Challenges in Marine UXO Surveying

Unexploded Ordnance (UXO) around the UK continues to cause problems for site developers, asset owners and contractors, as well as local communities.

Unexploded mines and bombs deployed during World War II are still present in the subsurface and seabed around the UK, as well as other smaller ammunition, and they continue to pose a threat after decades of inactivity.

Geophysical surveys are an important step in any marine UXO risk mitigation strategy, as they help to locate and identify potential UXO threats before any construction or development work takes place. However, these surveys have their own set of challenges that must be addressed in order to ensure accurate and reliable results.

How Are UXO Surveys Performed?

UXO (Unexploded Ordnance) surveys are a key part of the UXO risk management process, working to detect and map potentially hazardous unexploded munitions on land or underwater.

These surveys are typically specified by UXO consultants, who classify the minimum threat level expected based on historical records such as national archives and other relevant sources, environmental conditions, and project requirements. A UXO Risk Assessment is usually conducted to determine the potential hazards and prioritise areas for marine surveys.

In the survey phase, specialised geophysical instruments are deployed to detect anomalies that may indicate the presence of UXO. For underwater surveys, sensors are often towed behind a vessel in a planned grid pattern to ensure comprehensive area coverage.

Common tools include magnetometers, which sense variations in the magnetic field caused by ferrous objects, and sonar systems, which use sound waves to produce images of the seabed. Our UXO survey methodology involves towed sensor arrays configured to meet specific survey objectives, typically deploying high-precision tools such as caesium vapour magnetometers (e.g., Geometrics G882) and side scan sonars.

Collected geophysical datasets are processed through Oasis Montaj to interpret any anomalies and identify potential UXO targets, with results cross-referenced against side scan sonar and multibeam echosounder data. The findings are mapped and documented to support further investigation or safe disposal planning, allowing project teams to effectively mitigate UXO risk.

Achieving ALARP

An ALARP (As Low As Reasonably Practicable) sign-off for UXO (Unexploded Ordnance) is an essential document in the UXO risk management process for marine and offshore projects. This certification confirms that all reasonable measures have been taken to reduce UXO risk to a legally acceptable level, balancing safety and practicality. It’s not a guarantee that a site is entirely free of UXO, but it serves as proof that the risk has been reduced to the lowest level achievable within practical constraints.

The ALARP sign-off is a comprehensive record that details each step taken to assess and manage UXO risks, including risk assessments, mitigation actions, and specific criteria for ruling out certain targets as UXO. This documentation is clear, specific, and robust—designed to withstand scrutiny should a UXO incident occur.

By offering a structured account of all mitigation efforts, the ALARP sign-off gives project stakeholders confidence that the site has been managed to the highest safety standards, aligned with legal and regulatory requirements. It’s a key component in demonstrating that UXO risk has been approached with the thoroughness and responsibility clients expect.

Core Challenges in Marine UXO Investigation

There are many challenges associated with UXO, starting with survey and identification through to clearance and disposal. From the perspective of UXO survey companies, there are a few central challenges that we must overcome to deliver a quality survey result.

Positional Accuracy in Marine UXO Risk Management

Achieving accurate positioning is essential for effective UXO risk management, which, as we know, is a challenge in the marine environment. For UXO surveys, understanding the type and level of risk associated with the area is critical as it allows us to design our survey spread accordingly. Ensuring positioning accuracy is essential to this process and forms a key consideration at the conception stage of the project. Positional accuracy gains even more importance for small targets or targets with low ferrous content.

We work closely with UXO consultants, who conduct background research and assess UXO threat levels; we tailor our survey techniques and sensor configurations to the specific risks identified. This partnership ensures that, to the best of our ability, our survey spread is optimised to the site requirements.
To validate and refine our survey approach, SEP Hydrographic conducts Survey Verification Tests (SVTs) using test items that resemble anticipated UXO threats. SVTs confirm equipment performance, enabling us to adjust sensor arrangements and optimise the survey spread for maximum positional accuracy.
High-precision GNSS, RTK, INS, and USBL acoustic positioning systems are used in combination to provide continuous, accurate positioning across different environments. Our data processing integrates magnetometer results with additional datasets, including side scan sonar and multibeam echosounder data, to generate precise target listings. This data package empowers UXO consultants to perform reliable risk evaluations and helps clients implement an effective risk mitigation strategy.

Managing Background Noise

Background noise is a common challenge in UXO surveys, where natural geological formations and human-made structures—such as ports and harbours—can interfere with magnetometer readings and obscure potential UXO targets. To mitigate these effects, SEP Hydrographic employs advanced noise reduction techniques, ensuring that UXO detection remains precise, even in complex, high-noise environments.

One key approach is the use of gradiometer configurations, arranging magnetometer sensors to measure magnetic field gradients rather than absolute values. This setup increases sensitivity to localised anomalies, improving the distinction between UXO and background noise. SEP Hydrographic also partners with Kraken Robotics to deploy the Sub Bottom Imager (SBI) system, which uses 3D chirp sonar technology to generate detailed images of the sub-seabed. The SBI is particularly effective in built-up or noisy areas, creating high-resolution maps that support accurate UXO identification.

SBI Data in UXO Target Investigation

Kraken Robotics’ SBI technology enables high-accuracy mapping of UXO targets by generating a 3D image of potential UXO locations. This system is especially useful in shallow water areas, where it can be deployed from a shoreside crane or vessel, significantly boosting the efficiency of our target investigations.

SBI Data showing anomaly alongside as-found UXO

SBI Data showing anomaly

PanGeo GeoLink for SBI deployment

Innovations in Marine UXO Surveying

There is a drive for new and innovative survey techniques from marine UXO companies to help overcome some of the technical challenges while reducing the cost of data acquisition.

As with many other survey disciplines, autonomy will play a key role in achieving this aim. Whether the sensors are mobilised to Unmanned Surface Vessels (USVs), Unmanned Aerial Vehicles (UAVs) or Autonomous Underwater Vehicles (AUVs), there are clear benefits to embracing this technology.

Other advances include the increasing use of the Kraken Robotics SBI for non-ferrous UXO requirements and surveying in built-up areas. Kraken Robotics performs large-scale non-ferrous UXO surveys using their SeaKite ROTV equipped with the SBI. In the right conditions, the system can acquire accurate data at standard survey speed, vastly outperforming the standard ROV deployment technique.

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UXO Risk Mitigation at SEP Hydrographic

At SEP Hydrographic, our team has decades of experience in UXO risk management and has acquired, processed and interpreted thousands of line km of data.

We work closely with our clients so they receive end-to-end support for UXO risk mitigation in marine environments. Our geophysical survey experts collaborate with trusted partners, like Kraken Robotics, to provide a turnkey UXO risk management solution. Our proven risk mitigation strategies and robust marine services deliver safe, reliable results for projects across various marine environments.

For more information about our comprehensive UXO survey services, please contact info@sephydrographic.com.

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