Resinvm3d: a Matlab 3-d Resistivity Inversion Package a Manuscript for Submission to Geophysics

We present an open source 3-D, MATLAB based, resistivity inversion package. The package uses a finite volume discretization to solve the forward partial differential equation. We include a highly efficient approach to reducing boundary effects and source electrode singularities. Solving of the forward problem is achieved by using a preconditioned bi-conjugate stabilized gradient method. For the inversion algorithm we have implemented an Inexact Gauss-Newton solver, with the model update being calculated using a preconditioned conjugate gradient algorithm. Default regularization is Tikhonov, based on zero and first order derivatives. However, regularization type and magnitude can easily be changed. The code also has functions to include inversion constraints, such as borehole logs. We include two synthetic examples that demonstrate the usefulness of this code. 3 1. Introduction 3-D DC resistivity imaging is a geophysical method that is being increasingly used to assist hydrologists, engineers and geologists in the development of subsurface models. This method involves injecting current through a pair of electrodes, either on the surface or in boreholes, and measuring the resulting potentials at numerous (tens to hundreds of) electrodes located on the surface or in boreholes. The measurements yield values of resistance that can be inverted to determine the subsurface electrical resistivity structure. The electrical resistivity structure has been used, for example, to infer the presence of a contaminant, with electrical resistivity different from that of the background environment, (e.g.