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