Method for reducing induction noise in towed marine electromagnetic survey signals

1. A method for reducing effect of motion on electromagnetic signals detected while moving an electromagnetic receiver through a body of water, comprising:

towing a transmitter cable comprising an electromagnetic transmitter through the body of water;

towing a separate receiver cable comprising the electromagnetic receiver through the body of water

measuring a parameter related to an amount of current passed through the electromagnetic transmitter, wherein the measuring is done as close to the electromagnetic transmitter as possible;

measuring a magnetic field proximate the electromagnetic receiver, wherein the measuring a magnetic field proximate the receiver comprises measuring voltage induced in three mutually orthogonal wire coils;

estimating a transmitter portion of the measured magnetic field from the measured parameter related to the amount of current;

estimating a motion portion of the measured magnetic field from the estimated transmitter portion of the measured magnetic field;

estimating a voltage induced in the receiver from the estimated motion portion of the measured magnetic field; and

correcting voltage signals detected by the receiver using the estimated voltage.

2. The method of claim 1 wherein the parameter related to an amount of current comprises magnetic field proximate the electromagnetic transmitter.

3. The method of claim 1 wherein the electromagnetic transmitter comprises a bipole electrode.

4. The method of claim 1 wherein the electromagnetic receiver comprises a bipole electrode.

5. The method of claim 1 wherein the measuring a magnetic field proximate the receiver comprises measuring voltage induced in at least one wire loop.

6. The method of claim 5 wherein the measuring a magnetic field proximate the receiver comprises measuring voltage induced in at least two orthogonal wire loops.

7. The method of claim 1 wherein the estimating the voltage induced by the estimated motion portion comprises determining a filter that best estimates a component of the receiver signals that is correlated with the motion portion of the measured magnetic field.

8. The method of claim 7 wherein the filter comprises a Wiener filter.

9. The method of claim 1 wherein the estimating a transmitter portion of the magnetic field from the magnetic field measurements comprises determining a filter that best estimates a portion of the measured magnetic field that is correlated with the current passed through the electromagnetic transmitter.

10. The method of claim 9 wherein the filter comprises a Wiener filter.

11. A method for marine electromagnetic surveying, comprising:

towing a transmitter cable comprising at least one electromagnetic transmitter and a separate receiver cable comprising at least one electromagnetic receiver through a body of water;

at selected times passing an electric current through the at least one transmitter to induce an electromagnetic field in formations below the bottom of the body of water;

detecting signals at the at least one receiver in response to the induced electromagnetic field;

measuring a parameter related to an amount of the current passed through at least one transmitter, wherein the measuring is done as close to the at least one transmitter as possible;

measuring a magnetic field proximate the at least one electromagnetic receiver, wherein the measuring a magnetic field proximate the at least one receiver comprises measuring voltage induced in three mutually orthogonal wire coils;

estimating a transmitter portion of the measured magnetic field from the measured parameter related to the amount of current;

estimating a motion portion of the measured magnetic field from the estimated, transmitter portion of the measured magnetic field;

estimating a voltage induced in the at least one receiver from the estimated motion portion of the measured magnetic field; and

correcting the voltage signals detected by the at east one receiver using the estimated voltage.

12. The method of claim 11 wherein the parameter related to an amount of current comprises magnetic field proximate the electromagnetic transmitter.

13. The, method of claim 11 wherein the signals detected by the receiver comprise voltage imparted across a bipole electrode.

14. The method of claim 11 wherein the imparting the electromagnetic field comprises passing the electric current across a bipole, electrode.

15. The method of claim 11 wherein the measuring a magnetic field proximate the at least one receiver comprises measuring voltage induced in at least one wire loop.

16. The method of claim 15 wherein the measuring a magnetic field proximate the at least one receiver comprises measuring voltage induced in at least two orthogonal wire loops.

17. The method of claim 15 wherein the estimating the voltage induced by the estimated motion portion comprises determining a filter that best estimates a portion of the receiver signals that is cross correlated with the motion portion of the measured magnetic field.

18. The method of claim 17 wherein the filter comprises a Wiener filter.

19. The method of claim 11 wherein the estimating a transmitter portion of the magnetic field from the magnetic field measurements comprises determining a filter that best estimates a portion of the measured magnetic field that is correlated with the current passed through the electromagnetic transmitter.

20. The method of claim 19 wherein the filter comprises a Wiener filter.