Geoelectrical null-arrays

The term “geoelectric null-array” is introduced for those D.C. electrode configurations, where the measured or the interpreted potential difference is zero above a homogeneous half space. A classification of all known null-arrays is shown in Figure 1. While the arrays belonging to the second column (where not the measured, rather the derived quantities are zero) and to the third one (that is the focussed arrays) have already been used earlier, the arrays in the first column (where directly the measured value is zero) – in spite of their simplicity – were used at first by the authors. These latter arrays represent current fields at various typical distances from the current electrodes, and at the same time, they can be easily constructed from traditional arrays (Szalai et al., 2002). The pairs of traditional and non-traditional arrays, presented in Figure 2, are as follows. I) MN (M0N0) is close to one of the current electrodes. In this case the current lines are nearly radial around the current electrode. According to the analogy of the classical three-electrode configuration AMN, the AM0N0 configuration is called as the three-electrode null-array; II) MN (M0N0) is between the two current electrodes. Here the current lines are nearly parallel. A special situation in this group is the Schlumberger (or AM0N0B) null-array, when the center of M0N0 is in the connecting line between A and B, at an equal distance from both current electrodes; III) MN (M0N0) is far from the current electrodes. In this case the current source may be considered as a dipole. Two null-arrays can be easily constructed from well-known traditional configurations: IIIA) the dipole axial null-array; IIIB) the dipole equatorial null-array.