Multiple-gradient array measurements for multichannel 2D resistivity imaging

Two-dimensional resistivity imaging using multiple gradient, Wenner and dipole-dipole electrode arrays was carried out at two field sites in Sweden and one in Nicaragua, with the objective of confirming the practical applicability of results obtained with numerical modelling. The results support earlier numerical modelling studies that concluded that the gradient array, using multiple current electrode combinations, has resolution as good as or better than the commonly used Wenner array. The array behaved well in terms of sensitivity to noise at the test sites, and the results obtained generally agree with dipole-dipole array results, although the latter at two of the sites gave resistivities that differed significantly from the other arrays in the deeper parts of the inverted models. A formula proposed for pseudosection plotting works well for data quality assessment, where it can be an advantage to make separate plots for each m-factor (the distance between the midpoints of the current and potential electrode pairs) or for each a-spacing. The gradient array is well suited for multichannel data acquisition, and can significantly increase the speed of data acquisition in the field and at the same time give higher data density, but it is also an attractive option for single-channel data acquisition. The Wenner array, on the other hand, is not suitable for measuring in more than one channel. Compared to the dipole-dipole array, it offers lower sensitivity to noise which may be a major advantage in real data acquisition, and the remote electrode needed for the pole-dipole array is avoided, which is often a significant advantage for field logistics.