FEATURE SPONSOR
MARINE ORDNANCE
HIGH RESOLUTION UXO SURVEY
The uncontrolled detonation of UXO is still a present-day risk in areas affected by World War munitions such as the North Sea. As part of UXO risk management an appropriate geophysical survey is required to assess and mitigate UXO risks.
MINIMISING RISK
The purpose of the UXO geophysical survey is to minimise these risks to a tolerable level and enable investigative and installation works to be undertaken safely. In response to the constant demand for higher-resolution imaging of the seabed and sub-surface sediments, the marine survey industry has to continually evolve and develop new techniques to acquire and process geophysical data.
FUGRO GEOWING
In 2015 and 2016 Fugro used their newly developed Fugro GeoWing on several UXO survey projects for the offshore windfarm and cable industry. The GeoWing is a 5m wide gradiometer frame mounted behind a remotely operated towed system (ROTV).
The detection range depends on the type of object to be detected. A system like the GeoWing, using caesium vapour magnetometers, can detect anything with a ferrous content so long as the correct line spacing and altitude above object is used.
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The gradiometer configuration of the GeoWing differs from a magnetometer configuration as it allows the geophysicist to calculate the X, Y and Z gradients manually. The analytical signal is then derived from the 3 gradients. The result shows the analytical grid (tfas), the vertical gradient (dX), the transverse gradient (dY) and the longitudinal gradient (dZ).
ACCURATE TRACKING ABILITY The GeoWing’s ability to accurately track the seabed at a set altitude means that no heading or altitude compensation is applied to the data. This manual approach allows background noise to be minimised and the number of false positives to be reduced, ultimately reducing the number of potential UXO targets that will require further investigation.
Processed gradiometer data correlated to a German WWI minefield.
NEW APPROACH
Two GeoWings were deployed simultaneously on a project in the North Sea in March 2016. The swath of two systems was 13.5m, significantly reducing the number of survey lines normally required to collect high resolution data.
This new approach allows the acquisition of high resolution data whilst minimising the number of false positives, without compromising a client’s operational schedule.
Fugro GeoWing
www.windenergynetwork.co.uk 79
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