Multi-transient DC resistivity measurements

A system for measuring resistivity variations in the earth comprising a source for passing a current between two source electrodes;

• a measurement device for measuring the transient current at the source electrodes;
• a voltage measurement device for measuring the resultant transient voltage between at least one pair of receiver electrodes;
• and a processor configured to estimate one or more processing functions for applying to the measured input current to provide a step current profile;
• apply the same one or more processing functions to the measured voltage to provide an estimate of a step response voltage between the receivers, use both the step current and the estimated step response voltage to determine a resulting apparent earth resistance, and use the apparent earth resistance to determine the resistivity of the earth.