Geofisika & Geohazard
UXO Survey at Offshore Wind Farms: Finding What the War Left Behind
Much of the North Sea seabed now being built out for offshore wind was also, eight decades ago, a minefield and a bombing range. Before a single monopile can be driven into that seabed, a survey team has to find every piece of unexploded ordnance (UXO) still down there from two world wars — and prove, target by target, what it actually is before anyone decides whether it needs to be moved, avoided, or blown up in place.
Why the Detection Method Is Almost Always Magnetic
An offshore UXO survey is built around a magnetometer or gradiometer dataset, usually supported by side-scan sonar, multibeam, and sometimes a high-resolution sub-bottom profiler. The reason magnetometry leads is physical: a ferromagnetic object sitting on or buried in the seabed locally distorts the Earth's magnetic field, and that distortion — its amplitude and horizontal spread — depends on the object's mass and shape, its magnetic properties, and its distance from the sensor. Because buried ordnance can sit under sediment where sonar and cameras see nothing, magnetometry is the one method that can still flag a target the acoustic instruments would miss entirely.
Getting a clean signal out of that physics is a positioning problem as much as a sensing one. Survey tracks have to run in parallel lines spaced only a few metres apart, and the magnetic sensor's altitude above the seabed has to be tightly controlled and logged — typically flown at less than four metres, using an altimeter and depth sensor for active height control, with the whole array positioned by wide-area differential GPS at the surface and ultra-short baseline (USBL) tracking underwater. Multiple magnetometers are usually towed together, rather than a single sensor alone, to help resolve and localise each anomaly instead of just detecting that something is there.
From Anomaly to Confirmed Target
Modern inversion techniques model each magnetic anomaly's shape to estimate the size, depth, and orientation of the object producing it, which lets analysts rank targets by how strongly they resemble known ordnance before anyone commits to a physical investigation. That classification step has become reliable enough to change the economics of a survey: across three North Sea projects — Ørsted's Hornsea Offshore Wind Two and Borssele I and II, and ScottishPower Renewables' East Anglia ONE — data quality was good enough that classification was possible for most targets, and on Hornsea Two specifically, all 190 targets supplied could be classified without a physical dive or ROV inspection of each one.
Case in Point: Clearing the Inch Cape Site
The Inch Cape offshore wind farm off the Scottish coast ran a six-month pre-construction UXO campaign covering its 150-square-kilometre array site and 85-kilometre export cable corridor. The survey and investigation process identified 395 potential UXO targets, of which 86 were confirmed as genuine ordnance — a mix of projectiles, anti-submarine weapons, British buoyant sea mines, depth charges, and German Luftmine B parachute mines, the range of hazards a North Sea site can accumulate across two world wars. Of those 86 confirmed items, 38 required active disposal to clear a safe path for construction; the remainder could be avoided or left undisturbed. The disposal campaign was completed in October 2025, clearing the way for monopile foundation installation scheduled for December 2025.
Deciding How to Get Rid of What's Found
Once ordnance is confirmed, the disposal method itself carries its own risk trade-off. Traditional high-order detonation — placing a donor explosive charge next to the target and detonating both together — reliably destroys the ordnance but produces high underwater sound levels capable of injuring or killing marine mammals and other fauna, including organ damage and rupture of gas-filled cavities such as those in the auditory system. Low-order deflagration, the method used at Inch Cape via the Cobra System (one of only two disposal systems recognised by the UK Government for this purpose), instead ignites and burns out the explosive content in a controlled process without a full detonation. Field comparisons show low-order deflagration produces peak sound pressure and sound exposure levels roughly 20 decibels lower than high-order detonation of the same target.
Where Bubble Curtains Fit In
Bubble curtains — compressed air pumped through seabed hoses to form a screen of rising bubbles around a detonation site — are sometimes deployed for additional noise reduction, typically achieving a further 5 to 10 decibels of attenuation. Their track record is strongest for piling noise; evidence for their effectiveness specifically around UXO disposal is more limited and has mostly been tested in shallow water under about 30 metres. Every disposal, high-order or low-order, also requires its own permit and has to be scheduled around other noise-generating activity on site, such as pile driving, so that marine mammals are never exposed to two major noise events at once.
A Survey Phase With Its Own Timeline
A UXO campaign is not a quick pre-check bolted onto a geophysical survey — at Inch Cape it ran as long as many entire site characterisation surveys do, and it had to finish months before the first monopile could go in the water. For a wind farm developer, that timeline is now as much a planning input as wind resource data or grid connection dates: the seabed has to be proven clear, target by target, before construction risk can be signed off.
References
- Applied Acoustics, "What are UXO surveys? Marine Glossary," https://www.appliedacoustics.com/glossary/what-are-uxo-surveys/
- "Inverse modelling and classification of magnetic responses to improve marine unexploded ordnance rationalization," Geophysical Journal International, Oxford Academic, https://academic.oup.com/gji/article/237/1/123/7601866
- Inch Cape Offshore Limited, "Offshore Unexploded Ordnance Disposal Works Complete," https://www.inchcapewind.com/offshore-unexploded-ordnance-disposal-works-complete/
- Ocean Winds, "Low Order Deflagration of Unexploded Ordnance Reduces Environmental Impact" (Business Case), https://oceanwinds.com/wp-content/uploads/2024/05/OW-UXO-BusinessCase.pdf
- Joint Nature Conservation Committee (JNCC), "Marine Mammals and Noise Mitigation," https://jncc.gov.uk/our-work/marine-mammals-and-noise-mitigation/
- "In-situ Comparison of High-Order Detonations and Low-Order Deflagration Methodologies for Underwater Unexploded Ordnance (UXO) Disposal," Tethys / Marine Pollution Bulletin, https://tethys.pnnl.gov/publications/situ-comparison-high-order-detonations-low-order-deflagration-methodologies-underwater
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