KNOWLEDGE BASE ARTICLE

Zero Voltage Transmission (ZVT) Technology

Basics of the GPZ 7000 Technology:
Zero Voltage Transmission (ZVT)
By Bruce Candy

Basic Metal Detection Principles
1

All metal detectors transmit (generate)changing magnetic fields from their coils (1).
When these changing fields interact withmetal targets they cause electrical currents toflow in the targets called eddy currents (2).

2

3

4

Simplified
representation of thebasic principles ofmetal detection.

These eddy currents then generate theirown magnetic fields that are different to
the magnetic field transmitted by the metal
detector coil (3).
The transmitted field also causes soils togenerate magnetic fields different to thosetransmitted by the metal detector anddifferent to those from the target’s eddy
currents (4).
The metal detector coil senses (receives) themagnetic field from the target eddy currents,
and also the magnetic field generated by
the soils. The coil actually measures how fast
the magnetic field changes rather than how
strong the field is. For example, it will not
respond to a fixed unvarying magnetic fieldeven if it is strong.
The coil will also detect changing fields fromthe environment, such as electromagnetic
interference from nearby mains electricity andeven lightning from thousands of kilometresaway (4).

The soil signals have predictable types of
responses caused by the transmitted magnetic field, and thus
can be recognised by the processing electronics and cancelled
out; this is called ground balancing. Any received magnetic
signals in response to the transmitted field that are different tothose directly transmitted and also different to those from soils
are assumed to be caused by a metal target and are reported tothe operator.

Ground Balancing
The capability of metal detectors to effectively ground balancevaries considerably between technologies and models. Many
detectors only provide approximate ground balancing rather
than true ground balancing.

KNOWLEDGE BASE ARTICLE | KBA 24-1Signals generated in soils by the transmitted magnetic field
mostly consist of three different types:
Reactive Soil Component – By far the largest; a signal that
is identical in form to the transmitted signal. This is called
the Reactive Soil Component and often referred to as the
X component.
Viscous Remanent Magnetism Component – Signals frommicroscopic magnetic particles, about a mere 30 millionthsof a millimetre in size, produce a complex signal that can be
described as being a ‘delayed response’ from the transmitted
field and dependent on all of the past transmitted field, butmuch more related to the recent transmitted field than further
back in time. This is called Viscous Remanent Magnetism (VRM).
It is usually responsible for most of the ground signals that may
not be completely ground balanced, if any.
Saline Component – Eddy currents in saline soils often cause
most of the un‑ground balanced ground signals in higher
frequency VLF gold detectors (more so than VRM). However, in
PI and ZVT detectors, the receive signal processing is designed
so that signals from saline eddy currents are only noticeablefrom more highly saline soils. These eddy currents occupy alarge volume of ground and can be determined if present
because they cause un‑ground balanced (audio) signals if thecoil is moved up and down several feet above the soil surface(except if a Minelab GPX series or SDC 2300 gold detector is set
to a ‘salt cancelling’ ground balancing mode).

Ground Balance Technology Comparison
The biggest problem is trying to cancel out the signals from thesoil, which can be very large compared to the weak signals fromdeep metal targets.
Very Low Frequency (VLF) – VLF sinewave detectors have amajor disadvantage in having to ground balance out all of thesoil signals simultaneously (all three soil components above).
Pulse Induction (PI) – PI metal detectors have a majoradvantage of not even detecting the major soil signal; theX component, but only the saline components, but this ismostly insignificant, and the VRM component , which is justa very small percentage of the X component. This makes thecapability of PI technology to ground balance far more accurate
than VLF detectors. A disadvantage of PI is its lesser ability todetect very small nuggets compared to VLF detectors.

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Zero Voltage Transmission (ZVT) – ZVT has the same majoradvantage as PI for ground balancing in not detecting the
major soil component X, but has the same advantage of CW
metal detector technologies compared to PI for detecting verylarge nuggets because ZVT is CW (unlike PI that is not CW), andalso the same advantage as PI for being relatively insensitive tosaline soils compared to VLF.

What is the ZVT transmitted field, andhow does it differ to PI?

Magnetic Field

Both ZVT and PI measure a signal (receive signal) immediately
after a very rapid change in magnetic field. During the receiving(measuring) period, PI does not transmit a magnetic field atall, whereas ZVT transmits an exceptionally constant magnetic
field (until the next very rapid change in magnetic field occurs).
The ZVT transmitted field alternates between transmitting
very steadily in one direction; e.g. North Pole pointing into theground; then rapidly switches to South Pole pointing steadily
into the ground, then back to North and so on.

Time
Figure 1. ZVT time varying transmitted field.

Why the term ZVT?
A more complete definition actually would be Zero Reactive
Voltage Transmission. The transmit coil Reactive Voltage refers
to voltage associated with the transmit coil winding that is
proportional to changes in the transmitted magnetic field.
Signals detected directly from the transmit coil are given thesame name ‘reactive‘ because they are directly proportional to
the transmit coil reactive voltage.
The ZVT transmitted field is designed to be as constant(unchanging) as possible during the receive periods; this meansthat the transmit coil’s reactive voltage must be zero during
these constant field periods, and thus too is a receive signal
directly from the transmit coil’s field. Hence Zero Reactive
Voltage Transmission (ZVT) refers to this ultra‑stable transmitted
magnetic field period during which receiving occurs.

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Similarly, the signal from the soil reactive component, X (mostly
soil ‘ferrites‘), also produces zero receive signal if the transmit
coil reactive voltage is zero; just the same as during PI detector
receive periods.

Receive Signal (Resistive Component)

Another disadvantage of PI is its capability at detecting verylarge nuggets compared to CW metal detectors, such as ZVT.
CW means Continuous Wave and includes all technologies
that do not have zero transmit periods (almost all technologies
other than PI).

 ZVT VRM soil signal
 bipolar PI VRM of same frequency andrapid change in current periods

 large metal target (long TC) medium metal target (medium TC) small metal target (short TC)

Time
Figure 2. The receive signal from the sensing coil following a very rapid change inmagnetic field, measured during the constant current period. The Time Constant or
TC of a metal target is basically how quickly the eddy currents decay.

Figure 2 shows a comparison between the ZVT VRM soil signal
and the nearest equivalent PI soil VRM signal; both transmitted
magnetic signals being bi‑polar, with the same fundamental
frequency and same rapid change of magnetic field period. It
should be noted that the PI receive period is half that of the ZVTperiod, because the PI system shown in figure 2 transmits forhalf the time and receives for the other half of the time, unlike
ZVT that transmits and receives simultaneously just about all of
the time. The important difference in the VRM signals is that thePI signal decays away substantially faster than the ZVT system.
This indicates that the receive signal for PI is less sensitive to
longer Time Constant (TC) components compared to shorterTC targets than ZVT, and this is one of the main reasons why
ZVT technology is better at detecting large nuggets comparedto PI; another main reason being from the double length
receive period in ZVT compared to PI for the same fundamental
frequency.
A significant technical achievement of GPZ 7000 ZVTtechnology was in creating the ultra‑stable transmitted
magnetic field during the receive period, to ensure that thelarge reactive component of the soil signal, X, is not detected, inthe same way that it is not detected in PI.

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The GPZ 7000 coil must be symmetrical about the left‑right axis
because ZVT transmitted signals cause a type of signal to be
produced from the ground which is absent during PI receiving
periods. This ZVT receive signal is from something called soil
magnetic hysteresis. If an asymmetrical (un‑symmetric) coil likea Double‑D is used by a ZVT detector (asymmetrical because
the transmit coil is on one side and the receive coil on theother), a signal is produced which depends on the speed
at which the coil is swept over the ground (and how closethe coil is to the ground), and the audio tone will be higher
going in one direction (for example left to right), and lower
when sweeping in the other direction, which would clearly beunacceptable.

Two symmetric Double‑D
windings, with a central
transmit winding.

To avoid this problem, the GPZ 7000 uses
a coil consisting of two symmetricD‑shaped receive coils, one on the left
and one on the right of a central ovaltransmit winding. This Super-D coilwinding geometry means that fortargets close to the coil’s surface, thetargets produce a double audio response
as the coil is swept over the target, thesame response separately for each of thereceive coils. For targets further from thecoil’s surface, the coil behaves more likea traditional coil, with the peak audio
response occurring directly below thecoils central axis.

The GPZ 7000 coils are specificallydesigned to have minimal response to
scraping and knocks to the coil housing,which can cause annoying false signals
when in use; e.g. some PI coils suffer from this. While this knockand scraping insensitivity feature increases the weight of thecoil, it significantly improves performance through minimisingfalse signals.
Double audio response
for shallow targets, single
response for deeper
targets.

Performance advantage of the GPZ 7000
To measure the performance of any detector depends on
many factors, such as: particular detector settings, coil sizeand configuration, ground type, mineralisation levels andtype, electromagnetic interference, gold nugget size andcomposition, and of course, operator skill. Figure 3 shows thepercentage increase in depth of a GPZ 7000 compared to the
GPX 5000, using the same sized coils (14-inch). This data was
obtained at several different soil locations and conditions inAustralia.
For these measurements:•• A GPX 5000 using a Monoloop with Fine/Enhance timings isused when testing in (highly) mineralised soils.
•• A GPX 5000 using a Double‑D with either Normal or Sharp,
whichever gives the best depth for each nugget tested(referred to as ‘GPX 5000 Normal’), is compared to the
GPZ 7000 Difficult + General in moderately mineralised soils.

KNOWLEDGE BASE ARTICLE | KBA 24-1•• The GPZ 7000 General is compared to the GPX 5000 Sharp
or Normal (again referred to as ‘GPX 5000 Normal’) using amonoloop in moderately mineralised soils.
60

GPZ 7000 depth advantage compared to GPX 5000 (%)

The GPZ 7000 Super‑D Coil

50

40

30

20

10

0

-10
0

5

10

15

20

25

30

Nuggets with decreasing weight from left to rightGPZ 7000 High Yield + Difficult / GPX 5000 Fine + Monoloop (mineralised soils)
GPZ 7000 General + Difficult / GPX 5000 Fine + Monoloop (mineralised soils)
GPZ 7000 High Yield + Normal / GPX 5000 Normal + Monoloop (low mineralisation)
GPZ 7000 General + Normal / GPX 5000 Normal + Monoloop (low/moderate mineralisation)
GPZ 7000 General + Difficult /GPX 5000 Normal + Double-D (moderate mineralisation)

Figure 3. The percentage depth advantage of the GPZ 7000 versus the GPX 5000 for
the same diameter coils.

Here the vertical Y‑axis is the percent advantage, and thehorizontal X‑axis from left to right, is for a steadily decreasing
mass of the nugget tested, going from number 1 being for a 20
ounce nugget, to number 30 for a 0.13 gram nugget.
The X‑axis is not drawn to any scale, merely listing nuggetsthat were available for testing in decreasing weight order. The
depth advantages of nugget numbers above 30, (30–35 being
between 0.13 and 0.05g) are off scale; above 60%, and thus notshown on this graph.
The reason why the data is so scattered is because the twodifferent technologies respond differently to how fast theeddy currents change; due to different Time Constants (TC).
The Time Constants vary considerably between nuggets
even if they have the same mass; that is, whilst the X‑axis list
decreasing nugget mass, this does not necessarily correspondto continually decreasing Time Constant. As can be seen, thedepth advantage varies considerably from nugget to nugget,and setting to setting, but the general advantage of ZVT isclear; mostly distributed between 0–40% improvement,
and even more.

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It should be noted that the GPZ 7000’s Normal Ground Type is not
equivalent to the GPX 5000 Normal. Simplistically, the GPZ 7000
Normal behaves most similarly to a GPX 5000 Normal (or also Sharp)
timing when using a monoloop coil but with the ground noise
being typically less for the GPZ 7000 when using General Gold Modethan the GPX 5000 using a Monoloop. With the GPZ 7000 using theHigh Yield Gold Mode with the Normal Ground Type, the GPZ 7000
will have somewhat more ground noise than the GPX 5000 using a
Monoloop and Normal.

Default Settings
A significant improvement of the GPZ 7000 comparedto the GPX 5000 for most prospectors will probably beusing the default settings of High Yield plus Difficult compared to
the GPX 5000 most commonly used settings in average (Australian)mineralisation soils, namely, Fine plus a monoloop. These areshown as red circles in figure 3.

GPZ 7000 Gold Mode and Ground TypeGold ModeHigh Yield
High Yield is better for medium or small nuggets.
The transmitted field switches pole direction (N–S or
S–N) three times more often than in General mode.

GeneralGeneral is better for detecting large or medium
nuggets, however some very small shallow targets
may be missed.

Extra Deep
Extra Deep is better for detecting in highly
mineralised soils (soils that produce more audio
signals than average). However, it is not as sensitive
to very small nuggets as General or High Yield.

Ground Type
NormalThe Normal setting should only be used in lessmineralised soils. This gives the best depth, but atthe cost of significantly more ground noise. Normal
plus High Yield is suitable only for low mineralisation
ground types, but finds small/medium nuggets
deepest of all.
See ‘Saturable Soils’ tip.

Difficult
Difficult is best for average (Australian) mineralisedsoils. This plus High Yield will most likely produce
the most nuggets for average mineralisation and is
the default setting.

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SevereSevere is ideal for the most mineralised soils (where
the audio signals from the soil in other settings are
impractical to use). This setting is not recommended
for seeking very large deep nuggets.

Saturable Soils
In any of the above settings, it pays to swing thecoil an inch or so above the soil surface if the soils
are considered saturable (VRM). Saturable meansthat a detector ground balances well if the coil is raised
and lowered down to about a few centimetres above thesoil surface, and for the worst saturation, down to several
centimetres, but not if the coil is swung up and down to aheight lower than these saturation ‘height thresholds’ (e.g.
down to the soil surface.) This is discussed in further detailbelow.

Searching and Ground Balancing
The Instruction Manual tells you how to do this, but here issome insight into what is happening.
After the detector is switched on, it calibrates itself to theground within several seconds upon moving the coil near the
ground (see important tip below) and pressing the Quick‑Trak
trigger button. This initial fast calibration involves several
different aspects of the detector being calibrated, not just theusual ground balance.
Upon further searching, the ground balance continually
tracks or ‘updates’ the ground balance moderately quickly,
but the other parameters calibrated during the initial groundbalancing are far more slowly updated. Thus, if you happen to
initially ground balance on an atypical location (e.g. too close
to a metal target or over an unusual patch of soil), the initial
calibration may end up being inaccurate, and this will cause
extra audio signals when detecting. This is because the slowly
updating calibration is taking a longer time to become moreaccurate to the typical conditions (rather than the initial atypicalconditions).

Ground Balancing - VERY IMPORTANT
The best way to ground balance initially after switch onwith the Quick‑Trak trigger depressed, is to sweep thecoil in a typical side‑to‑side search mode at the expected operatingheight above the soil surface, e.g. 2–3 cm or whatever the soil
saturation or terrain will allow, but at the same time move forward
at slightly faster than normal walking speed, so as to cover as muchdifferent ground as possible in the first 10 to 12 seconds. The idea
during this initial period after turn on, is to expose the detector toas much different soil data as reasonably possible to improve theinitial calibration.
IMPORTANT! DO NOT ground balance using a coil up and down
motion (like one would do using a PI detector) initially after turnon with the Quick‑Trak trigger depressed. This does not give theinitial calibration enough different soil information to calibrate
optimally. If ground balance is required again sometime later (butwithout turning the detector off ), ONLY THEN is the usual (PI typeof ) ground balancing up‑and‑down coil motion OK.

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Ground Balance ResetIf you think the detector is producing an
unexpected level of soil signals, switch thedetector off, then on again, and re‑ground balance. Usually
this time your detector will be correctly calibrated from therestart.
If your detector is still producing an unexpected level ofsoil signals, select Quick Start on the Detect page, and thenselect Reset Audio and Detector settings when prompted.
The guide will then prompt you to perform a groundbalance again.

frequencies: Rather, if the resistive VRM component is measured
and plotted on a graph with (linear) resistive signal magnitude
on the Y‑axis, versus the logarithm of frequency for the X‑axis,
the graph is an almost horizontal straight line, but slightly
tilted away from the horizontal. Graphs with tilted straightlines plotted with a linear Y‑axis versus a logarithm scale of avariable for the X‑axis are referred to as ‘log‑linear.’ The relative
slope of tilt varies for each different soil location, sometimes
considerably over even short distances, sometimes hardly at alleven over longish distances. Hot rocks have different slope tilts
compared to the soil matrix in which they are buried; this is why
they are detectable.
1.2

Shallow Soils

To find deeper larger nuggets, you need to seek out deeper soils.
Generally, many of the most well known gold fields that haveoften been detected with the GPX 5000 tend to be shallow (butby no means all), whereas the deeper fields are generally not sowell‑known because overall fewer nuggets (mostly fewer small/medium nuggets) have been found in these fields.

Saline Soils
Whilst the GPZ 7000 does not have a dedicated ‘salt’detection setting (saline soils), the best Gold Modesetting for salt soils is Extra Deep.

1

VRM Resistive Magnitude

When searching in previously well-detected shallow
soils that have been previously detected with a
GPX 5000, it is likely all of the larger gold nuggets have been foundbecause it is easily detectable at shallow depths, and only thesmaller pieces remain. Hence in these locations, only operate theGPZ 7000 using High Yield, and do not use Extra Deep because thisdoes not detect small gold well.

0.8

0.6

0.4

0.2

0
0.1

1

10

100

1000

Frequency in kHz (logarithmic scale) Standard model of soil VRM resistive component; log-uniform Minelab model of soil VRM resistive component; log-linear.
Example 1; slightly less low frequency than high Minelab model of soil VRM resistive component; log-linear.

What is the technical difference between
Difficult or Severe Ground Types, andNormal?
Difficult and Severe are considerably less sensitive to spatialdifferences in ground balance conditions compared to Normal;that is, the optimal ground balance setting does not change
from meter to meter as one passes the coil over different soils
using Difficult or Severe, whereas the optimal ground balancesetting varies considerably more using Normal for different soil
locations, even as little as centimetres apart. In addition, thedegree of (VRM) soil saturation is considerably less for Difficult
or Severe than Normal.
Minelab has patents for the Difficult and Severe ground balancemethod. Simplistically, the standard ground balance methods
and the scientific literature assumes that the VLF (sinewave) soil
VRM received signal that is different to the transmitted signal,
called the ‘resistive’ signal, has the same relative magnitude
for all transmitted frequencies. The GPZ 7000 Normal groundbalance method, and all PI ground balance methods indetectors prior to the GPX 4000, assumed that all soils behaved
as such. However, the reality is that soils do not have resistive
VRM signals that are the same for different VLF transmitted

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Example 2; slightly less low frequency than high, and less overall
log-uniform component than the graphs above

 Minelab model of soil VRM resistive component; log-linear.

Example 3; slightly more low frequency than high, and more overall
log-uniform component than the graphs below

Figure 4. Different resistive VRM soils.

In figure 4, the X‑axis is frequency in kilohertz drawn as alogarithmic scale. The Y‑axis is the magnitude of the VRM
resistive component measured using a VLF metal detector
with variable transmit frequency. The standard (old) model
is a horizontal line with zero slope and is called ‘log‑uniform’,
whereas in reality, soils exhibit log‑linear resistive VRM
responses. The examples shown are exaggerated for illustrative
purposes.
The relative variations in the slope of the log‑linear resistive soil
components cause the VRM signal decay signals following the
rapid change in transmitted field direction to vary. This is shownin figure 5.

Page 5Received VRM signal for different
resistive log-linear soils

10.9

 ZVT VRM soil signal for resistive log-uniform soil;
the standard soil model

0.8

 Resistive VRM log-linear example 1 (exaggerated); slightly

0.7
0.6

more longer TC soil components than short Resistive VRM log-linear example 2 (exaggerated); slightly
more short TC soil components than long

0.5
0.4
0.3
0.2
0.1
0

Time following rapid change in magnetic field
Figure 5. GPZ 7000 VRM receive signal decay signals

In figure 5, the standard model of Log‑uniform resistive decay
is shown as the red curve. The purple curve is the response fora negative slope tilt for figure 4; more low frequency than high,or, in other words, relative more long time constant signals thanshort, whereas the blue curve example is the opposite. The
examples shown are exaggerated for illustrative purposes.
The Normal ground balance method in both the GPZ 7000
and GPX 5000, and all ground balanced methods prior to the
GPX 4000 did not balance out these log‑linear resistive signal
relative tilt variations simultaneously, whereas, the Difficult
and Severe GPZ 7000 ground balance settings, and Fine,KNOWLEDGE BASE ARTICLE | KBA 24-1

Enhance and Smooth for the GPX 4000, 4500, and 5000, and
SDC 2300 do balance out these log linear relative tilt variations
simultaneously. If for example, Normal is ground balanced toany one of the soils exhibiting the decay curves of figure 5, thenit will not be ground balanced to either of the other two; rather,
it will require re‑ground balancing. But Difficult or Severe will besimultaneously ground balanced to all three soil types in figure
5, without any re‑ground balancing required. Hence Difficult
and Severe (and Fine, Enhance and Smooth) typically produce
far less ground noise than the Normal type of ground balancemodes.
As the metal detector coil is moved towards a soil, thetransmitted magnetic field in the soil gets stronger. This causesa (very) small degree of VRM signal ‘saturation’ that happens tocause the resistive signal relative slope of the tilt to change. This
is why the amount of VRM soil saturation is far less for Difficult
and Severe than Normal. Soil saturation often requires the user
to operate the coil several centimetres above the soil surface forbest results (see ‘Saturable Soils’ tip).
However, whilst soils do have resistive signal that are very
accurately log‑linear, unfortunately this is not perfectly accurate
for some soils, and, because the GPZ 7000 has such very highsensitivity, even miniscule deviations in the straightness of theline of the log‑linear resistive signal will cause ground noise
signals. Severe is less sensitive to these miniscule deviations
than Difficult.

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