Three-dimensional inversion of EM coupling contaminated spectral induced polarization data

The spectral induced polarization (SIP) response of rocks and soils is a complex function of biological, chemical, and mineral subsurface processes. Its measured geophysical attribute, the complex resistivity (CR), represents a macroscopic parameter to relate SIP data to structural, mineral and hydraulic information of porous media and fluids contained therein. The complexity of subsurface CR distributions necessitates accounting for the three-dimensionality of the Earth and the development of adequate data interpretation algorithms (cf. Oldenburg and Li, 1994; Weller et al., 2000; Yang et al., 2000). In this work we describe the development and testing of a three-dimensional SIP inversion algorithm for CR based on the non-linear conjugate gradient method and finite-difference (FD) forward modeling. A hierarchical parallel architecture of the algorithm and an optimal FD mesh design allow for an economic use of today’s parallel computing capabilities to process large field data sets. Two different types of forward modeling operators provide a tradeoff between computational speed and maximal SIP data simulation accuracy, the latter achieved by properly accounting for electromagnetic coupling effects. We demonstrate the benefits of directly inverting data containing EM coupling effects on synthetic data that represent a mineral exploration survey.