On airborne IP effects in standard AEM systems: tightening model space with data space

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ABSTRACTIP effects in AEM data are subject of current research around the world. There have been success stories and it is now practical to model AIP. It is widely accepted that failure to account for IP effects, when present, will produce artefacts in the resistivity model. However there still is a need to study more accurately the boundaries of the effect on AEM data and of its relevance, beyond common past acceptance. This paper provides a clear and extensive overview of detectable AIP effects in the data space, without imposing simplistic assumptions (e.g. fixing some parameters to arbitrary values or limited boundaries), beside using a 1D approach. We produce forward responses with dispersive resistivity for hundreds of thousands of combinations of Cole–Cole model parameters (different rock types) and AEM system transfer functions. The results are analysed using various metrics (e.g. sum of negative voltages, exponential fitting), presented with a series of 3D plots that capture different AIP signatures in the transients. Experiments include half spaces, 2 and 3 layer models, combined with different waveforms, Rx types (dB/dt and B), Tx-Rx geometries, flying heights, base frequencies. The results allow a clear assessment of the different aspects of AIP effects over a wide range of geological and geophysical situations. Measured AIP effects are mostly focused in the range of τ from 10−2 s to 10−4 s. Measurable AIP effects depend on AEM system’s specs, are often unpredictable, can originate from chargeable layers at considerable depth, are heavily affected by layering and can occur over a wide range of situations. Deeper chargeable layers do not necessarily produce fainter AIP anomalies. What can be generalised is that AIP effects are increased most often by the presence of a resistive bedrock, often using slow turn-off of the waveform, are generally better observed with B field instead of dB/dt and lowering base frequencies. They can vary abruptly, due to the rapidly changing relative contribution of pure EM and pure IP responses. AIP effects can occur more often than previously thought and should not be discarded a-priori from any AEM survey.