The dipole-dipole array (Figure 5.4c) is normally configured to have a relatively large separation between the pair of current electrodes on one side of the array and the pair of potential electrodes on the other side of the array. The dipole-dipole array is employed both for mapping lateral changes in pa and for assessing the variation of resistivity with depth. When using this array for areal mapping of pa, the spacing between electrodes remains constant as the array is moved from one location to another, but for measuring resistivity changes with depth, the array midpoint stays at the same location and the current electrode pair and the potential electrode pair are moved further apart. As deduced from inspection of Figure 5.4, if the overall electrode array length is large, the dipole-dipole array has a decided advantage over other arrays, due to the lesser amount of electric cable that is needed for transferring current or measuring voltage. The amount of electric cable required by the Schlumberger or Wenner arrays, in particular, can become quite unmanageable for long electrode arrays. An additional advantage of the dipole-dipole array is that the cables for the current electrodes are more easily kept separate from the cables for the potential electrodes, which reduces the electric potential noise due to electromagnetic coupling (Sharma, 1997). The apparent resistivity for the dipole-dipole array (Figure 5.4c) is expressed as follows:
Field resistivity surveys carried out with one of the more common electrode arrays (Shlumberger, Wenner, or dipole-dipole) utilizes equipment that is fairly basic. All that is actually needed is an electric current power source, a transmitter to regulate and measure the electric current, a receiver to determine the electric potential difference (essentially a high-impedance voltmeter that draws very little current), four insulated single-core copper wire cables, and four electrodes. The transmitter and receiver are often but not always combined into a single unit. Iron, steel, or copper stakes are commonly used for the electrodes that are inserted into the soil at the ground surface.
One problem does occur with the use of metal stakes for electrodes, and it involves the formation of unwanted electric potentials due to electrode polarization caused by contact between the metal in the stake and the electrolytic aqueous solution present within the soil. This problem is alleviated by using low-frequency alternating electric current (AC) to cancel out these spurious electric potentials. Typically, AC with a frequency less than 10 Hz is used for conventional resistivity surveys, so that these simple metal stake electrodes can be employed. Lower-frequency AC is used instead of higher-frequency AC, because higher-frequency AC reduces the electric current density with depth and, in turn, the depth of investigation (Sharma, 1997). Direct electric current (DC) is employed for very deep investigations and requires the use of nonpolarizing electrodes. A nonpolar-izing electrode is typically composed of a copper rod inserted through the lid of a container that is porous at its base and filled with an aqueous copper sulfate solution (Milsom, 2003).
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