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The Square Array
Resistivity surveys have been performed using a square array of electrodes. The square array has been used for shallow environmental surveys, for assessment of joint orientation, and especially for archeological surveys. Appendix Y contains several references for the square array. A common “a” spacing is one meter for archeological surveys. The apparent resistivity is plotted at the center of the array.
Apparent resistivity is
Ra = p R a (2 + Ö2 ) = 10.73 R a
The square array is very sensitive to anisotropy. If the anisotropy of the site is to be evaluated, each observation can be repeated with the array rotated 90 degrees. The transverse anisotropy is the ratio of the two ororthogonal apparent resistivities at each site. Another method of plotting the anisotropy would be to plot:
A = (Ra1 – Ra2)/( Ra1 + Ra2)
The square array can be used for determining the orientation of fractures and other linear features. See F. Boadu, Sept 2005 Geophysics.
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