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Underburden
08-10-2011, 06:51 PM
I'm experiencing a loss of threshold and don't know what is causing it. I'll be sweeping an area with a solid threshold then suddenly the sound is gone completely until I pass beyond... I believe they call it NULL.
Hard to locate the source...what mineral would rob you of the threshold? I'm using a discontinued 5x7 coil Steve had left over years ago...the thing finds the tinniest pieces at good depth.
Appreciate your opinions on what is causing the null.

Bob

little man
08-11-2011, 05:19 PM
when you over target you got DIS out you get the null in the threshold hold this help good luck and good hunting

Steve Herschbach
08-11-2011, 05:54 PM
I assume you are in all metal since there is no threshold in the Lobo disc mode. In all metal the unit will null when you get over a spot where the ground is negatively offset from the current ground balance setting. The Lobo ST has automatic tracking so if you work that spot with the coil the threshold will come back, but then you will get a false positive signal when you move off the spot as your ground balance is now mis-adjusted for the general ground condition. Anyway it is either ground mineralization or buried "cold rocks" , the opposite of "hot rocks".

Underburden
08-12-2011, 07:01 AM
Steve, good explanation. Yes, all metal mode. I was going to use the term 'cold rocks' but I didn't think there was such a thing. Thought I would be making up a new word :^)
The material I'm working would be classified as tailings if it had been previously processed but this stuff is raw. BLM hired a contractor to create salmon pools on my claim. The excavator dug over 2 dozen pools right down to bedrock and layed the raw material up on the bank. No motorzied equipment is allowed for mining so we're using hand suckers to work the bedrock. I plan on spending more time with the detector on the raw material just to see what's there.
Thanks for the reply

Bob
Bedrock or Bust (http://bedrockorbust.blogspot.com)

Derek W
08-14-2011, 10:10 PM
I own a super trac also and have experienced the same thing. I believe it was in the literature that came with the unit where it explained how to identify what kind of target I may be swinging over. The null, as you say, I have found to be mineralized rock or ground I guess, every time.

On the opposite side with a positive signal and using the pinpoint switch to accurately locate where to dig, the super trac will go silent when over a mineralized rock.

That's my experience.

Jim Hemmingway
08-15-2011, 08:23 PM
Hi Underburden...your question below...

"Hard to locate the source...what mineral would rob you of the threshold?"

The mineral causing your problem with losing threshold is magnetite whether in the form of concentrated black sand or as negative hotrocks containing magnetite, for example basalt. For a more detailed explanation of ground minerals and detector responses have a look at the link below re: Phase Measurement & Magnetic Susceptibility...

http://www.akmining.com/forums/showthread.php/1345-Phase-Measurement-and-Magnetic-Susceptibility

Jim.