Fiberoptic systems and methods detecting EM signals via resistive heating
Fiberoptics can be employed to detect downhole electromagnetic signals via resistive heating. A disclosed electromagnetic energy detector embodiment includes an optically-interrogated temperature sensor; and a conductive element thermally coupled to the sensor, the conductive element having a temperature response to incident electromagnetic energy. The optically-interrogated temperature sensor may be a length or coil of optical fiber to which a distributed acoustic sensing (DAS) or distributed temperature sensing (DTS) system is attached. The conductive element may be a metal coating on the fiber that experiences resistive heating in response to electromagnetic energy and creates an optically-measurable thermal response in the sensor.
1. A formation conductivity sensor that comprises:
a transmitter that radiates electromagnetic energy into a subsurface formation;
a receiver having:
one or more loops of an optical fiber thermally coupled to a surrounding conductive element to yield an optically-detectable thermal response to electromagnetic energy from the formation, said electromagnetic energy inducing an electrical current in the conductive element along the one or more loops and through an electrical short across the ends of the one or more loops, and said thermal response resulting from the electrical current; and
an optical source and optical detector that measure said thermal response; and
a processing unit that derives a measure of formation conductivity from said thermal response.
2. The sensor of claim 1 , wherein the processing unit extracts an AC response at twice a frequency of the radiated electromagnetic energy.
3. The sensor of claim 1 , wherein the measure of formation conductivity is based at least in part on a phase of the thermal response relative to the radiated electromagnetic energy.
4. The sensor of claim 1 , wherein the measure of formation conductivity is based at least in part on an attenuation of the radiated electromagnetic energy.
5. The sensor of claim 1 , wherein the transmitter and receiver are spaced apart in a borehole.
6. The sensor of claim 5 , wherein the one or more loops are wound on a casing string.
7. The sensor of claim 5 , wherein the one or more loops are wound on a production tubing string.
8. The sensor of claim 5 , wherein the one or more loops are wound on a drill string.
9. The sensor of claim 5 , wherein the one or more loops are attached to a casing string, production tubing string, or drill string, without enclosing said string.
10. The sensor of claim 5 , wherein the one or more loops are part of a wireline logging tool.
11. A method comprising:
transmitting electromagnetic energy into a formation, thereby inducing an electrical current into a shorted looped segment of a conductively-clad optical fiber in a borehole of the formation in response to the electromagnetic energy;
detecting a thermal response of the conductively-clad optical fiber produced by the electrical current; and
deriving information from the thermal response.
12. The method of claim 11 , wherein the information is telemetry data modulated onto the electromagnetic energy.
13. The method of claim 12 , wherein the electromagnetic energy is transmitted from a borehole other than the borehole in which the optical fiber is positioned.
14. The method of claim 11 , wherein the information is a measure of formation conductivity.
15. The method of claim 14 , wherein the measure is based at least in part on attenuation or phase shift of the transmitted electromagnetic energy.
16. The method of claim 11 , wherein the information includes a distance to a formation boundary or existing well.
17. The method of claim 11 , wherein said deriving includes extracting an AC response at twice a frequency of the transmitted electromagnetic energy.
18. An electromagnetic energy detector that comprises:
a looped optically-interrogated temperature sensor; and
a conductive element thermally coupled to and surrounding the sensor, the conductive element being electrically shorted at opposites ends of a sensor loop and having a temperature response to incident electromagnetic energy,
wherein said temperature response is produced by an electrical current that flows through the conductive element along the sensor loop and through the shorted ends; and
wherein said electrical current is induced by the incident electromagnetic energy.
19. The detector of claim 18 , wherein the conductive element is a conductive coating on the sensor.
20. The detector of claim 18 , wherein the sensor comprises one or more loops of an optical fiber.
21. The detector of claim 20 , wherein the conductive element is a conductive coating on the fiber in the loops, each loop being electrically insulated from adjacent loops.
22. The detector of claim 21 , further comprising a ferromagnetic material inside the loops.
23. The detector of claim 18 , wherein the sensor includes a fiber Bragg grating.
24. The detector of claim 18 , wherein the sensor comprises an optical fiber coupled to a distributed acoustic sensing system.
25. The detector of claim 18 , further comprising sensor electronics that extract an AC response at twice a frequency of the incident electromagnetic energy.