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A temperature sensor that has an elongated sensing element having a length of at least 10 m, measured at a temperature of 20° C. The elongated sensing element includes an elongated jacket and an optical fiber mounted in the jacket and having an EFL of at least 0.35%, wherein the elongated sensing el

A temperature sensor that has an elongated sensing element having a length of at least 10 m, measured at a temperature of 20° C. The elongated sensing element includes an elongated jacket and an optical fiber mounted in the jacket and having an EFL of at least 0.35%, wherein the elongated sensing element has an average temperature error of less than 2° C.

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1. A method for measuring a physical parameter at a plurality of spaced apart locations in a subterranean formation where the temperature is in excess of 200 degrees Celsius, the method comprising: a. inserting into a well extending into the subterranean formation an elongated sensing element, the e

1. A method for measuring a physical parameter at a plurality of spaced apart locations in a subterranean formation where the temperature is in excess of 200 degrees Celsius, the method comprising: a. inserting into a well extending into the subterranean formation an elongated sensing element, the elongated sensing element including: i. an outer jacket having a coefficient of thermal expansion CTEjacket;ii. an optical fiber in the jacket defining an optical path through the plurality of locations, the optical fiber having an excess fiber length EFL at 20 degrees Celsius of at least 0.35%, the optical fiber having a coefficient of thermal expansion CTEfiber, a ratio CTEjacket/CTEfiber being greater than 1.0;b. injecting an optical signal in the optical fiber, the optical fiber producing a measurable response to the optical signal, the measurable response being dependent on” a dimensional change induced in the optical fiber as a result of thermal expansion or contraction of the optical fiber;c. processing the measurable response to derive physical parameter information at the plurality of spaced apart locations. 2. A method as defined in claim 1, wherein the optical fiber has a length of at least 100 m. 3. A method as defined in claim 1, wherein the optical fiber has a length of at least 500 m. 4. A method as defined in claim 1, wherein the optical fiber has a length of at least 1 km. 5. A method as defined in claim 1, wherein the excess fiber length EFL at 20 degrees Celsius is at least 0.50%. 6. A method as defined in claim 1, wherein the excess fiber length EFL at 20 degrees Celsius is at least 0.70%. 7. A method as defined in claim 1, wherein the optical path manifesting an interaction with the optical signal, the interaction occurring only at a plurality of discrete areas spaced apart along the optical fiber and corresponding to the plurality of spaced apart locations. 8. A method as defined in claim 1, wherein the optical fiber includes a plurality of spaced apart discrete sensing devices having a temperature dependent response to optical interrogation. 9. A method as defined in claim 8, wherein each discrete sensing device is responsive to a dimensional change imparted to the optical fiber as a result of thermal expansion. 10. A method as defined in claim 9, wherein each discrete sensing device includes a grating. 11. A method as defined in claim 10, wherein the grating includes a Bragg grating. 12. A method as defined in claim 1, wherein the optical path manifesting an interaction with the optical signal, the interaction being continuous along the optical fiber. 13. A method as defined in claim 12, wherein the interaction produces back-scattering. 14. A method as defined in claim 13, including sensing the back-scattering to derive the temperature information. 15. A method as defined in claim 1, wherein the ratio CTEjacket/CTEfiber is at least 10. 16. A method as defined in claim 15, wherein the ratio CTEjacket/CTEfiber is at least 20. 17. A method as defined in claim 1, wherein the jacket is made of metallic material. 18. A method as defined in claim 1, wherein the jacket includes ceramic material. 19. A method as defined in claim 1, wherein the jacket includes a polymeric material. 20. A method as defined in claim 1, wherein the subterranean formation contains oil for extraction to the surface. 21. A method as defined in claim 20, wherein the well is either one of a steam injection well and an oil production well of a steam assisted gravity drainage (SAGD) oil production installation. 22. A method as defined in claim 21, including the step of inserting the elongated sensing element in the steam injection well. 23. A method as defined in claim 21, including the step of inserting the elongated sensing element in the oil production well. 24. A method as defined in claim 1, wherein the jacket has an internal cross-sectional area Aj, the elongated sensing elemen AjEFL ng a parameter S=of at least 1 mm2 per percent of EFL. 25. A method as defined in claim 24, wherein the parameter S is at least 6.25 mm2 per percent of EFL. 26. A method as defined in claim 25, wherein the parameter S is at least 10 mm2 per percent of EFL. 27. A method as defined in claim 1, wherein the physical parameter is temperature. 28. A method as defined in claim 27, wherein the elongated sensing element has an average temperature error of less than 2 degrees Celsius. 29. A sensor for measuring a physical parameter at a plurality of spaced apart locations in a subterranean formation where the temperature is in excess of 200 degrees Celsius, the sensor having an elongated sensing element for insertion in a well extending into the subterranean formation, the elongated sensing element comprising: a. an outer jacket having a coefficient of thermal expansion CTEjacket;b. an optical fiber in the jacket, the optical fiber having an excess fiber length EFL at 20 degrees Celsius of at least 0.35%, the optical fiber having a coefficient of thermal expansion CTEfiber, a ratio CTEjacket/CTEfiber being greater than 1.0, variations in the physical parameter causing localized dimensional changes in the optical fiber at the spaced apart locations, the localized dimensional changes being measurable via an optical signal injected in the optical fiber. 30. A sensor as defined in claim 29, wherein the optical fiber has a length of at least 100 m. 31. A sensor as defined in claim 29, wherein the optical fiber has a length of at least 500 m. 32. A sensor as defined in claim 29, wherein the optical fiber has a length of at least 1 km. 33. A sensor as defined in claim 29, wherein the excess fiber length EFL at 20 degrees Celsius is at least 0.50%. 34. A sensor as defined in claim 29, wherein the excess fiber length EFL at 20 degrees Celsius is at least 0.70%. 35. A sensor as defined in claim 29, wherein the optical path manifesting an interaction with the optical signal, the interaction occurring only at a plurality of discrete areas spaced apart along the optical fiber and corresponding to the plurality of spaced apart locations. 36. A sensor as defined in claim 29, wherein the optical fiber includes a plurality of spaced apart discrete sensing devices having a temperature dependent response to optical interrogation. 37. A sensor as defined in claim 36, wherein each discrete sensing device is responsive to a dimensional change imparted to the optical fiber as a result of thermal expansion. 38. A sensor as defined in claim 37, wherein each discrete sensing device includes a grating. 39. A sensor as defined in claim 38, wherein the grating includes a Bragg grating. 40. A sensor as defined in claim 29, wherein the optical path manifesting an interaction with the optical signal, the interaction being continuous along the optical fiber. 41. A sensor as defined in claim 40, wherein the interaction produces back-scattering. 42. A sensor as defined in claim 41, including sensing the back-scattering to derive the temperature information. 43. A sensor as defined in claim 29, wherein the ratio CTEjacket/CTEfiber is at least 10. 44. A sensor as defined in claim 29, wherein the ratio CTEjacket/CTEfiber is at least 20. 45. A sensor as defined in claim 29, wherein the jacket is made of metallic material. 46. A sensor as defined in claim 29, wherein the jacket includes ceramic material. 47. A sensor as defined in claim 29, wherein the jacket includes a polymeric material. 48. A sensor as defined in claim 29, wherein the jacket has an internal cross-sectional area Aj, the elongated sensing element having a parar AjEFL S=of at least 1 mm2 per percent of EFL. 49. A sensor as defined in claim 48, wherein the parameter S is at least 6.25 mm2 per percent of EFL. 50. A sensor as defined in claim 49, wherein the parameter S is at least 10 mm2 per percent of EFL. 51. A sensor as defined in claim 29, wherein the physical parameter is temperature. 52. A sensor as defined in claim 51, wherein the elongated sensing element has an average temperature error of less than 2 degrees Celsius. 53. A method for measuring a physical parameter at a plurality of spaced apart locations in a subterranean formation that contains oil for extraction to the surface, where the temperature is in excess of 200 degrees Celsius, the method comprising: a. inserting into a well extending into the subterranean formation an elongated sensing element, the elongated sensing element including: i. an outer jacket having a coefficient of thermal expansion CTEjacket;ii. an optical fiber in the jacket defining an optical path through the plurality of locations, the optical fiber having an excess fiber length EFL at 20 degrees Celsius of at least 0.35%, the optical fiber having a coefficient of thermal expansion CTEfiber, a ratio CTEjacket/CTEfiber being greater than 1.0;b. injecting an optical signal in the optical fiber, the optical fiber producing a measurable response to the optical signal, the measurable response being dependent on” a dimensional change induced in the optical fiber as a result of thermal expansion or contraction of the optical fiber;c. processing the measurable response to derive physical parameter information at the plurality of spaced apart locations,d. wherein the well is either one of a steam injection well and an oil production well of a steam assisted gravity drainage (SAGD) oil production installation, the method including inserting the elongated sensing element such that it runs at least 50% of the well. 54. A method as defined in claim 53, wherein the optical fiber has a length of at least 100 m. 55. A method as defined in claim 53, wherein the optical fiber has a length of at least 500 m. 56. A method as defined in claim 53, wherein the optical fiber has a length of at least 1 km. 57. A method as defined in claim 53, wherein the excess fiber length EFL at 20 degrees Celsius is at least 0.50%. 58. A method as defined in claim 53, wherein the excess fiber length EFL at 20 degrees Celsius is at least 0.70%. 59. A method as defined in claim 53, wherein the optical path manifesting an interaction with the optical signal, the interaction occurring only at a plurality of discrete areas spaced apart along the optical fiber and corresponding to the plurality of spaced apart locations. 60. A method as defined in claim 53, wherein the optical fiber includes a plurality of spaced apart discrete sensing devices having a temperature dependent response to optical interrogation. 61. A method as defined in claim 60, wherein each discrete sensing device is responsive to a dimensional change imparted to the optical fiber as a result of thermal expansion. 62. A method as defined in claim 60, wherein each discrete sensing device includes a grating. 63. A method as defined in claim 62, wherein the grating includes a Bragg grating. 64. A method as defined in claim 53, wherein the optical path manifesting an interaction with the optical signal, the interaction being continuous along the optical fiber. 65. A method as defined in claim 64, wherein the interaction produces back-scattering. 66. A method as defined in claim 65, including sensing the back-scattering to derive the temperature information. 67. A method as defined in claim 53, wherein the ratio CTEjacket/CTEfiber is at least 10. 68. A method as defined in claim 57, wherein the ratio CTEjacket/CTEfiber is at least 20. 69. A method as defined in claim 53, wherein the jacket is made of metallic material. 70. A method as defined in claim 53, wherein the jacket includes ceramic material. 71. A method as defined in claim 53, wherein the jacket includes a polymeric material. 72. A method as defined in claim 53, wherein the jacket has an internal cross-sectional area Aj, the elongated sensing element having a parameter S=AjEFL of at least 1 mm2 per percent of EFL. 73. A method as defined in claim 72, wherein the parameter S is at least 6.25 mm2 per percent of EFL. 74. A method as defined in claim 72, wherein the parameter S is at least 10 mm2 per percent of EFL. 75. A method as defined in claim 53, wherein the physical parameter is temperature. 76. A method as defined in claim 75, wherein the elongated sensing element has an average temperature error of less than 2 degrees Celsius.