Systematic Errors In Resistivity Measurement Systems

The greatest obstacles in achieving higher resolution of electrical resistivity tomography (ERT) data are systematic errors that result from the non-ideal nature of measurement systems or procedures. Systematic errors are correlated to the system functions and cannot be removed by data averaging. This paper discusses a number of sources of systematic errors including: Errors in the system gains or calibration of voltage and current monitors, Leakage and coupling in multiplexers, cables and wires, and Electrode aging and impedance effects. Of particular concern are leakage currents within the types of cable used on sites in the past. These errors were found to be larger than expected. It is also important to investigate errors related to the electrodes used in ERT in part since errors such as the cable leakage depend partly on electrode impedance and in part as errors arise from the electrode themselves. There is no simple, clear relation between electrode errors and such factors as material type, history of use, and waveform time/frequency. Preliminary results indicated that the best electrodes were copper-copper sulfate even when they are used as both transmitters and receivers. The best, simple metal electrodes were welding steel.