Frequency dependent complex resistivity inversion in 3D from Controlled-Source Electromagnetic data
Résumé
In some rocks and soils, induced polarization (IP) phenomena are occurring when an electric perturbation is applied. These mechanisms are described by a frequency dependent complex resistivity (CR). The study of relaxation model parameters describing these phenomena allows to access indirectly to several properties of interest of the underground, as properties linked to the pore space geometry, fluid content or presence and discrimination of disseminated metallic particles. Complex resistivity is usually studied using electrical method with a direct current hypothesis, in time or frequency domain. Electromagnetic induction coupling with the ground or cable layout are then neglected or considered as a source of noise, as they increase with frequency and larger offset. Thus, strong limitations appear to recover a complex resistivity image of buried target. It leads several authors to rely on empirical EM induction retrieval techniques (Routh and Oldenburg, 2001) to manage IP data contaminated by EM effects. Lately, other authors worked to implement the problem of CR to Controlled-Source Electromagnetic method (CSEM), a resistivity imaging technique using multi-frequency electromagnetic signals fully taking into account EM induction. It uses thus the geometrical and the frequency sounding information of the ground for a larger investigation depth. Ghorbani et al (2009) developed, CR1Dinv, a 1D inversion code taking into account EM induction and IP effects for SIP data. Some codes at larger dimension exists but usually take into account only a constant complex resistivity as the work of (MacLennan et al, 2014) in 2.5D. Nevertheless, there still are a strong need of a method able to manage both effects in this domain. In this work, we implemented frequency dependent complex resistivity inversion in POLYEM3D, a 3D finite-difference modelling and inversion code for controlled-source electromagnetic data (CSEM) (Bretaudeau et al, 2021) in order to fully recover IP information contained in EM data. Through a three layers synthetic example, we present the inversion method developed to constrain the multi-parameters problem using an 1D inversion module fully implemented to the 3D code for methodological developments. Secondly, a 3D synthetic example is showed to illustrate a transposition of the developed inversion method to the 3D code.
Domaines
Géophysique [physics.geo-ph]
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