the 5" showed 2 bars and the 7"x11" was about 50-50 divided by 2 bars and 3 bars.  The measurements made with the Gold Bug were nearly identical in terms of the bars shown (2 with 5" and 2-->3 with 7"x11") for ground phases of ~64.5 and ~60 respectively)  even though the scales on the two detectors don't really align in terms of the units of measure.

Now here's a possibly interesting and serendipitous observation:  for my teststand the readings were 3 bars in three of the four cases (I missed the measurement with the GB Pro and 7"x11" coil).  If you refer back to the linked thread there is a drawing which shows a concrete layer at 15" depth.  This was made with a store bought bag of concrete mix.  Those contain not only the cement but also sand and gravel filler.  Where did that sand and gravel come from?  I don't know but it sure appears that it came from a site more mineralized than my back yard!

I vaguely recall previous measurements I've made (but failed to record -- my bad) in my back yard where the F75 read 4 bars.  One thing I wonder about is how much soil moisture affects the measruement.  (We haven't had rain for a couple weeks so the ground is considerably drier than typical for the late autumn, winter, early spring months here when I likely took those earlier measurements.)  But there are other variables so I wouldn't conclude that moisture does have an effect.  One would hope not, but certainly dectectors are known to perform differently when detecting in various soil moisture conditions which is why I wonder.

As always I'd be interested in seeing other measurements (regardless of detector model) and comments.

 

 

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