A review of inductive induced polarisation effects on electromagnetic data: cause, effect and applications

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ID: 331571
2026
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Abstract
Summary Inductive electromagnetic (EM) methods induce currents in the ground with a changing magnetic field associated with time-varying current flowing in a transmitter loop. Geophysicists use measurements of the secondary magnetic field to estimate the conductivity structure of the ground. These estimations often assume that conductivity is constant, but in some cases, the conductivity can be dispersive and vary as a function of frequency. In this case, the dispersion is called Induced Polarisation (IP), and with an inductive (EM) source is called Inductive Induced Polarization abbreviated as IIP. IP and IIP phenomena are usually caused by electrochemical diffusion associated with small capacitor-like components in the ground. These components may be metallic particles, graphite, clay or local restrictions in current flow paths. Capacitor-like elements result in induced electrical polarisation or charge separation associated with current flow. When the injected or induced currents have turned off or decayed, the separated charges return to their original equilibrium state. This results in a net flow of current, called polarisation current, in the opposite direction to the original secondary current flow. The most common way of characterising IP and IIP is to estimate the parameters of a mathematical model that fits the measured data. The Cole-Cole model is the most common. Reversed or return polarisation current flow manifests itself in time-domain (transient) EM data as negative amplitudes or as positive decays that decay faster at late-time decays than is possible from a non-polarizable source. Depending on whether excitation is from a current or a voltage source, observed decays may have time constants that differ by orders of magnitude. For inductive sources, characterisation is best achieved by fitting a model that is a perturbation from the high-frequency conductivity rather than a perturbation from a low-frequency resistivity, with the associated Cole-Cole conductivity time-constant most directly associated with grain size. Sign reversals or negative responses from return current flow have been recorded since the 1970s or earlier in ground central-loop EM data, and in airborne EM data since the 1990s. Prior to the development of inversion methods, IIP negative transients were often simply removed from field data, leading to inaccurate conductivity models and interpretation. Since the turn of the century, modelling and inversion of Cole-Cole parameters have been common and applied to measured ground, semi-airborne and airborne transient EM data to parameterise any IIP effects. In order to explain the distortion of decays by IIP phenomena, IP-incorporated modelling and inversion of both the source and its host materials are critical for valid characterisation and interpretation. However, overparameterization and the limited bandwidth can lead to model uncertainty and ambiguous results. This is particularly the case if large-scale 3D inversion is attempted. We review IIP field examples for minerals of economic interest that may have IP associations, including disseminated sulphides, gold-associated alteration and kimberlite pipes under permafrost conditions. Of importance in near-surface, hydrogeological and contaminant mapping are other polarizable sources, and IIP has been applied to mapping clays, hydrocarbons and permafrost.
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Authors Hesham M. El‐Kaliouby, Juntao Lu, Richard Smith, James C. Macnae
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Journal geophysical journal international
Year 2026
DOI
10.1093/gji/ggag409
URL
https://doi.org/10.1093/gji/ggag409
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