Method and system for removing effects of conductive casings and wellbore and surface heterogeneity in electromagnetic imaging surveys

1. A method for characterizing a reservoir, comprising the steps of:

normalizing measured voltage data by transmitter moment;

sorting the normalized voltage data into common receiver profiles;

densely resampling transmitter locations using common positions for the common receiver profiles;

coarsely resampling the normalized voltage data at discrete transmitter locations;

defining a starting model for inversion;

weighting the normalized voltage data by a factor;

converting the normalized voltage data to ratios;

calculating a conductivity image using a ratio inversion method; and

verifying that an inversion has converged.

2. The method as recited in claim 1 , further comprising the step of removing outlier results from each of the receiver profiles.

3. The method as recited in claim 2 , further comprising the step of displaying the conductivity image.

4. The method as recited in claim 3 , wherein the step of normalizing measured voltage data by transmitter moment does so by a value that tracks the transmitter moment, wherein the value is measured transmitter current.

5. The method as recited in claim 3 , wherein the step of normalizing measured voltage data by transmitter moment does so by a value that tracks the transmitter moment, wherein the value is a voltage produced in a secondary solenoid wrapped around a transmitting solenoid.

6. The method as recited in claim 3 , wherein a common receiver profile comprises measurements made during one of a crosswell, surface-to-borehole, or borehole-to-surface electromagnetic survey with a receiver fixed at a given depth.

7. The method as recited in claim 3 , wherein the step of densely resampling transmitter locations using common positions for the receiver profiles further comprises the steps of:

defining a common transmitter depth interval for all of the receiver profiles;

determining a minimum spatial sampling interval among all of the receiver profiles and using them to define transmitter positions that will be employed for all receiver profiles; and

using a spline technique to interpolate the data from actual measurement locations to global locations.

8. The method as recited in claim 3 , wherein the step of coarsely resampling the data at discrete transmitter locations further comprises selecting a subset of values output from the step of densely resampling transmitter locations using common positions for the receiver profiles using a depth-interval between adjacent transmitter points that is approximately equal to a minimum receiver sampling interval that is employed.

9. A system for characterizing a reservoir, comprising:

a plurality of electromagnetic transmitters and receivers, one or more of which are disposed within a wellbore positioned relative to the reservoir to be characterized;

means for normalizing measured voltage data by transmitter moment;

means for sorting the normalized voltage data into common receiver profiles;

means for densely resampling transmitter locations using common positions for the receiver profiles;

means for coarsely resampling the data at discrete transmitter locations;

means for defining a starting model for inversion;

means for weighting the data by a factor;

means for converting the normalized voltage data to ratios; and

means for calculating a conductivity image using a ratio inversion method.

10. The system as recited in claim 9 , further comprising means for displaying the conductivity image.

11. The system as recited in claim 9 , wherein the means for normalizing measured voltage data by transmitter moment does so by a value that tracks the transmitter moment, wherein the value is measured transmitter current.

12. The system as recited in claim 9 , wherein a common receiver profile comprises measurements made during a one of a crosswell, surface-to-borehole, or borehole-to-surface electromagnetic survey with a receiver fixed at a given depth.

13. The system as recited in claim 9 , wherein the means for densely resampling transmitter locations using common positions for the receiver profiles further comprises:

means for defining a common transmitter depth interval for all of the receiver profiles;

means for determining a minimum spatial sampling interval among all of the receiver profiles and using them to define transmitter positions that will be employed for all receiver profiles; and

means for using a spline technique to interpolate the data from actual measurement locations to global locations.

14. A computer program product embodied in a computer storage medium, the computer program product operable for characterizing a reservoir, comprising the program steps of:

normalizing measured voltage data by transmitter moment;

sorting the normalized voltage data into common receiver profiles;

densely resampling transmitter locations using common positions for the receiver profiles;

coarsely resampling the normalized voltage data at discrete transmitter locations;

defining a starting model for inversion;

weighting the normalized voltage data by a factor;

converting the normalized voltage data to ratios;

calculating a conductivity image using a ratio inversion method; and

verifying that an inversion resulting from the ratio inversion method has converged.

15. The computer program product as recited in claim 14 , further comprising the program step of removing outlier results from each of the receiver profiles.

16. The computer program product as recited in claim 14 , further comprising the program step of displaying the conductivity image.

17. The computer program product as recited in claim 16 , wherein the program step of normalizing measured voltage data by transmitter moment does so by a value that tracks the transmitter moment, wherein the value is measured transmitter current.

18. The computer program product as recited in claim 16 , wherein the program step of normalizing measured voltage data by transmitter moment does so by a value that tracks the transmitter moment, wherein the value is a voltage produced in a secondary solenoid wrapped around a transmitting solenoid.

19. The computer program product as recited in claim 16 , wherein the program step of densely resampling transmitter locations using common positions for the receiver profiles further comprises the program steps of:

defining a common transmitter depth interval for all of the receiver profiles;

determining a minimum spatial sampling interval among all of the receiver profiles and using them to define transmitter positions that will be employed for all receiver profiles; and

using a spline technique to interpolate the data from actual measurement locations to global locations.

20. The computer program product as recited in claim 16 , wherein the program step of coarsely resampling the data at discrete transmitter locations further comprises selecting a subset of values output from the step of densely resampling transmitter locations using common positions for the receiver profiles using a depth-interval between adjacent transmitter points that is approximately equal to a minimum receiver sampling interval.

21. A method, comprising:

positioning a plurality of electromagnetic transmitters and receivers within a wellbore positioned relative to a reservoir to be characterized;

normalizing measured voltage data by transmitter moment;

sorting the normalized voltage data into common receiver profiles;

densely resampling transmitter locations using common positions for the receiver profiles;

coarsely resampling the data at discrete transmitter locations;

defining a starting model for inversion;

weighting the data by a factor;

converting the normalized voltage data to ratios; and

calculating a conductivity image using a ratio inversion method.