Resolution of dc resistivity-thickness equivalences by inclusion of induced polarization data

The principle of equivalence is known to cause non-uniqueness in interpretations of direct current (DC) resistivity data. Here, we show that these equivalences can be significantly reduced by combining DC data with time-domain induced polarization (IP) measurements. An understanding of model equivalences requires a comprehensive investigation for each model when using gradient-based inversion methods. Instead, we apply a 1D Monte Carlo inversion that makes it possible to investigate the space of equivalent models. We invert synthetic DC data of a model with low-resistivity equivalences. We then include a contrast in chargeability, modelled in terms of spectral Cole-Cole IP parameters, and invert the DC and IP data together. The results show that the inclusion of IP data resolves the equivalences. The degree of resolution depends on the contrast in the chargeability and/or the other IP parameters. The contrasts required are easily expected in the field, which is justified with a field example where DC and IP data are measured on a sand formation with an embedded clay layer. Inversion results show that the resistivity data alone does not resolve the clay layer due to equivalence problems, but by adding the IP data to the inversion, the layer is resolved.