Comparison Of Airborne Magnetic And Electromagnetic Systems For Ordnance Detection And Mapping

Magnetic and electromagnetic data collected by helicopter boom-mounted systems at the same survey sites permit direct comparison of the systems as to their suitability for buried ordnance detection. The ORAGS-TEM system is an airborne electromagnetic system developed specifically for unexploded ordnance mapping and detection. Although airborne boom-mounted magnetic systems are at a more advanced stage of development than their electromagnetic counterparts, in basaltic terrain, electromagnetic systems have proven capable of detecting buried ordnance, whereas magnetic systems sometimes fail altogether. Magnetic systems use passive sensors and these can be distributed along the boom structure such that dense data can be collected with sensors spaced 1-2 m apart over a broad swath, up to 12 m wide. Current airborne electromagnetic systems are unable to match this data density. The ORAGS electromagnetic system, having only two receivers, must rely on interleaved flight lines to obtain data of a spatial density approaching that of the airborne magnetic systems. Active electromagnetic fields decay at greater rate than magnetic fields, and this constrains current electromagnetic systems to practical survey altitudes of no more than about two meters for ordnance detection. Tests at the Badlands Bombing Range indicate that in some circumstances, the signal-to-noise for the ORAGS electromagnetic system exceeds that of airborne magnetic systems.