Inversion of 2D spectral induced polarization imaging data

Laboratory measurements of various materials suggest that more information can be obtained by measuring the in-phase and out-of-phase potentials at a number of frequencies. One common model used to describe the variation of the electrical properties with frequency is the Cole-Cole model. Apart from the DC resistivity (rho) and chargeability (m) parameters used in conventional induced-polarization (IP) surveys, the Cole-Cole model has two additional parameters, i.e. the time (tau) and relaxation (c) constants. Much research has been conducted on the use of the additional Cole-Cole parameters to distinguish between different IP sources. Here, we propose a modified inversion method to recover the Cole-Cole parameters from a 2D spectral IP (SIP) survey. In this method, an approximate inversion method is initially used to construct a non-homogeneous starting model for the resistivity and chargeability values. The 2D model consists of a number of rectangular cells with constant resistivity (rho), chargeability (m), time (tau) and relaxation (c) constant values in each cell. A regularized least-squares optimization method is then used to recover the time and relaxation constant parameters as well as to refine the chargeability values in the 2D model. We present results from tests carried out with the proposed method for a synthetic data set as well as from a laboratory tank experiment.