Factors That Affect VDI Readings of Non-Ferrous Targets, page 8

Even though this article is included into a section on features and modes of the XP Deus metal detector, information presented on this page is the "must-know" for ALL metal detectorists using other brands of advanced metal detectors.

1. Effect of Operating-Frequency Change on VDI Readings of Coins (Formula) and Other Non-Ferrous Targets

If you plan to use different detection frequencies (operating frequencies) in your search programs, it is extremely important for you to understand this: a VDI (Visual Display Indication) value of a coin (or any NON-FERROUS target) VARIES from one detection frequency to another (please read my note #1 further below on this page). This phenomenon is reflected by the following SIMPLIFIED formula:

According to this formula, the coin's VDI value is proportional to DETECTOR'S OPERATING FREQUENCY times COIN'S CONDUCTIVITY VALUE times COIN'S DIAMETER times COIN'S THICKNESS. It is obvious from this proportionality that the lower the frequency, the lower the VDI number if other variables remain unchanged. This relation can be observed when the operating frequency is changed during both an Air Test and actual metal detecting. For example, a clad dime (US 10-cent coin, 17mm in diameter) is indicated with a VDI number of 91 at 18 kHz, 88 at 12 kHz, 83 at 8 kHz, and 73 at 4 kHz.

After Air Testing (Bench Testing) other coins of different sizes and conductivities, I noticed almost the same difference between any coin's VDI values obtained at 18 kHz and 4 kHz frequencies - near 20 points. This difference may not be critical for high-conductive and large-sized coins and other non-ferrous valuables, but it certainly is "fatal" for LOW-CONDUCTIVE, small-sized specie and other non-ferrous targets as they can be easily turned into the "drop-out" targets by the 20-point reduction in their VDI values.

For example, if a 14k gold ring has a VDI value of 43 at 18 kHz, its VDI would change to 29 if the frequency is switched to 4 kHz. If you set a second tonal zone ranging from 10 to 30 for low-conductive non-ferrous junk targets and assign a low "iron" tone (100-200 Hz) to it, the ring with a "newly attained" VDI value of 29 will be most likely ignored by you because an audio response to the ring will sound "iron".

If you do not lower the Threshold T2/T3 to 25 after selecting the 4 kHz frequency, you will pass up all gold rings that had VDI values below 43 at 18 kHz.

Of course, this is an extreme example because only an unwise enthusiast would use the 4 kHz frequency for detecting the low-conductive gold rings and small silver coins. However, the example describes exactly what may happen in reality: thin gold chains and their fragments as well as other tiny valuables simply "drop out" of the third tonal zone and "fall" into the second zone. And they may not even get visual indication when current OPERATING frequency is changed to 4 kHz.

The worst situation may occur when the Multi-Notch mode of Conventional Discrimination is utilized. For instance, if one switches from the 18 kHz to the 12 kHz frequency to increase the Detection Depth Range (increase of up to 2 inches is possible) for certain coins (or other targets with similar properties), some coins with VDI values near the notch threshold(s) may end up within the adjacent rejecting notch(es), and the detector's audio responses to them would be muted. By the way, poor Notch-Discriminators are usually incapable of establishing sharp notch thresholds; thus, creating "Rejection Plumes" - expanded Rejecting ranges which discriminate valuable targets with VDI's located in neighboring Accepting zones on the Discrimination/Conductivity scale.

In the same case, if you use the 3-, 4-, or 5-Tone modes of Tonal Discrimination to designate tonal zones for various NON-FERROUS targets, you should change the operation frequency with caution. Just like in the above-described case, desirable coins with VDI's occupying the Threshold-adjoining area may turn up in the lower conductivity zone and, therefore, "sound off" like the unwanted targets when the frequency is changed form 18 kHz to 12 kHz. This change may be quite counterproductive, and this example could probably be another good argument in favor of using the 2-Tone mode of Discrimination.

To sum everything up, if you search for small, both shallow and deep, desirable targets, using the Multi-Tone mode of Tonal Discrimination and/or Multi-Notch mode of Conventional Discrimination, it is recommended to operate your Deus on the 18 kHz frequency and do not switch for lower frequencies unless you really have to... However, here comes my note!

NOTE 1: Since a great feature - ID Norm (in the "Profile" section of a menu), was added to the XP Deus firmware beginning with a version v3.0, all the above-described info concerns only those Deus users who do not set the ID Norm to "yes". The ID Norm was designed specifically for solving the above-described issue with deflation of the VDI values during frequency switching.

If you do not want to utilize the ID Norm, the best way to deal with the Frequency Change effects is to run an Air Test (Bench Test) with samples of coins you are going to search for and write down their VDI readings for each operating frequency. If you memorize them all before detecting these coins, you would not have any of the above-described problems during the search.

If it is problematic for you to air-test various targets due to not having them at hand, you might want to use my 100-Target VDI Chart, in which you will find the VDI values for common US targets that were air-tested with all four operating frequencies.

Operating Frequency Change Has Opposite Effect on Some Iron Targets

Due to ferromagnetic properties of ferrous targets and strong Halo Effect surrounding them, the operating frequency change does not affect them in the same way as it affects the non-ferrous targets. In fact, the effect is quite the opposite! Large/medium ferrous targets and targets made of thin sheet iron, such as beer bottle caps and fragments of roofing, are registered with the higher VDI numbers (95-97) at 4kHz than at 18kHz (83-95) as shown below, even after they get dug up!

Although VDI readings of the dug small- and medium-sized rusty square nails are always in the lowest zone of the Conductivity scale, the VDI readings of the undug iron square nails, which are obtained during real metal detecting, not air-testing (!), might reach the mid-90s. Two US coins with the highest VDI values, a Silver Kennedy Half Dollar and a Silver "Peace" Dollar, are placed into this example chart to show where the most conductive silver coins are positioned on the Conductivity scale in relation to a rusty Bottle Cap and a rusty fragment of roofing (you can see its picture on this page). You can see that a VDI readout of the latter is higher than the silver coins' VDIs, but, unlike the coins' VDIs, it is positioned at the opposite end (4kHz) of the 4-frequency range.

This is just another phenomenon related to the operating frequency change, which could be a manifestation of the Wrap Around Effect. However, many detectorists worldwide found a good use for this phenomenon. The "Switch-To-4kHz" technique is based on it and used for quick identification of the rusty bottle caps, rusty fragments of roofing, large square nails, railroad screw-nuts, horseshoes, axe heads, other large- and medium-sized iron objects, etc. But it works only when the ID Norm is set to "no"!

Unfortunately this technique does not work for the small- and medium-sized square nails that were machine-made in the 19th century. They might fool you! This technique will be described in my article "Switch-To-4kHz Method To ID Bottle Caps and Large Iron Objects" which will be posted soon.

NOTE 2: In the following portion of my article, I will continue to use the term "VDI" even though it might not refer to the VISIBLE VDI's of targets, but rather to their PHASE SHIFT values which are received and registered by the detector's curcuitry upon the target detection. The VDI numbers just visually express the received Phase Shift values of targets. Deep targets usually do not trigger exhibition of their VDI values on the detector's display... Well, this subject is out of scope of this article. Just please keep in mind that the targets' Phase Shift values get affected by negative effects or factors desrcibed below.

2. Mixed VDI Value Effect

The "Mixed-VDI-Value" effect is another type of the VDI value reduction. But, unlike the previously described Effect of Operating-Frequency Change, this effect can not be simply switched off by simply adjusting one setting in your detector's menu. The Mixed-VDI-Value effect is primarily caused by two external factors: 1) Ferromagnetic Disturbance of a Signal, and 2) Hot Rock Disturbance of a Signal.

The first case involves mostly the low-conductive, non-ferrous targets and deep high-conductive targets that are partially masked by iron objects or in close proximity to them. The second case involves the same targets that are partially masked by hot rocks or positioned close to them.

The Mixed-VDI-Value effect is a major "contributor" to the Target Drop-Out occurrence (also described in my article - "Detection Depth Test" (a link to it will be posted soon)) - the "all-the-time-happening" type of Target Missing. The Target Drop-Out occurrences cause the following: many desirable coins and other valuables are either not heard or ignored by users of all brands of metal detectors operated at hunt sites containing high concentration of iron junk and/or hot rocks, and/or excessive ground mineralization.

What does usually happen when a buried coin is in close proximity to a rusty nail or any small iron target? In simple words, the coin's VDI value gets "mixed" with the nail's low VDI value, and a resulting VDI for the coin shown on a display is lower than it is supposed to be. In the "hot rock" cases, the target's VDI value gets affected in the same way.

If the resulting VDI "lands" into an adjacent tonal zone on the Conductivity scale, and this zone is assigned a lower audio tone, not the one to pay attention to, an operator ignores it and... passes up the coin. The same happens when the Rejecting notches of Conventional Discrimination are established on the Conductivity/Discrimination scale, and the coin's "mixed" VDI value "falls" into one of them.

The worst case is when ample Discrimination is utilized, and numerous VDI's of "goodies" fall into a vast Rejecting zone, the good targets are "silenced" and left for the more experienced detectorists. And also, if you use a multi-tonal feature of your detector, e.g. FULL TONES (Tonal Discrimination) or Multi-Tones (Audio Target ID), at the trashy or highly mineralizaed sites, a number of the Target Drop-Out occurrences will increase dramatically, and you will be missing lots of targets without even knowing about it!

Below is an example chart of a few non-ferrous targets' VDIs registered during air-testing these targets both with and without a nail. All targets were placed next to a rusty nail head (shown on this page). Since there are no two identical dug square nails in reality, the resulting VDIs in the pink cells are relative and would be slightly different if the targets were tested with different types of nails.

One should notice that, in this chart, targets of either a small size (a clad Dime) or low conductivity (a Nickel and, therefore, the medium-sized gold rings) have a wider "gap" between their normal and "mixed" VDIs. This explains why such targets, even if not rejected by the Conventional Discrimination settings, may be easily "rejected" by the Tonal Discrimination settings and, therefore, are usually missed at the hunt sites littered with square nails. This is a reason why the low-conductive targets are always present at the "pounded" hunt sites.

How to avoid or minimize the Target Drop-Out occurenses caused by the Mixed-VDI-Value effect?

The first thing you should do is to assign a high-pitched tone to a Tonal Zone on the Conductivity/Discrimination scale, which "harbours" VDI's of desirable targets. Then consider increasing the Recovery-Speed (Reactivity, Recovery Fast, etc.) level, however, this is a "stick with two ends": increasing Reactivity always causes some loss in the Detection Depth. You will be able to improve your detector's audio responses to shallow targets, but the audio responses to some deep targets will become "silent". You can try to increase Reactivity just by one level, and carefully listen for "lifts" - squeaks of the high-pitched tone at the end of "suspicious" low-pitched tone signals.

3. Mineralization Interference Effect

It is also called "MI Effect", "Mineral Effect" and "High Mineralization Effect". This effect takes place in areas containing high levels of mineralization, both natural and man-made, in soil. Because the MI Effect negatively affects both the metal detector's Penetration Depth ability and VDI readings of the non-ferrous targets, it is considered another big cause for accidental rejection of coins and other non-ferrous valuables at the above-mentioned hunt sites.

Metal detectors with less potent Discriminate and Visual Target ID circuits usually suffer the most from this effect, if not disabled by it at all. But even for the most advanced metal detectors (excluding Pulse Induction (PI) metal detectors which generally are not affected by mineralization), analyzing and processing responses to non-ferrous targets through heavy mineralization can be quite challenging. Cases when the target's VDI value turns out to be 10-20 points lower than a "normal" readout are not as bad as the cases when the non-ferrous target's VDI reading can not "bail out" of the iron range on the Conductivity/Discrimination scale. As a result, such low-conductive valuables end up being rejected.

Obviously, to avoid leaving coins and other non-ferrous "keepers" behind in the high-mineralized areas, one should run a "Mineralization Interference/Depth Penetration Test" (a link will be posted soon) for desirable targets prior to actual metal detecting. And then one should factor the obtained test results in while creating any reject/accept pattern(s) on the Conductivity/Discrimination scale in one's custom search program. And this specially concerns areas with relatively uniform mineralization (naturally mineralized ground), which may contain unexpected "hot red spots" - clusters of sporadic or diverse mineralization (resulted from former human activities in the past) containing concentrations of hot rocks, slag, pottery and brick fragments. Sometimes such clusters might be the former sttlement sites and can occupy sizable sections of the field or pasture. A special custom program (similar to the main one for the area) is created for these situations and switched on every time the detector indicates challenging conditions. If one continues detecting with the initially set search program, massive Target Missing will take place.

4. High Salinity Effect

Not being as common as the "Mixed VDI Value Effect", the "High Salinity Effect" nevertheless is another negative factor affecting the non-ferrous target's VDI value. On a beach, in wet sand of high salinity, the target's depth can affect a VDI reading big time! The clad dime lying on the wet sand surface would have a VDI value of 91. If you bury it 4" deep, the VDI read-out would be in the 70's. And at 7" depth, the dime's VDI number would drop to 30! Other coins were tested in the same manner, and similar effects were observed.