Marine EM exploration

1. A method for estimating the effects of an airwave in marine electromagnetic data measured using a source and at least one receiver, the method comprising measuring the electromagnetic response at two different under water source-receiver separations; using one or more measurements at the larger separation to estimate the air wave response at that separation, and using this to estimate the air wave effect in the response measured at the shorter separation.

2. A method as claimed in claim 1 comprising scaling the measurement at the larger separation by a factor that is a function of the ratio of the larger separation to the smaller separation, thereby to determine an estimate of the airwave at the shorter separation, and subtracting this from the measurement at the shorter separation.

3. A method as claimed in claim 2 wherein the ratio of the separations is approximately five or more.

4. A method as claimed in claim 2 wherein the function is equal or related to the cube of the ratio of the larger separation distance to the smaller.

5. A method as claimed in any of claims 1 to 4 wherein both separations are large compared with the water depth.

6. A method as claimed in claims 1 to 4 wherein the depth of the source (a) is the same in both measurements and the depth of the receiver (b) is the same in both measurements.

7. A method as claimed in any of claims 1 to 4 wherein for one measurement the depth of the source is (a) and the depth of the receiver is (b), and for the other measurement the depth of the source is (b) and the depth of the receiver is (a).

8. A method as claimed in claims 1 to 4 wherein the source is a current bi-pole or a magnetic loop.

9. A method as claimed in claims 1 to 4 wherein the receiver is a bi-pole receiver.

10. A method as claimed in claims 1 to 4 wherein the electromagnetic measurements are transient electromagnetic measurements, for example MTEM, or continuous electromagnetic measurements.

11. A method as claimed in claims 1 to 4 wherein measurements are taken at more than two different under water source-receiver separations.

12. A system for measuring the earth response to electromagnetic signals applied in a shallow water environment, the system comprising a source and at least one receiver in the water arranged to allow measurements to be taken at two or more different source receiver separations; and means for using a measurement at a larger one of the separations to estimate an air wave contribution in the measurement at the shorter separation.

13. A system as claimed in claim 12 comprising means for scaling the measurement at the larger separation by a factor that is a function of or related to the ratio of the larger separation distance to the smaller separation distance.

14. A system as claimed in claim 13 comprising means for subtracting the scaled measurement from the measurement at the shorter separation.

15. A system as claimed in claim 13 wherein the function is equal or related to the cube of the ratio of the larger separation distance to the smaller.

16. A system as claimed in any of claims 12 to 15 wherein the larger separation is approximately five or more times greater than the shorter separation.

17. A system as claimed in any of claims 12 to 15 wherein both separations are at least five times greater than the depth of the water.

18. A system as claimed in any of claims 12 to 15 wherein the larger separation is selected so that the earth response measured at the receiver is negligible, preferably substantially zero.

19. A method, for removing the effects of an airwave from shallow water marine electromagnetic data as measured by at least one source and at least one receiver, using two measurements taken at two different source receiver separations to estimate the air wave contribution at the larger separation and using this to determine the earth response at the shorter separation.

20. A method as claimed in claim 19 comprising scaling the measurement at the larger separation by a factor that is a function of or related to the ratio of the larger separation distance to the smaller, thereby to estimate the air wave contribution to the measurement at the shorter separation.

21. A method as claimed in claim 20 comprising subtracting the scaled measurement from the measurement at the shorter separation.