Africa Surveyors May-June issue 2023 digital

UXO SURVEYING
Stephen Wilson, Director of
Business Development & UXO
Manager - EPI Group
Alexey Dobrovolskiy, Chief
Technology Officer - SPH
Engineering
Thomas Mennerdahl, Technology
Director Subsea - Reach Subsea
AS
Vincent van Santen, Business
Development Manager Offshore
Wind and UXO - Fugro
the site's particular conditions, the target's features, and the survey's
goals,” says Vincent. “To achieve reliable and accurate identification of
UXO artefacts, automated detection approaches are often combined
with human experience and visual confirmation. Multiple detection
techniques to identify potential UXO objects can be applies in
the seabed, electromagnetic signals being one of them. The latest
developments in data processing algorithms can also be used to
improve processing time and ensure methods are objective, repeatable
and auditable,” he adds.
“The utilisation of this emerging broadband electromagnetic
technology could allow for superior detections when compared to
most technologies currently available. The purpose of UXO survey
techniques is to produce a master target list of potential pUXO, which
will need to be investigated prior to ALARP. It is imperative that we
do not miss pUXO and that we do not unnecessarily add pUXO to
the master target list, as investigation is a very expensive process,”
observed Stephen.
During the initial phase of a technical UXO survey, a range of
different sensor methods are deployed. This phase requires a deep
understanding of the type of munitions, geophysical survey methods,
data processing and management skills.
Innovative technologies for UXO surveys
When it comes to best technologies for land-based UXO surveys,
Allison affirms that Ground Penetrating Radar is complementary to
other detection methods. “GPR uses low energy, high frequency radio
waves to image metallic and non-metallic objects in the subsurface,
making it a natural technology to use for locating buried UXO. There is
a wide selection of GPR systems and configurations available, making
GPR adaptable to different terrains for UXO applications”, she adds.
Moreover, magnetometers are subject to systematic errors inherent to
the sensor itself and the magnetic environment it is placed in. These
errors are induced by hard and soft iron effects, zero bias errors, scale
factors and non-orthogonality- which will show up as an orientationand
movement induced noise on the measured total magnetic field.
The sources of these magnetic influences are the sensor, the sensor
platform and the surface vessel (in shallow water); this is with regards
to Thomas.
“In late 2000s ROV based gradiometer systems were introduced which
has advantages but also some challenges. The main reason why
magnetometers measuring total field are very complicated to use on
ROVs, is due to the noise induced by the motion of the ROV/vessel
in relation to the magnetometer which will change the measured
magnetic field,” he adds. Additionally, Thomas is certain that, “The
main solution to this is the use of two or more magnetometers in a
gradiometer configuration. In such a setup, the influence on the total
field from the surface vessel is assumed to be equal on all sensors due
to the relative distance between the ROV and the surface vessel, and
the distance between the magnetometers in the array. As this is not
the case for the targets located on the seabed, this method does not
suppress any data of interest in an archaeological or sea based UXO
survey.”
“When there is a risk for non-ferrous UXOs or there are subsea
structures and/or geology/boulders that could mask the magnetic
signature of a possible UXO target a ROV based electromagnetic
system or 3D chirp system can be an appropriate solution. In shallow
water the 3D chirp could be fitted to the surface vessel without
requirement for ROV or towed solutions,” adds Thomas.
“Once the fact that intervention into a UXO contaminated area, for
survey or clearance, is determined, factors have to be taken into
consideration for the best suited and most practicable technologies or
techniques required. For example, the most up to date third generation
detection equipment may not be suitable for very simple small-scale
contamination; or time and speed of clearance may be the precedence
over cost of such high-end technologies (and techniques such as
mechanical clearance with armoured earth moving machines)” clinches
Steve.
In recent decades, a number of new and innovative technologies have
been developed to improve the detection of UXOs. These include
digital geophysics and a combination of electromagnetic sensors.
Combined with data analysis workflows, these technologies can
greatly improve the accuracy of locating UXOs.
The opinions expressed by individual experts are their sole
responsibility and do not imply any endorsement or support
by other participants in this context.
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May-June issue l 2023 15