3-D Induced Polarization Inversion Using Complex Electrical Resistivit Ies

Induced polarization (IP) is a current-stimulated electrical phenomenon observed as a delayed<br>voltage response in earth materials. The basic mechanism of the IP response is mainly driven<br>by electro-chemical processes. It has been indicated that the IP method is an effective means of<br>detecting and mapping environmental contaminants in many situations where conventional d.c.<br>resistivity has been ineffective. In this paper we present an inversion technique for interpreting<br>frequency-domain IP data in terms of complex electrical resistivity in the subsurface. The amplitude<br>of the observed voltage and its phase shift from the injected current are inverted to recover a<br>3-D subsurface resistivity structure. Extended from a 3-D d.c. resistivity inversion technique,<br>the algorithm uses the complex bi-conjugate gradient method in both the forward modeling and<br>inversion, The IP inversion procedure we have developed can be applied to l-D, 2-D, or 3-D earth<br>models and to any configuration of current and potential electrodes. We illustrate our methods by<br>inverting synthetic data taken over a contaminated plume embedded in a homogeneous background<br>medium. The model parameters are based on laboratory measurements.