Time-domain-induced polarization: Full-decay forward modeling and 1D laterally constrained inversion of Cole-Cole parameters

Time-domain-induced polarization has significantly broadenedits field of reference during the last decade, from mineralexploration to environmental geophysics, e.g., for clay and peatidentification and landfill characterization. Though, insufficientmodeling tools have hitherto limited the use of time-domaininducedpolarization for wider purposes. For these reasons, anew forward code and inversion algorithm have been developedusing the full-time decay of the induced polarization response,together with an accurate description of the transmitter waveformand of the receiver transfer function, to reconstruct thedistribution of the Cole-Cole parameters of the earth. The accuratemodeling of the transmitter waveform had a strong influenceon the forward response, and we showed that thedifference between a solution using a step response and a solutionusing the accurate modeling often is above 100%. Furthermore,the presence of low-pass filters in time-domain-inducedpolarization instruments affects the early times of the acquireddecays (typically up to 100 ms) and has to be modeled in theforward response to avoid significant loss of resolution. The developedforward code has been implemented in a 1D laterallyconstrained inversion algorithm that extracts the spectral contentof the induced polarization phenomenon in terms of the Cole-Cole parameters. Synthetic examples and field examples fromDenmark showed a significant improvement in the resolution ofthe parameters that control the induced polarization responsewhen compared to traditional integral chargeability inversion.The quality of the inversion results has been assessed by a completeuncertainty analysis of the model parameters; furthermore,borehole information confirm the outcomes of the field interpretations.With this new accurate code in situ time-domaininducedpolarization measurements give access to new applicationsin environmental and hydrogeophysical investigations,e.g., accurate landfill delineation or on the relation betweenCole-Cole and hydraulic parameters.