Crucial Concepts in Metal Detecting
(Reprinted from “Metal Detecting for the Beginner” with permission from the author.)
1. Discrimination and notch filtering
Discrimination is the ability of the detector to distinguish between
one target and another. In most cases this translates into being
able to distinguish between a coin and trash, such as between a
quarter and a bottle cap. The composition of the object being
scanned has an effect on the signal received by the metal detector.
This effect is called phase shift. A positive, or leading, phase
shift indicates a highly conductive target, such as a silver dime. A
negative, or lagging, phase shift generally indicates junk, such as
a rusty nail. An adjustable discrimination dial lets the operator
tune out and reject signals from the junk targets.
The discriminator dial, in effect, blocks any response from the
detector for targets with a phase shift at or below the level you
select. Unfortunately, when you tune out the pull-tabs from soda
cans you also tune out nickels and some jewelry. Experience
adjusting the discriminator setting then becomes an important part
of the learning curve for the novice coin-shooter. Some advanced
metal detectors (more expensive) let you tune out only certain
portions of the phase shift spectrum. This is called “notch
filtering.” For example, you could tune out most of the pull tabs
and still get nickels with careful notch filtering.
As you might guess, discrimination is extremely valuable in the
field, where you don’t want to spend a lot of time digging up
garbage. In some situations, such as relic hunting, discrimination
might not be as crucial, since relic hunters will often choose to
dig up everything made of metal.
Detector controls with DISCrimination and NOTCH filtering dials shown at left. This detector uses sounds for target identification.
2. Target identification (TID)
Target Identification is closely related to discrimination. Modern
discriminating metal detectors can tell you if the target is a
copper penny (minted before 1982) or a zinc penny. It can tell
between a silver dime (minted before 1965), and what we call a
“clad” dime, one that is a composite of copper and nickel.
The output of the discrimination circuitry can be an audible tone,
with a high pitched tone for valuables and a low grunt for junk.
This is called tone identification. The output is more commonly a
meter reading, or a numeric value called a VDI number which appears
on a screen. VDI stands for Visual Discrimination Indicator.
Discrimination output sometimes uses multiple modes to alert the
operator. A high-pitched tone will indicate a high-conductivity
target, while the LDC display shows both a VDI number and a probable
target. On White’s metal detector, for example, a nickel might
appear as “VDI = 18 Nickel” on the display, or “VDI = 80 Dime,
Penny.”
Target Identification is not always accurate. The error rate tends
to go up when the target is deeper in the ground.
This type of detector uses a display to show target identification using VDI numbers.
3. Sensitivity
Sensitivity refers to the ability to detect metal objects from a
distance. It is something akin to the volume control knob on an
amplifier. Set too low, you won’t hear the music. Set too high and
you get distortion and chatter. Most detectors with sensitivity
control have a mark on the control panel where the manufacturer
recommends you set the dial. Under certain conditions, such as areas
of high mineral content, or near power lines, it may be necessary to
lower the sensitivity to cut down on noises and false signals.
4. Search depth
The depth that your machine is able to search for coins depends on a
number of factors. These include:
• Detector operating frequency. The lower frequencies have better
depth.
• The strength of the magnetic field created by the detector; the
stronger the field, the deeper the
penetration.
• The size and shape of the search coil. Generally the larger the
coil, the deeper it can search. Some coils
employ two separate D-shaped windings. These “DD” coils distort the shape
of the magnetic field and enable
deeper searching.
• The composition of the ground being searched and its electrical
characteristics. This is usually referred to
simply as “mineralization”, but it may include such variables as how
tightly the ground is compacted, the
presence of water, and the
chemical makeup of the soil.
• The shape, size, and orientation of the object. A dime buried on
its edge, for example, would be more
difficult to detect than one laying flat (parallel to the surface).
"How Deep Will This Coil Go?"
The truth is, nobody can really say with certainty, due to the
variables mentioned above. To be on the safe side, consider the half
and half idea: You can easily find coins down to a depth of half
your coil diameter. If you’re using an 8-inch circular coil, you’ll
easily find coins at 4 inches. The other half is tricky. Under ideal
circumstances – good ground conditions, finely tuned detector,
experienced operator, a flat-lying silver coin, and fresh batteries
– you might make up the other half, so you could get down to a full
8 inches. It’s in this borderland region, between half the coil
diameter and the full diameter, where Target Identification suffers,
the signal becomes weak or intermittent, and your skill and patience
at detecting pays off. This of course is just a general observation.
There are experts in this hobby who will swear they can find coins
much deeper than 8 inches using and 8-inch coil.
5. Ground balance
All soils contain some degree of metals, such as iron, which can
interfere with a detector’s ability to find coins. Ground balancing
allows the detector to see past these minerals. There are different
ways to set ground balance:
• Factory preset ground balance. Here the manufacturer sets the
ground balance at a predetermined level,
which they estimate will
work adequately in most searching environments. This “fixed” ground
balance may
be a less desirable compromise, but it lowers costs and
works well in normal soil conditions.
• Automatic ground balance. Many modern detectors now employ
circuitry which detects and adjusts for
ground effects
automatically.
• Manual ground balance. Manual ground balance requires the user to
adjust for ground balance at the start
of each searching session.
This is a simple procedure where the user adjusts a setting on the
detector
with the coil alternately up in the air, then near the
ground. Some detectorists prefer manual ground
balance, at least as
an option, since it provides greater control in prospecting, where
changes in ground
conditions are important for assessing the
presence of gold-bearing ore.
• Mixed or combined methods of ground balance. Manufacturers often
allow owners to select manual or
automatic ground balancing, simply
by flipping a toggle switch on the detector. As mentioned above, in
some circumstances, such as prospecting for gold, the owner may want
more control over settings on the
detector. Think of it as automatic
transmission versus a manual clutch.
Detectors use very low frequency sine wave oscillations in the coil
to detect their targets. The vast majority of detectors on the
market today use an operating frequency between 3,000 and 20,000
cycles per second, or hertz. The typical land-oriented detector
might use an oscillator at 7,000 cycles per second, abbreviated 7k
Hz.
For purposes of selecting a detector, the crucial point to remember
is:
• The higher the frequency, the easier it is to detect small
objects, such as BB-sized relics or minute specs
of gold. Higher frequencies, however, are less able to penetrate deep
into the ground.
• The lower the frequency, the deeper your detector can scan, with
the same sized coil. Lower frequencies,
however, are less efficient at picking up very small objects.
Detector operating frequency is not like a computer clock speed,
where the faster it goes the better. The different frequencies are
more suited for specific purposes. For high frequencies, imagine
shooting sewing needles into the sand. They are good for picking up
tiny objects, but they don’t go very deep beneath the surface. Next
shoot hundreds of nails into the sand. They go deeper than the
needles, and are good for detecting coin-sized objects. Finally,
shoot a few large tent stakes into the sand. They penetrate much
deeper, but are not good at finding tiny objects. They are suited
for finding larger objects.
Some manufacturers now produce detectors which generate multiple
simultaneous frequencies for the same coil. This type of detector
will span a much greater spectrum of both depth and object size. The
Minelab Safari metal detector, for example, boasts 28 frequencies.
Although this improves your detecting options, it also adds to the
cost of the detector.
Target ID using VDI numbers. Here a +22 indicates a nickel or a ring.