Phase angle based EMI object discrimination and analysis of data from a
commercial differential two frequency system
Claudio Bruschini*a,b, Hichem Sahlib
aÉcole
Polytechnique Fédérale de Lausanne, EPFL–LAMI, CH–1015 Lausanne, Switzerland
bVrije
Universiteit Brussel, VUB-ETRO-IRIS, Pleinlaan 2, B-1050 Brussels, Belgium
ABSTRACT
The EPFL and the VUB have been investigating the response of metal detectors within the framework of humanitarian
demining research activities, in particular frequency domain systems. A simple circuit model has been looked at first,
followed by the analysis of a more complete model. As has also been stressed before, this analysis indicates the possibility of
identifying some metallic objects. In addition, the phase shift of the received signal turns out to be a continuous,
monotonically decreasing function of the object size; this leads to the idea of imposing a “phase threshold” in order to reduce
the amount of detected clutter. This discrimination-based approach is less ambitious than object identification, but is likely to
be more robust and to work when looking for metallic objects of a certain size, e.g. non minimum-metal mines or UXO.
A first series of measurements was therefore carried out using a commercially available, differential two frequency metal
detector, the Förster MINEX 2FD. The detector’s internal signals have been recorded in a laboratory setup along linear scans
varying different object parameters for several representative objects. The collection of data as a function of movement
enables the possibility of analysing the data in the complex plane, and makes it possible to exploit global object properties.
Some representative results are presented and the limits of such discrimination/identification approaches briefly outlined.
Keywords: Humanitarian demining, metal detector, complex plane, phase angle, Förster MINEX, metallic object
discrimination, electromagnetic induction, mines, UXO
1. INTRODUCTION
The École Polytechnique Fédérale de Lausanne (EPFL) and the Vrije Universiteit Brussel (VUB) have been investigating the
response of metal detectors within the framework of humanitarian demining research activities. The details and peculiarities
of mine detection and clearance in humanitarian demining – see Figure 1 for examples of typical AP (antipersonnel) mines –
can be found in a number of references1,2,3 and will not be repeated here.
1.1. Frequency domain metal detectors (electromagnetic induction devices)
Electromagnetic induction devices (“metal detectors”) work usually in the VLF (Very Low Frequency) part of the
electromagnetic spectrum, up to a maximum of some hundred kHz3. Here we will simply recall that the secondary field
B sec ( r, t ) generated by the target object depends, both temporally and spatially, on a large number of parameters (see also 4):
the problem’s geometry, the object’s properties, the primary field, and the background signal (in particular the ground itself!).
Metal detectors can be subdivided in Frequency Domain, or Continuous Wave (CW), and Time Domain systems; the first
make use of a discrete number of sinusoidal signals, very often just one, the second work by passing pulses of current through
a coil5. In the following we will concentrate on the analysis of data from a Frequency Domain instrument which employs
separate transmit/receive circuits, analysing in particular the behaviour of the phase response.
In the case of single frequency Frequency Domain instruments the secondary (or received) signal V sec ( t ) = A sec sin ( ωt + ϕ )
(see for example 6) will vary sinusoidally as a function of time at the angular frequency ω of the transmitted (or primary)
signal, but in general phase shifted by ϕ with respect to the latter. The received signal can also be written as:
V sec ( t ) = A sec sin ( ωt + ϕ ) = V R sin ωt + V X cos ω t where V R = A sec cos ϕ and V X = A sec sin ϕ .
*
(1)
C. B.: Tel: +41 21 693 3911, Fax: +41 21 693 5263, E-mail: Claudio.Bruschini@epfl.ch. H. S.: Tel: +32 2 629 2916, Fax: +32
2 629 2883, E-mail: hsahli@etro.vub.ac.be. Web: http://diwww.epfl.ch/lami/detec/ & http://etro.vub.ac.be/minedet/
Copyright 2000 Society of Photo-Optical Instrumentation Engineers.
This paper was published in [SPIE AeroSense 2000, Detection and Remediation Technologies for Mines and Minelike Targets V, 24-28
April 2000, Orlando, FLA, USA, Proc. SPIE Vol. 4038, paper [4038-156]] and is made available as an electronic reprint with permission of
SPIE. One print or electronic copy may be made for personal use only. Systematic or multiple reproduction, distribution to multiple locations via electronic or other means, duplication of any material in this paper for a fee or for commercial purposes, or modification of the
content of the paper are prohibited.
Figure 1: Metallic content of a Russian PMN (left) and a LI11 minimum metal mine (right, with detail of striker pin)
A
sec
VR is the component of the received signal in phase with the transmitted one (“in-phase” or resistive component), VX is the
component of the received signal dephased by 90 degrees with respect to the transmitted signal (“quadrature-phase” or
reactive component); both representations are clearly equivalent. It is therefore customary to represent the received signal
Vsec, seen as the complex quantity V sec = V R + iV X , i= – 1 , bidimensionally in a complex plane (or impedance plane) as
a point of coordinates Vsec=(VR,VX), as shown in Figure 2.
The amplitude Asec, and often
V(t)
REACTIVE Component
Vprim(t)
the
phase shift ϕ too, will in
(QUADRATURE-PHASE)
fact
vary when the detector and
VX=Asec sinϕ
the target object are moved
Asec
Vsec = VR + i VX
with respect to each other,
Vsec(t)
containing information on the
tanϕ= VX/VR
target’s nature. Equivalently,
t
the point corresponding to
Vsec=(VR,VX) will move in the
ϕ
RESISTIVE
complex
plane
along
Component
VR=Asec cosϕ
ϕ
characteristic
lines,
which
we
(IN-PHASE)
will
analyse
in
the
following.
Complex (Impedance)
Transmitted and Received
Plane representation
signals as a function of time
Figure 2: Representation of the received signal as phase vector in the complex (impedance) plane
1.2. Background signals
The target signal is accompanied by an omnipresent background signal, made up of contributions due to electromagnetic
(EM) background and to drift effects. Both do not seem critical in the analysis of the Förster MINEX data, also because of its
differential setup. What seems important is the effect of the soil itself, in particular in areas such as sea beaches or otherwise
conductive ground, or strongly mineralized regions which can be conductive or iron rich. The soil’s conductivity is indeed
small compared to that of metals, but is distributed over a much larger volume. The resulting signal can therefore be far from
negligible. In such regions the detector’s performance can be seriously degraded, especially when looking for small signals;
background rejection becomes essential and often relies on measuring the ground signal when no metallic object is present,
to then “follow” and suppress it as well as possible. Background rejection is usually more difficult in nonhomogeneous areas.
1.3. A simple circuit model
A theoretical analysis based on a simple circuit model5,7,8 already points to the possibility of identifying some metallic
objects based on their characteristic phase response. It also shows that the phase shift of the received signal is a continuous,
monotonically decreasing function of the circuit’s response parameter α. In the following we will investigate a more realistic
model, whose response parameter will depend on the object size; this leads to the idea of imposing a “phase threshold” in
order to reduce the amount of detected clutter (in particular the one due to very small or poorly conductive objects) when
operating in certain scenarios. Both approaches (identification and discrimination) will be in the back of our minds.
2. RESPONSE OF A SPHERE IN THE FIELD OF A COIL
(s)
The response V of a homogeneous sphere of radius a,
conductivity σ and permeability µ (µ=µ0µr), placed in the
field of a circular transmit coil of radius RT, as measured
PRIMARY LOOP
by a circular receive coil of radius RS, can be calculated
RT
exactly (“quasi-static” solution, only displacement currents
are ignored7). The coils are coaxial and placed on the
SECONDARY
sphere’s axis at a distance dT and dS from the sphere’s
RS
LOOP
r0
centre as shown in Figure 3. The operating frequency is ω,
dT
and the transmit coil is traversed by a constant current I.
Time dependence is implicitly assumed as eiωt.
dS
Similar theoretical considerations are also given in 9.
Simulations for linear scans and variations of the
geometrical and material parameters have also been
Y
reported in 10, using technical specifications representative
σ, µ, a
of the Förster MINEX metal detector.
X
Note that throughout this analysis we will assume a
Figure 3: Geometry for the calculation of the response of a sphere
constant relative permeability µr. In fact this is far from
in the field of a coil (adapted from 7). Size and position of the
obvious as µr can depend on the operating frequency and
primary and secondary loops are not representative.
in particular on the strength of the applied field11.
(s)
It can be shown that the induced voltage V can be
written as a rather complicated sum of products of geometry and frequency dependent terms as follows ( P n1 are associated
Legendre polynomials and In+1/2(ka), In-1/2(ka) are modified Bessel functions of the first kind):
Z
V
(s)
2
2 1⁄2
2
2 1⁄2
∞
2n + 1
P n1 ( d T ⁄ [ d T + R T ] )P n1 ( d S ⁄ [ d S + R S ] )
RS RT
a
------------------------------------------------------------------------------------------------------------------------ X n ( ka ) + iY n ( ka ) ,
= 2πiµ 0 Iω ---------------------------2
2 1⁄2
2
2 n⁄2
2
2 (n + 1) ⁄ 2
( d T + R T ) n = 1 2n ( n + 1 )
( dT + RT ) ( dS + RS )
∑
[ ( n + 1 )µ r + n ]I n + 1 ⁄ 2 ( ka ) – kaI n – 1 ⁄ 2 ( ka )
- and k 2 a 2 = iσµωa 2 = iα .
with χ n ( ka ) = X n ( ka ) + iY n ( ka ) = -------------------------------------------------------------------------------------------------n ( µ r – 1 )I n + 1 ⁄ 2 ( ka ) + kaI n – 1 ⁄ 2 ( ka )
(2)
(3)
The adimensional expression χ n ( ka ) plays the role of response function and contains all the frequency dependence (apart
from the external ω factor) as well as the dependence on the object properties (apart from the a2n+1 term). The response
parameter is k2a2=iσµωa2=iα (adimensional and purely imaginary). Note that χ n ( ka ) is parametrized by the relative
permeability µr. The rest of the previous equation contains the geometrical factors, i.e. the relative geometry of the sphere and
the coils. We shall define in particular as “geometry term” gn the adimensional expression:
2 1⁄2
2 1⁄2
2n + 1
P n1 ( d T ⁄ [ d T + R T ] )P n1 ( d S ⁄ [ d S + R S ] )
a
-.
g n ( d T, R T, d S, R S ;a ) = ----------------------- -------------------------------------------------------------------------------------------------2
2 n⁄2
2
2 (n + 1) ⁄ 2
2n ( n + 1 )
( dT + RT ) ( dS + RS )
2
2
(4)
Note also the presence of the ω factor, which means that the induced voltage grows linearly with frequency (apart from the
χ n ( ka ) behaviour). Table 1 lists typical values of the response parameter α=σµωa2 for spheres of radius 1 mm, 5 mm, 1 cm
and 5 cm, at the two operating frequencies of the Förster MINEX metal detector (f1=2.4 kHz and f2=19.2 kHz), which are
typical of the frequency range of interest. Note that α increases quadratically with the object radius, and linearly with its
conductivity and permeability, and with the frequency. We will now analyse in more detail Eq. 2 for the most important cases.
Material
σ (107 S/m)
Perm. µr
α for a=1mm
α for a=5mm
α for a=1cm
α for a=5cm
Copper
5.8
1
1.10 (8.8)
27 (2.2x102)
1.10x102 (8.8x102)
2.7x103 (2.2x104)
Aluminium
3.54
1
0.67 (5.4)
17 (1.3x102)
0.67x102 (5.4x102)
1.7x103 (1.3x104)
Brass (yellow)
1.5
1
0.28 (2.2)
7.1 (57)
0.28x102 (2.2x102)
0.71x103 (5.7x103)
Steel (typical)
(µr=150)
0.63
150 (300)
18 (1.4x102)
4.5x102 (3.6x103)
1.8x103 (1.4x104)
4.5x104 (3.6x105)
Table 1: Typical conductivity, relative permeability, and response parameter α for spheres of different sizes at f1=2.4 kHz (and f2=19.2 kHz)
2.1. Multipole terms (geometry term gn)
Higher order terms decrease
more rapidly with distance
0.1
n=1
0.2 n=1
than lower order terms (this
n=2
is easier to see for d=dT=dS,
0.15
0.01
a=R=0.1
the
coplanar
case).
a=R=0.1
0.1
Conversely,
as
the
coils
n=3
n=2
n=4
0.001
n=3
approach the sphere higher
0.05
order
terms
become
0
0.0001
d
d
0.01 0.02 0.05 0.1 0.2
0.5 1
significant
and
the
0.01 0.02 0.05 0.1 0.2
0.5 1
secondary field in the sphere
Figure 4: First 4 multipole expansions of the geometry term gn for a “large” sphere (a=R=0.1, d=dT=dS
receives contributions from
and R=RT=RS)
higher order multipoles.
This is illustrated in Figure 4, representing the geometry term gn (see Eq. 4) for n=1 to n=4 in the case of overlapping coils
(d=dT=dS and R=RT=RS) and a rather large sphere, of radius a equal to the coil radius R (a=R=0.1). Units are in fact
indifferent, but thinking in terms of meters for example provides realistic size and depth estimates. The plots are interesting
for us for d>a (top of the sphere below the coil); there the n=1 term clearly dominates, with an initially substantial
contribution from the n=2 term (up to a distance of 2-3 times the sphere diameter) and a smaller contribution for n=3. Note
that the gn term scales as can be easily seen when rewriting gn in terms of a/d and R/d.
Geometry Term Hlog scaleL
Geometry Term
2.1.1. Multipole terms for a non-ferromagnetic object (response function χn, µr=1)
-Re@ ΧD HΜr =1L
1
0.8
0.6
n=1
n=4
0.4
0.2
0
Α
10-1 100 101 102 103 104
Abs@ ΧD HΜr =1L
1
0.8
0.6
n=1
n=4
0.4
0.2
0
Α
10-1 100 101 102 103 104
-Im@ ΧD HΜr =1L
0.35
0.3
n=1
n=4
0.25
0.2
0.15
0.1
0.05
0
Α
10-1 100 101 102 103 104
0
Phase@iΧD HΜr =1L
-20
-40
-60
n=1
n=4
-80
10-1 100 101 102 103 104
Α
Figure 5 illustrates the behaviour
of the first four terms of the
response function χ for a nonferromagnetic object (µr=1). The
phase has in fact been calculated
over iχ (compare also with Eq. 2)
in order to reproduce the overall
phase of the induced voltage,
rather than the phase of the
induced currents which generate
the
secondary
field.
The
multiplication by i = – 1 is
due to time differentiation of the
eiωt factor, and is equivalent to a
counter-clockwise rotation of 90˚
in the complex plane (i.e.
Re(iχ)=–Im(χ) and Im(iχ)=
Re(χ)).
Figure 5. First 4 multipole expansions of the response function χ for a non-ferromagnetic object
The first term (n=1) has the largest absolute value and the strongest phase contribution. The phase decreases steadily as α
increases, but can increase at a given α, with respect to the n=1 value, if the higher order multipole terms are relevant (see
also the previous discussion of the geometry term gn).
2.1.2. Multipole terms for a ferromagnetic object (response function χn, µr=10)
Figure 6 illustrates the behaviour of three of the first four terms of the response function χ for a ferromagnetic object, having
taken as example µr=10. Note that the absolute value is finite for α → 0 (for which the phase approaches +90˚) due to an
induced magnetization effect, and that there are values of α for which the dipole term (n=1) is not dominating (but it might
very well do taking into account the contribution of the geometry term gn). For α → ∞ the absolute value saturates and the
phase approaches –90˚, exactly as for the non-ferromagnetic case discussed above (inductive limit).
Abs@ ΧD HΜr =10L
1.6
1.4
1.2 n=2
1
0.8 n=4
0.6
0.4
0.2
Phase@iΧD HΜr =10L
75
50
25
0
-25
-50
-75
n=1
101
102
103
104
105
Α
100
n=1
101
102
n=4
103
104
105
Α
Figure 6. First multipole expansions of the response function χ for a ferromagnetic object (µr=10)
As in the case of the nonferromagnetic object, the
phase decreases steadily as α
increases, but can increase at a
given α, with respect to the
n=1 value, if the higher order
multipole terms are relevant.
With the notable difference
that the phase changes sign
here.
2.2. Dipole approximation (n=1) – general considerations
If the sphere is far from the coils compared to its size (d>>a), or much smaller than the coil diameter (a<<R), it only “sees” a
homogeneous primary field and only the first term (n=1) is relevant (Dipole Approximation). In this case the sphere behaves
like a simple magnetic dipole with magnetic moment m ( ẑ is the unit vector in the z direction):
2
m = – 2πa H 0 ( X + iY )ẑ
3
IR
2
2
with H 0 = --- -----3T- , r 0 = d T + R T and
2 r0
[ µ 0 ( 1 + k a ) + 2µ ] sinh ( ka ) – ( 2µ + µ 0 )ka cosh ( ka )
X + iY = -------------------------------------------------------------------------------------------------------------------------------- = – χ 1 ( ka ) .
2 2
[ µ 0 ( 1 + k a ) – µ ] sinh ( ka ) + ( µ – µ 0 )ka cosh ( ka )
(5)
2 2
(6)
where χ 1 ( ka ) = X 1 ( ka ) + iY 1 ( ka ) has been defined in Eq. 3 and H0 is the transmit field at the sphere’s centre.
The induced magnetic moment m of a sphere in the dipole approximation is in fact always aligned along the transmit (or
primary) field HT, i.e. we can simply replace ẑ with HT in Eq. 5. It is also worthwhile to note that the sphere’s (dipole) phase
response X+iY depends on its parameters (µ, σ, a) and on ω, but not on the problem’s geometry (i.e. the sphere’s position
with respect to the coils), and is therefore constant for a given object (as long as µr does not change obviously) and working
frequency ω12. As we have seen this is not necessarily true when higher order multipole contributions are relevant.
Table 2 lists values (in degrees) of the phase of the induced voltage in the dipole approximation for the response parameter
(α=σµωa2) values of Table 1, again at f1=2.4 kHz and f2=19.2 kHz.
Material
Phase for a=1mm
Phase for a=5mm
Phase for a=1cm
Phase for a=5cm
Copper
–6.0 (–38.8)
–63.3 (–81.4)
–77.6 (–85.8)
–87.6 (–89.2)
Aluminium
–3.7 (–26.8)
–54.8 (–78.7)
–73.9 (–84.6)
–87.0 (–88.9)
Brass (yellow)
–1.5 (–11.8)
–33.3 (–72.4)
–63.8 (–81.4)
–85.3 (–88.4)
Steel (typical)*
+88.4 (+85.4)
+81.8 (+68.2)
+74.2 (+49.2)
+18.4 (–52.8)
Table 2: Phase of induced voltage in the dipole approximation at f1=2.4 kHz (and f2=19.2 kHz) for spheres of different sizes (*µr=150)
2.2.1. Dipole approximation for a non-ferromagnetic object (response function χ1, µr=1)
The dipole’s approximation response function (Eq. 6) further reduces, in the case of µr = 1 (non-ferromagnetic object), to:
X + iY
µr = 1
1
1
cosh ( ka )
= 3 --------- + --- – ---------------------------- = – χ 1 ( ka )
2 2
3
ka
sinh ( ka )
k a
µr = 1
.
(7)
Its behaviour is reproduced in Figure 7, the term “Dipole” and X + iY µ = 1 being equivalent; the phase has been calculated
r
here over (–i Dipole) in order to again reproduce the overall phase of the induced voltage
Re, Im, Abs@DipoleD
1
0.8
Abs
0.6
Complex plane representation
Im@-i DipoleD vs. Re @-i DipoleD
Re
0.1
Α®0
0.4
Im
0.2
0
10-1 100
0
101
102
103
104
Α
0.2
0.3
-0.2
Phase@-i DipoleD
Α=10
-20
-0.4
-40
-60
-80
10-1 100
0
As expected the plots correspond to
those of the first term in Figure 5; they
are indeed qualitatively similar to what
expected for a simple circuit model.
Examples of working points at the
Förster MINEX operating frequencies
are shown as circles and squares for
copper spheres of radius 1 mm (very
small object) and 1 cm respectively
(see also Table 1 and Table 2).
101
102
103
104
Α
-0.6
Phase@-i DipoleD Hlinear scaleL
-20
-0.8
Α=102
-40
-60
Α®¥
-80
100 200 300 400 500
Α
(example) Copper sphere, 1mm radius:
-1
@f1 1cm radius:
@f2
@f1
@f2
Figure 7. First multipole term (n=1: dipole approximation) of the response function χ for a
non-ferromagnetic object (µr=1).
For the small object there is a very
large difference in the absolute value
of the response function at the two
frequencies, whereas for the larger one
the ratio of the two absolute values
approaches unity, as we are already
close to the inductive limit (the overall
induced voltage features in fact an
additional factor ω, see Eq. 2).
Considering the absolute value of the
response
function
for
nonferromagnetic objects higher operating
frequencies are therefore favoured,
which is not necessarily true when
phase-based
identification
is
attempted, as the phase response curve
flattens8 when approaching –90˚
(different objects tend to show similar
phase behaviour). From the point of
view of discriminating smaller debris
from larger pieces a value of the phase
threshold in the range –60˚ to –80˚
could represent a good starting point
for further investigations (for nonferromagnetic objects).
2.2.2. Dipole approximation for ferromagnetic objects (response function χ1, µr>1)
The dipole’s approximation response function (Eq. 6 or equivalently Eq. 3 with n=1) is reproduced in Figure 8 for two
different ferromagnetic objects (µr=10 and µr=100) as well as for µr=1 (non-ferromagnetic) for comparison. The plots for
µr=10 correspond to those of the first term (n=1) in Figure 6. The complex plane representation is here shown for µr=100
only. The behaviour of the phase includes again the factor i = – 1 in order to reproduce the overall phase of the induced
voltage. As we already pointed out, the response function for ferromagnetic objects is parametrized by µr (i.e. there is one
curve for each value of µr).
For α → ∞ the absolute value saturates and the phase approaches –90˚, as for the non-ferromagnetic case (inductive limit);
the phase approaches +90˚ and the absolute value is finite for α → 0 due to an induced magnetization effect7,8. This wellknown fact could be used to determine if an object with negative phase response is ferromagnetic or not. Indeed, it is clear
that a single phase measure resulting in a negative value is per se not sufficient to state that the object is non-ferromagnetic.
For a given µr the phase decreases steadily as α increases, and changes sign. On the other hand the phase increases when
increasing µr whilst keeping the other object parameters (σ, a) and ω constant (follow for example the arrow labelled “1” in
Figure 8).
-Re@ ΧD,-Im@ ΧD HΜr =1,10,100L
1
Im@iΧD vs. Re@iΧD HΜr =100L
2
10
0.5
Α®0
100
0
-0.5
100
101
102
103
105
104
106
107
Α
-1
-1.5
-2
Α=6 102
1.5
10
100
Abs@ ΧD HΜr =1,10,100L
Α=2 103
1
1.75
Μr =100
1.5
1.25
Μr =10
3
1
Α=104
0.5
0.75
0.5
Μr =1
0.25
100
101
2
102
103
105
104
106
107
Α
Α=2 10
0.1 0.2 0.3 0.4 0.5 0.6
4
Phase@iΧD HΜr =1,10,100L
75
50
1
25
Α=105
Μr =100
-0.5
0
-25
-50
Μr =10
Μr =1
-75
100
101
102
103
104
105
106
107
Α
(example) Steel sphere (µr=100), 1mm radius:
For a given µr the absolute value of the
response function decreases as α
increases and then slowly rises again;
this seems to favour lower operating
frequencies (again, keeping in mind
that the overall induced voltage
features an additional factor ω (Eq. 2)).
Α®¥
-1
@f1 1cm radius:
@f2
@f1
@f2
If instead we increase µr whilst
keeping the other object parameters (σ,
a) and ω constant, at low values of α
the absolute value of the response
function increases (follow for example
the arrows labelled “2” and “3” in
Figure 8) and the difference between
non-ferromagnetic and ferromagnetic
objects can be significative. At higher
values of α this trend is reversed.
Examples of working points at the
Förster MINEX operating frequencies
are again shown as circles and squares
for mild steel spheres, with µr=100, of
radius 1 mm (very small object) and
1 cm respectively (see also Table 1 and
Table 2, where the values are in fact
calculated for µr=150). From the point
of view of discriminating smaller
debris from larger pieces a value of the
phase threshold around +75˚ could
represent a good starting point for
further investigations (but in particular
the case of objects with high µr values
has to be studied more attentively).
Figure 8. First multipole term (n=1: dipole approximation) of the response function χ for
different ferromagnetic objects as well as for µr=1 (for comparison; arrows: see text).
2.3. Summary of theoretical analysis
We will briefly summarize the basic findings of the previous theoretical analysis (these general trends should often still hold
also for non-spherical objects, especially for smaller ones for which the dipole approximation is more likely to be
satisfactory):
• The induced voltage can be expressed as the sum of an infinite number of multipole terms, which can be represented
as the product of a geometry dependent term gn (real) and a response function χn (complex). The latter is a function
of the response parameter α=σµωa2, which depends on the object’s properties (σ, µ, a) and on ω.
•
Higher order multipole components are not important if the sphere is far from the coils compared to its size (d>>a),
or much smaller than the coil diameter (a<<R) (dipole approximation, homogeneous primary field).
•
Non-ferromagnetic objects behave similarly to what predicted by a simple circuit model. The behaviour of
ferromagnetic objects is parametrized by µr; they exhibit a finite response for α → 0 , and their phase ranges from
+90˚ to –90˚. In both cases the phase decreases monotonically with α, but it can increase when higher order
multipole terms are important or for increasing µr.
•
In the dipole approximation the sphere’s response does not depend on its position, only on the object’s parameters.
•
Higher operating frequencies are particularly important for the detection of small non-ferromagnetic objects; the
opposite tendency is true for ferromagnetic ones (thinking of the absolute value of the response function).
•
In principle one can differentiate between ferromagnetic and non-ferromagnetic objects (only ferromagnetic objects
exhibit a positive phase response). Note that a single phase measure resulting in a negative value is per se not
sufficient to state that the object is non-ferromagnetic.
3. PROPOSED APPROACH
5,8,9,13,14,15
As has been stressed for example in
and in 16,17, this theoretical analysis and other similar approaches indicate
the possibility of distinguishing between ferromagnetic and non-ferromagnetic objects, and point to the possibility of
identifying some metallic objects based on their characteristic phase response (as already implemented up to a certain level
for other applications, such as Non-Destructive Testing or treasure hunting5). The Geophex GEM-3 detector has for example
been used to acquire data at discrete frequencies over a large spectrum (“Electromagnetic Induction Spectroscopy”9). Förster
has also proposed an extension of its two frequency MINEX detector to a three frequency system with the aim of providing
information on the type of metal under analysis18. Investigations are obviously being carried out on time domain (pulse)
systems as well. Some work on background effects, in particular due to the ground itself, has been published in 19,20,21.
In addition, it has been noted that the phase shift of the received signal turns out to be a continuous, monotonically decreasing
function of the object size (all other parameters being kept constant). It has been suggested to exploit this relationship for
example for the characterisation of UXO12, to obtain a very rough indication of the object size.
The idea we will pursue here goes more into the direction of imposing a “phase threshold” in order to reduce the amount of
detected clutter. This discrimination-based approach is less ambitious than object identification, but is likely to be more
robust (which is of fundamental importance in humanitarian demining tasks), at least for general applications. In fact some
think that even a reduction of the false alarm rate of a metal detector by a factor of 2 or 3 would be highly beneficial,
obviously without compromising on the detection efficiency. This approach is likely to work when looking for metallic
objects of a certain size, such as those contained in non minimum-metal mines (e.g. the PMN or PMN2 antipersonnel mines)
or UXO, and when the clutter is mostly represented by small metallic objects. An application in the time domain aimed at
discriminating metallic mines from clutter is for example detailed in 22.
From what we saw in the previous theoretical analysis, this type of discrimination (phase threshold) looks feasible on paper
and indicative values have been given in the corresponding sections, even if in particular the case of objects with high µr
values has to be studied more attentively. We will therefore now have a look at it from the experimental point of view.
4. DATA COLLECTION STRATEGY
In order to test these ideas and to get qualitative indications of a CW metal detector response, we have carried out a first series
of measurements using a commercially available, differential two frequency metal detector, the Förster MINEX 2FD. We
have recorded the detector’s internal signals, varying different object parameters, for a minimum metal mine, some mine
components as well as a few reference objects (cylinders) and clutter, in a laboratory setup with the objects either flush or
buried. Linear scans, and in some cases series of parallel scans as well, have been carried out with a high density of points in
the scan direction, placing the detector on a Cartesian gantry16 (which is now at the VUB in Brussels).
The collection of data as a function of movement enables the possibility of analysing the data in the complex plane (in-phase
vs. quadrature-phase component). This method resembles measurements carried out in Non-Destructive Testing (NdT)6,
from where it was inspired, and makes it possible to exploit global object properties rather than only local ones. Similar work
has also been carried out independently by Szyngiera5. Some multifrequency data collected at discrete spatial locations with
the GEM-3 sensor has also been represented in the complex plane21.
5. THE FÖRSTER MINEX METAL DETECTOR
The Förster MINEX 2FD metal detector (see Figure 9) generates two continuous wave frequencies, f1 and f2, at 2.4 kHz
(especially for the detection of ferromagnetic objects) and 19.2 kHz (especially for the detection of stainless steel and alloys)
respectively. The transmitter consists of a conventional copper coil (diameter 230 mm), whereas the receiver is composed of
an eight-layer printed circuit board (diameter 200 mm), coaxial and coplanar with the transmitter, arranged as a gradiometer.
differential
metal detector
V (amplitude)
2
y
object center
x=0
1
2
object center
3
x
1
3
x
scanning line
SCANNING
SETUP (top view)
DIFFERENTIAL
RESPONSE
Figure 9: Förster MINEX 2FD detector head on gantry support frame (left). Differential response for a linear scan along x (right).
V (mVolt)
This results in a differential left-right system, its output audio signal vanishing when the detector’s mid axis crosses the
object’s center, thus allowing a very precise spatial localisation along one dimension. The differential setup also helps in
background suppression and enhances the separation power (localizing nearby objects). The physical quantity which the
detector measures is therefore an approximation of the gradient of the induced magnetic field (actually its vertical
component) along a direction perpendicular to the detector’s axis, i.e. ∂B zsec ⁄ ∂x when working as shown in Figure 9.
To fully exploit the detector’s capabilities we intercept, at the output of the receiver-transmitter module, five signals (f10˚,
f190˚, f20˚, f290˚, Delta) corresponding respectively to the real (f10˚, f20˚, or equivalently f1 REAL and f2 REAL) and
imaginary parts (f190˚, f290˚, or equivalently f1 IMAG and f2 IMAG) (in the complex plane) of the analog signals V 1( s ) and
V 2( s ) induced at f1 and f2 in the receiver, and the difference of the imaginary parts multiplied by a constant (Delta=c(f190˚–
f290˚)), which turns out to be quite insensitive to changes in (back)ground conditions. The phase is measured with respect to
the transmitted signal shifted by a constant amount (about 5˚), representing a reference angle which corresponds to a typical
magnetic soil. This phase shift stays constant unless a “ground cancellation (balancing)” procedure is explicitly performed. A
slow “ground following” adaptation function is also implemented. Typical signals are shown in Figure 10 and Figure 11
taking as example the object represented in Figure 1 on the right (striker pin of a minimum metal mine).
The induced voltages V 1( s ) and V 2( s ) are
Processed Amplitudes vs. Distance along Scan
10
amplified by a factor F1 and F2
f1 REAL
LeftPeak
f2 REAL
respectively, which are chosen to satisfy the
5
following relationship (I1 and I2 are the
0
Right
Left
Object
currents in the primary coil at f1 and f2):
−5
Centre
F1 ⋅ ω1 ⋅ I1 = F2 ⋅ ω2 ⋅ I2 .
RightPeak
−10
200
300
400
500
600
700
800
900
1000
1100
400
500
600
700
800
900
1000
1100
20
V (mVolt)
10
0
−10
−20
200
f1 IMAG
f2 IMAG
DELTA
300
400
V (mVolt)
AUDIO
300
200
100
0
200
300
400
500
600
700
X (mm)
800
900
1000
1100
Figure 10: Typical processed (i.e. filtered and centered) internal and Audio signals
from the Förster MINEX 2FD metal detector
(8)
This scaling effectively removes the linear
dependency on ω of the induced voltage
(see Eq. 2) and makes it possible to use the
Delta signal defined before to suppress the
soil influence. It also allows us to compare
the absolute values of the induced voltages
with the absolute value of the response
function χ (in fact their ratios and trends,
rather than the actual values).
Data collection has been carried out in a
straight line across the object’s center,
recording the signals with a dedicated 16
bit conversion card over 5 Volt every 0.56
mm (about 2 points per mm). Typical scan
lengths were between 60 and 100 cm with
the detector at 2-3 cm above the ground
unless otherwise specified. The data was then lowpass filtered with a simple sliding window 30 to 45 data points wide, and
centered (subtraction of mean value to remove offsets).
The detector’s differential response is clearly visible in Figure 10, where “Left” has been assigned to lower value coordinates.
When moving in the direction of increasing coordinates a first peak is traversed to the left of the object (“LeftPeak”); the
object centre then comes at about x=600 mm, followed by a second peak to the right of the object (“RightPeak”). Note that
the signal for such a small and shallow object is about as wide as the detector head itself. The imaginary components show
large fluctuations outside the zone of interest, probably due to soil inhomogeneities; these fluctuations are strongly correlated
and the imaginary signals practically overlapping. The resulting difference (Delta) is therefore non zero only in
correspondence of the target. The real components are on the other hand clear in the given experimental conditions. The
Audio signal results from a thresholding of the sole Delta signal. The object’s center is in a position corresponding to the dip
in between the peaks of the audio signal, in correspondence of the zero crossing of Delta.
6. DATA COLLECTION RESULTS
RAW data
PROCESSED data, around area of interest
18
20
f1
f2
Left
16
LP: LeftPeak
10
RP: RightPeak
Left
ObjectCentre
−10
f2
−20
f1
−30
Right
f1 IMAG (mVolt), f2 IMAG (mVolt)
f1 IMAG (mVolt), f2 IMAG (mVolt)
14
0
LP1
ObjectCentre
12
10
LP2
8
6
RP2
4
2
−40
0
Right
−50
−2
−80
−70
−60
−50
f1 REAL (mVolt), f2 REAL (mVolt)
RP1
−5
0
5
f1 REAL (mVolt), f2 REAL (mVolt)
Results
are
presented
for
several
representative objects (23, with some of the
measurements also reproduced in 16, 17). No
particular attempt at background subtraction
has been done other than the subtraction of
the mean values already described (to
remove offsets).
For each data point in the complex plane we
can obviously calculate a magnitude and a
phase. When the phase response of the
received signal changes little over the whole
scan we get diagonal lines and the overall
phase is nothing else but their slope. When
this is not the case there might still exist a
general trend which can be used to calculate
an “average phase”. Another possible choice
consists in taking the phase where the signal
is strongest5. We will not insist on this point
and concentrate on the qualitative aspects of
the received signal instead (the curves’
slopes and shapes).
Figure 11. Raw and processed internal signals (see also Figure 10) plotted in the
complex plane
6.1. Raw vs. processed data in the complex plane and background fluctuation effects
The right half of Figure 11 shows the processed data from Figure 10, plotted this time directly in the complex (impedance)
plane and taken around the area of interest only (from x=450 mm to x=750 mm). The left half of Figure 11 shows the raw
data from the same acquisition, that is all data unprocessed; the sets of data points at the two frequencies are well separated,
due to the signals having different offsets. These offsets seem to be rather constant. In this particular case the background
fluctuations, probably due to soil inhomogeneities, are non negligible and the corresponding curves in the complex plane are
distorted, in particular at f2. The target signals, corresponding to the diagonally lying sets of points, are however still strong
enough and we can rather clearly separate them in the left half of the image from the background signals (the fluctuating
imaginary component which translates into vertically distributed data points). This is unfortunately not always the case.
6.2. Non-ferromagnetic vs. ferromagnetic object
Figure 12 represents well the difference between non-ferromagnetic objects and (small) ferromagnetic ones. It shows that the
phase angle decreases from f1 to f2 in both cases and also that the absolute value of the response at f1 is larger than at f2 for
the ferromagnetic object, the inverse for the copper piece. These trends are consistent with the previous theoretical analysis of
the response function’s behaviour. In detail, looking first at a non-ferromagnetic object and then at a ferromagnetic one:
f1 REAL,IMAG f2 REAL,IMAG
complex plane representation
f1 IMAG (mVolt), f2 IMAG (mVolt)
400
f1
f2
V (mVolt)
200
100
0
−100
−200
−300
600
800
1000
1200
f1
f2
200
V (mVolt)
100
0
−100
−200
400
600
800
X (mm)
f1 IMAG (mVolt), f2 IMAG (mVolt)
300
f1
f2
300
200
100
0
−100
Non−
ferromagnetic
object (ex.
copper scrap)
−200
−300
−400
−200
0
200
200
400
f1
f2
100
0
Ferromagnetic
object (ex.
mild steel
cylinder)
−100
−200
1000
−200 −100
0
100 200
f1 REAL (mVolt), f2 REAL (mVolt)
Figure 12. Typical response of non-ferromagnetic (top) vs. ferromagnetic (bottom) objects
As an example of the first we
have taken as an irregularly
shaped, long and slender piece
of copper (upper half); the real
and imaginary components
have opposite polarity, which
translates into a negative slope
in the complex plane (the phase
angle of interest here is the one
measured in the fourth quadrant
(+,–)).
As an example of the second
we have taken a small mild
steel cylinder of diameter 7 mm
and height 30 mm, placed flatly
in the horizontal (xy) plane and
transversally (i.e. along y) to
the scanning direction, which is
the default setup (lower half of
the image). The real and
imaginary components have the
same polarity, which translates
into a positive slope (the phase
angle of interest here is the one
measured in the first quadrant
(+,+)).
f1 IMAG (mVolt), f2 IMAG (mVolt)
6.3. Phase vs. axial offset (parallel scans)
Parallel scans over a live minimum metal mine
5
f1
f2
C − 1cm (scan 13)
6
2
0
0
−2
−4
C − 5cm (scan 11)
−6
−5
−5
0
5
−5
6
4
0
5
C + 7cm (scan 17)
C + 3cm (scan 15)
f1 IMAG (mVolt), f2 IMAG (mVolt)
f1
f2
4
f1
f2
4
f1
f2
2
2
0
0
−2
−2
−4
−4
−6
−5
0
5
f1 REAL (mVolt), f2 REAL (mVolt)
−5
0
5
f1 REAL (mVolt), f2 REAL (mVolt)
Figure 13. Response at f1 and f2 to a live mine (flush), for four parallel scans (the scan
running directly over the object being in between the second and third)
The behaviour of scans not carried out
exactly over the target has been analysed
by means of series of parallel scans along
the x axis (at constant detector height and
object position).
In the case of the large and irregularly
shaped piece of copper already
encountered, there are some changes in
the phase response within each scan,
especially at f1, which could be due to
the object’s irregular shape and/or higher
order multipole terms. From one scan to
the other there are no remarkable
differences, apart from the response at f1
at the outer edges of the object, which
also features a reduced absolute value.
In Figure 13 the target is a composite
object, the real (live) version of the
minimum metal mine shown to the right
of Figure 1, i.e. with the addition of the
detonator cap (will be discussed in more
detail later). The distance between the
scans plotted is 4 cm, with the scan
running directly over the object being in between the second and third. There are some changes in the phase at f2 and the ratio
of the absolute values of the induced voltages at f1 and f2 is not constant, in particular for the last scan shown (C+7cm, scan
17).
Summarizing, for the objects studied there are some changes for different axial offsets, but this does not seem to represent a
major effect. The case of ferromagnetic objects needs to be looked at in more detail.
6.4. Phase vs. orientation in the horizontal plane
f1 IMAG (mVolt), f2 IMAG (mVolt)
f1 IMAG (mVolt), f2 IMAG (mVolt)
f1&f2 REAL,IMAG (mVolt)
f1&f2 REAL,IMAG (mVolt)
Using small cylinders of the same shape as described before (diameter 7 mm, height 30 mm) it was checked how the response
depends on the object’s orientation in the horizontal plane (xy), the plane into which the metal detector moves. The cylinders
were successively placed perpendicularly (transversally) to the scanning direction, i.e. along y (the default), parallel
(longitudinal), i.e. along x, and at +45˚ and –45˚.
The non-ferromagnetic cylinder, of aluminium in this case, does not show any noticeable difference. The mild steel cylinder’s
response is on the contrary strongly orientation dependent (Figure 14, only two orientations shown), with the curves’ shapes
not being always symmetric probably due to a non perfect positioning of the object.
The shape of the real and the
Parallel to scanning direction (−> x)
complex plane representation
imaginary part curves is not
150
f1
f1
150
necessarily the same any
f2
f2
more, with the real part which
100
100
seems to be the more
50
50
irregular one; there are also
parts of the curves in which
0
0
the two have opposite
polarity, corresponding to a
−50 Ferromagnetic
−50
object (ex. mild
negative phase. The peaks do
−100 steel cylinder)
also not necessarily occur at
−100
the same spatial position. A
200
400
600
800
−100
0
100
closer look at the curves
shows that it is the central
Perpendicular to scanning direction (−> y)
100
section between the first and
f1
f1
last peak which exhibits the
f2
f2
50
greatest variation; the phase
50
is rather constant to the left
and to the right of it, i.e.
0
0
while approaching the object
and while moving away from
it (not when over it). Similar
−50
−50
plots are in fact well known
in Non-Destructive Testing
−100
200
400
600
800
−100
−50
0
50
applications.
Figure 14. Response at f1 and f2 to a mild steel cylinder (flush), oriented parallel and orthogonal to the
scanning direction in the horizontal plane
This behaviour can be explained qualitatively as follows (K. Ausländer, Institut Dr. Förster, Private Communication, March
2000), thinking for example of the situation in which the cylinder is parallel (longitudinal) to the scanning direction: far away
from the coil it sees basically a horizontal primary magnetic field aligned along its axis, which emphasizes induced
magnetization effects (and possibly increases µr). The object’s ferromagnetic nature prevails. Right over the coil the primary
magnetic field is vertical and therefore transversal to the cylinder axis; eddy current effects can prevail and the object can
appear to behave as non-ferromagnetic. The field orientation effect is well illustrated in 22.
Results for different orientations in the vertical plane are reported in 13,24 for GEM-3 UXO data; 4 also reports significant
differences for prolate steel spheroids, using time domain equipment. Horizontal vs. vertical orientation differences are
reported in 8 for several mines using laboratory frequency domain equipment. Most data is acquired for the target in a fixed
spatial position, 21 representing an exception.
6.5. Response for different versions of a PMN mine
f1 IMAG (mVolt), f2 IMAG (mVolt)
f1 IMAG (mVolt), f2 IMAG (mVolt)
The Russian PMN is a widely diffused non minimum-metal AP mine (see Figure 1), of which we initially obtained a replica
version from MILTRA Engineering in the UK. This replica mine contains a steel striker, believed to be an exact copy of the
original, composed of two cylinders aligned along the same axis, with diameter 9 mm and 5 mm and length 19 mm and 39
mm respectively. This is the mine’s largest metallic object apart from a ferromagnetic cover retaining ring, also visible in
Figure 1, which was not used in these tests but which would certainly contribute in a substantial way to the overall signal.
The phase response to this steel
Parallel to scanning direction (−> x)
Perpendicular to scanning direction (−> y)
striker is plotted in Figure 15, which
f1
again shows how the signal strongly
100
PMN AP
f2
100 (replica mine)
depends on the orientation (not
steel striker
f1
50
surprisingly, given that the target
f2
0
object is composed of two coaxial
0
steel cylinders and that it is
−50
−100
ferromagnetic).
−100
Later one we also had access to a
−200
mine, coming from Cambodia, which
−200 −100
0
100
200
−100
0
100
looked very much like a PMN but
which turned out to have a slightly
At +45 degrees (−> y=x)
At −45 degrees (−> y=−x)
smaller non-ferromagnetic (!) striker
200
150
f1
f1
pin. This is in fact the mine actually
150
f2
f2
100
shown in Figure 1 to the left. The
100
50
response to its striker pin resembles
50
Figure 12, top right, and is obviously
0
0
very different to the one shown in
−50
Figure 15. The mine could be a
−50
−100
Chinese copy of the PMN, but this
−100
−150
episode was actually very useful in
−100
0
100
200
−200
−100
0
100
200
reminding us of the limits of a target
f1 REAL (mVolt), f2 REAL (mVolt)
f1 REAL (mVolt), f2 REAL (mVolt)
identification based approach.
Figure 15. Response at f1 and f2 to a PMN AP mine steel striker (flush), at four different
orientations (parallel and orthogonal to the scanning direction, and at +45 and –45 degrees
6.6. Phase vs. depth (distance)
Variations of the response with increasing distance from the object were checked on the small mild steel cylinder described
before, placed flush with the surface and again perpendicularly to the scanning direction. The detector head was raised in 1
cm increments without displacing the object; this is definitely easier than changing burial depth, even if obviously less
realistic, and has the advantage that the influence of the background is reduced too as the height increases, not only the target
signal. Plots similar to the one in Figure 12, bottom right, were obtained at all distances up the limit of detection, the phase of
the object under analysis staying constant (both frequencies). In fact it would be necessary to check if this kind of behaviour
holds also for other object orientations. Similar effects have been reported using a time domain system in4.
6.7. Response for a live minimum-metal mine
Figure 16 features the response of a live (real) minimum metal mine (Figure 1, right) and of its metallic components
separately.
It is very interesting to see that the detonator, placed just above the striker pin, delivers a significant and absolutely non
negligible non-ferromagnetic signal (perhaps due to the thin capsule containing the primary explosive) with an f2 component
much larger than the f1 component. This would indeed be indicative, according to the previously studied model, of a rather
small object. It is in any case a reminder to include whenever possible the detonator in the study of minimum-metal mines.
f1 IMAG (mVolt), f2 IMAG (mVolt)
Detonator only
Mine without Detonator
f1
f2
6
4
4
2
2
0
0
−2
−2
−4
−5
0
5
−5
0
5
Live Mine, MD @ 10cm
Live Mine, MD @ 5cm
1
6
4
2
The signal of the mine without
detonator is on the other hand
ferro-magnetic, the mine containing
in this case only the steel striker pin
(actually a vertically placed steel
nail, of diameter 1.5 mm, length 11
mm and weight 0.1-0.2 g).
−4
−6
−10
f1 IMAG (mVolt), f2 IMAG (mVolt)
f1
f2
0.5
f1
f2
0
f1
f2
0
−2
−0.5
−4
−6
−5
0
5
f1 REAL (mVolt), f2 REAL (mVolt)
−1
−0.5
0
0.5
1
f1 REAL (mVolt), f2 REAL (mVolt)
Figure 16. Minimum metal mine: response to a detonator cap, to the mine without detonator
(striker pin only), and to the live (real) mine at two different detector heights (all objects flush)
The signal of the real complete
(live) mine is an interesting
combination of the two – actually
simply the vector sum of the first
two plots – looking like the
response of a ferromagnetic object
at f1 and of a non-ferromagnetic
one at f2; this seems quite
characteristic indeed. All scans
were carried out on flush objects
with a detector height of 5 cm. The
live mine was also scanned at a
detector height of 10 cm, and the
corresponding responses look quite
similar to those at 5 cm.
6.8. Summary of experimental results
A number of elements of the theoretical model we had a look at seem to be confirmed (at least for the objects and
experimental conditions described before) by the experimental results obtained with the CW two frequency system:
• The phase of (small) ferromagnetic and non-ferromagnetic objects shows a sign difference, but features similar
trends: in both cases the phase decreases for increasing frequency.
•
The absolute value of the induced voltages increases for increasing frequency considering non-ferromagnetic
objects, the contrary for (small) ferromagnetic ones.
•
The phase response is quite stable with distance (but more checks are needed, e.g. for different object orientation).
•
Axial offsets (scans not carried out exactly over the target): the model does not provide a general answer as it
assumes that the sphere is coaxial with the circular coils. We nevertheless know that in the dipole approximation the
phase response does not depend on the object’s position. Experimentally, for the objects studied there are some
changes for different axial offsets, but this does not seem to represent a major effect. The case of ferromagnetic
objects needs nevertheless to be looked at in more detail.
The experimental results also tell us that:
• The response of ferromagnetic objects can be strongly orientation dependent (tests were carried out only in the
horizontal plane).
•
The components of similarly looking mines can have completely different responses.
•
The detonator can produce a non negligible response in a minimum-metal mine.
•
Fluctuations in the background signal, in particular due to the soil itself, can be far from negligible, especially when
the target signals are weak; similar conclusions have been reached for example in 19. In our experimental conditions
the real component was stable, the imaginary one fluctuated (probably due to soil inhomogeneities).
7. FINAL DISCUSSION
In a certain sense some deminers are already discriminating metallic objects themselves, or some of their parameters, using
as input “only” the audio signal and relying on their experience. Whether or not they take decisions based on their
interpretation depends a lot on the operating context. The answer ranges from a policy of strictly investigating each alarm to
some degree of freedom of decision. What we are basically investigating is if we can provide similar reliable (and operator
independent?) information directly and reproducibly from the detector’s internal signals. Without pretending to give any final
response, and without repeating in detail the contents of the “Summary” paragraphs, we can highlight a number of issues:
7.1. Object Identification:
Experimental measurements on objects such as the live minimum-metal mine have highlighted some quite characteristic
behaviour, which could probably be even better exploited by a system working at a large number of frequencies. On the other
hand an object’s “signature”, defined here as its behaviour in the complex plane, has shown some dependency on the axial
offset, little with distance, and strong dependency on object orientation for ferromagnetic objects. Concerning the shape of
the curves in the complex plan, it is not clear if it is practically exploitable. Practical problems will be discussed below.
7.2. Object Discrimination:
The idea of imposing a “phase threshold” to reduce the amount of detected clutter seems viable from the theoretical point of
view and still holds after the experimental measurements, but more data needs to be collected, especially with larger
(ferromagnetic) objects, to clearly define how to calculate an “average phase” and where to put the threshold, and to quantify
the corresponding results. Practical problems will be again be discussed below. This approach is likely to work when looking
for metallic objects of a certain size, such as those contained in non minimum-metal mines (e.g. the PMN or PMN2 AP
mines) or UXO, and when the clutter is mostly represented by small metallic objects. Other possible application scenarios
include for example the use of metal detectors after having employed mechanical devices and possibly dogs, as certain teams
look at this point only for larger metallic pieces. Quality Control applications might also be considered (this is true for an
identification-based approach as well).
7.3. Practical Problems:
Needless to say, some problems might complicate a practical implementation. For example (“D” means applicable to the
Discrimination approach, “I” to the Identification):
• (I/D) The object parameters (σ, µ, perhaps even the shape) can vary. These variations can be small, for ex. within a
single batch of metallic pieces, or larger, between different batches. Copies of mines might also use different pieces.
•
(I) There might be a metallic piece close to a minimum-metal mine, faking its response. Unforeseen objects can also
be found.
•
(D) The effect of co-located ferromagnetic and non-ferromagnetic objects needs to be studied more attentively.
•
(I/D) Background signals, in particular due to the soil itself, could become THE problem in a number of ground
conditions, in particular when weak signals (small and/or deeply buried objects) or ground inhomogeneities are
present. How to overcome them whilst guaranteeing robust identification or discrimination is probably far from
trivial. We recall that most objects were buried flush with the surface in the experimental data shown.
Apart from technical issues there are certainly practical problems to deal with, such as making sure that a sufficient number
of application scenarios do actually exist, and ensuring acceptance by the end users (who accepts responsibility for certain
choices for example?).
ACKNOWLEDGMENTS
First of all we would like to thank the EPFL, in particular Prof. Jean-Daniel Nicoud (DI-LAMI) under whose DeTeC project
this work was started, and the members of the former DeTeC team who made the actual data acquisition and collection
possible, in particular Frédéric Guerne and Bertrand Gros. Many thanks go also to Klaus Ausländer and Thomas Himmler
from the Institut Dr. Förster for their very positive and cooperative attitude.
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