In resistivity and IP prospecting, the arrangement of electrodes, also called configuration. Several array types are shown in Figures A-18 and A-19. Resistivity array types include the following: (a) Azimuthal array; see azimuthal survey. (b) Dipole-dipole array and axial-dipole array, an array in which one dipole (a connected pair of electrodes) sends current into the ground and the other dipole serves as the potential-measuring pair. The separation between pairs is often comparable to (or only a few times greater than) the spacing within each pair, so the electrode pairs are not ideal dipoles; in deep resistivity sounding, the separation is larger. The dipole pairs are usually collinear (in line) but other orientations are also used (Figure A-19). Resistivity and IP data from this array often are displayed as on Figure P-14. (c) Gradient array or AB rectangular array, an arrangement in which a pair of potential electrodes measure the voltage between points of a rectangular grid between two distant, fixed current electrodes; a variation of the Schlumberger array. (d) Pole-dipole array, a voltage-measuring pair of grounded potential electrodes separated successively from one current electrode (pole) while traversing a survey line. The second current electrode (the infinite electrode) is so far away that its location has negligible effect on the measurements. Data can be plotted below the mid-point between the current and the near potential electrode on a pseudosection. Called a half-Schlumberger array and, if the electrodes are equally spaced, a three array. (e) Pole-pole array or two array, one current and one potential electrode (poles) are traversed or successively expanded on a survey line. The other current and potential electrodes are located so far away that their location has negligible effect on the measurements. Data are plotted either at the potential electrode or halfway between the two poles. Also called a half-Wenner array. (f) Radial array; see azimuthal survey. (g) Schlumberger array, with the inner voltage-measuring pair of potential electrodes closer together than the outer current electrode pair, by a factor of about 6. MN is usually but not necessarily at the center of AB. Also called a gradient array. In a half-Schlumberger array, one of the current electrodes is very far away. (h) Wenner array, four equally spaced inline electrodes; either the electrodes are all moved along a traverse or their separation is successively expanded. The usual or α-configuration has the center two electrodes as the potential electrodes; the β-configuration has the first two electrodes as potential electrodes; and the γ-configuration alternates current and potential electrodes.
FIG. A-18. Colinear arrays used in resistivity surveying. The equation for the apparent resistivity ρa is given for each array.
FIG. A-19. Dipole-dipole arrays used in resistivity and IP exploration.
FIG. P-14. A processing flow chart for seismic data. (After Sheriff, 1980, 405.)