Causes and effects of the AIP trap in AEM data

In the presence of shallow chargeable layers and resistive basement, IP effects in AEM data can severely alter the relationship between depth and voltage measured in the receiver at a given time. The contribution of the IP currents from the shallow layers can overcome that of the downward moving EM currents. As a result, the contribution of the total currents to the entire recorded transient may effectively remain trapped in the near-surface chargeable layer, with a phenomenon we call the "AIP trap". The AEM data therefore become more sensitive to the near-surface geology. They also become more sensitive to AEM systems' altitude variations. These effects can be espedally relevant for AEM systems with slow ramps, which otherwise display limited near-surface resolution. The implication is a larger range of possible applications of AEM systems to mapping of, e.g., bedrock topography, permafrost, days in regolith, and other layers relevant to geotechnics, where these layers demonstrate some chargeability. SkyTEM 12.5 Hz data from southern Spain are particularly affected by IP in areas of conductive cover. By reducing artefacts in the resistivity models derived from inversion, modelling IP improves the prediction of depth to resistive basement within a certain range of cover thickness. (C) 2020 Elsevier B.V. All rights reserved.