Aim4res, an open-source 2.5D finite differences MATLAB library for anisotropic electrical resistivity modeling

Abstract Electrical Resistivity Tomography (ERT) is one of the oldest geophysical techniques, and due to the advances of numerical techniques along with computational resources, it is widely used for geophysical prospecting. It has found various domains of application as it is easy to implement and fast to image the ground resistivity heterogeneity. However, anisotropy, which is another key resistivity parameter, is seldom considered. Although being a well-known phenomenon, its consideration in the characterization process is only recent. Amongst the reasons behind this is the absence of available anisotropic resistivity modeling tools. We present aim4res (Anisotropic Inverse Modeling for RESisitivity) to that end. This open-source MATLAB library allows for 2.5D forward and inverse anisotropic resistivity modeling based on a finite differences scheme. The inverse problem is solved with a Gauss–Newton algorithm. The regularization coefficient, initial model and constraints can be adjusted from prior knowledge in order to avoid local minima during optimization. Analytical and synthetic studies have been carried out to prove the reliability of aim4res . The results demonstrate its ability to identify anisotropy, along with the correct geometry and resistivity amplitude. It is also able to correctly detect isotropy, as the inversion comparison with a previous toolbox already proven working showed. A real case study inversion is carried out to demonstrate that aim4res is a relevant tool to use on the field, able to reveal strong anisotropy fields even at short scales.