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 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.