Electromagnetic surveying

• US 20060001428A1
• Filed: 06/29/2005
• Published: 01/05/2006
• Est. Priority Date: 07/02/2004
• Status: Active Grant

• Current Assignee: OHM Limited (PGS ASA)
• Original Assignee: OHM Limited (PGS ASA)
• Inventors: Bennett, Mark, Milne, James, Sykes, Chris, Rust, Jennifer
• Granted Patent: US 7,683,625 B2
• Time in Patent Office: Days
• US Class Current: 324/360
• 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

Abstract

A submersible electromagnetic (EM) field generator for seafloor electromagnetic surveying comprising an AC to DC converter operable to generate a DC output from an AC input and a switching module operable to generate a waveform driving signal by selectively switching the DC output. The EM field generator also comprises an antenna that is operable to generate an EM field in response to the waveform driving signal. This design approach of using a switched DC source allows square or rectangular wave EM signals to be generated which have sharp transition characteristics and which are substantially independent of the AC input characteristics.

Claims

1. A submersible electromagnetic (EM) field generator for seafloor electromagnetic surveying, comprising:

an AC to DC converter operable to generate a DC output from an AC input;

a switching module operable to generate a waveform driving signal by selectively switching the DC output; and

an antenna operable to generate an EM field in response to the waveform driving signal.

2. The submersible electromagnetic field generator according to claim 1, wherein the AC to DC converter is operable to generate the DC output from a three-phase AC input.

3. The submersible electromagnetic field generator according to claim 1, wherein the switching module comprises at least one semiconductor switch.

4. The submersible electromagnetic field generator according to claim 1, wherein the AC to DC converter is further operable to generate at least one additional DC output so that there is a plurality of DC outputs, and the switching module is further operable to generate the waveform driving signal by selectively switching the plurality of DC outputs.

5. The submersible electromagnetic field generator according to claim 1, further comprising surge protection circuitry.

6. The submersible electromagnetic field generator according to claim 1, further comprising a controller that is operable to control selective switching of the switching module so as to generate the waveform driving signal.

7. The submersible electromagnetic field generator according to claim 6, wherein the controller is operable to receive an external synchronization signal and, in response thereto, to control the operation of the switching module so as to start generation of the waveform driving signal.

8. The submersible electromagnetic field generator according to claim 6, wherein the controller is operable to receive an external command signal and to control the switching module to generate one of a plurality of predetermined waveform driving signals in dependence upon the external command signal.

9. The submersible electromagnetic field generator according to claim 1, further operable to receive an external clock signal.

10. The submersible electromagnetic field generator according to claim 9, wherein the switching module is operable to generate the waveform driving signal in synchronization with the external clock signal.

11. The submersible electromagnetic field generator according to claim 1, further comprising a data gathering module.

12. The submersible electromagnetic field generator according to claim 1, further comprising a data gathering module operable to receive an external clock signal and to time-stamp data gathered by the data gathering module with a time-stamp generated from the external clock signal.

13. The submersible electromagnetic field generator according to claim 11, wherein the data gathering module is further operable to transmit data gathered by the data gathering module from a submersible vehicle to topside.

14. An electromagnetic (EM) field generation system for seafloor electromagnetic surveying, comprising:

an AC to DC converter operable to generate a DC output from an AC input;

a switching module operable to generate a waveform driving signal by selectively switching the DC output;

an antenna operable to generate an EM field in response to the waveform driving signal;

a controller that is operable to control selective switching of the switching module so as to generate the waveform driving signal;

a power source operably coupled to the AC to DC converter of the electromagnetic field generator; and

a control module operably coupled to the controller of the submersible electromagnetic field generator, wherein the control module is operable to control the waveform driving signal that drives the antenna.

15. The electromagnetic field generation system according to claim 14, wherein the control module is operably coupled to a global positioning satellite (GPS) receiver, and is further operable to generate a clock signal in synchronization with a GPS transmitter signal.

16. The electromagnetic field generation system according to claim 15, wherein the control module is operable to supply the controller with the clock signal.

17. The electromagnetic field generation system according to claim 15, wherein the control module is operable to supply a data gathering module with the clock signal.

18. The electromagnetic field generation system according to claim 17, wherein the data gathering module is operable to time-stamp gathered data with a time-stamp generated from the clock signal.

19. The electromagnetic field generation system according to claim 14, wherein the control module is operably coupled to the controller of the submersible electromagnetic field generator using a fiber optic cable.

20. A method of generating an electromagnetic (EM) field for seafloor electromagnetic surveying, comprising:

providing an AC signal;

generating a DC signal from the AC signal;

generating a waveform driving signal by switching the DC signal; and

driving an electric dipole with the waveform driving signal to generate the EM field.

21. The method of claim 20, further comprising generating at least one additional DC signal so that there is a plurality of DC signals, and generating the waveform driving signal by selectively switching the plurality of DC signals.

22. A method for controlling the generation of the EM field according to claim 20, comprising synchronizing generation of a waveform driving signal produced at a submersible electromagnetic field generator signal with a clock signal provided from topside.