Magnetic properties of unrusted steel drums from laboratory and field-magnetic measurements

Abstract Laboratory-derived magnetic properties from samples of steel drums appear to be lower than bulk magnetic properties required to produce observed magnetic anomalies over the same drums. The origin of this discrepancy is perhaps in the shape demagnetization experienced by samples used in the laboratory study. Laboratory observations of magnetic susceptibility in different directions suggest that the demagnetization mechanism may have significantly attenuated the laboratory-derived magnetization values from small samples of drums. Field observations and computer modeling indicate that even though the effect of demagnetization is important for drum-shaped objects, demagnetization is less pronounced in the shape of the drum than in the samples cut for laboratory measurements. Therefore, laboratory-derived magnetizations from samples of steel drums cannot be used to model magnetic anomalies of steel drums. If laboratory-derived magnetizations were used to model steel drums, the models would underestimate the resulting magnetic anomalies considerably and, in turn, would overestimate the number of buried drums at an environmental investigation site. Apparent bulk magnetization values for unrusted vertically oriented 55 and 30 gallon drums have been calculated (i.e., the values corrected for the effect of shape demagnetization of the drums). These range from ~90 to ~125 SI units (~7 to ~10 cgs units) for volume susceptibility and from ~325 to ~2750 A/m (~0.325 to ~2.75emu/cm3) for remanent magnetization (based on eight 55 gallon and four 30 gallon drums). Further deviations in these values could arise from the type and thickness of the steel and variations in manufacturing conditions affecting magnetizations. From the point of view of modeling the drums, at most source-to-observation distances applicable to environmental investigations, the equivalent source method is able to approximate the observed anomalies of steel drums better than the 3-D modeling method. With two years of rusting, magnetic anomalies of some of the drums have reduced, while in other drums, they have slightly increased. The overall magnetic changes caused by rusting appear to be more complex than anticipated, at least in the initial phase of rusting.