Techniques and methodologies required for UXO detection vary depending on a number of factors including the type of ordnance expected in the area, the presence of man-made structures, water depths and site specific environmental conditions. Nearshore surveys pose particular operational difficulties - especially when multiple towfish sensors are deployed – whilst UXO surveys in deeper water (approximately 60 metres) face a different set of challenges.
Since pioneering the first multiple-magnetometer array utilising USBL positioning techniques in 2009, Fugro has conducted over 100 UXO surveys. The experience and expertise developed since then allows us to provide a sophisticated technical approach to any challenging UXO project.
In 2008 Fugro pioneered the four sensor array, a highly-efficient technique that has become an established method for surveying large areas of seabed in a single vessel pass. Widely used in pre-construction surveys on wind farm sites, this method typically involves towing four individual marine magnetometers behind a vessel. Specially adapted winches on the vessel deck control the altitude of the towfish, enabling them to ‘fly’ as close to the seabed as possible and detect anomalies with a high degree of accuracy.
In shallow water the main challenge is to maintain the altitude of the sensors whilst towing the magnetometers at a sufficient distance to ensure that the magnetic signature of the vessel does not interfere with the data which can introduce noise into the dataset, masking potential UXO. To alleviate this problem Fugro developed a surface towed system which enables the acquisition of high quality data in extremely shallow waters. This system enables the tow distance to be tightly controlled and configured to provide vertical and horizontal gradiometer solutions.
Fugro’s advanced operational and processing techniques enabled it to win a contract at RWE’s Gwynt y Môr offshore wind farm site in the Irish Sea, UK. During spring 2014 Fugro conducted UXO surveys amongst the site’s existing wind turbine foundations. The considerable magnetic interference associated with these 400-tonne steel structures meant that standard magnetometer survey techniques were unsuitable. Fugro’s solution was to operate both a four-magnetometer towed array and a towed gradiometer array, comprising two magnetometers attached to a non-ferrous metal frame, towed vertically through the water column. Expertise in processing magnetometer and gradiometer data produced exceptional results. Both the acquisition and the processing significantly reduced the ‘masking’ effect of the turbines’ ferrous content, which was seen in the foursensor array data. The magnetic targets could be clearly discriminated within 20 metres of the steel structure.
UXO targets with significant associated risk were then inspected by an ROV equipped with a multi-sensor gradiometer array and advanced processing techniques were adapted to meet site-specific requirements.