Multi-transient DC resistivity measurements

1. A method for measuring resistivity variations in the earth comprising passing a transient current between two source electrodes; measuring the transient current at the source electrodes; measuring the resultant transient voltage between at least one pair of receiver electrodes; estimating one or more processing functions for applying to the measured input current to provide a step current profile, and applying the same one or more processing functions to the measured voltage to provide an estimate of a step response voltage between the receivers, using both the step current and the estimated step response voltage to determine a resulting apparent earth resistance, and using the apparent earth resistance to determine the resistivity of the earth.

2. A method as claimed in claim 1 wherein the step of applying the estimated processing function to the measured voltage involves convolution of the received voltages with the estimated processing function to find the corresponding step responses.

3. A method as claimed in claim 1 wherein the processing function is a filter for converting the measured input current to the step profile.

4. A method as claimed in claim 1 wherein the one or more processing functions comprise a filter for converting the measured current to an impulse and means for integrating the result to obtain the step input profile.

5. A method as claimed in any of claims 1 , 2 , 3 or 4 comprising removing cultural noise.

6. A method as claimed in any of claims 1 , 2 , 3 or 4 comprising estimating a steady state voltage from a step response.

7. A method as claimed in any of claims 1 , 2 , 3 or 4 wherein the source and receiver electrodes are provided in a marine environment.

8. 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.

9. A system as claimed in claim 8 wherein the processor is operable to apply the estimated processing function to the measured voltage by convolution of the received voltages with the estimated processing function to find the corresponding step responses.

10. A system as claimed in claim 8 wherein the processing function is a filter for converting the measured input current to the step profile.

11. A system as claimed in claim 8 wherein the one or more processing functions comprise a filter for converting the measured current to an impulse and means for integrating the result to obtain the step input profile.

12. A system as claimed in any of claims 8 , 9 , 10 or 11 wherein the processor is operable to remove cultural noise.

13. A system as claimed in any of claims 8 , 9 , 10 , or 11 wherein the processor is operable to estimate a steady state voltage from a step response.

14. A system as claimed in any of claims 8 , 9 , 10 or 11 wherein the source and receiver electrodes are provided in a marine environment.