Method for attenuating air wave response in marine electromagnetic surveying

1. A method for measuring the electromagnetic response of formations below the bottom of a body of water, comprising:

positioning at least one electromagnetic transmitter and at least one electromagnetic receiver in the body water each at a selected depth below the water surface;

conducting a transient electric current through the at least one transmitter;

detecting an electromagnetic signal at the at least one electromagnetic receiver;

wherein the depths are selected so that substantially all electromagnetic response to the current passed through the transmitter from the air above the body of water in the detected electromagnetic signal occurs before the beginning of a response originating in the formations below the water bottom.

2. A method as claimed in claim 1 wherein the at least one transmitter is an electric dipole.

3. A method as claimed in claim 1 wherein the transient electric current comprises a step change in current.

4. A method as claimed in claim 3 wherein the transient electric current comprises a pseudo-random binary sequence.

5. A method as claimed in claim 1 wherein the at least one receiver is an electric dipole.

6. A method as claimed in claim 1 further comprising estimating a response of the body of water and removing the water response from the detected electromagnetic signal.

7. A method as claimed in claim 6 wherein the water response is estimated by measuring a resistivity of the water and a depth of the body of water.