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WENNER SOUNDINGS: This is the most common method of sounding in the United States and has the greatest signal to noise ratio and is least susceptible to near-surface homogeneities. The four electrodes are in a straight line and are at uniform separation. The separation between adjacent electrodes is designated “a”.
If the maximum depth of investigation is to be D, then observations should be taken with “a” spacings from approximate one-fifth D to about five times D.
The data are usually plotted on a log-log graph. To have a uniform spacing of data points, the following “a” spacings are suggested: 1.0, 1.4, 1.9, 2.7, 3.7, 5.0, 7.0, 10, 14, 19, etc.
In Appendix I there is an example of a field form for collecting Wenner sounding data.
Apparent resistivity, Ra, is given by the following formula where p is 3.14, a is the electrode spacing and R is the resistance read on the MiniRes.
Ra = 2 p a R = 6.283 a R
If the dimensions of a are in meters, the apparent resistivity is in Ohm-meters. If the a dimensions are in feet, the apparent resistivity is in Ohm-feet.
LEE ARRAY: There is a variation on the Wenner array called the Lee Array. Five electrodes are used with the fifth electrode located at the center of the array. The distance from the center electrode to each of the potential electrodes is a/2. Current is applied to the end electrodes as usual. First the potential is observed between one potential electrode and the center electrode and the resistance is measured. Then the resistance is observed between the center electrode and the remaining potential electrode. The resistivity of each half is calculated as:
Ra = 4 p a R = 12.57 a R
The average of the two observations should be identical to the value that would be obtained by the standard Wenner observation. An advantage of the Lee technique is that it allows a check on lateral variability of earth resistance.
OFFSET OR BARKER METHOD This method uses five evenly spaced electrodes. Like the Lee array it yields a good check on lateral inhomogeneities. The method was devised to work with a multiconductor cable and a switching box. The details of the offset array are published by Barker in Geophysical Prospecting, 1981, vol. 29, pp.128-143.
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