Metal detecting technologies for gold prospecting

This essay pertains to modern name-brand metal detectors designed and marketed specifically for gold prospecting. There are two basic metal detection technologies in common use for gold prospecting -- single-frequency VLF induction balance (“VLF”), and pulse induction (“PI”).  

VLF advantages

In general, the following statements will apply to VLF gold machines, as compared to PI gold machines:

1. Finds tinier nuggets.  In general, the higher the operating frequency, the smaller the gold which can be found.
2. Lower cost
3. Lighter weight, more ergonomic
4. Easier to use
5. Longer battery life
6. Easier to demo in urban environment (less susceptible to electrical interference)
7. Don’t have a “dead zone” in target response causing nuggets in a particular size range to lose sensitivity

Most VLF models offer some type of iron discrimination for dealing with iron trash and/or hot rocks.  Some offer a multipurpose discriminator suitable for relic hunting and coinshooting. 

Note: in electronics, the term “VLF” applies to frequencies below 30 kHz.  However in the metal detector industry we use the term “VLF” to apply to higher frequencies also, because the technology is the same as is used below 30 kHz.

PI advantages

1. Operate in heavy concentrations of magnetite without overloading (saturating) the circuit
2. Greatly reduced response to "hot rocks" and to so-called "red dirt"
3. Most PI's give greater depth on larger nuggets esp. in heavily mineralized conditions

Both PI and VLF have their place in gold prospecting

VLF and PI are very different.  Quite a few gold prospectors use both types of machines depending on conditions. Sometimes a prospector will search an area with a pulse induction to detect what it will detect, and then go over the same area with a VLF to find what it will find that the PI missed.  (Or, may do the VLF first and then follow up with the PI.)

The least expensive PI’s start at about $1500 list price.  If you haven’t got that much money, your choice will be VLF, which runs from about $500 to $1100.  If you want to do relic hunting and coinshooting in addition to gold prospecting, your best bet may be to get a multipurpose VLF which is designed to have acceptable performance in gold prospecting.

If you’re really serious about gold prospecting and can afford a PI, you may decide you prefer a PI to a VLF.  If your budget can withstand it, you might get both a PI and a VLF.  If you have (or get) a PI, the best kind of VLF to complement it will be one with a frequency greater than 40 kHz in order to have the best response to the tiny stuff that the PI misses. 

Searchcoils used in gold prospecting

Concentrics are used on VLF's but not (to my knowledge) on PI machines.  They tend to be a little hotter on the smallest nuggets and to have a less confusing response pattern.   

Monoloops are used only on PI machines.  They tend to be hotter on small nuggets than a DD and are easily manufactured in various sizes and shapes.

DD's tend to be quieter in noisy ground than either concentrics (VLF) or monoloops (PI). Less ground noise makes it easier to hear the signals from gold nuggets.  The search pattern tends to be longer (broader sweep) and narrower (crisper response).  The advantages of a DD are seen mostly in the medium and large size searchcoils.  In small searchcoils there is not so much difference between a concentric and a DD.

Large searchcoils cover a lot of ground and may go slightly deeper on large nuggets.  However they tend to have more ground noise and so the theoretical depth advantage may be lost.  They are not as sensitive to small nuggets, which of course means most nuggets.  (If most gold nuggets were large, gold would be 15 cents a pound and we’d leave the hard work of digging the stuff to giant mining companies.)  Users of PI’s tend to favor large searchcoils.

Small searchcoils are quieter in noisy ground, and therefore go nearly as deep (or even as deep) as a medium size searchcoil.  Small searchcoils are more sensitive to smaller nuggets, and make it easier to pinpoint the location of targets needing to be dug.  In rocky or shrubby areas, a small searchcoil can get into places where larger searchcoils won’t go.  Users of VLF gold prospecting machines tend to favor medium and small size searchcoils.

Elliptical searchcoils tend to give a broader sweep and a crisper response than a round searchcoil.

Round DD searchcoils tend to go a little deeper than an elliptical in mineralized ground, but their response is not as crisp and they have more tendency to lose the smallest nuggets. 

Round concentrics, while popular for coinshooting, are not often used for gold prospecting except where a round concentric happens to be the standard searchcoil furnished with the detector and the user doesn’t have a more suitable accessory coil.

Ground balance in VLF's: manual, "grab", and "tracking"

The advantage of manual ground balancing is that it is under user control at all times. The disadvantage is that accurate manual ground balancing requires some skill, and it's a skill that some people find it difficult to learn. 

"Tracking" is in a sense an attempt to automate what a skilled user would do manually.  It is however not the same thing.   A good tracking system "tracks" fairly fast into changed ground conditions, yet is moderately resistant to being thrown off balance by hot rocks or metal.  The tracking systems presently on the market range from primitive to fairly sophisticated. Even the sophisticated ones tend to be thrown off balance by hot rocks or metal when you hover over them to "check the target".  The response changes with each sweep and the target may disappear entirely, making it impossible to tell what's there.  ……The primary advantage of "tracking" is that it unloads the task of ground balancing from the user.  Advertising which implies that tracking is always continuously balanced to the matrix and is therefore superior to manual balancing, is "misleading" (to use a polite term, I had a different word in mind).

"Grab" systems are constantly collecting ground data in a computer, but not doing anything with it until you push the button or pull the trigger.  The ground data are then moved to the part of the software that does ground balancing.  This approach eliminates the problem with mistracking on hot rocks and metal targets. In highly variable ground you may be fairly busy pushing the button or pulling the trigger, but at least you can get a consistent response because you've eliminated all the questions about what the tracking system thinks is down there and how fast it's changing trying to catch up and to catch up to what.  

Manual, tracking, or "grab"-- none of these is "right" or "wrong", it's a matter of preference – just like some people prefer a car with automatic transmission, other people prefer manual shift.  For the last several years the engineers at FTP-Fisher have preferred "grab" systems: most of these have also provided manual ground balancing capability as well.  This preference is the result of the Chief Designer’s prior experience designing what are probably the best "tracking" systems on the market today, and having seen in the field what their limitations are.

Ground balance in PI's is based on principles different from those of VLF's.  Modern gold prospecting PI's balance signals from two different delays in order to cancel response to lossy magnetic minerals.  The balance  may be fixed internally, adjustable manually, or "tracking".  The scheme does not work perfectly and causes some loss of sensitivity to metal signals, but is nonetheless advantageous in ground which contains substantial concentrations of iron minerals of varying composition. 

Threshold control on VLF units

Most VLF prospecting is done in an all-metals mode which may be variously described as "single-filter", "first derivative", "autotune", or by trademarked names.  In this mode, a metal target gives a "zip" sound. Nearly all VLF gold machines provide a threshold control to vary the loudness of the zero-signal background hum.  This is usually set high enough that normal background noise does not cause a valid signal (if present) to become inaudible.  If there's a lot of ground noise that can't be eliminated by ground balancing, a few users will adjust the threshold into the negative region so that the machine runs silently until a signal strong enough to poke its head above the threshold is present.  This is generally inadvisable because of the loss of sensitivity to valid signals which results, and because without hearing the ground you can't tell if you're properly ground balanced. Reducing the gain (which may be labeled “sensitivity”) is usually a better tactic than running below threshold.

Discrimination on VLF units

Most VLF gold machines offer some type of discrimination for identifying iron or hot rocks.  Discrimination is usually by a distinct iron tone, rather than by silencing as would typically be the case in relic hunting or coinshooting.  Discrimination usually entails some loss of sensitivity: therefore one doesn't normally search in discrimination mode, the discrimination mode is used primarily for checking a target already located in all metals mode. 

Some VLF machines are designed for general purpose metal detecting use including gold prospecting.  These machines have a discrimination system designed primarily for relic hunting and coinshooting.  How usable it is for gold prospecting varies between models.

Threshold control and discrimination in PI’s

The various models of PI’s are so different from one another, it is difficult to generalize about how threshold should be used or what kind of discrimination they offer.  It can be said that none of the PI’s from major manufacturers offers the sort of discrimination features which are found on many VLF’s because PI’s do not readily distinguish between ferrous and nonferrous metals.  It can also be said that the ground balancing methods used in PI’s which makes them so effective in “hot ground” tend to reduce response to metallic iron and also to reduce response to nonferrous metal targets in a certain size range.   The poor discrimination characteristics of PI’s is a primary reason why no general purpose metal detectors use this principle.

What’s in the future?

This essay reflects the industry state of the art as of April 2009.   In the last few years the metal detector industry has seen major advances in basic PI, single-frequency VLF, and multifrequency VLF detection technology.  There have also been major improvements in how signals are processed in software and in how information is presented to the user.  Progress hasn’t stopped: in the future there will undoubtedly be models which overcome some of the limitations of today’s technologies.  Sorry, I can’t say exactly what or how!

-- Dave Johnson

   Chief Designer, First Texas Products and Fisher Research Labs