Metal Detectors

Question 1

Metal detectors

Answer 1

• Uses Man-made (active) electromagnetic field which is
  applied to the ground via a coil
• Measures induced Magnetic field in metal
• The Man-made electromagnetic field is made by
  alternating currents in a coil
• Described by frequency f in Hz or wavelength λ in m
• The electromagnetic field induces a secondary current
  in metal objects which drains energy from the original
  electromagnetic field
• This alters the original current in the original coil
• Induction - occurs over short distances and varies with
  metals
• Accuracy in position = 10 cm
• Maximum depth = 50 cm
• Light weight and reliable
• Active method - limited distance operation
• Field weakens by a factor of (Distance)^3
• Signal weakens by factor of (Distance)^6

Question 2

Uses of Metal detectors

Answer 2

• Originally designed for the detection of land mines
• Used to detect metal weapons
• Can be used to find coins
• Detection of Archaeological artefacts
• Detection of a gun buried in the woods
• Locate small metal objects through, vegetation soil,
  concrete
• Can detect all common metals
• Able to detect a 0.22 calibre bullet at 5cm depth
• Can only find a body if there is an associated metal
  object
• Target size pertains to surface area facing metal
  detector (not mass)

Question 3

How are different metals distinguished?

Answer 3

• The Balanced induction on the original coil is disrupted
  due to the presence of a metal
• The disturbance type is related to resistivity of the metal
• Which can then be used to distinguish between metals
  found

iron = 10.1 μΩ.cm
Aluminium = 2.7 μΩ.cm
Copper = 1.7 μΩ.cm

Question 4

How does a metal detector work?

Answer 4

• There is a stem (pole), control box and a search “head”
• The search head contains a transmitter coil and receiver
  coil
• An alternating current is sent to the transmitter coil and
  generates an electromagnetic field
• The electromagnetic field travels into air, ground and
  metal targets
• Induction in metal targets causes power loss and a
  secondary field to be created
• The effects of this are measured by the receiver coil and
  converted into an audible signal

Question 5

Types of Metal Detector

Answer 5

1. Pulsed Induction (PI)
2. Beat frequency oscillator (BFO)
3. Very low frequency (VLF)

Question 6

Searching with a metal detector:

Answer 6

• Detection area = cone shape under search head
• Overlapping sweeping required to avoid targets passing
  outside cone
• Sweep lines = 1/3 to 1/2
• Sweeps should be conducted in one direction and then
  the transverse direction
• Search head should be 3 to 5 cm above ground to
  reduce noise from soil

Question 7

Search Head types:

Answer 7

MAIN FACTOR = SIZE OF SEARCH HEAD
- Large = e.g 60cm - good for seeking deep in the ground
Can only detect large objects
- Medium = 20cm - Recommended for forensic searches
Jewellery or coins
- Small = < 10cm - Can pinpoint small objects at shallow
depths
MOST SENSITIVE PART = target centre:
- Co-axial = stacked coils - reduces noise, target centre
directly under centre of search head
- Co-planar = overlapping coils - reduces size, target
centre is towards front of search head

Question 8

Pulsed Induction (PI)

Answer 8

• After the pulse, the decay of secondary field is picked
  up by the receiver
• Limited duration 0.5 millisecond pulse
• Suitable for all metals and all terrains (including
  underwater)
• Unable to distinguish between metals and minerals
• Good sensitivity, similar to that of VLF metal detectors
• Extremely sensitive to iron and iron-containing metals
• Suffer from high power consumption
• Not good for pinpointing small targets
• Good depth penetration
• Can detect single coins at depths of 20 to 30 cm

Question 9

Beat frequency oscillator (BFO)

Answer 9

• Two frequency oscillators, one of which is a control
  frequency, other frequency oscillator is in the search
  head
• Secondary field from a metal causes change in
  frequency
• The beating of “control” and “search” frequencies
  produces an audible signal
• Low cost and simple to use
• Suitable for general use under all conditions
• Uses a high frequency so depth penetration is limited
• Can discriminate between metals and minerals
• Less sensitive to iron than other detectors, so less affected by background
• Can detect single coins at depths of 10 to 20 cm

-Used more in mineral prospecting than metal detection

Question 10

Very low frequency (VLF)

Answer 10

• Contains two coils, transmitter and a receiver coil in
  which induction occurs
• In the absence of metal, the two coils are balanced
  (balanced induction)
• Secondary field from a metal will cause imbalance
• Very good sensitivity to non-ferrous metals (i.e. not iron)
• Can discriminate between metals and minerals
• Can distinguish different types of metals
• Can be set to screen out background noise due to soil
• Commonly used in coin hunting
• Cannot be used in water or wet conditions
• Very low frequency = good depth penetration
• Can detect a single coin at depths of 30 to 40cm
• Recommended for Forensic use