Square array resistivity anomalies and inhomogeneity ratio calculated by the finite-element method

In this work the square array configuration is studied. As with any four-electrode measuring configuration, three different resistances can be measured directly and, in particular, by using the square array these resistance values can be used to obtain a measure of the apparent anisotropy: this is the so-called azimuthal inhomogeneity ratio (AIR). The AIR is used widely to derive information regarding the directional variation of the subsurface resistivity. Similar types of information can be obtained using any collinear array but would need to be used in a crossed mode. This gives an operational advantage to the square array. Here, the AIR and the square alpha and beta apparent resistivities have been calculated for a number of simple subsurface models. Moreover, the apparent resistivity responses for several different linear arrays were calculated to allow useful comparisons. The forward modeling has been carried out via a 2.5-D finite-element scheme and an existing approach has been modified in order to calculate the potential variations parallel to the strike direction. Here, the use of AIR as a pattern recognition tool is investigated. The AIR anomalies are shown to delineate the edges of the targets successfully, retaining the same pattern when the model was shifted in depth, and indicating a satisfactory resolving ability. The AIR can be used as complementary information to the apparent resistivity measures and can improve the interpretation. However, careful consideration has to be given to the data noise.