Microsoft Word - Magnetometer Systems for EOD on Land.doc

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Specialising in:
•  explosive ordnance detection
      and remediation
•  minerals exploration
•  project management
•  environmental geoscience
•  engineering geoscience

MAGNETOMETER SYSTEMS

FOR

EXPLOSIVE ORDNANCE DETECTION ON LAND

John M. Stanley, PhD

Malcolm K. Cattach, PhD

Conventional  magnetic  locators  were  analogue  instruments.  These  have  now  been  greatly
outperformed  by  digital  systems.  Analogue  locators  rely  on  the  operator  watching  a  meter  and
listening to an audio tone as the sensing probe is systematically scanned across the survey area. These
systems  depend  heavily  on  operator  concentration  and  subjective  judgements  during  the  search.
Digital  magnetometers  outperform  their  analogue  predecessors  in  survey  speed  and  detection
performance.  They  verify  the  integrity  of  the  search  and  enable  a  detection  assurance  level  to  be
quantified.

Contents

List of Figures

iii

List of Tables

iii

ABSTRACT

THE USE OF MAGNETICS IN ORDNANCE LOCATION

REQUIREMENTS OF AN EOD MAGNETOMETER

TOW CONVENTIONAL ANALOG MAGNETOMETERS

The Ferex 4.021

The Varian V92 / Mk 22

DIGITAL EOD MAGNETOMETERS

RELATIVE PERFORMANCE OF CONTEMPORARY MAGNETOMETERS

THE TM-4 IMAGE PROCESSIN MAGNETOMETER SYSTEM

The Data Acquisition Unit

The Transportable Data Processing Unit

THE TM-4 OPERATING PROCEDURE

The Field Procedure

The Data Processing Procedure

TM-4 SEARCH ASSURANCE LEVELS

CASE EXAMPLES WHRE THE TM-4 MAGNETOMETER WAS USED

CONCLUSIONS

APPENDIX

Technical Specifications of the TM-4 Magnetometer

List of Figures

A typical magnetic profile showing “signal' and "noise".

Magnetic anomalies intensities over a selection of ordnance devices

showing the limit of sensitivity for each EOD magnetometer.

Total field intensity and gradient anomaly amplitudes over a

selection of ordnance devices.

The TM-4 magnetometer showing the optically pumped magnetic

sensor and the data acquisition and control unit.

The TM-4 data acquisition and control unit.

An example of It TM-4 search assurance graph for a particular area

where the soil type was classified as "sandy" and the magnetic noise was 2.5 nT.

Isoplot images of magnetic anomalies measured with a TM-4

magnetometer over a five inch rocket head and four practice bombs, all buried at two metre depth.

An Isoplot of magnetic anomalies measured with a TM-4 over a

contaminated World War II artillery range.

An enhanced colour image of a 'contaminated site showing only

magnetic anomalies due to UXO.

10.

An Isoplot showing the TM~4 record after: a contaminated area

had been cleared.

List of Tables.

Detection limits for: an 81 mm Mortar in conditions of zero magnetic

noise and conditions where geological magnetic noise was 10 nT.

Detection limits for a 1000lb HE Bomb in conditions of zero

magnetic noise and conditions where geolt,'lgic81 magnetic noise was 10 nT.

An example of an interpretation record with the anomaly "source"

entry completed by the field excavation crew.

iii

MAGNETOMETER SYSTEMS

FOR EXPLOSIVE ORDNANCE DETECTION ON LAND

John M Stanley, PhD

Malcolm K Cattach, PhD

ABSTRACT

The  use  of  magnetometers  to  detect  sub-surface,  unexploded  ordnance  is  not  new.  Conventional  magnetic
locators  were  analogue  instruments.  These  have  now  been  greatly  outperformed  by  digital  systems.  Analogue
locators  depend  upon  the  operator  watching  a  meter  listening  to  an  audio  tone  as  the  sensing  probe  is
systematically  scanned  across  the  survey  area.  These  systems  depend  heavily  upon  operator  concentration  and
subjective  judgements  the  time  of  searching.  Further  more,  analogue  detectors  do  not  produce  a  record  of  the
magnet data from which the effectiveness of the search may be verified.

Digital magnetometers outperform their analogue predecessors in survey speed and detection performance. They
provide verification of the integrity of the search and they enable a detection assurance level to be quantified.

Two distinct types of magnetic sensors are used for detection in Explosive Ordnance Disposal (EOD). One
measures the gradient of the vertical component of the magnetic field (the difference between two fluxgate type
sensors)- The other type measures the intensity of the total magnetic field. Gradiometers are shown to be much
less sensitive to deeper targets.

A  comparison  was  made  between  the  performance  of  gradiometer  and  total  field  intensity  analogue  locators
(Forster 4.021 and US Navy's Mk22) and gradiometer and total field intensity, digital recording systems (Forster
CAST and TM-4). Each instrument was compared when used both in areas of high, medium and low levels of
magnetic  interference  from  geological  sources.  In  low  noise  areas  the  depth  to  which  a  UXO  target  may  be
detected  was  shown  to  always  be  3  times  greater  with  the  TM-4  than  with  the  Mk22,  and  at  least  3.5  times
greater than with the CAST or 4.021 systems. In medium noise areas, the detection depth was 1.5 times greater
with the TM-4 than with the Mk22 and up to 2.5 and 3 times greater than with the CAST and 4.021 systems. In
areas of high noise, the detection depth was up to 2.5 times greater with the TM-4 than with the CAST system
and analogue systems failed completely.

The  most  sensitive  and  best  performing  magnetic  EOD  system  presently  available  is  the  model  "TM-4"
manufactured  by  Geophysical  Technology  Pty.  Ltd.  This  magnetometer  system  enables  both  the  position  and
total  magnetic  field  intensity  to  be  automatically  determined.  Measurements  to  0.01  nT  resolution  can  be
recorded at up to 100 per second. Typically, two hectares (about 4.5 acres) can be surveyed on foot per day, or
up to 25 hectares (ha) using the TM-4 with multi-sensors mounted on a quad cycle All Terrain Vehicle (ATV).
At least foul measurements per square metre ( 40,400 per ha) are recorded. This very high density of data makes
image processing practical and effective. Computer aided interpretation tabulates the position, depth and size of
ferrous items requiring investigation. Assurance depths are defined objectively and quantitatively. The results of
the entire search process are fully documented and are verifiable from the data record.

An example of TM-4 data sets collected before and after the clearance of a contaminated artillery site
demonstrate the effectiveness of the search and verification procedure.

THE USE OF MAGNETICS IN ORDNANCE LOCATION

The location of dangerous, unexploded ordnance by the detection of anomalies in the earth's magnetic field is
inherently attractive because the magnetic sensor is totally passive. It can be scanned over a target zone without
emitting radiation capable of causing detonation. However, the magnetic method is only applicable to ordnance
which contains ferromagnetic materials such as iron.

Ferromagnetic materials become magnetic by induction from the earth's magnetic field. Hard iron also retains
remanent magnetization acquired when it was last heated above the Curie temperature of about 650 deg C. The
magnetic field associated with an object is the vector sum of the induced and remanent magnetization.

The magnetic field of a bar shaped body is dipolar, with a north and south magnetic pole. Iron ordnance devices
have a magnetic field which approximates that of a dipole. The intensity of a dipolar magnetic field decreases
with the inverse cube of the distance from the source. The intensity of the vertical magnetic gradient drops off
even faster, as the inverse fourth power of the distance from the source. The magnetic anomaly due to a small
iron object may only be detectable within a small distance. In order to locate a magnetic anomaly, several
measurements must be made within the area of measurable influence. If, for example, the magnetic detector is
operated half a metre above ground, then a minimum sample density of four uniformly spaced measurements per
square metre is required.

Magnetic anomalies in the earth's magnetic field can also arise from variation in the chemistry of the rocks and
soil from one position to another. Even at any one place, the Earth's field is forever changing by a small amount
with  time  due  to  natural  causes  and  it  may  also  change  as  a  result  of  electromagnetic  interference  from  mains
electricity.  Anomalies  due  to  changes  in  the  ground  geology  or  due  to  temporal  sources  can  be  referred  to  as
"magnetic noise" in the context of ordnance location.

Magnetic anomalies may be located by mapping the total magnetic field intensity, a component of this field or
by measuring the difference (gradient) between two total field or component sensors. The earliest magnetometers
could only measure a single component of the magnetic field. Modern atomic devices enable the total field
intensity to be measured with extreme precision.

REQUIREMENTS OF AN EOD MAGNETOMETER.

There  are  three  fundamental  requirements  for  a  magnetometer  to  be  suitable  for  verifiable  ordnance  clearance
operations.  The  device  must  be  sufficiently  sensitive  to  resolve  the  target  anomaly,  it  must  be  capable  of  very
rapid  measurement  of  both  magnetic  field  and  sensor  position  and  it  must  automatically  record  the  data
measured.

The analogue magnetometers used in the past for ordnance location do not meet these requirements and their
sole use for area clearance during peacetime could be deemed negligent in court. Analogue locators produce an
output which may be a meter deflection, an audible tone or both. The output varies with a change in the magnetic
intensity or its gradient. The tone may be continuously present, or it may be activated only when a preset
anomaly amplitude or rate of change is exceeded. As data is not recorded with these detectors, the target signal
must be recognized at the time of the survey. It must be assumed that the operator is diligent.

and that the equipment is performing properly. A target signal can only be recognised if it significantly exceeds
the background magnetic noise level characteristic of the sit. There is a high dependence upon the concentration
and subjective judgements of the operator who may be working under adverse conditions. On completion of the
search, the only record that the search was performed effectively is the word of the operator.

Digital recording magnetometers avoid the above problems. To make efficient use of digital data, both the value
of  the  measurement  and  its  position  must  be  automatically  recorded.  While  high  resolution  digital
magnetometers have been available since the mid 1960’s, the positioning and recording time of at least several
seconds per measurement has excluded their use in high definition surveying except where very small areas were
to  be  examined.  Digital  data  in  large  quantities  require  automatic  digital  processing,  preferably  onsite.  High
speed  data  acquisition  and  on-site  data  processing  are  essential  to  the  efficiency  of  verifiable  magnetic  EOD
operations.

It is only since the development of automatic resonance type magnetic detectors and digital microelectronic that
an efficient, digital magnetometer system suitable for ordnance detection could be built.

Before describing and comparing existing magnetic locating devices, consider a typical profile of magnetic field
variations  0.5  m  above  the  ground  (Figure  1  ).  The  magnetic  "noise"  on  this  profile  can  be  defined  as  those
variations  which  are  not  of  interest.  They  may  be  due  to  geological  or  temporal  sources.  In  this  example,  the
noise  envelope  has  an  amplitude  of  about  20  nanoTeslas  (uT).  The  "signal"  can  be  defined  as  the  magnetic
anomaly due to the target of interest that is to be located. In the profile shown, the signal amplitude is 100 nT.
The  "signal-to-noise"  ratio  of  this  data  is  therefore  five.  The  objective  of  the  magnetic  search  is  to  locate  and
identify the "signal" from within the "noise", preferably even when the signal-to-noise ratio is small.

TWO CONVENTIONAL ANALOG MAGNETOMETERS.

The two work-horses of magnetic EOD during the last 20 years have been the Foerster "Ferex" and the Varian
"V92" (known to the US Navy as the "Mk22"). The detection limitations of these instrument types can be
described in tennis of signal detection capability as follows:

The FEREX 4.021.

The  Ferex  locator  consists  of  a  pair  of  fluxgate  type  sensors  mounted  400  mm  apart,  in-line.  When  operated
vertically,  it  measures  the  difference  in  the  vertical  component  of  the  magnetic  field  between  the  sensors.  For
source depths greater than about 2 m, this difference approximates the vertical gradient of the vertical component
of  the  anomaly.  A  switch  enables  a  sensitivity  to  be  selected  from  a  ran~  between  3  nT  and  10.000  nT.  The
choice of sensitivity setting that can be used is determined by the amplitude of the noise envelope in the area of
the search.

The magnetometer output is provided by an audio tone and an analogue meter. The tone is emitted only when the
magnetic field exceeds a threshold which is 20% of full scale range. The frequency of the tone increases the
further this threshold is exceeded. The meter displays the magnetic intensity and its polarity.

The Varian V92 / Mk22.

This  Magnetometer  was  developed  by  Varian  Associates  and  is  now  manufactured  by  Scintrex  in  Canada  and
Geometrics in the USA. It is also known in the US Navy as the Mk22 locator.  The sensor probe is of the atomic
resonance type (optically pumped caesium) and it measures the total magnetic field intensity. A digital display is
provided with a resolution of 1nT. However as it is impractical to conduct a UXO search by mentally registering
changes  in  the  digital  display,  the  instrument  had  been  designed  to  emit  a  continuous  tone  of  which  the
frequency  changes  by  fixed  7  Hz  per  nT.  It  is  therefore  used  as  an  analogue  instrument  in  this  application.  A
good ear may  resolve a change of 15 or 20 Hz and therefore can detect a  magnetic field change of 2 or 3 nT.
However “signal” can only be recognised audibly if it exceeds the noise envelope by a factor of at least three. A
high  dependence  upon  operator  concentration  is  a  major  disadvantage  of  this  unit.  In  areas  where  there  is
appreciable  geological  magnetic  interference,  the  audio  tone  becomes  incomprehensible  because  the  tone
sensitivity is fixed.

Note: Neither the Ferex 4.021 nor the Varian V92 record data. Consequently, there is no way of demonstrating
that the search was effective, and there is no record that would facilitate a second opinion or withstand a legal
challenge

DIGITAL EOD MAGNETOMETER

As with the analogue locators, digital EOD magnetometer may be of the total field intensity type or they may be
vertical component gradiometers.

The inherent limitation of gradiometer type of instruments is the fact that the gradient anomaly over an item of
UXO attenuates more rapidly with depth below the sensor than does the total intensity anomaly. Therefore the
detection depth for a given size of object is always less for this type of instrument than for a total field intensity
magnetometer.

The Forster CAST system consists of a Ferex 4.021 gradiometer to which has been added a digital recording
option. Apart from the fundamental limitation of detection depth, the absence of an in-built odometer in the
CAST system severely impedes the survey coverage rate and the integrity of data collected in rough terrain.

The TM-4 is a total field intensity magnetometer. It has applied the latest technology to best solve the logistical
problems associated with EOD. Its performance has been compared directly with the other magnetometer
systems previously discussed.

RELATIVE PERFORMANCE OF CONTEMPORARY

MAGNETOMETERS.

In defining the performance of a magnetic technique for locating sub-surface ferromagnetic objects, the
important parameter to consider is the depth to which an object of a given size can be confidently detected, or the
minimum size of object that can be detected to a given depth. These parameters are related and are a function of
the resolution of the magnetometer, the intensity of magnetic noise, the spacing between adjacent passes of the
magnetic sensor and, in the case of non-recording detectors, the performance of the operator.

The measurement of the magnetic properties of a very large number of ordnance items has enabled a quantitative
measure to be made of the depth to which they can be detected using each of the magnetometer systems. The
same data also enables the detection depth to be defined for any given amplitude of magnetic interference.

The anomalous magnetic field form an ordnance device is dipolar. The amplitude of this anomaly is known to
attenuate as the inverse cube of the distance from the device. Therefore, a plot of the cube root of the reciprocal
of  the  measured  total  magnetic  field  anomaly  against  the  distance  from  the  target  is  a  straight-line  graph.  The
slope of the graph is dependent upon the size and intensity of magnetization of the magnetic source. In Figure 2,
data  from  a  sample  of  ordnance  devices  has  been  plotted.  The  exact  slope  of  each  line  varies  a  little  with  the
composition  of  the  steel  used  in  manufacture,  the  time  that  the  device  has  been  in  a  static  limit  of  detection,
determined only by the sensitivity of each of the four magnetic locator types being considered. In the case of the
Forster  4.021  and  CAST  instruments,  the  limit  of  detection  is  a  curved  line  due  to  the  more  rapid  decrease  in
anomaly amplitude with depth associated with the magnetic difference (or gradient). We can determine from this
graph what is the maximum depth to which the given target may be detected with each of the instruments.

Consider  first  the  performance  of  each  magnetometer  in  an  ideal  geological  environment  where  the  magnetic
noise  is  zero.  With  reference  to  the  limits  of  detection  in  Figure  3,  the  81  mm  Mortar  for  example  can  be
detected to a depth of about 2.3 m with the Forster Ferex 4.021 and CASE, 3.0 m with the Varian V 95 / Mk 22
and  9.8  m  with  the  TM-4.  These  figures  show  the  relative  resolution  of  each  instrument  but  they  do  not
necessarily  reflect  the  practical  detection  depths  because  the  assumption  of  zero  magnetic  noise  is  never
justified.

Consider now the effect of geological noise. It is the signal-to-noise ratio that will determine the depth to which
a target object may be detected in practice. It had been determined that for effective operation the Ferex 4.021
gradiometer required a signal-to-noise ratio greater that about 2 and that the V 92 / Mk 22 required a ratio greater
than 3. With the advantages of image processing, the digital systems required a signal-to-noise greater than 1 (in
cases where the noise source is shallow, deep signals may be detected even when the signal-to-noise ratio is less
than 1). With these values we can determine the signal amplitude that will be required, given a particular value
of the noise amplitude characteristic of the area being searched. For example if the near surface noises amplitude
is 10nT then the V 92 / Mk 22 requires a signal greater than 30nT, the Ferex 4.021 requires a signal amplitude
greater than 20nT while the CAST and TM-4 requires a signal greater than 10nT. With reference to Figure 3, we
can use these signal anomaly values to determine the practical depth of detection for this given noise situation. In
the case of the 1000 lb bomb, detection to a depth of 4.0 m would be expected with the Ferex 4.021, 4.7 m with
the CAST, 5.6 m using the V 92 / Mk 22 and 8.0 m with the TM-4

These latter figures for comparing the four instruments are realistic given the practical conditions encountered in
the field. Although the sensitivity of the V 92 / Mk 22 and the Ferex 4.021 is similar, the Ferex is better able to
tolerate magnetic noise. (A signal-to-noise ratio better than 3 is required for effective use of the V 92 / Mk 22
whereas a ratio of only 2 is required by the Ferex 4.021). This would suggest that the Ferex would out-perform
the V 92 /Mk 22 but this is not always the case. Only for very shallow and small objects is the superior noise
tolerance of the Ferex significant. Because the gradient anomaly decreased more rapidly with depth than the total
field anomaly (the curved lines on Figure 3 compared with the horizontal lines), the V 92 /Mk 22 in fact
performs better than the Ferex whenever the target is deeper than about one meter below the sensor.

Although the much higher absolute sensitivity of the TM-4 cannot always be used in the presence of noise, the
depth to which a target may be detected with the TM-4 must always be more than 1.5 times greater than when
using an analogue system, and it may be several times more after the use of digital signal enhancement. By
virtue of the characteristics of gradient and total intensity anomalies in geologically noisy locations, the TM-4
and CAST will detect equally small object only a “zero” depth. However, by 5 meters depth, the TM-4 will be
able to locate an object that is only half the size that could be located with the CAST in the same noise situation.

TABLE 1.

Detection Limits for an 81 mm Mortar in Conditions of Zero Magnetic Noise

and 10nT Magnetic Noise.

INSTRUMENT

ZERO NOISE

10 nT NOISE

Ferex 4.021
Forster CAST
V 92 / Mk 22
TM-4

2.3 m
2.3 m
3.0 m
9.8 m

1.1 m
1.3 m
1.4 m
2.1 m

TABLE 2.

Detection Limits for a 1000 lb HE Bomb in Conditions of Zero Magnetic Noise

And 10nT Magnetic Noise

INSTRUMENT

ZERO NOISE

10nT NOISE

Ferex 4.021
Forster CAST
V 92 / Mk 22
TM-4

6.2 m
6.2 m
11.0 m
40.0 m

4.0 m
4.7 m
5.6 m
8.0 m

THE TM-4 IMAGE PROCESSING MAGNETOMETER SYSTEM.

The proceeding data has demonstrated the TM-4 magnetometer to be the best performing magnetic EOD system
presently available. The TM-4 is not just a magnetometer, but a complete data acquisition, processing,
interpretation and documentation system.

The great advantages of the TM-4 system over contemporary ordnance locating magnetometers are that it
measures the total magnetic intensity and not just its gradient, it has a higher sensitivity and resolution, it
achieves this resolution in the very short measurement time of only 2.5 milliseconds per measurement, and most
importantly, it automatically records digital data including the position of each magnetic field measurement.

During its development, approximately 40,000 line kilometres of data sampled at 0.25 m intervals were recorded
and  processed.  This  considerable  quantity  of  data  was  recorded  in  a  diverse  range  of  geological  environments
and  in  temperatures  ranging  from  sub-zero  to  nearly  50 deg  C.  From  this  data-base  the requirements  of  signal
processing  software  have  been  extensively  researched  and  a  comprehensive  software  package  applicable  to
magnetic UXO identification and interpretation had been developed.

The combination of high resolution, very fast digital measurement rate and automatic position determination has
made very high definition magnetic surveys efficient and economically viable.

The TM-4 package consists of two units, one of which is used for data acquisition and the other for data
processing. Each unit is packaged in robust, light, alloy clad transit case for convenient and secure transport into
field locations.

The Data Acquisition Unit

The data acquisition unit may be either hand-carried or operated from a quad cycle ATV. The main components
for hand-held operation of this unit are:

•

An optically pumped, atomic resonance type magnetic sensor, with a sensitivity of 0.005nT.

•

Magnetometer electronics capable of recording the magnetic field values to a resolution of 0.01nT at a
rate of up to 100 per second, or to 0.1nT, 400 per second.

•

A DGPS is routinely used for positioning and navigation control. The type, either Standard (Mapping)
or Real Time Kinematic (Geodetic), is chosen according to the anticipated target size and therefore the
level of accuracy required.

•

An odometer, which measures the distance traversed by the sensor and which electronically initiates
each magnetic field measurement at programmable distance increments.

•

A combination of the DGPS and odometer control systems can be used in areas where satellite coverage
is problematic such as thick canopies in wooded areas and inside buildings.

•

Solid-state memory with the capacity to record survey line header information, operator comments and
up to 100,000 measurements.

•

Multi-tasking  operating  system  software  to control and  monitor  data  acquisition. Additional  functions
include:  display  of  the  magnetic  field  profile  as  it  is  being  recorded,  generation  of  an  audio  tone  at
selectable  frequency  ranges,  digital  filtering  of  interference  from  mains  electricity  and  a  facility  for
recording notes during a survey.

If the TM-4 is to be operated from a quad cycle ATV, then the following optional accessories are available:

• Additional magnetic sensors for multiple sensor operation.

• A tri-axial fluxgate compensator for heading, pitch and roll corrections.

• Receivers and expansion RAM for differential GPS positioning.

The Transportable Data Processing Unit

The unit typically consists of:

• A high performance, 32 bit laptop computer with colour monitor

•

A3 size “PaintJet XL300” colour printer with HPGL vector plotting emulation.

•

Comprehensive data processing software package for data validation, filtering, griding, contouring,
profile plotting, isometric presentation, colour image processing, interactive computer-aided
interpretation and search documentation.

• A spreadsheet data base for storing data and interpretation statistics

THE TM-4 OPERATING PROCEDURE

The Field Procedure

The usual field procedure for hand-held ordnance location is first to peg the corners of the block to be surveyed.
For convenience these blocks should be about one hectare in area, and should be square or rectangular. If part of
such an area is not accessible, then the area may be a polygon. The local, or map grid coordinates of the corners
of the block are entered through the keypad of the magnetometer. Next, the line spacing and chosen sample
interval (usually 1 m and 0.25 m but closer line spacing may be required if very small items are to be located)
and the starting point of the survey (usually one of the corners) are entered.

Data  may  be  automatically  recorded  at  eh  prescribed  interval  while  walking  at  any  convenient  speed  along
straight-line  traverses.  The  sensor  is  usually  held  0.5  m  above  ground,  or  0.25  m  if  the  location  of  very  small
items is of concern. Constant speed is not required. The coordinates of the start and end points of each survey
line are automatically determined from the coordinates of the survey block. Intermediate coordinates, if known,
may also be manually entered if increased positional accuracy is desired. By walking to and fro along successive
grid lines at the prescribed spacing, an area may be covered quickly. While conducting the search, the operator
may  inspect  the  current  magnetic  profile  as  it  is  plotted  to  the  magnetometer  screen.  The  audio  monitor  also
alerts the operator of the presence of magnetic objects.

While conducting the survey the operator may note objects or features on the ground surface that influence the
measurements. The position of these may be automatically recorded in the data file by keying in an appropriate
note as the object is passed. Noteworthy objects may be scraps of iron, fences, creeks and gullies, etc.

Surveys conducted in this manner have regularly been performed at a rate of one hectare in less than three hours.

A quad cycle mounted, multi sensor TM-4 system without GPS positioning is suited to situations where the
terrain is reasonably flat and cleared of scrub and trees. The survey procedure requires the system to be operated
along a regular grid pattern of straight lanes with position control at no greater then 500 m intervals.

Differential GPS positioning is available for the quad cycle mounted system. This obviates the need for straight
line surveying and position control points. With GPS, the system is ideal for searching long strips such as
roadways, cable or pipe trench easements etc. Traverse speed is only limited by the ability of the ATV to
negotiate the terrain.

The Processing Procedure

On completion of recording a block of data (one hectare should take between 2.5 and 3 hours on foot) or a day’s
traversing  over  a  larger  grid,  the  data  acquisition  unit  is  connected  to  the  field  computer  processing  system.
Transfer of the data to the computer takes a few minutes. The operator has access to a menu of data processing
options  from  a  “Windows”  environment.  These  include  a  choice  of  image  processing  and  signal  enhancement
filters  applicable  to  different  geological  situations  and  the  desired  output  presentation.  Also  selected  are  the
plotting scales, the orientation of the view of the data and information that is required to be printed with the plot
to identify the survey area and filters used. A default menu may be specified for a particular location so that all
the data from that area are processed identically.

The processed magnetic data may be represented in several output forms. The most useful of these are colour
maps, isometric images and contour maps.

Interpretation of the magnetic data in terms of potential UXO targets has been semi automated and can be
performed by operators who have not been trained in magnetic field theory. All of the interpretation can be
conducted interactively on the computer screen images.

A most important aspect of an EOD search and clearance operation is the ability to be able to specify the
confidence level with which an assurance can be given that a search has been effective. The interpretation
software automatically defines the assurance levels appropriate to each area searched.

The computer-aided ordnance detection and interpretation software computes the position, and approximate size
and  depth  of  all  anomaly  sources  recognised  as  requiring  excavation.  This  information  is  tabulated  leaving  a
blank column  to record the actual items recovered upon excavation. An example of a completed interpretation
form is given as Table 3.

TM-4 MAGNETIC ORDNANCE DETECTION

SURVEY INTERPRETATION

AND EXCAVATION RECORD

INTERPRETER:                JMS                     JOB NO:                           9056                           PAGE:    1 OF 1
DATE:                                 10.04.90                GRID ID:                          J42
SENSOR ELEVATION:   0.5 M                    NOISE THRESHOLD:   2.5 nT

COORDINATES

E (m) N(m)

MASS

DEPTH

SOURCE CONFIRMED

1.5
2.7
3.2
3.8
5.7
8.4
9.2

11.4
12.7
14.6
17.5
18.0
19.2
21.5
23.0

68.5

3.6
7.8

24.8
80.0
72.5
90.6
12.7
18.5
73.2
35.4
70.3

5.2

63.5
87.4

0.2
0.2
5.0

0.4

180

5.0

0.1
0.2
0.1
5.0
2.5
0.2

0.6
0.8
0.8
0.6
0.7
3.8
2.3
0.7
0.9
0.6
0.5
0.5
1.8
1.2
0.8

FRAG
3 INCH MORTAR TAIL
STEEL FENCE POST
25LB SMOKE SHELL
VEHICLE TOW BALL
500LB HE BOMB
5 INCH ROCKET HEAD
ANTITANK PROJ. (INERT)
ROCKET MOTOR (INERT)
37 MM HE SHELL
FRAG
DRINK CAN
81 MM MORTAR
5 LB SOLID
37 MM SOLID

Table 3. An example of an interpretation record with the anomaly “source” column completed by the field

excavation crew. Note that the “depth” measurement refers to the depth below the magnetic sensor,
which in this search was 0.5 m above the ground.

SEARCH ASSURANCE LEVELS

It is improbable that anyone would claim that their magnetic explosive ordnance disposal program has been
100% effective. Rarely would it be possible to support such a claim. However, it is possible to define a magnetic
search procedure, which is 100% effective in detecting all UXO down to a quantifiable depth below ground.

The  depth  to  which  a  magnetic  search  can  be  considered  100%  effective  is  a  function  of  many  variables
associated with both the ordnance to be detected and the environment where it is buried. In particular, the type
and  orientation  of  the  ordnance  and  the  time  it  has  been  in  the  ground,  each  affect  the  characteristics  of  the
magnetic  anomaly  to  be  located.  The  magnetic  properties  even  vary  from  unit  to  unit  as  the  composition  and
manufacturing history of each is no identical. The ability to locate magnetic ordnance is also determined by the
intensity of magnetic interference from cultural sources, the magnetic properties of the ground, the sensitivity of
the  magnetometer,  the  interval  at  which  magnetic  measurements  are  made  and  the  elevation  of  the  magnetic
sensor above ground.

The  magnetic  properties  of  ordnance  items  have  been  the  subject  of  exhaustive  analysis  by  the  authors.  As  a
result,  the  complicated  relationship  between  all  the  search  variables  has  now  been  quantified.  The  level  of
magnetic  interference  characteristic  to  each  search  locality  and  the  size  of  the  smallest  UXO  required  to  be
located at the site, determine the required survey parameters of sensor elevation, acceptable elevation tolerance,
sample interval, traverse line spacing and acceptable line spacing tolerance. It is essential that these parameters
be determined prior to conducting a search, and that they be strictly adhered to if the search effectiveness is to be
quantified.

The  TM-4  UXO  detection  software  is  unique  in  that  it  has  been  designed  with  knowledge of  the  relationships
between each of the magnetic UXO detection variables. A simple analysis of reconnaissance data determines the
required  survey  parameters.  The  analysis  of  data  collected  to  this  specification  is  then  interactive  and  semi-
automatic.  The  complicated  relationships  are  reduced  to  a  simple  graph  showing  the  depth  limits  to  which  a
given  UXO  object  can  be  detected  at  the  locality  searched.  The  most  important  depth  is  that  to  which  the
magnetic  search  was  capable  of  detecting  100%  of  buried  ordnance.  The  (greater)  depths  to  which  less  than
100% of buried objects can be detected can also be defined.

The analysis of the data from each area searched results in a list of targets to be investigated as potential UXO
contamination (as in Table 3) and a graph (as figure 6) showing the depth to which the location can be expected
to be 100% effective in that area.

Figure 6 contains an example of a TM-4 assurance graph for a search area where the magnetic noise threshold
was 2.5 nT. The graph plots the ‘mode’ value of the magnetic anomaly distribution associated with each item
type. The graph also shows the noise level and the 100% detection confidence level appropriate to the particular
locality, sample interval and line spacing.

CASE EXAMPLES WHERE THE TM-4 MAGNETOMETER WAS USED

Figure 7 shows an isometric image, (isoplot) of the magnetic field recorded with a TM-4 magnetometer, over an
area where five items of ordnance were buried at a depth of 2 m. A 5 inch rocket head oriented north-south, and
BDU 33 practice bombs, two oriented north-south, on east-west one and vertical, were located within this survey
area. The characteristic dipolar nature of these magnetic anomalies is evident from the isoplot.

Figure 8 shows an isoplot from a half-hectare area within a World War II artillery range. The area was obviously
heavily contaminated. A very large number of dipolar anomalies of different amplitudes and sizes are evident.
Near the centre of the area is large, complex anomaly that was due to an old, rusted car body.

Figure  9  shows  a  colour  picture  of  this  same  area  after  image  processing  the  data.  The  computer  aided
interpretation software has computed the geological noise level encountered at this locality. All  magnetic field
variations  with  in  this  noise  range  have  been  presented  as  blue.  The  anomalies  due  to  potentially  hazardous
ferrous objects are highlighted in colour with red being high field values and blue being low field values. With a
picture  such  as  this  on  the  computer  monitor,  the  operator  simply  “clicks’  the  mouse  on  each  target  to  be
excavated.  Automatically  the  coordinates,  depth  and  size  of  the  target  are  tabulated  in  a  spreadsheet  file.  The
noise rang allows the assurance depth appropriate to the area to be automatically determined for a selection of
ordnance items.

In the particular half hectare are mapped in Figures 6 and7, a digging team, using the interpretation table and
magnetic field images for guidance, unearthed a harvest of almost two hundred pieces of scrap steel. These
included over fifty items of ordnance, which had been fired into the area as part of wartime activates.

The increased sensitivity of the TM-4 imaging magnetometer system permits the detection of ordnance items at
depths that are often greater than that at which they could be expected to penetrate. By providing a permanent
record of the instrument’s output, the system no longer requires the judgement of a single human operator as to
whether an anomaly needs to be investigated by excavation.

An  additional  benefit  offered  by  the  TM-4  magnetometer  is  its  ability  to  produce  a  final  record  upon  the
completion  of  the  search  and  recovery  process.  Figure  10  shows  the  same  area  as  above  after  the  scrap  and
ordnance  had  been  excavated  and  removed.  The  remaining  area  of  contamination  near  the  centre  of  the  plot
contains only rust and scale flakes resulting from the decay of the car body, which had been dumped on the site
some years previously. In normal operations, the removal of the affected soil would eliminate this anomaly.
It is readily apparent that all other anomalies likely to result form the presence of UXO have been removed. The
assurance  that the  area  is  free  of  UXO  contamination  is  therefore  a  confident  one,  based  as  it  is  on  verifiable,
documented  evidence.  By  employing  the  most  sensitive  and  capable  system  available  in  the  world  today,  the
land can then by safely put to a wide variety of uses.

CONCLUSIONS

•

Because the analogue magnetometers used in the past for ordnance location do not record data, there is
no way of demonstrating that the search was effective or diligently performed. The use of these
locators in peacetime could be deemed negligent in court

•

Vertical difference or gradiometer type locators are inherently less sensitive to anomalies from buried
UXO than are total field intensity instruments.

•

In areas of very low geological magnetic noise, the depth to which a given target may be detected with
a TM-4 is about 5 times greater than that using an analogue system, and up to 3.5 times greater than
when using the CAST system.

•

In magnetically noisy areas, the depth to which a target may be detected with a TM-4 must always be
more than 1.5 times greater than when using an analogue system, and it may be several times more by
virtue of the use of digital signal enhancement. The detection depth using the TM-4 is up to 2.5 times
greater than that when using the CAST system in these conditions.

•

In all noise situations, the depth to which exploration for a given size of target has been effective, can
only be quantitatively defined from digital data.

•

The TM-4 coverage rate of four hectares per day on foot (and very much more where vehicle access
permits) is serval times greater than can be achieved with either the CAST or analogue systems.

•

Computer  aided  interpretation  of  image  processed;  digital  data  can  take  place  in  a  favourable
environment  and  involve  input  from  the  experience  of  more  than  one  operator.    In  other  words,  the
interpretation  is  not dependent  upon  the  concentration  of a  single  field operator who may  be  working
under adverse conditions.

•

A permanent, digital record of the search data (before and after clearance if required) provides
verification of the thoroughness and effectiveness of the search.

•

The confidence level at which UXO can be considered all to have been detected and removed can be
specified. The search depth to which such confidence applies varies with projectile size and local
geological conditions but in each search area the assurance level can be quantitatively defined by way
of a single graph.