Technical Note No. I.03.168 Development of Tests for Measuring the Detection Capabilities of Metal-Detectors T.J. Bloodworth November 2003 i
Distribution List Internal (Humanitarian Security Unit) Sieber A.J. Bloodworth T.J. (5 copies) Lewis A. Littmann F. Logreco A. Pike M.A. Secretary, Humanitarian Security Unit (5 copies) External Gülle D. (ITEP Secretariat, c/o JRC Ispra) Borry F. (ITEP Secretariat, c/o JRC Ispra) Das Y. (Canadian Centre for Mine Action Technologies, DRDC, Canada) Schoolderman A. (TNO-FEL, The Netherlands) Reidy D. (CECOM NVESD, USA) Müller C. (BAM, Berlin, Germany) The information contained in this document may not be disseminated, copied or utilized without the written authorization of the Commission. The Commission reserves specifically its rights to apply for patents or to obtain other protection for matter open intellectual or industrial protection. ii
Technical Note Development of Tests for Measuring the Detection Capabilities of Metal-Detectors Tom Bloodworth (JRC Ispra/IPSC/HS Unit) November 2003 Abstract Metal detectors are an essential tool used by people performing humanitarian mine clearance operations. In many cases, the performance of such detectors is not well known. In particular, users want to know the detection capability. There has therefore been a demand from metaldetector users for testing standardization. This Technical Note describes a series of experimental tests on various commercial metal detectors that have been made to support the development of standardized tests of detection capability. Many of the results of these tests have provided evidence for CEN Workshop 7, which is an international group working to standardize the testing of metal detectors for humanitarian demining. Tests of in-air maximum detection height using parametric target sets of metal balls allow detection capability to be defined in terms of a minimum detectable ball diameter at a given height. The relative detection capability of the detectors for different metals is also studied. Evidence is given that supports the use of chrome steel balls as a standard parametric target set. Extending the idea of using sets of metal balls, the detection capability in soil can be readily measured and compared to the capability in-air. Comparison of in-soil and in-air capability has been made in tests using a "noisy" magnetic soil. This gives a test for giving quantitative measurements of the influence of soil on the capability to detect buried metal targets. Finally; initial results are given of a study to determine whether it is feasible to grade targets according to an "equivalent" steel ball diameter. 1
CONTENTS 1 Introduction 4 2 Purpose and Principles of the Tests 4 2.1 Maximum Detection Height 4 2.2 Detection Capability Curves 5 2.3 Soil Tests 5 2.4 Target Grading 5 3 Test Targets 5 3.1 Metal Balls 5 3.1.1 Chrome steel UNI 100Cr6 5 3.1.2 Stainless steel AISI 420 6 3.1.3 Stainless steel AISI 316 6 3.1.4 Aluminium 6 3.1.5 Copper 6 3.1.6 Bronze 6 3.1.7 Brass 6 3.1.8 Bearing balls 6 4 Detectors Tested 7 4.1 Detector Characteristics 7 4.1.1 Continuous Wave and Pulsed Techniques 8 4.1.2 Dynamic and Static Mode 8 4.1.3 Coil Configuration 8 4.1.4 Ground Compensation 8 4.2 Detector Models 8 4.2.1 Ceia MIL-D1 8 4.2.2 Ebinger 421 GC 9 4.2.3 Foerster Minex 2FD 4.500 9 4.2.4 Guartel MD8 9 4.2.5 Minelab F3 (Prototype) 9 4.2.6 Schiebel AN 19/2 9 4.2.7 Vallon ML1620C and VMH-2.1 9 5 Test Equipment 10 5.1 In-air Test Jig 10 5.2 Soil Test Box 11 6 In-air Tests 12 6.1 Overview of Detection Capability Curves 12 6.2 Ceia MIL-D1 In-air Curves 13 6.3 Ebinger 421 GC s/n 518 In-air Curves 13 6.4 Ebinger 421 GC s/n 724 In-air Curves 13 6.5 Foerster Minex 2FD 4.500 (s/n 172) In-air Curves 14 6.6 Foerster Minex 2FD 4.500 (s/n 536) In-air Curves 14 6.7 Guartel MD8 In-air Curves 15 6.8 Minelab F3 (Prototype) In-air Curves 15 6.9 Schiebel AN 19/2 In-air Curves 16 2
6.10 Vallon ML1620C In-air Curves 16 6.11 Vallon VMH-2.1 In-air Curves 17 6.12 Summary of In-air Maximum Detection Capability 17 7 Soil Box Tests 18 7.1 Ceia MIL-D1 In-soil Detection Capability 18 7.2 Ebinger 421GC s/n 518 In-soil Detection Capability 19 7.3 Ebinger 421 GC s/n 723 In-soil Detection Capability 19 7.4 Foerster Minex 2FD 4.500 (s/n 172) In-soil Detection Capability 19 7.5 Foerster Minex 2FD 4.500 (s/n 536) In-soil Detection Capability 19 7.6 Guartel MD8 In-soil Detection Capability 19 7.7 Minelab F3 (Prototype) In-soil Detection Capability 20 7.8 Schiebel AN 19/2 In-soil Detection Capability 20 7.9 Vallon ML1620C In-soil Detection Capability 20 7.10 Vallon VMH-2.1 In-soil Detection Capability 20 7.11 Summary of In-soil Detection Capability 20 8 Equivalence of Targets: Preliminary Study 21 9 Conclusions 24 10 Acknowledgements 25 11 References 25 12 Results Tables and Graphs 26 3
1 Introduction Metal detectors are an essential part of the toolkit of humanitarian demining. Metal detection is the main non-contact method available to detect mines in most of the areas of the world where humanitarian mine clearance operations are carried out. Modern metal detectors are extremely sensitive, being able to detect small amounts of metal in their vicinity. Most detectors are designed to be simple to use, with few user adjustments and only an audio alarm to indicate the presence of metal. Following demands for common testing practice, a CEN Workshop (CW07) was created to produce standard requirements for testing and evaluation of metal detectors. A CEN Workshop Agreement (CWA 14747:2003 [1]) has been produced. Experiments are described below that have been used to support the CEN Workshop, helping the development of the tests within the CEN Workshop Agreement. It is stressed that the tests described do not however constitute a controlled trial for the purpose of selecting detectors and the results should not be used in that way. 2 Purpose and Principles of the Tests 2.1 Maximum Detection Height One of the aspects where standardization is needed is in the method used to measure the detection capability of a detector. The approach most often employed in the field to quantify the detection capability is to measure the maximum height above a given target at which the target still causes an alarm indication. This idea has been developed in the present work, using a set of targets to define the way that detection capability varies with detector height above the target. Spherical metal balls of different diameters were used. Detection capability curves are produced with the measurements. Metal detectors for demining are very sensitive instruments, which show a response to tiny changes in the field produced by their coils. These instruments are however designed for use in difficult conditions, by personnel who may have little, if any technical education. Metal detectors are therefore made very simple to operate. There is usually no output signal level display, either in an analogue, graphical or digital form, just an audible alarm signal whose tone and/or amplitude increases with increasing signal. It is therefore difficult to quantify directly the response from a detector to a metal object by measuring the signal level. One of the objectives of this work was to establish a test that gives quantitative measurements of the detection capability. A scheme not based upon signal level output is needed to make such measurements. Following the common practice of metal detector operators in the field, the maximum height above the target at which an alarm indication is triggered is used here as a measure of detection capability. The higher the detection capability, the higher is the maximum detection distance for a given target, or the smaller the minimum detectable target at a given height. With some detectors the threshold height between detection and non-detection is very clear. For other detectors the alarm is such that the transition can take place over a range of some tens of millimetres. Therefore a criterion is defined for what constitutes detection and this criterion has been adopted by CW07. The alarm signal has to be repeatable and not intermittent at the test height/depth for it to be considered that detection has occurred. In the in-air and in-soil tests described below, the optimum detection capability is always measured. The maximum detection height or depth may occur when the centre of the coil is over the target, 4
when the limb of the coil is over the target (when close), or under some other point on the coil. In whatever position it occurs, the maximum value is always recorded. 2.2 Detection Capability Curves A spherical ball is the ideal target for detection capability tests, because the response will not vary with orientation, so the results are repeatable. A set of balls of the same material and different diameters can be used as a "parametric target set", where the ball diameter is a parameter describing detection capability at a given detector height above target. The choice of material is considered important for standard targets because the relative responses of all metal detectors to different materials (in particular, any difference between magnetic and non-magnetic materials) may not be the same. For this reason, balls of both magnetic and non-magnetic steels were used for these tests, as well as balls of other metals. One reason that metal balls are useful targets to use is that predicting the response of such targets in the fields of circular coils is readily theoretically calculable. Theoretical models and comparison of their results with experiment are reported elsewhere [2], [3]. 2.3 Soil Tests An important aspect of testing a metal detector is measuring the detection capability in soil. This test may be made for two reasons; either to compare different detectors against a common general reference soil in order to evaluate the detectors, or to find out what effect a particular soil (perhaps representative of a mined area) has on a detector. The idea of detection capability curves has been extended to testing for targets in soils, where capability is defined in terms of ball diameter and the depth below the soil surface. 2.4 Target Grading The metal components in real landmines have diverse geometric shapes and orientations and they are made of many different metals. It would be useful if landmines could be graded in some quantitative way according to how easy or difficult they are to detect. One of the ideas investigated in this work is whether the detection capability curve produced for balls of one type of metal (chrome steel) can be used to define an arbitrary reference scale of ease of target detection. Maximum detection height for a target is translated into "equivalent" metal ball diameter. 3 Test Targets 3.1 Metal Balls One of the aims of this work was to investigate the feasibility of using spherical metal balls as test targets for measuring the detection capability of metal detectors. Balls of several sizes and materials were used. A range of material properties was covered; high and low electrical resistivity, ferromagnetic and non-magnetic. Some of the balls were obtained during the course of the work, so were not used in tests for all of the detectors. The materials used are listed below: 3.1.1 Chrome steel UNI 100Cr6 (Equivalent designations are UNS G52986, AISI 52100, DIN 1.3505). This is the most commonly used type of steel for ball-bearings. This steel is ferromagnetic it is strongly attracted to a magnet. The listed electrical resistivity [4] for this steel is 21.9 10-8 Ωm (conductivity 4.6 10 6 Sm -1 ). In general, the magnetic permeability of steel varies with the magnetic field strength applied and the frequency of the field. The low-field limit (initial permeability) is the property of interest for metal 5
detectors. Unfortunately this property is rarely measured and listed for different steels. The relative initial permeability for most steels is usually of the order of 100 [5] (free space or air =1). Many components in minimum metal mines are made of steel, such as firing pins and helical springs. 3.1.2 Stainless steel AISI 420 (Equivalent designation is UNS S42000). The listed electrical resistivity [4] is 55 10-8 Ωm (conductivity 1.8 10 6 Sm -1 ), i.e. a higher resistivity than the chrome steel. Although a "stainless steel", AISI 420 is ferromagnetic and it is assumed that the relative initial permeability is of the order of 100 as for the chrome steel. 3.1.3 Stainless steel AISI 316 (Equivalent designation is UNS S31600). The listed electrical resistivity [4] is 74 10-8 Ωm (conductivity 1.4MSm -1 ), i.e. a higher resistivity than the chrome steel and the AISI 420. This steel is basically non-magnetic, with a listed relative permeability [4] of 1.008. In fact, the cold working of the metal surface during machining can produce metallurgical changes at the surface that result in weak ferromagnetic effects. The balls can be weakly attracted by magnets, for example. For the purpose of these tests, this material is considered a non-magnetic, high-resistivity metal. 3.1.4 Aluminium Aluminium has a low electrical resistivity; about 2.8 10-8 Ωm in its pure state (conductivity 36MSm -1 ), increasing to about 6 10-8 Ωm for various levels of alloying. Aluminium is nonmagnetic (relative permeability of 1). Aluminium is widely used in landmines, for example in detonator bodies. 3.1.5 Copper Copper has a low electrical resistivity; about 1.7 10-8 Ωm in its pure state (conductivity 58MSm -1 ). Copper is non-magnetic (relative permeability of 1). 3.1.6 Bronze The balls used were CuSn8 (8% tin) phosphor bronze (0.2-0.3% P). The electrical resistivity for this bronze is approximately 13 10-8 Ωm (conductivity 7.7MSm -1 ). Bronze is non-magnetic (relative permeability of 1). 3.1.7 Brass The balls used were CuZn37 (37%Zn) yellow brass. The listed electrical resistivity [4] for this brass is 6.39 10-8 Ωm (conductivity 15.6MSm -1 ). Brass is non-magnetic (relative permeability of 1). 3.1.8 Bearing balls These balls are of unknown ferromagnetic steel from various sources. They are probably of similar steel to the chrome steel, since this is a common material for making ball bearings. These balls were included to see whether detection capability is sensitive to the minor differences in material properties (particularly permeability variations), that are likely to be represented in this assortment from various sources. A list of all the sizes and materials used is given in the table below. 6
Table 1 Metal Ball Diameters (mm) and Materials 100Cr6 AISI 420 AISI 316 Aluminium Copper Bronze Brass Bearing 2.4 3 3.2 4 4 4 4 4.7 4.76 4.7 5 5 5 5.5 5.6 5.6 5.56 5.6 6 6 6 6.35 6.35 6.35 6.5 6.35 7 7.1 7 7.4 8 8 8 8.7 8.6 9 9 9.5 9.5 9.5 10 10 10 10 10.3 11.1 11.9 12 12 12.7 12.7 12.7 13.5 13.49 14 14 14 14.3 15 15.1 15.9 15.88 15.88 16 16 17.46 17.46 18 18 19.05 19 20 20 20 20 23.8 22.23 25 25.4 25.4 25.38 4 Detectors Tested Several detectors in common use in humanitarian demining (plus one prototype) were used for these tests. There was no attempt to make this a comparative trial of detectors, but as wide a range of models as possible was tested. If the tests conceived by this work are established as standards, they need to be valid for all of the detectors commonly in use. Some of the detectors belong to JRC, others became available on loan. The tests were therefore made as the detectors became available and the measurements reported here span a period from October 2002 to July 2003. 4.1 Detector Characteristics The range of metal detectors commonly used for demining can be characterized according to the way that they function. 7
4.1.1 Continuous Wave and Pulsed Techniques Some detectors use a continuous sinusoidal current to excite the transmit coil of the sensor head. A single frequency or multiple frequencies can be used the latter to help reject noise from soils, for example. The detection signal is obtained by phase-sensitive detection of the receive-coil voltage, or of the impedance change of a single transmit/receive coil. Other detectors excite the transmit coil with a short broad-band pulse. The received signal is based on the decay of this pulse as detected in the receive coil. 4.1.2 Dynamic and Static Mode Many detectors can be held stationary over a metal target and the alarm will sound continually, this is known as a static alarm mode. Other detectors employ high-pass filter or periodic auto-balancing as a way of rejecting constant or slowly-varying background noise. This means that if the detector is held stationary over a target, the alarm will die away. The detector therefore needs to be swept continuously over the suspect area (or a test target) to give optimum detection capability. These features are known in general as dynamic modes of operation. 4.1.3 Coil Configuration The coils used to generate and detect magnetic fields in the sensor heads of metal detectors have a variety of designs. The main feature that the detector operator needs to be aware of is whether or not the coil has a simple or "absolute" spatial response or a "differential" spatial response. The latter type of design is called variously "double-d coil" and "gradiometer coil". In certain detectors, the coil is realized in such a way that there are two lobes of the sensor head which produce equal and opposite responses when a target is underneath them. This design aims to reduce noise from soil or other external sources that is approximately constant over both lobes. The output of the detector is the difference between the signals "seen" by each lobe. In the centre of the detector, between the lobes, there is a line, where the influence from a target on both lobes is equal and so there is poor detection capability. This feature can be used to locate metal objects accurately; the detector is swept from side to side, alarming as first one lobe, then the other passes over the metal object. The null between the alarm locations is at the position of the object. 4.1.4 Ground Compensation It has been recognized that the effect of certain soils on metal detectors is a major factor limiting their performance. Many manufacturers have therefore developed functions to reduce or eliminate the effect of soil on their detectors. These ground (or soil) compensation functions often require a the operator to perform an optimization for the soil under test. Where no such functions are provided, it may be necessary to reduce the sensitivity setting of the detector to prevent the soil giving spurious alarms. 4.2 Detector Models 4.2.1 Ceia MIL-D1 The MIL-D1 is a continuous-wave, multi-frequency detector with a differential ("gradiometer") sensor coil, alarming when there is a difference between the signals from the two sides of the coil. This detector has a static alarm response (the alarm is maintained if the sensor is held stationary over the target). There is a ground compensation function that can be initiated by the operator to suppress noise from soils. Two MIL-D1 detectors running with version 3.30 firmware were tested (s/n 20314002045, 20314002179). 8
4.2.2 Ebinger 421 GC The 421 GC is a pulsed-induction detector with a simple (non-differential) sensor design. Two versions of this detector were tested. One version (s/n 518) had a dynamic alarm response, the others (s/n 723, 724) gave the choice of static or dynamic response. In both cases there are separate sensitivity and ground compensation controls, both of these controls having continuous adjustment. 4.2.3 Foerster Minex 2FD 4.500 The Minex 2FD 4.500 is a continuous-wave twin-frequency detector with a differential ("gradiometer") sensor coil. There is one elliptical excitation coil (290mm 210mm) and two semi-elliptical receive coils in opposition, giving a differential spatial response (the difference between what is "seen" by the two lobes of the coil). The coil is excited by two simultaneous continuous wave frequencies (2.4kHz and 19.2kHz). The output signal is derived from the phasesensitive detected voltages at these frequencies on the receive coils. The alarm sounds when the output signal exceeds positive or negative thresholds, according to whichever of the semi-elliptical receive coils is nearer to the target. Different sound pitches are used for the positive and negative alarms. There is no auto-zero or high-pass filter function on the output, so the detector can be used stationary - it has a static alarm response. There is a function that can be initiated by the operator to suppress noise from soils, using a weighted difference of the quadrature components. There is also a three-step sensitivity control (Llow, M-medium, H-high). Two examples of model 4.500.01, batch 166 645 2 were tested, with serial numbers 172 and 536. Serial number 172 is non-standard, because it has been adapted for JRC by the manufacturer so that it has an analogue output signal (not used in this work). 4.2.4 Guartel MD8 The MD8 is a pulsed-induction detector with a differential sensor coil. There is a 320mm-diameter circular transmit coil and two semi-circular receive coils, connected differentially. The detector has a dynamic response the alarm signal dies away if the detector is held over a target for several seconds due to an auto-zero function that operates every few seconds. There is no specific soil compensation function. If soil noise is a problem, the sensitivity must be reduced with the threestep control. 4.2.5 Minelab F3 (Prototype) The F3 is a pulsed-induction detector with a simple (non-differential) sensor design. The alarm response is static and there is a soil compensation function that can be initiated by the user. Sensitivity can be changed by changing the colour-coded caps on the end of the detector. A black cap is used for high sensitivity (normal) and a red cap for lower sensitivity. 4.2.6 Schiebel AN 19/2 The AN 19/2 (also known by its US Army designation AN/PSS-12) is a pulsed-induction detector with a simple coil (non-differential) sensor design. The search head is circular, with a 260mm diameter. An audio alarm sounds when the output signal exceeds a variable threshold the sensitivity control. The sensitivity control has a continuous adjustment, i.e. no pre-selectable positions. The alarm response is static. There is no specific soil compensation function, other than reducing the sensitivity control. Two examples were tested, a pre-production version (s/n TEST2) and a "Mode-7" production version (s/n 92748). 4.2.7 Vallon ML1620C and VMH-2.1 The ML1620C and the VMH-2.1 are pulsed-induction detectors with simple coil (non-differential) sensor designs. The shape of the search head is elliptical, being longer in the "fore-and-aft" direction, but is slightly flattened at the ends. The search head has length 300mm & width 170mm. 9
The alarm response is dynamic. There are soil compensation settings operating slightly differently in the two models. There is a coarse sensitivity control, allowing the user to select the low sensitivity M ("metal-mine") setting or the high-sensitivity P ("plastic-mine") setting. There is also a fine control knob. The serial number of the ML 1620C used is 1018, that of the VMH-2.1 is 1185. 5 Test Equipment 5.1 In-air Test Jig For measurements of the maximum detection height of detector above a target in air, a non-metallic jig was used. This jig includes a platform that can be slid up and down on three pillars to vary the target height. The detector is swept or held on a fixed upper platform. The height of the mechanism that supports the target can be adjusted relative to the sliding platform in such a way that the top of the target can be set to a reference height. The height of the detector above the target can then be read directly from the scales on the pillars for any target. The jig enables maximum detection height to be measured from 0 to about 300mm. For greater heights, plastic blocks are placed on top of the jig and the detector swept on these. In-air detection height measurements were always made in a low-metal environment, to avoid signals from metal objects other than the test target. detector target height above target adjustable-height platform (about 0,3 m travel) height scale target height zerosetting adjustment Figure 1 Non-Metallic Jig for In-air Measurement of Maximum Detection Height 10
a b Figure 2 In-air Measurement Jig (a); Jig in Use (b) 5.2 Soil Test Box In order to measure the capability for detecting targets in soil, a soil-filled non-metallic box 1m 1m 0.5m deep was used. The soil box was filled with a soil known to have an influence on metal detectors. It originates from the Naples area of Italy and is of volcanic origin. The soil properties are not claimed to be representative of any particular soil that may be encountered by deminers. The experiments using this soil were performed to investigate whether such a test could be used to evaluate the capability of detectors to detect metal targets in noisy soil. The box was installed in a low-metal laboratory to avoid any signals from metal objects other than the test target. The soil susceptibility was measured using a Bartington MS2 susceptibility meter and MS2D surface coil (operating at 958Hz [6]) to be 450 10-5 in SI units. The susceptibility as measured on 10cm 3 samples with the MS2 and an MS2B 2-frequency enclosing coil is between about 680 10-5 and 780 10-5, depending on the packing of the sample and the frequency used. The measured susceptibility at low frequency (465Hz) is higher by about 10 10-5 in SI units than it is at high frequency (4.65kHz). Another way to express the soil susceptibility is in terms of the "mass susceptibility". This is the susceptibility as measured above (sometimes called "volume susceptibility") that is normalized to soil density. This gives a property that is more a characteristic of the soil and less dependent on the sample packing. The mass susceptibility is 450 to 500m 3 kg -1 (SI). Targets were inserted into a plastic tube extending from the bottom of the box to the top surface and the depth of targets below the soil surface could be varied. The target was positioned in the tube on a plastic rod. The position of the target within the tube could be measured by measuring the length 11
of the rod protruding from the bottom of the tube. The tube outer diameter is 30mm. For each detector used, it was checked that this "void" in the soil did not cause a detector signal. Detectors were swept on a board over the soil surface, to maintain constant sweep height. The effective sweep height above the surface was 15mm. a b Figure 3 Soil Test Box; (a) Overhead View, (b) Side View 6 In-air Tests Each of the detectors was set up according to the manufacturers' instructions to give the highest sensitivity in air. The detectors were each swept manually over the target using the jig described in 5.1 above. The rate at which the detector was moved over the target and the time between successive sweeps over the target were adjusted to produce the best conditions for detection, depending on the individual characteristics of the detector. The maximum detection height was measured for the sets of balls of different metals. The results were plotted as curves of maximum detection height against ball diameter. The measurement error for determining the maximum detection height is estimated to be approximately 5mm. These in-air tests extend the scope of previous work [6] with a larger range of metals, ball diameters and detector models. These curves may be used to define the detection capability in terms of a minimum detectable ball size at a given detector height above target. The results of these measurements are given in Table 6 to Table 16 and Figure 4 to Figure 14. For clarity the error bars are not shown on the Figures. 6.1 Overview of Detection Capability Curves The results show a considerable variation of detection capability for different detectors, but the general pattern is the same for most detectors. Larger targets are required to trigger the alarm at greater heights, but when the height becomes much more than the sensor coil diameter, the curve flattens off and it needs very big metal objects to trigger the alarm. In all cases the curve for the 100Cr6 chrome steel balls is used as a reference, since this is the material adopted as a reference standard in CWA 14747. The maximum detection height of a 10mm-diameter ball of 100Cr6 varies between 140mm and 240mm for the different detectors. Using minimum detectable ball diameter as a parameter to express detection capability, the smallest detectable 100Cr6 ball at 100mm height varies between 4 and 7mm for the detectors tested. 12
6.2 Ceia MIL-D1 In-air Curves The transition between alarm/no alarm for the MIL-D1 is very clear, making it easy to define the maximum detection height. Detection was deemed to have occurred if a consistent alarm was heard for one or other of the lobes of the sensor head. It was possible to set the continuous sensitivity adjustment knob to maximum for these in-air tests (i.e. beyond the red-spot on the dial that is the "recommended" level for normal use). The in-air detection capability measurements are listed in Table 6 and shown in Figure 4. The curve for the chrome steel balls shows a high detection capability, a 10mm ball being detected at 240mm. At 100mm a 4mm ball is the minimum target detected. The AISI 420 ferromagnetic stainless steel balls follow the chrome steel quite closely, with some evidence that AISI 420 is slightly less detectable. Small balls of AISI 316 stainless steel are much less easy to detect than the chrome steel, a 5mm ball only being detectable when right next to the detector. At 100mm height, the minimum detectable target is about 8mm for AISI 316. With bigger balls and greater heights, the AISI 316 measurements approach but remain below those of the chrome steel. Small balls (5 to 7mm diameter) of aluminium are slightly easier to detect than chrome steel balls of the same size. In contrast, the larger balls (16mm) have a significantly smaller maximum detection height. 6.3 Ebinger 421 GC s/n 518 In-air Curves The 421GC serial number 518 was adjusted in air by turning the soil compensation knob fully anticlockwise and using the maximum level of the sensitivity knob that is possible without getting spurious signals. The in-air detection capability measurements are listed in Table 7 and shown in Figure 5. The curve for chrome steel balls shows a curious behaviour for this detector. For small balls, the sensitivity maximum detection height increases steadily with increased diameter, but the curve flattens very quickly (above 12mm diameter) and even appears to dip at 25mm diameter. A 10mm ball is detected at 150mm and the minimum detectable target at 100mm height is 7mm. Small balls of AISI 316 stainless steel are much less easy to detect than chrome steel balls of the same diameter. At 10mm diameter, the curve for the 316 meets that for the chrome steel and at larger diameters the 316 balls are easier to detect than the chrome steel, with the curve flattening at higher diameters than the chrome steel, but also dipping at 25mm. Apart from the smallest ball (5mm), which falls well below, the aluminium balls lie near to the chrome steel curve. The reason for the flattening and dipping of the curves at large diameters is not known. It may be that the characteristic response times of such large balls fall outside of the bandwidth of this detector. 6.4 Ebinger 421 GC s/n 724 In-air Curves The 421GC serial number 724 was adjusted on setting 4 (static response, no ground compensation) and the sensitivity knob adjusted to the highest level possible without spurious signals. The in-air detection capability measurements are listed in Table 8 and shown in Figure 6. 13
The curve for chrome steel balls does not show the same reduced response for large balls as s/n 518. The detection capability is quite high, with a 10mm ball detectable up to 180mm. The minimum detectable ball at 100mm height is about 6mm diameter. At the time of writing, no measurements had been made for balls of other metals. 6.5 Foerster Minex 2FD 4.500 (s/n 172) In-air Curves The Minex 2FD 4.500.01 s/n 172 (non-standard) was tested at the highest sensitivity setting (H) for these in-air measurements. Detection was deemed to have occurred if a consistent alarm was heard for one or other of the lobes of the sensor head. The in-air detection capability measurements are listed in Table 9 and shown in Figure 7. The maximum detection height for a 10mm chrome steel ball is 185mm and the minimum detectable ball at 100mm is about 4.5mm diameter. The detection capability for ferromagnetic AISI 420 stainless steel is close to that of the chrome steel for the whole range measured, with some evidence that AISI 420 is slightly less detectable. The non-magnetic AISI 316 stainless steel has a lower maximum detection height for most of the range of diameters, except for the largest balls, for which the detection capability approaches that for the chrome steel. For the very smallest aluminium balls (5, 6mm), the maximum detection height is the same as the chrome steel. The max detection heights for larger balls fall below the chrome steel curve. The bronze balls fall consistently below the chrome steel in maximum detection height, as do the brass balls, although the smaller brass balls are closer to the steels. The steel bearings of unknown specification have maximum detection heights falling between those of the chrome steel and the AISI 420 balls. 6.6 Foerster Minex 2FD 4.500 (s/n 536) In-air Curves The Minex 2FD 4.500.01 (s/n 536) was tested at the highest sensitivity setting (H) for these in-air measurements. A signal was required from one or other of the lobes of the sensor head before detection was recorded. The in-air detection capability measurements are listed in Table 10 and shown in Figure 8. The maximum detection height for a 10mm chrome steel ball is 195mm and the minimum detectable ball at 100mm is 4mm in diameter. At the H setting, s/n 536 can be seen to have a slightly higher detection capability than does s/n 172. The detection capability for ferromagnetic AISI 420 stainless steel is close to that of the chrome steel for the whole range measured, with no clear evidence in this case that AISI 420 is any less detectable. The non-magnetic AISI 316 stainless steel has a lower maximum detection height for most of the range of diameters, except for the largest balls, for which the detection capability approaches that for the chrome steel; i.e. the same behaviour as for the other model. 14
For the very smallest aluminium balls (5, 6mm), the maximum detection height is the same as the chrome steel. The max detection heights for larger balls fall below the chrome steel curve. The bearing balls of unknown steel behave similarly to the chrome steel balls. The small bronze and brass balls have maximum detection heights similar to the ferromagnetic steels, but above about 9mm diameter the max detection height starts to fall below that of the chrome steel and the AISI 420. Note: These two examples of the 4.500 were nominally identical at manufacture, but they show different in-air detection capability on setting H. It may be that the modification of the s/n 172 model has increased noise levels that required the "H level" sensitivity to be reduced slightly. 6.7 Guartel MD8 In-air Curves The Guartel MD8 was tested at its highest sensitivity setting (III) for these in-air measurements. Detection was deemed to have occurred if a consistent alarm was heard for one or other of the lobes of the sensor head. The in-air detection capability measurements are listed in Table 11 and shown in Figure 9. The maximum detection height for a 10mm chrome steel ball is 190mm and the minimum detectable ball at 100mm is about 5mm diameter. The detection capability for ferromagnetic AISI 420 stainless steel is consistently below that of the chrome steel for the whole range measured. The non-magnetic AISI 316 stainless steel has a lower maximum detection height than the chrome steel for all of the range of diameters. Near the middle of the diameter range (about 10 to 12mm), the detection capability approaches that for the AISI 420. For the very smallest aluminium balls (5mm), the maximum detection height is just below that of the chrome steel. The max detection heights for larger balls fall well below the chrome steel curve. The maximum detection height for the bronze and brass balls is consistently below that of the chrome steel, although for the smallest balls (5 to 6mm), only just below. The steel bearings of unknown specification have maximum detection heights similar either to those of the chrome steel or the AISI 420 balls. 6.8 Minelab F3 (Prototype) In-air Curves The Minelab F3 prototype was set up for optimum in-air sensitivity with the black cap (highsensitivity) in place. The in-air detection capability measurements are listed in Table 12 and shown in Figure 10. The maximum detection height for a 10mm chrome steel ball is 155mm and the minimum detectable ball at 100mm is 6mm in diameter. The detection capability for ferromagnetic AISI 420 stainless steel is close to that of the chrome steel for the whole range measured, with no evidence that AISI 420 is any more or less easy to detect. 15
Small balls of the non-magnetic AISI 316 stainless steel have a much lower maximum detection height than the chrome steel. Above 8mm diameter, the maximum detection height rapidly increases and by 12mm it is equal to that of the chrome steel. For the largest balls, above 14mm, the maximum detection height is above that of the chrome steel. For the small aluminium balls (5 to 9mm), the maximum detection height is the same as the chrome steel. The max detection heights for larger balls fall slightly below the chrome steel curve. The small bronze balls are slightly more detectable than the chrome steel and the larger balls are about the same as the chrome steel. The brass balls of all sizes follow the chrome steel curve fairly closely. The bearing balls of unknown steel behave similarly to the chrome steel balls. 6.9 Schiebel AN 19/2 In-air Curves The sensitivity was set to the maximum possible without the detector ever alarming when held in air. Care was required when comparing maximum detection height between different targets for these detectors as some drift in detection capability seems to occur with temperature change. The in-air detection capability measurements for the Mode 7 (s/n 92748) version are listed in Table 13 and shown in Figure 11. The in-air detection capability measurements for the "TEST2" preproduction model are listed in Table 14 and shown in Figure 12. As set, the Mode 7 detects a 10mm chrome steel ball at 165mm maximum and the minimum detectable chrome steel ball at 100mm is 6.3mm. The prototype version needed to be adjusted to a lower sensitivity, the 10mm chrome steel ball having a maximum detection height of 135mm and the minimum target diameter being 7mm at 100mm height. For both the prototype and the Mode 7 versions, the AISI 420 stainless steel shows a maximum detection height that is close to, but slightly below that of the chrome steel balls for all sizes/heights. Small balls of AISI 316 stainless steel have much smaller maximum detection heights than their chrome steel counterparts, however the difference is less marked for larger balls. Small balls of aluminium have a similar maximum detection height to the same size chrome steel balls, but larger balls (16mm) fall well below the chrome steel curve. Small bronze and brass balls have maximum detection heights close to those of chrome steel balls, but the larger balls (above about 12mm) fall below the chrome steel curve. The bearing balls of unknown steel are indistinguishable from the chrome steel. 6.10 Vallon ML1620C In-air Curves For the in-air tests, the ML 1620C was set up in "P" mode, with the 50Hz mains filter, in program 1 (max sensitivity) and with the sensitivity knob on 6 - the maximum possible whilst avoiding the alarm being triggered when the detector is in the air, away from metal. The ML1620C works in a dynamic mode, so it was swept over the target at a speed and with a length of "rest time" away from the target after each sweep that gave best detection capability. The in-air detection capability measurements are listed in Table 15 and shown in Figure 13. 16
With the ML1620C, the 10mm chrome steel ball is detected at 177mm maximum height and at 100mm the minimum detected chrome steel ball is about 5.8mm diameter. The AISI 420 balls are marginally less easy to detect than the chrome steel at all diameters/ heights with the ML1620C. Small balls of AISI 316 have small maximum detection heights compared to the chrome steel, but for larger balls this difference diminishes. Small balls of aluminium (and copper) are marginally easier to detect than the chrome steel equivalent. Larger balls (about 8 to 16mm) are however less easy to detect. The max detection height for bronze balls fall close to, or just below that of the chrome steel balls at small diameters, but at larger diameters this difference is more marked. A similar behaviour is seen with the brass balls. The bearing balls of unknown steel behave similarly to the chrome steel balls. 6.11 Vallon VMH-2.1 In-air Curves The VMH-2.1 was set up similarly to the ML 1620C; "P" mode, 50Hz mains filter, soil mode knob at position 1 (max sensitivity) and with the sensitivity knob at the 1 o'clock position. The in-air detection capability measurements are listed in Table 16 and shown in Figure 14. In these settings, the VMH-2.1 has a higher detection capability than the ML1620C. The 10mm ball is detected up to 230mm and the minimum chrome steel ball at 100mm is 4.5mm in diameter. The AISI 420 balls have broadly equivalent maximum detection height to those of chrome steel. The maximum detection height for small balls of AISI 316 is small compared to chrome steel, but for larger balls this difference diminishes. Small balls of aluminium (and copper) are as easy to detect as the chrome steel equivalent. Larger balls (about 7 to 16mm) are however less easy to detect. Small brass and bronze balls are detectable at the heights greater than or close to those at which chrome steel balls are detected. At larger diameters (above 10mm) they are much less detectable however. The bearing balls of unknown steel behave similarly to the chrome steel balls. 6.12 Summary of In-air Maximum Detection Capability The results of the detection capability defined by the in-air measurements are summarized in Table 2 below. Note that this only shows the capability as set in air and should not be used to indicate the detection performance in soil. 17
Detector Table 2 Detection Capability at Maximum In-air Sensitivity Maximum detection height of 10mm 100Cr6 ball (mm) Minimum detectable 100Cr6 ball diameter at 100mm height (mm) Ceia MIL-D1 240 4.0 Ebinger 421GC s/n 518 150 7.0 Ebinger 421GC s/n 724 180 6.0 Foerster Minex 2FD s/n 172 185 4.5 Foerster Minex 2FD s/n 536 195 4.0 Guartel MD8 190 5.0 Minelab F3 prototype 155 6.0 Schiebel AN 19/2 m7 s/n 92748 165 6.3 Schiebel AN 19/2 s/n TEST2 135 7.0 Vallon ML 1620C 177 5.8 Vallon VMH 2.1 230 4.5 7 Soil Box Tests Each detector was adjusted so that it would not give an alarm from the soil itself when brought into contact with the soil surface in the soil box, away from the edge. Soil compensation functions were used where available to give the best sensitivity possible according to the manufacturer's operating instructions. In some cases the only adjustment possible is a reduction in sensitivity. In one case (AN 19/2 Mode 7), it was not possible to reduce the sensitivity control to a sufficiently low level to prevent alarms from the soil, so it was not possible to test this detector. For other detectors it was not possible to eliminate the signal from the soil-air discontinuity at the sides of the box. As long as no alarm occurred when the sensor head was raised and lowered on the soil, the measurements were made. Once the detector was set up for the soil, the maximum detection depth in the soil was measured as a function of target size using 100Cr6 steel balls. These results are plotted as maximum detection depth vs. ball diameter and compared with the previous in-air result, correcting for the 15mm sweep height above the soil surface. For some detectors the in-air measurement was also repeated with the detector as set up for the soil. For most detectors, adjustment to the soil produced a reduction in detection capability, although for some detectors it is maintained. At a depth of 100mm and sweep height of 15mm, the minimum detectable ball size varies from 6 to 14mm diameter. Note that this is an experiment to determine whether this test gives valuable information about a detector's in-soil performance. The actual results are specific to the particular soil in which tests were made and cannot be assumed to apply generally to all soil types. 7.1 Ceia MIL-D1 In-soil Detection Capability The soil signal was eliminated using the ground compensation function. It was possible to leave the sensitivity knob on maximum. The maximum detection depth for chrome steel balls using the Ceia MIL-D1 is shown in Figure 15 and in Figure 16 after completing the ground compensation procedure on two occasions. There is a large reduction in the detection capability compared to that in air, the minimum target detected at a depth of 100mm and a sweep height of 15mm being 8.5mm to 10mm in diameter. 18
The in-air measurements at the same setting as used for the soil measurements give approximately the same effective detection depth as for the tests in the soil itself. For this detector, this suggests that setting up the detector on the soil and measuring the in-air detection capability would be a valid test of the in-soil capability. It is clear that the effectiveness of the ground compensation depends to an extent on exactly how the set-up procedure is performed. 7.2 Ebinger 421GC s/n 518 In-soil Detection Capability The ground compensation and sensitivity controls were used together to eliminate the soil signal whilst maintaining the highest sensitivity possible. The maximum detection depth for chrome steel balls using the Ebinger 421GC s/n 518 is shown in Figure []. There is no reduction in the detection capability compared to that in air, in fact the detection capability is slightly improved. The minimum target detected at a depth of 100mm and a sweep height of 15mm is 7mm in diameter. The flattening of the in-air detection curve at large diameter is not so marked on the in-soil curve. 7.3 Ebinger 421 GC s/n 723 In-soil Detection Capability Setting 4 (static response, ground compensation) was used. The ground compensation and sensitivity controls were then used together to eliminate the soil signal whilst maintaining the highest sensitivity possible. The maximum detection depth for chrome steel balls using the Ebinger 421GC s/n 723 is shown in Figure 18. There is a reduction in the detection capability compared to that in air. The minimum target detected at a depth of 100mm and a sweep height of 15mm is 9.5mm in diameter. 7.4 Foerster Minex 2FD 4.500 (s/n 172) In-soil Detection Capability The ground compensation function was used to reduce the soil signals, however it was also necessary to reduce the sensitivity control to the "L" (low) position to eliminate soil signals. The maximum detection depth for chrome steel balls using the Minex 2FD 4.500.01 s/n 172 is shown in Figure 19. There is a reduction in the detection capability compared to that in air. The minimum target detected at a depth of 100mm and a sweep height of 15mm is 12mm in diameter. Measuring the detection capability in air at the same setting gives approximately equal results to the in-soil measurements. For this detector, this suggests that setting up the detector on the soil and measuring the in-air detection capability would be a valid test of the in-soil capability. 7.5 Foerster Minex 2FD 4.500 (s/n 536) In-soil Detection Capability The ground compensation function was used to reduce the soil signals, however it was also necessary to reduce the sensitivity control to the "L" (low) position to eliminate soil signals. The maximum detection depth for chrome steel balls using the Minex 2FD 4.500.01 s/n 536 is shown in Figure 20. There is a reduction in the detection capability compared to that in air. The minimum target detected at a depth of 100mm and a sweep height of 15mm is 11mm in diameter. 7.6 Guartel MD8 In-soil Detection Capability There is no ground compensation function on the MD8. To prevent the detector from alarming constantly on the soil, the sensitivity was set at the lowest (I) setting. The maximum detection depth for chrome steel balls using the MD8 is shown in Figure 21. There is a large reduction in the detection capability compared to that in air. The minimum target detected at a depth of 100mm and a sweep height of 15mm is 18mm in diameter. Measuring the detection capability in air at the soil setting gives approximately equal results to the in-soil measurements. For this detector, this suggests that setting up the detector on the soil and measuring the in-air detection capability would be a valid test of the in-soil capability. 19
7.7 Minelab F3 (Prototype) In-soil Detection Capability The ground compensation function was used to eliminate signals from the soil. The maximum detection depth for chrome steel balls is shown in Figure 22. There is no change in the detection capability compared to that in air. The minimum target detected at a depth of 100mm and a sweep height of 15mm is 6mm in diameter. 7.8 Schiebel AN 19/2 In-soil Detection Capability The sensitivity control on the AN 19/2 detectors was reduced until no signal was obtained from the soil. This was not possible, for the soil used, with the Mode 7 detector it was not possible to reduce the sensitivity far enough. The maximum detection depth for chrome steel balls as measured using the AN 19/2 s/n TEST2 is shown in Figure 23. There is a reduction in detection capability compared to that measured in air. The minimum target detected at a depth of 100mm and a sweep height of 15mm is 13.5mm in diameter. With the detector still adjusted for the soil, the in-air measurement of the chrome steel balls was repeated. The maximum detection height so measured is less than that measured in the soil. This demonstrates that it is not valid (for this detector) to adjust the detector to the soil and then measure the in-air detection capability under the assumption that this is equal to the in-soil detection capability. Such a test would underestimate the detection capability in soil. This result suggests that the soil and target signals are additive in some way. The sensitivity control is basically an alarm threshold. When the soil is absent, the target signal needs to be bigger before it can break the alarm threshold. 7.9 Vallon ML1620C In-soil Detection Capability The ML 1620C was adjusted to the soil by switching to program 3 and making adjustment with the fine compensation control. The "P" setting was used and the sensitivity knob kept at 6, as for the in-air tests. The maximum detection depth for chrome steel balls is shown in Figure 24. There is a reduction in detection capability compared to that measured in air. The minimum target detectable at a depth of 100mm and a sweep height of 15mm is 12.5mm in diameter. With the detector still adjusted for the soil, the in-air measurement of the chrome steel balls was repeated. The maximum detection height so measured is approximately the same (perhaps slightly less for large balls/depths) as that measured in the soil. This suggests that adjusting the detector to the soil and measuring the in-air capability may be a valid way to tests for the effect of soil on this detector. 7.10 Vallon VMH-2.1 In-soil Detection Capability The VMH 2.1 was adjusted to the soil by switching to soil setting 2 and making fine adjustment with the push-button compensation control. The "P" setting was used and the sensitivity knob kept at the 1 o'clock position, as for the in-air tests. The maximum detection depth for chrome steel balls is shown in Figure 25. There is a reduction in detection capability compared to that measured in air. The minimum target detectable at a depth of 100mm and a sweep height of 15mm is 7.8mm in diameter. 7.11 Summary of In-soil Detection Capability Table 3 below summarizes the detection capability of all of the detectors tested for the particular soil type used in the soil box tests. 20