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The Ticking Brownfield Timebomb:
BACTEC survey exposes flaws in non-intrusive
magnetometry to detect buried ordnance
The detection of potential unexploded ordnance (UXO) is
critical to the safe development of some sites in many parts
of the UK. Intrusive and non-intrusive surveys are not capable
of detecting explosives but are used to detect buried objects
that may be casings of potential UXO and warrant further
investigation. Yet Clearance Certificates or risk reduction
reports issued by some UK Explosive Ordnance Disposal (EOD)
contractors may be meaningless, having failed to assess and
recognize the limitations of their survey equipment. EOD
specialist BACTEC International explains why and calls on the
industry to abandon the use of non-intrusive surveys on
brownfield sites when it is clear that a distinction cannot be
made between made ground with ferrous contamination and
a buried unexploded bomb (UXB), and that the survey cannot
penetrate or filter out the contamination to provide accurate
survey results to identify deeper UXBs.
A recent situation highlights the potential problem. Following a desk top
study, a site in London was deemed to have a UXO/UXB risk. Another EOD
contractor advised the client after a site visit that deploying their nonintrusive survey system would provide clearance from a potential buried
UXB to a depth of 10 metres below existing ground level, despite the
presence of ferrous contaminated made ground.
Location of Buried Bomb
Bomb in place in the ground ~3.5m deep
More than 60 years after the end of the Second World War, the German
bombs that rained down on London, the UK’s major cities, military bases
and industrial areas continue to ply their deadly trade. For most they are
out of sight and out of mind, relegated to a footnote in the history books.
The Luftwaffe’s ordnance was very different to the precision-guided
“smart” weapons that we see on our TV screens today. Unlike their
modern counterparts, they contained no computer-controlled guidance
systems that enabled them to be dropped with pin-point accuracy,
therefore blanket bombing techniques were employed. Nor did the pilots
have the advantage of radar to take them to their destination. Buffeted
around the sky by anti-aircraft fire and fighter planes, the pilots would
often drop their bombs miles from their targets. With a typical failure rate
of 10% many UXBs remain unrecorded and buried but if disturbed may
function as originally designed with potentially disastrous results.
contamination ranging from fired bricks through reinforced concrete to
discarded scrap metal and buried services. Man-made ambient effects on
magnetic fields also include moving plant, vehicles, HV power cables,
electric trains or individuals from clothing, mobile phones, etc.
The post-war rush to rebuild London and other bomb-damaged cities saw
derelict buildings razed. A 2m deep layer of made ground was routinely
deemed sufficient to providing a working base for housing and other lowrise buildings. Many of those buildings are now reaching the end of their
own lifespan and with other brownfield sites such as former industrial
areas are also earmarked for regeneration. The buildings on these sites are
generally much taller with deeper basements, and therefore require deeper
foundations than previously provided. It is this new development that is
increasing the likelihood of discovering – and disturbing - buried ordnance.
For example, an object with a field strength of 100nT at a distance of 0.5m
from a detector will only have a field of 0.2nT at a distance of 4m.
UXO can often remain buried and present no risk. Most are unlikely to
function spontaneously as long as the surrounding stratum is not
disturbed. But it is essential that prior to construction possible UXBs are
investigated so serious injury – or even death – can be avoided. Operators
of piling rigs, drilling rigs and excavators together with groundworkers
using heavy earth moving equipment are most at risk.
With a typical failure rate of 10% many UXBs remain
unrecorded and buried but if disturbed may function as
originally designed with potentially disastrous results.
Contractors have a legal responsibility to comply with Duty of Care,
Health and Safety and CDM Regulations. From April 2007 this responsibility
was also extended to developers and employers. This means sites should
be assessed for unexploded ordnance hazards, which is common practice in
other European Countries. For EOD specialists such as BACTEC International,
this begins with desktop studies which are tailored to health and safety
plans and land quality assessments. These detail the wartime history of
the site, the likely nature of contamination such as explosive residues or
explosive ordnance in soils or structures, weapon characteristics and
bomb penetration depth modeling. The study indicates the level of threat
and recommends the risk mitigation measures such as explosive ordnance
surveys necessary to eliminate the risk or reduce to ALARP. These surveys
can be either intrusive, non-intrusive or a combination, and their
effectiveness is completely dependent on site conditions and the
equipment used.
A non-intrusive survey is a “surface sweep” system using one or more
sensors to detect buried ferrous objects, typically with the capability to
detect a 50kg bomb to a depth of 4m in magnetically clean ground. An
intrusive survey involves the use of probes to penetrate the ground to a
detection depth beyond the capability of non-intrusive systems, or where
the site / made ground is contaminated with ferrous objects.
BACTEC believes that non-intrusive magnetometry surveys of magnetically
contaminated brownfield sites for the detection of buried ordnance are
being promoted by a number of EOD contractors using the theoretical
capability of non-intrusive magnetometry survey (which is based on
optimum parameters) instead of accounting for the site conditions, the
potential UXB size and the penetration depth. This is seriously flawed, and
their use could have potentially fatal consequences.
Non-intrusive magnetometry or electromagnetic surveys are used in the
search for buried UXO and rely on the detection of small variations in the
Earth’s magnetic field caused by the presence of ferro-magnetic objects.
The technique operates very successfully in environments where there
is minimal ground contamination from other sources of magnetism.
Brownfield sites are almost always contaminated since the made ground
will invariably contain multiple sources of ferro-magnetic/magnetic
The level of magnetic field variation caused by typical items of ordnance is
measured in Nanoteslas, or one billionth of a Tesla (nt) and magnetometers
used on ordnance survey work can normally detect magnetic field changes
of the order of tenths or hundredths of Nanoteslas. Magnetic fields from
discrete objects are generally present as dipoles (positive/negative or a
north and south pole) or a monopole if the object is vertical. Magnetic field
strengths vary over a vast dynamic range of many orders of magnitude and
the field strength intensity varies with distance. The strength of the signal
decreases as the distance from the object increases.
Distance
from source
(m)
Distance
multiplier
Cube
of distance
Example of
effect of
distance on
magnetic field
0.5m
1.0m
2.0m
4.0m
1
2
4
8
1
8
64
512
100nT
12.5nT
1.5nT
0.2nT
Two types of magnetometer sensor are commonly used in ordnance survey
work. The most sensitive (which can detect magnetic fields below 0.01nT) is
the “total field” optically pumped caesium sensor while the fluxgate
gradiometer which can typically be used to sensitivity levels of below 0.1nT.
When using magnetometers to detect ordnance, results are usually
displayed as false colour drawings showing magnetic “contours” in which
one colour is used to represent a neutral field and other colours to
represent the positive and negative poles. Software is used to identify
individual dipoles originating from discrete objects.
Background magnetic field variation “noise”, the level of which varies
dramatically in different environments, is filtered out to aid data
interpretation. In a truly “green-field” environment with minimal manmade influences this filtering can be set at 1nT or even lower. In other
environments where there are man-made influences, such as moving
vehicles, the level may have to be set at 5nT or higher.
The level of filtering will determine the effectiveness of detecting buried
ordnance. In a “clean“ magnetic environment (1nT background), the
smallest German WWII air-dropped high explosive bomb (nominally 50kg),
can be detected at a depth of up to 5-6m. In a noisy environment (20nT),
the detection depth is less than 2m (Source: AGS GmbH).
Filtering is normally used to remove or reduce the effects of ambient
magnetic fields due to surrounding man-made influences such as fencing,
nearby buildings, vehicles and plant. Since these effects are often below
the 5nT level this process can be used successfully without adversely
effecting the detection capability of the survey.
Above: Results from a Non-intrusive
Magnetometer Survey
Right: Type SC-50 (without tail)
weight ~50kg
When BACTEC was invited to tender for the proposed non-intrusive survey,
it declined and expressed concern over the suitability and effectiveness of
a non-intrusive survey based on the site conditions. However, the client
accepted BACTEC’s offer to verify the survey by burying the casing of an
inert WWII German 50kg bomb on part of the site due to be piled at 3.5m
deep. When the results of the non-intrusive survey were analysed with a
200nt filter it was evident that the survey had failed to distinguish between
the ferrous contaminated made ground and the buried bomb casing. Also
on this site as with many other brownfield sites the made ground severely
limited the effectiveness of the survey which prevented the acquisition of
data on potential deeper targets.
Subsequent to the unsuccessful non-intrusive survey, BACTEC successfully
completed an intrusive UXB survey and provided the necessary clearance
to pile. This ensured that the Health & Safety requirements were met by
deploying the appropriate technology and risk mitigation strategy for
the project.
It is clear that searching for buried ordnance using nonintrusive magnetometry has significant limitations when used
on the majority of brownfield sites. BACTEC does not, and will
not, use non-intrusive magnetometry on brownfield sites
unless an on site assessment has been carried out to ensure
that the survey technology deployed will provide valid results
and calls on the rest of the industry to follow its lead.
If the site contains made ground, buried structures or
services. Since these fields are orders of magnitude greater
than the field produced by buried ordnance.
A more difficult situation arises if the site contains made ground, buried
structures or services. Since these fields are orders of magnitude greater
than the field produced by buried ordnance non-intrusive surveys are
normally inappropriate since it will be impossible to detect the small
discrete signal produced by a buried bomb compared to the very large field
from the made ground. This is particularly true if the made ground is at the
surface and the bomb is at depth.
In order to produce any meaningful false colour drawing from the survey of
such a site a level of filtering must be applied which automatically removes
the colour contrast produced by the ordnance, therefore eliminating
potential smaller targets that may be UXBs.
BACTEC International Limited
37 Riverside, Sir Thomas Longley Road
Rochester, Kent ME2 4DP UK
Tel: +44 (0)1634 296757
Fax: +44 (0)1634 296779
www.bactec.com