Marine UXO surveying: Overcoming the Challenges

Posted 12 October 2020

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. 

How are UXO surveys performed?

For standard ferrous-UXO surveying, we use caesium vapour magnetometers such as the widely popular Geometrics G882. Depending on the UXO risk identified and the objective of the survey, the sensors are towed in various configurations. The data is brought into Oasis Montaj for processing before being cross-correlated with other datasets such as side scan sonar and multibeam echosounder data to allow the creation of target listings.

What are the challenges faced?

There are many challenges associated with UXO, starting with survey and identification through to clearance and disposal.

From the perspective of a survey company, there are a few central challenges that we must overcome to deliver a quality survey result.

 

Positional Accuracy

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. For small targets or targets with low ferrous content, positional accuracy gains even more importance. 

One way that we ensure the positional accuracy is sufficient is to perform a Surrogate Item Trial (SIT) in the area using an item as close to the identified threat item as is practicable. The results of the SIT optimise the survey spread further to maximise positional accuracy. 

Background Noise

Background noise can lead to interference of the magnetometer and can result from both natural and human-made features. Background geological noise is generally known based on where the survey is taking place, meaning we can attempt to mitigate this risk during the planning phase.

Background noise can also result from built-up areas and is a common problem when surveying around ports and harbours. The materials used are detected by the magnetometer, causing interference which can potentially mask any potential UXO targets. 

There are a couple of techniques we can adopt to reduce the impact of the background noise on the UXO survey. For magnetometer surveys, we can arrange the sensors in either a vertical or horizontal fixed gradiometer frame, which allows us to measure the gradient of the magnetic field, increasing sensitivity to localised changes. Alternatively, for surveying in built-up areas, we work with PanGeo Subsea using their patented Sub Bottom Imager (SBI) system. A sophisticated 3D chirp, the SBI builds up a 3D image of the sub-seabed, allowing the identification of targets within the surveyed area. The system can be deployed in shallow water either from a vessel or a shoreside crane. The data is then processed, with a target listing produced for further intrusive investigation.

SBI Data showing anomaly alongside as-found UXO
SBI Data showing anomaly alongside as-found UXO
SBI Data showing anomaly
SBI Data showing anomaly
PanGeo GeoLink for SBI deployment
PanGeo GeoLink for SBI deployment

UXO survey innovation

There is a drive for new and innovative UXO survey techniques to help to 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 Autonomous Surface Vessels (ASVs), 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 PanGeo SBI for non-ferrous UXO requirements as well as surveying in built-up areas. PanGeo has recently embarked on a large-scale non-ferrous UXO survey in the North Sea using their custom-built ROTV equipped with the SBI. In the right conditions, the system can acquire up to 130km a day; vastly outperforming the standard ROV deployment technique. 

At SEP Hydrographic, our team has decades of experience in UXO survey, and have acquired, processed and interpreted thousands of line km of data. We work closely with trusted partners including PanGeo Subsea to ensure a turnkey UXO risk management solution.

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

UXO Survey Services

Related Pages: UXO Survey Hydrographic and Geophysical Survey