A Comparison Of Towed And Mounted Helicopter Magnetometer Systems For Uxo Detection

Traditional helicopter magnetic surveys have always utilised a towed platform, or bird, to<br>remove the need to compensate for the magnetic effects of the helicopter. Such platforms have<br>evolved to include total field, vertical gradient and triaxial gradient configurations. For detection<br>of small objects in the near surface, the principle limitation of towed systems is positional control<br>in three dimensions. In particular, the sensor height above the ground (Z) is critical for small<br>objects, and consistency as much as the actual altitude is an important consideration for<br>interpretation.<br>For environmental surveys with towed systems, nominal survey heights are reduced from 30 to 5<br>or 10 metres above ground level, tree cover permitting. In order to achieve these altitudes safely,<br>air speed is reduced which subsequently reduces aerodynamic stability. Speed changes also alter<br>the distance between helicopter and platform making control more difficult. Higher speeds drag<br>the bird back and upwards, and usually into a nose down orientation. Platform altitude is<br>monitored by laser altimeter and displayed to the pilot, but most pilots find this insufficient<br>warning, preferring direct visual observation. Depth perception from the top of a 30m cable is<br>still marginal, and minimum survey heights are limited by caution to 5m clearance under ideal<br>conditions (+/-3m) with 7-10m being more typical altitudes. For positioning in X&Y, lines can<br>theoretically be flown as close as the budget will allow. In practice, X&Y control is limited to<br>10m line spacing due to +/-5m of bird swing.<br>One solution to this problem is to mount sensors directly onto the helicopter in the tips of<br>stingers or booms. This provides a significant improvement in the pilot’s control, but requires a<br>more sophisticated data acquisition and processing system. When the mounting platform is a<br>direct extension of the helicopter and is placed at eye level for perspective, positional control can<br>be improved to +/-lm in any direction.<br>IABG in Germany has supported several years of airborne platform development through<br>European field projects. In 1997/98, the first phase of a research project consisted of comparison<br>surveys of Aerodat’s towed triaxial gradiometer and their boom mounted system. Tests were<br>conducted over a controlled test site, which included targets such as UXO, barrels, UST’s, pipes<br>and debris in varying amounts and configurations. This paper presents some of the results of<br>these comparative tests. Other tests by IABG at this site included multi-spectral and<br>photogrammetric surveys, but are not discussed within the scope of this paper.