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

摘要

IP effects in AEM data are subject of current research around the world. There have been successstories and it is now practical to model AIP. It is widely accepted that failure to account for IPeffects, when present, will produce artefacts in the resistivity model. However there still is a needto study more accurately the boundaries of the effect on AEM data and of its relevance, beyondcommon past acceptance. This paper provides a clear and extensive overview of detectable AIPeffects in the data space, without imposing simplistic assumptions (e.g. fixing some parametersto arbitrary values or limited boundaries), beside using a 1D approach. We produce forwardresponses with dispersive resistivity for hundreds of thousands of combinations of Cole–Colemodel parameters (different rock types) and AEM system transfer functions. The results are analysedusing various metrics (e.g. sum of negative voltages, exponential fitting), presented with aseries of 3D plots that capture different AIP signatures in the transients. Experiments include halfspaces, 2 and 3 layer models, combined with different waveforms, Rx types (dB/dt and B), Tx-Rxgeometries, flying heights, base frequencies. The results allow a clear assessment of the differentaspects of AIP effects over a wide range of geological and geophysical situations. MeasuredAIP effects are mostly focused in the range ofτ from 10~(−2) s to 10~(−4) s. Measurable AIP effectsdepend on AEM system’s specs, are often unpredictable, can originate from chargeable layers atconsiderable 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 begeneralised 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 ofdB/dt and lowering base frequencies. They can vary abruptly, due to the rapidly changing relativecontribution of pure EM and pure IP responses. AIP effects can occur more often than previouslythought and should not be discarded a-priori from any AEM survey.