Testing Weak Gold Targets & Ground / Salt Settings

[Steve Herschbach]

Gold targets overlap the ground balance range, and eliminating either salt or any ground, eliminates a certain class of gold target. In PI nugget hunting circles this is known as “the hole”, when nuggets get eliminated by a certain ground balance setting. This happens with low end targets like a small gold nuggets, thin gold chains, single post ear rings, and broken gold rings. The results you get depends on the ground balance or salt elimination settings (when available - many detectors have a salt setting, most do not). You can air test for this by using a single digit target id item, and testing with salt balance on or off. Also test same item, running ground balance to both extremes. Test both settings separately, and in combination. Note that at certain ground balance settings the item may get weaker, or disappear entirely.

This does not apply just to small targets, but any target that has a weak signal. Any large item buried deep enough becomes a weak signal, so in theory all targets regardless of size are affected by this, if they are at fringe detection depth.

Single frequency detectors without a salt setting can balance to salt, or to ground, but not both at the same time. Whether they can balance to salt depends on how far the range of the ground balance control has been extended. A machine that is ground balanced to eliminate a salt response may get a weaker, or no response, to many single digit id targets.

Multi frequency machines can eliminate both salt and ground at the same time, but this can actually work against you, by doubling the chance of some targets being missed, as you now have two “holes” to deal with instead of one. Advanced processing can compensate for this to some degree, however, by comparing results in both channels, and accepting targets that exceed certain parameters, bringing some missed items back into the game. This stuff can get tricky and complex on the processing side, invisible to all but the engineers who designed the circuit.

What this means is a person on a low mineral locations will see different results that people in high mineral locations, not just because of the mineralization, but because the more aggressive ground cancellation settings may be affecting the target. Most metal detector testing and reporting is inherently flawed, because people do not report Fe3O4 results, and use ground balance settings as a flawed methodology for determine how bad the ground is. Comparing detector results without knowing the exact ground composition is a crap shoot at best, and often almost a waste of time.

A White’s MXT example, many machines similar….

 

From the manual:

The Salt position provides an extended ground balance and tracking range to compensate for conductive salts also called alkali. Ground rejection against salt/alkali slightly overlaps the lower end of the conductive target (metal) range. In other words if you ground balanced against significant salts, some loss of sensitivity to lower conducting metals (metals low on the VDI target scale) can be expected. The advantage and performance improvements of rejecting the salt, however, far outweighs any loss. Because the Salt TRAC setting can track well into the target range it is not recommended for normal conditions, only for areas known to contain salt. Salt water beaches for example or alkali desert regions. The Ground setting will not track into the nonferrous metal region. The Salt setting will. The MXT 300 ground rejection system is capable, in both the Ground and the Salt settings, of considering some iron a ground mineral. (Emphasis added).

More on gold and saltwater

[mcjtom]

On 2/14/2022 at 11:09 PM, Steve Herschbach said:

Multi frequency machines can eliminate both salt and ground at the same time, but this can actually work against you, by doubling the chance of some targets being missed, as you now have two “holes” to deal with instead of one.

Would it be safe to assume that the GB adjustments on MF detector such as the Nox would mostly adjust the location of the ferrous ground type hi-pass filter, as opposed to adjusting the range of the the other filter trying to ignore the non-ferrous salt water ground response, in general?

I guess my logic is that, depending on the composition of the ferromagnetic minerals in the ground, the location of their 'time-constant' changes, but the wet salty ground signal time constant is relatively stable - just the intensity differs?

[Steve Herschbach]

2 hours ago, mcjtom said:

Would it be safe to assume that the GB adjustments on MF detector such as the Nox would mostly adjust the location of the ferrous ground type hi-pass filter, as opposed to adjusting the range of the the other filter trying to ignore the non-ferrous salt water ground response, in general?

I guess my logic is that, depending on the composition of the ferromagnetic minerals in the ground, the location of their 'time-constant' changes, but the wet salty ground signal time constant is relatively stable - just the intensity differs?

You can speculate as you wish or as seems logical. Unless the engineers involved reveal the process, which they won’t, all we can do is speculate. Personally, I work more in the realm of observing what adjustments work best for me in the field. The Tarsacci is a good example of a detector where people are using the salt balance for things I’m not sure it was ever intended before. Now that things are done in microprocessors via what are essentially computer programs, some of the classic thinking as applied to analog circuits no longer holds. If these things are of great interest to you, I’d suggest joining Carl Morelands Geotech forum, for more in depth discussion by a decidedly more tech oriented crowd. At the end of the day I’m certainly no engineer, and don’t much care how the guts gets things done as long as they get done. But maybe somebody else will chime in with their thoughts on the question.

A long winded way of saying “I don’t know”