Crucial Concepts in Metal Detecting
(Reprinted from “Metal Detecting for the Beginner” with permission from the
author.)
1. D
iscrimination 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 thepenetration.
• 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 forground 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.
6.
Detector operating frequency
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 specsof 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.