System and method for electromagnetic wavefield resolution

• US 20060103387A1
• Filed: 05/21/2003
• Published: 05/18/2006
• Est. Priority Date: 05/24/2002
• Status: Active Grant

• Current Assignee: Electromagnetic Geoservices ASA
• Original Assignee: Electromagnetic Geoservices ASA
• Inventors: Amundsen, Lasse
• Granted Patent: US 7,319,330 B2
• Time in Patent Office: Days
• US Class Current: 324/337
• CPC Class Codes: G01V 3/083 Controlled source electromagnetic [CSEM] surveying
• CPC Class Codes: G01V 3/12 operating with electromagnetic waves operating with millimetre waves G01V8/005
• CPC Class Codes: G01V 3/38 Processing data, e.g. for analysis, for interpretation, for correction

Abstract

A method of processing an electromagnetic wavefield response in a seabed logging operation. The wavefield is resolved into upgoing and downgoing components. The downgoing component represents reflections from the sea surface while the upgoing component represents reflections and refractions from subterranean strata. The upgoing component is then subjected to analysis.

Claims

1. A method of determining the nature of strata beneath the seabed comprising applying an electromagnetic (EM) wavefield to the strata;

detecting an EM wavefield response; and

processing the wavefield by resolving the wavefield to produce upgoing and downgoing components then analysing the upgoing component, thereby deriving the nature of the strata.

2. A method as claimed in claim 1, including the step of applying the EM field is applied by means of a transmitter located at or near the seabed.

3. A method as claimed in claim 1, wherein the EM wavefield is transmitted at a frequency between 0.01 and 20 Hz.

4. A method as claimed in claim 1, wherein the EM wavefield is transmitted at a wavelength between 1S and 50S, where S is the thickness of the overburden above the considered strata.

5. A method as claimed in claim 1, wherein the transmitter is a dipole antenna.

6. A method as claimed in claim 1, wherein the wavefield response is detected by means of a receiver located at or near the seabed.

7. A method as claimed in claim 1, wherein the receiver is a dipole antenna.

8. A method as claimed claim 1, wherein the receiver comprises a detector pair consisting of means for detecting the electromagnetic wavefield and means for detecting a magnetic field.

9. A method as claimed in claim 8, wherein the detector pair is housed in a single unit.

10. A method as claimed in claim 8, wherein data from one of the detector pairs is used to resolve the field detected by the other of the detector pairs.

11. A method as claimed in claim 1, wherein the wavefield is detected using a plurality of receivers arranged over an area of the seabed.

12. A method as claimed in claim 11, wherein the receivers are arranged in a line.

13. A method as claimed in claim 11, wherein data from an array of receivers is used to resolve the wavefield.

14. A method as claimed in claim 11, wherein data from each receiver independently is used to resolve the wavefield.

15. A method as claimed in claim 6, wherein the receiver is moved to a different location while the transmitter is maintained stationary, and a further EM wavefield is applied, detected and processed.

16. A method as claimed in claim 1, wherein the wavefield is resolved using the 2.5D assumption that there are no variations in the medium parameters in the cross-profile direction.

17. A method as claimed in claim 1, wherein the resolution and analysis are carried out on the assumption of vertically travelling plane waves.