Fiber optic system for electromagnetic surveying

Abstract

An electromagnetic survey sensing device includes at least two electrodes disposed at spaced apart locations. An electrical to optical converter is electrically coupled to the at least two electrodes. The converter is configured to change a property of light from a source in response to voltage imparted across the at least two electrodes. The device includes an optical fiber optically coupled to an output of the electrical to optical converter, the optical fiber in optical communication with a detector.

1 . An electromagnetic survey sensing device, comprising:
 at least two electrodes disposed at spaced apart locations;   an electrical to optical converter electrically coupled to the at least two electrodes, the converter configured to change a property of light from a source in response to voltage imparted across the at least two electrodes; and   an optical fiber optically coupled to an output of the electrical to optical converter, the optical fiber in optical communication with a detector.   
   
   
       2 . The device of  claim 1  wherein the electrical to optical converter comprises means for changing an optical path length in response to the voltage imparted across the at least two electrodes. 
   
   
       3 . The device of  claim 2  wherein the means for changing optical path length comprises a sensing fiber wound around a piezoelectric element, the piezoelectric element electrically coupled to the at least two electrodes such that the voltage causes change in shape of the piezoelectric element. 
   
   
       4 . The device of  claim 3  further comprising a reference fiber associated with the sensing fiber. 
   
   
       5 . The device of  claim 3  wherein the piezoelectric element is separated into two longitudinal segments, each segment capable of changing shape substantially independently of the other segment, the segments each electrically coupled to the at least two electrodes in opposed polarity to the other segment, and wherein each segment includes a sensing fiber wound therearound and coupled to an optical interferometer. 
   
   
       6 . The device of  claim 2  wherein the means for changing length comprises a first piezoelectric element electrically coupled to the at least two electrodes, a first mirror functionally associated with the first piezoelectric element and disposed proximate a first end of an optical path associated with the optical fiber, such that electrical actuation of the first piezoelectric element by the voltage causes corresponding change in distance between the end of the first optical path and the first mirror. 
   
   
       7 . The device of  claim 6  further comprising a second piezoelectric element electrically coupled to the at least two electrodes, a second mirror functionally associated with the second piezoelectric element and disposed proximate a second end of an optical path associated with the optical fiber, such that electrical actuation of the second piezoelectric element by the voltage causes corresponding change in distance between the end of the second optical path and the second mirror opposite to the change in distance between the end of the first path and the first mirror. 
   
   
       8 . The device of  claim 2  wherein the means for changing length comprises an electric field sensitive etching on an optical fiber and electrically coupled to the at least two electrodes, and a mirror functionally associated with the etching such that imparting voltage from the at least two electrodes to the etching causes movement of the associated mirror. 
   
   
       9 . An electromagnetic survey system, comprising:
 a receiver cable having an optical fiber associated therewith and coupled at one end to a recording device and at another end to at least one electrical to optical converter, the recording device including a light source and a photodetector therein in optical communication with the associated fiber;   at least two electrodes disposed at spaced apart locations along the receiver cable;   the electrical to optical converter electrically coupled to the at least two electrodes, the converter configured to change a property of light from the source in response to voltage imparted across the at least two electrodes; and   the optical fiber optically coupled to an output of the electrical to optical converter, the optical fiber in optical communication with the detector in the recording device.   
   
   
       10 . The system of  claim 9  wherein the at least one electrical to optical converter comprises means for changing an optical path length in response to the voltage imparted across the at least two electrodes. 
   
   
       11 . The system of  claim 10  wherein the means for changing optical path length comprises a sensing fiber wound around a piezoelectric element, the piezoelectric element electrically coupled to the at least two electrodes such that the voltage causes change in shape of the piezoelectric element. 
   
   
       12 . The system of  claim 11  further comprising a reference fiber associated with the sensing fiber. 
   
   
       13 . The system of  claim 11  wherein the piezoelectric element is separated into two longitudinal segments, each segment capable of changing shape substantially independently of the other segment, the segments each electrically coupled to the at least two electrodes in opposed polarity to the other segment, and wherein each segment includes a sensing fiber wound therearound and coupled to an optical interferometer. 
   
   
       14 . The system of  claim 10  wherein the means for changing length comprises a first piezoelectric element electrically coupled to the at least two electrodes, a first mirror functionally associated with the first piezoelectric element and disposed proximate a first end of an optical path associated with the optical fiber, such that electrical actuation of the first piezoelectric element by the voltage causes corresponding change in distance between the end of the first optical path and the first mirror. 
   
   
       15 . The system of  claim 14  further comprising a second piezoelectric element electrically coupled to the at least two electrodes, a second mirror functionally associated with the second piezoelectric element and disposed proximate a second end of an optical path associated with the optical fiber, such that electrical actuation of the second piezoelectric element by the voltage causes corresponding change in distance between the end of the second optical path and the second mirror opposite to the change in distance between the end of the first path and the first mirror. 
   
   
       16 . The system of  claim 10  wherein the means for changing length comprises a piezoelectric element, the piezoelectric element configured to change shape in response to voltage applied thereto from the antenna. 
   
   
       17 . The system of  claim 10  wherein the means for changing length comprises an electric field sensitive etching on an optical fiber and electrically coupled to the at least two electrodes, and a mirror functionally associated with the etching such that imparting voltage from the at least two electrodes to the etching causes movement of the associated mirror. 
   
   
       18 . A method for sensing an electromagnetic field, comprising:
 exposing an electric dipole antenna to the electromagnetic field;   conducting voltage imparted to the antenna to an electrical to a device that changes a property of light imparted thereto in response to the voltage;   changing a property of light conducted from a light source to the device and from the device to a photodetector along an optical fiber, so that a signal corresponding to the voltage is optically communicated to the photodetector.   
   
   
       19 . The method of  claim 18  wherein the changing the property of light comprises causing a phase change therein by changing a length of an optical path between the light source and the photodetector. 
   
   
       20 . The method of  claim 19  wherein the changing path length comprises changing a length of an optical fiber wound around a piezoelectric element, the piezoelectric element configured to change shape in response to voltage applied thereto from the antenna. 
   
   
       21 . The method of  claim 19  wherein the changing path length comprises moving a mirror by changing a dimension of a piezoelectric element, the piezoelectric element configured to change shape in response to voltage applied thereto from the antenna. 
   
   
       22 . The method of  claim 19  wherein the changing path length comprises moving a mirror by actuating a piezoelectric element, the piezoelectric element configured to change shape in response to voltage applied thereto from the antenna. 
   
   
       23 . The method of  claim 19  wherein the changing path length comprises imparting voltage to an electric field sensitive etching on an optical fiber and electrically coupled to the antenna, and moving a mirror functionally associated with the etching such that imparting voltage from the at least two electrodes to the etching causes movement of the associated mirror. 
   
   
       24 . An electromagnetic survey system, comprising:
 a survey vessel configured to tow a receiver cable through a body of water;   a receiver cable having an optical fiber associated therewith and coupled at one end to a recording device on the vessel and at another end to a plurality of optical magnetic field sensors disposed at spaced apart positions along the cable, the sensors configured to change a property of light from source associated with the recording device; and   a signal detector associated with the recording device configured to convert the changed property of light into a signal corresponding to a property of a magnetic field proximate each sensor.   
   
   
       25 . The system of  claim 24  wherein each sensor comprises a reference optical fiber and a sensing optical fiber, the sensing optical fiber coupled to a magnetostrictive material such that dimensional change in the magnetostrictive material causes corresponding change in a length of the sensing fiber. 
   
   
       26 . The system of  claim 25  wherein an end of each of the sensing fiber and the reference fiber distal from the recording system comprises a mirror coupled thereto. 
   
   
       27 . A method for electromagnetic surveying of formations in the Earth's subsurface, comprising:
 imparting an electromagnetic field into the formations;   exposing a magnetostrictive material to an electromagnetic field produced in response to the imparted electromagnetic field;   transferring a change in dimension of the magnetostrictive material to an optical fiber to cause a change in a property of light conducted from a light source to a photodetector along the optical fiber, so that a signal corresponding to the responsively produced electromagnetic field is optically communicated to the photodetector.   
   
   
       28 . The method of  claim 27  wherein the causing change the property of light comprises causing a phase change therein by changing a length of an optical path between the light source and the photodetector in response to the change in dimension of the magnetostrictive material.