An innovative interpretation of a challenging helicopter electromagnetic survey

In 1991 a helicopter electromagnetic (HEM) survey was acquired at the U.S. Department of Energy Idaho National Engineering and Environmental Laboratory (INEEL). Data were collected by Ebasco Inc., an environmental consulting company, using the Aerodat HEM system. The strategy was to increase the resolution by reducing the flight height. In principle, a good idea, but standard interpretational procedures are not applicable at flight height approximately twice the transmitter-receiver coil separation. Hence the dataset was released to the U. S. Geological Survey for interpretation. The survey targets were buried waste sites of trenches and pits containing drums, ordinance and other objects, which are very conductive and highly three-dimensional (3D) in nature. An additional complication was the magnetic signature of the waste, basalt and soil that hosts the waste.