Enhanced resolution for long electrode ERT

The resolution of the long electrode electrical resistivity tomography method is investigated through the comparison of arrays. The investigations included a synthetic model study and a pilot-scale field experiment, in which data from the 2-pole and 4-pole arrays were used to reconstruct known targets through inverse modelling. The results confirmed that the 2-pole array maps conductive targets with low lateral resolution and no vertical resolution. The 4-pole array performs extremely well or extremely poorly depending on the specific subset of data used in the inversion modelling. The worst performance was observed from using a randomized 4-pole subset. In this case, the reconstructed target was offset from the known location. The best performance came from evaluating the comprehensive data set comprising all possible 4-pole combinations and choosing favourable subsets that minimized outliers in transfer resistance, geometric factor, data error and apparent resistivity. These favourable 4-pole subsets were capable of resolving both conductive and resistive targets with higher fidelity than the 2-pole array. Unfortunately, it may not be possible to acquire the comprehensive 4-pole data set, especially for a large number of electrodes. A viable alternative is to acquire the comprehensive 2-pole data set and calculate any desired 4-pole subset using superposition. In this way, the geophysicist will also have full advantage of signal strength and shorter measurement cycle that accompanies the 2-pole array.