Practical example of ERT field data analysis with x2ipi software.
The field data were from SuperSting R4 unit. It was Schlumberger array. Ground resistance was good and AB current was from 90 up to 300 mA. You can download a STG file from here. Inter-electrode distance was 3 meter. 
Apparent resistivity, Ohm.m
Measured voltage, mV
AB current, mA
We see that apparent resistivity pseudosection is smooth and data quality is good. There are duplicate readings. To see them we need to use Menu-Load-Extract duplication option, which splits one dataset in two.
 [D-transf] button shows the difference between two data set in percent. We see that maximal data error   corresponds to the maximum spacings and minimal measured voltage.  We can see also dependence data error on one plot, but it need some manipulation.
  1. Menu-Load-Make control. It saved the file "2901G.ctrl.txt", which keeps all duplicate readings. 
  2. This file keeps apparent resistivity and voltage if it is available in data file.
  3. We can calculate data error and plot the results. 
Thus, the following conclusions can be drawn about the data set.
  • The field data has good accuracy. 
  • Minimum required voltage is about 10 mV.