Why Pulse Induction metal detectors work so well for gold detecting

22 Nov 2010
So as not to make this article enormous, if you see apparently strange new terms or acronyms, please consult our Terminology reference.
By now, I would imagine that many metal detector operators have come up against the problem of mineralised ground when looking for gold. This is ground that produces noise from a conventional detector. Many gold fields around the world contain mineralised ground to varying degrees. In Australia, the gold fields contain exceptional levels of mineralisation.
CW or VLF metal detectors such as a Eureka Gold or X-TERRA 705 are able to ground balance in this mineralised or hot ground but with some limitations. This ground balance works fine in so-called homogeneous ground, that is ground that shows very consistent mineralisation. In these grounds the detector can operate quietly and the operator hears relatively little ground noise.

A pulse induction machine however can deal more effectively with much more difficult, variable ground conditions. Unlike a CW machine that transmits and receives simultaneously, a Pulse Induction detector generates a periodic magnetic pulse that excites the ground. During transmission of a pulse, the sensitive receiver is turned off, then turned on a short time after the transmitter is turned off. This means that the very strong instantaneous response from the ground, to the changing magnetic field, is avoided as the receiver is turned off during this time. A target is then much more obvious amid the much smaller remanent magnetic response of the ground. Because of this a PI detector is not exposed to many of the ground issues that a CW detector attempts to deal with. In particular a PI detector can operate in more variable ground together with hot rocks etc.

R&D metal detecting
Even with this huge PI advantage in hot, variable ground, your GPX detector then makes use of some unique technologies including DVT and MPS to remove this remanent magnetic response whilst still maintaining maximum depth. The timings, or sequence of transmission and reception are created in such a way as to exploit the fundamental physics of the ground response and so cancel the majority of the remanent response. The ground balance process in the detector is then responsible for mopping up the last vestiges of ground noise, leaving a clean signal free of ground noise so that even small, deep targets can be clearly heard.


G'day Phil as always an excellent blog. It's interesting to see the detector I use through a technicians eyes, every time I turn on my Minelab I know there is more going behind the scenes than just what I'm hearing through the speaker/headphones. Your blog helps cement some of those concepts more firmly in my mind.

It is interesting your comments RE Homogenous ground types, I've revisited areas as you have described from the old XT 17000 days (one area only just recently with the GPX 5000) and was surprised by the amount of gold still missed by the VLF detectors!

Is there a reason for this or was the ground a lot noisier than I first thought?
Posted By: Jonathan Porter on November 23, 2010 07:04pm
I just gained access to 80 acres of land outside Fox Alaska that are covered w/ tailings piles from the old dredging days here in fox. Through research, I discovered that the old dredge screens were set @ 1" or larger! This means that any gold 1" or less went directly into the tailings piles!!! What is the best detector to purchase and scan these piles with? ( They are high Iron, quartz and heavier metals content.
I am in the process of attempting to lease all 80 acres from BLM here, as I anticipate having a blast, and maybe even finding some "Tossed Aside" gold due to the Fed. not being interested in 1" or smaller nuggets! L.O.L.
Any suggestions? DR.
Posted By: on January 23, 2011 03:10pm
Sorry for the delay in responding I did not know Phil was out on holiday. You have what sounds to be an excellent opportunity for finding some real lunkers, who knows that retirement nugget may be in those piles as well.
I would love to hunt those piles with the GPX 5000 and an 18inch DD coil. The reason being is that the 18in DD coil will offer you excellent depth and allow for some discrimination capabilities along with being able to handle the concentrated iron soils that are sometimes related to that type of hunting.

I have hunted piles in Alaska and other Western states and I have done quite well over the years. I have generally found that the piles offer great depth for a detector as many of them are pretty well devoid of soil from the washing. I have however found some piles that were extremely concentrated in black sands (iron soils) that have always presented a challenge to many detectors.
The GPX 5000 should walk right through the piles with no problems at all. Plus having a little discrimination is very useful in these piles as there always seems to be relics, ok trash from the past like blade shavings belt parts and blade teeth. Not to mention the occasional tin can tossed in.

I would never advocate the complete and total use of discrimination but it does come in very handy in a way to check a dig sometimes before opening it up into a creator.
Starting settings for the GPX 5000 in this type of hunting;
Set the detector in the Sharp timing moving to Sensitive Extra only if the ground minerals prove to be to challenging, set threshold, auto tune, and ground balance. Match the gain to the ground, set your tone to your optimal hearing point, set discrimination level and go hunt. That all takes about a minute and a half then send us pictures of the massive nuggets you find.
Best regards,

Posted By: Kevin Hoagland on January 28, 2011 07:21am
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