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Comparison of Performance of Airborne Magnetic and Transient EM Systems for Ordnance Detection and Mapping
JEEG, September 2008, Volume 13, Issue 3, pp. 291–305
Les P. Beard*, William E. Doll, T. Jeffrey Gamey, Jacob R. Sheehan* and Jeannemarie Norton, Battelle-Oak Ridge Operations, Oak Ridge, Tennessee;
J. Scott Holladay, James L.C. Lee, Geosensors Inc., Toronto, Ontario;
Nathan W. Eklund, 3AMEC, Minneapolis, Minnesota
Magnetic and electromagnetic data collected by helicopter boom-mounted systems at three different sites permit direct comparison of the systems as to their suitability for buried ordnance detection, mapping, and discrimination. Airborne boom-mounted magnetic systems are at a more advanced stage of development than their electromagnetic counterpart. However, in basaltic terrain, transient electromagnetic systems have proved capable of detecting buried ordnance, whereas magnetic systems may fail to detect ordnance 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. The ORAGS-TEM 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. The total magnetic
fields from unexploded ordnance decays at 1/R3 for total field and its gradient at 1/R4. This permits adequate signal-to-noise levels to be easily attained for larger ordnance types at survey heights up to seven meters. Active electromagnetic fields decay at between1/R4 and 1/R6, depending on ordnance type and sensor geometry, and this constrains current electromagnetic systems to practical survey altitudes of less than three meters. Tests at the Badlands Bombing Range indicate that, in some circumstances, the signal-to-noise for the airborne electromagnetic system exceeds that of airborne magnetic systems, and even ground electromagnetic systems. Because time must be allowed for transmitter current buildup and decay, ORAGS-TEM is not capable of sampling along line at the same spatial density as can magnetic systems. However, the temporal signal decay permits greater opportunity for ordnance discrimination than magnetic measurements.