SCHLUMBERGER SOUNDINGS: This is the most common method of sounding in Europe and is common in the United States. It saves moving the potential electrodes every time the current electrodes are moved. The electrodes are in a straight line and like the Wenner array, the outer electrodes are the current electrodes and the inner electrodes are the potential electrodes. The potential electrodes, usually designated M and N, should never be separated by more that one-fifth the separation between the current electrodes. The current electrodes are usually designated A and B.
Apparent resistivity is given by the following formula where p is 3.14, AB is the distance between the current electrodes, MN is the distance between the potential electrodes and R is the resistance read on the MiniRes. MN can also be designated a and the distance between a current electrode and the nearest potential electrode designated as na.
Ra = p R (AB)(AB)/4(MN) = p R a n (n+1)
As the current electrodes are expanded farther and farther, the potential difference between the fixed potential electrodes becomes smaller and smaller. Finally the signal to noise ratio becomes noticeably small. Then the potential electrodes are expanded and an observation is made with the current electrodes at the same spacing. Theoretically, the apparent resistivities should be the same. However, they will always differ by at least a small amount. This may be due to lateral inhomogeneities in the earth or to a localized irregularity near one of the potential electrodes. The survey is resumed with the several more observation made with the current electrodes being placed at greater and greater separation. For a single sounding there may be three, four or even five separations used for the potential electrodes. This, in turn, generates three, four or five segments of sounding curve, each with at least a small offset from the adjacent segments.
The field data are traditionally plotted on log-log graph paper. Because of the offset of the segments, some adjusting of the data is necessary. The adjusted data can then be entered into a resistivitiy inversion program.
The Schlumberger array has an advantage over the Wenner array. The potential electrodes are moved less often and therefore there is less fieldwork in collecting the data for a sounding. Despite this timesaving, there are great advantages to the Wenner sounding technique. First, the Wenner array gives the highest signal-to-noise ratio. Second, if there is an inhomogeneity in the vicinity of the inner (potential) electrodes, a whole segment of the apparent resistivitiy curve will have an erroneous offset. With the Wenner method, there would be only one data point having an erroneous offset. If there are numerous surficial inhomogeneities, the Wenner method would show the interpreter this important feature of the survey and the smoothed curve fitted through the Wenner data will reflect a truer value for the subsurface.
In Appendix I there are examples of work sheets for collecting Schlumberger sounding data.