Array effects in electromagnetic surveying: misplacement of anomaly locations in 1D inversion

Laterally distributed conductive bodies and intervening resistive regions within the transient electromagnetic (TEM) sensitivity region can produce anomalously enhanced responses over resistive zones. When interpreted with standard one-dimensional (1D) inversion, this response pattern can lead to laterally misplaced conductive anomalies. We refer to this family of inversion artefacts as array effects, because they arise when a subsurface array of conductive and resistive features jointly contributes to the measured TEM response, causing 1D inversion to recover conductive anomalies at incorrect lateral positions. The underlying physical cause is the conductive–resistive alternation mechanism: spatially separated conductive regions, interleaved with resistive gaps, can jointly influence the measured response, causing 1D inversion to place conductive anomalies into, or towards, intervening resistive regions. This produces severe artefacts in inversion models and can mislead geological interpretation. Such conditions may occur in geological settings characterized by repeated lateral electrical heterogeneity, provided that the electrical contrast and spacing are appropriate. Using smoke-ring theory and 3D numerical modelling, we investigate how these effects arise and propose the Instantaneous Smoke Ring Footprint (InSR-Footprint) as a practical criterion for model design and effect identification. Finally, we present field evidence for array effects in SkyTEM data acquired over a coastal dune environment in the Netherlands.