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This experimental study investigates how the distance between a target and a frequency-domain electromagnetic (FDEM) multi-coil instrument (EM38-MK2) influences sign reversal in data measurement, a phenomenon often observed in archaeological contexts. Experiments were conducted by moving various objects above the instrument, including magnetite powder (high magnetic susceptibility, low conductivity) and an aluminum plate (high conductivity, low magnetic susceptibility). The key parameters tested include the nature of the object, its position relative to the instrument, and its size relative to the inter-coil spacing. Although the selected materials are not typical archaeological remains and the air-based configuration simplifies soil structure, their strong electromagnetic responses make them suitable for studying signal behavior. Results from these controlled tests reveal that shallow targets (less than one meter deep) can produce complex signals with multiple peaks, occasionally of opposite signs, which could potentially lead to misinterpretation regarding the number or depth of buried features. In our experimental setup, sign reversal between the two coil spacings depends on the object's properties: it occurs in the in-phase component for magnetic materials and in the quadrature for conductive ones. The in-phase response often overestimates object size, especially near the surface or for targets with strong electromagnetic properties. Conversely, the quadrature component allows for more accurate localization. Operator movement speed also impacts signal symmetry, sometimes generating asymmetric anomalies that complicate bidirectional survey interpretation. These results highlight the need to replicate such experiments using more representative archaeological analogues to improve the evaluation of depth ranges and refine the interpretation of electromagnetic data in archaeological contexts.
Tempere et al. (Tue,) studied this question.