U.S. patent application number 10/176858 was filed with the patent office on 2003-12-25 for method for measuring and calibrating measurements using optical fiber distributed sensor.
Invention is credited to Mullins, Oliver C., Ramos, Rogerio T., Schroeder, Robert J., Yamate, Tsutomu.
Application Number | 20030234921 10/176858 |
Document ID | / |
Family ID | 29734236 |
Filed Date | 2003-12-25 |
United States Patent
Application |
20030234921 |
Kind Code |
A1 |
Yamate, Tsutomu ; et
al. |
December 25, 2003 |
Method for measuring and calibrating measurements using optical
fiber distributed sensor
Abstract
Methods for calibrating and making measurements using fiber
optic sensors are disclosed using backscattered wavelengths and
independent sensors. The disclosure sets outs methods applicable
with fiber optic sensors either in a deployed in a loop and in a
linear configuration and useful for measurements including
temperature.
Inventors: |
Yamate, Tsutomu;
(Yokohama-shi, JP) ; Schroeder, Robert J.;
(Newtown, CT) ; Ramos, Rogerio T.; (Hampshire,
GB) ; Mullins, Oliver C.; (Ridgefield, CT) |
Correspondence
Address: |
Schlumberger K.K.
Intellectual Property and Legal Department
2-2-1 Fuchinobe, Sagamihara-shi
Kanagawa-ken
229-0006
JP
|
Family ID: |
29734236 |
Appl. No.: |
10/176858 |
Filed: |
June 21, 2002 |
Current U.S.
Class: |
356/73.1 ;
374/E11.015 |
Current CPC
Class: |
G01K 15/00 20130101;
G01K 11/32 20130101; G01D 5/35364 20130101; G01K 11/3206 20130101;
G01K 15/002 20130101 |
Class at
Publication: |
356/73.1 |
International
Class: |
G01N 021/00 |
Claims
1. A method of determining temperature using a fiber optic
distributed temperature sensor, comprising: a) providing optical
energy at the Stokes wavelength to a fiber optic distributed
temperature sensor; b) receiving backscattered signal at Stokes
wavelength and measuring its intensity; c) providing optical energy
at the anti-Stokes wavelength to said fiber optic distributed
temperature sensor; d) receiving backscattering signal at
anti-Stokes wavelength and measuring its intensity; e) calculating
the attenuation ratio between the backscattered signal at Stokes
and anti-Stokes wavelengths at selected positions along the optical
fiber; f) repeating steps a through e at a different time; g)
calculating the change in the attenuation ratio with time of the
Stokes and anti-Stokes wavelengths; h) calculating a corrected SAR
by multiplying the measured SAR and said change in attenuation
ratio with time; and h) using the corrected SAR to adjust
temperatures measured by said fiber optic distributed temperature
sensor.
2. The method as claimed in claim 1, wherein a tunable light source
provides said optical energy at Stokes wavelength and said optical
energy at anti-Stokes wavelength.
3. The method as claimed in claim 1, wherein said fiber optic
distributed temperature sensor is deployed in a borehole.
4. A method of calibrating fiber optic distributed temperature
sensor measurements comprising: a) providing optical energy at the
Stokes wavelength to a fiber optic distributed temperature sensor;
b) receiving backscattered signal at Stokes wavelength and
measuring its intensity; c) providing optical energy at the
anti-Stokes wavelength to said fiber optic distributed temperature
sensor; d) receiving backscattering signal at anti-Stokes
wavelength and measuring its intensity; e) calculating the
attenuation ratio between the backscattered signal at Stokes and
anti-Stokes wavelengths at selected positions along the optical
fiber; f) repeating steps a through e at a different time; g)
calculating the change in the attenuation ratio with time of the
Stokes and anti-Stokes wavelengths; h) measuring temperature using
the fiber optic distributed temperature sensor; and i) calculating
a corrected SAR by multiplying the measured SAR and said change in
attenuation ratio with time.
5. The method as claimed in claim 4, wherein a tunable light source
provides said optical energy at Stokes wavelength and said optical
energy at anti-Stokes wavelength.
6. The method as claimed in claim 4, wherein said fiber optic
distributed temperature sensor is deployed in a borehole.
7. A method of determining temperature along a fiber optic
distributed temperature sensor, comprising the steps of: a)
measuring temperature using a fiber optic distributed temperature
sensor; b) measuring temperature using an independent temperature
sensor located along said fiber optic distributed temperature
sensor; c) calculating the difference in temperature .DELTA.T.sub.1
between the temperature measured by said independent temperature
sensor and the temperature measured by said fiber optic distributed
temperature sensor at the location of said independent temperature
sensor; and d) using .DELTA.T.sub.1 to adjust temperatures measured
by the fiber optic distributed temperature sensor.
8. The method as claimed in claim 7, wherein said fiber optic
distributed temperature sensor is deployed in a borehole.
9. The method as claimed in claim 7, wherein said fiber optic
distributed temperature sensor further comprises a fiber Bragg
grating.
10. A method of calibrating fiber optic distributed sensor
measurements, comprising the steps of: a) measuring a parameter of
interest along a fiber optic distributed sensor; b) measuring said
parameter of interest by an independent sensor; c) determining the
difference .DELTA.T.sub.1 between said parameter of interest
measured by said independent sensor and said parameter of interest
measured by said fiber optic distributed sensor at the location of
said independent sensor; d) using .DELTA.T.sub.1 to adjust said
parameter of interest as measured by said optical fiber distributed
sensor.
11. The method as claimed in claim 10, wherein said fiber optic
distributed sensor is deployed in a borehole.
12. The method as claimed in claim 10, wherein said fiber optic
distributed sensor further comprises a fiber Bragg grating.
13. The method as claimed in claim 10, wherein said parameter of
interest is temperature.
14. The method as claimed in claim 10, wherein said parameter of
interest is pressure.
15. The method as claimed in claim 11 wherein the independent
sensor is placed at a location of particular interest in said
borehole.
16. A calibrated fiber optic distributed temperature sensor
deployed in a borehole, comprising a fiber optic distributed
temperature sensor comprising at least one fiber Bragg grating and
a least one independent temperature sensor deployed in said
borehole.
17. A method for accurately determining a borehole parameter
comprising: a) deploying a fiber optic distributed sensor in a
borehole; b) providing an independent sensor in said borehole along
said fiber optic distributed sensor; c) measuring a borehole
parameter using said fiber optic distributed sensor; d) measuring
said borehole parameter using said independent sensor, wherein said
independent sensor provides a more accurate measurement than said
fiber optic distributed sensor; e) calculating the difference
between the borehole parameter measurement made by the independent
sensor and the borehole parameter measurement made using the fiber
optic distributed sensor at the location of the independent sensor;
and f) adjusting the borehole parameter measurements made using the
fiber optic distributed sensor by said difference.
18. The method as claimed in claim 17, wherein said borehole
parameter is temperature.
19. The method as claimed in claim 17, wherein said borehole
parameter is pressure.
20. The method as claimed in claim 17, wherein said fiber optic
distributed sensor further comprises FBG.
21. A method of determining a parameter of interest along an
optical fiber distributed sensor, comprising the steps of: a)
providing optical energy to a fiber optic distributed sensor; b)
measuring a parameter of interest using said fiber optic
distributed sensor; c) measuring said parameter of interest using
at least one fiber Bragg grating in the optical fiber distributed
sensor; d) determining the difference in said parameter of interest
.DELTA.T.sub.2j between the parameter of interest measurement by
the at least one fiber Bragg grating and the parameter of interest
measured using said fiber optic distributed sensor at the location
of the at least one fiber Bragg grating reference, wherein j is the
number of fiber Bragg gratings; and e) using .DELTA.T.sub.2j to
adjust parameter of interest measurements made using the optical
fiber distributed sensor.
22. The method as claimed in claim 21, wherein said fiber optic
distributed sensor is deployed in a borehole.
23. The method as claimed in claim 21, wherein said borehole
parameter is temperature.
24. The method as claimed in claim 21, wherein said borehole
parameter is pressure.
25. The method as claimed in claim 21, wherein said fiber optic
distributed sensor is a single mode fiber.
26. The method as claimed in claim 21, wherein said fiber optic
distributed sensor is a multimode fiber.
27. The method as claimed in claim 21, wherein said fiber optic
distributed sensor is a combination of single mode and multimode
fibers.
28. A method of determining temperature along an optical fiber
distributed temperature sensor, comprising the steps of: a)
providing optical energy to a fiber optic distributed temperature
sensor; b) measuring temperature using said fiber optic distributed
temperature sensor; c) measuring temperature using at least one
independent temperature sensor located along said fiber optic
distributed temperature sensor; d) measuring temperature using at
least one fiber Bragg grating temperature in the optical fiber
distributed temperature sensor, e) determining the difference in
temperature .DELTA.T.sub.1i between said at least one independent
temperature sensor and the temperature measured by said fiber optic
distributed temperature sensor at the location of said at least one
independent temperature sensor, wherein i is the number of
independent temperature sensors; f) determining the difference in
temperature .DELTA.T.sub.2j between the temperature measured by
said at least one fiber Bragg grating and the temperature measured
by said distributed temperature sensor at the location of said at
least one fiber Bragg grating reference, wherein j is the number of
fiber Bragg gratings, and g) using .DELTA.T.sub.1i and
.DELTA.T.sub.2j to adjust temperatures measured by said optical
fiber distributed temperature sensor.
29. The method as claimed in claim 28, wherein adjustments to
temperatures measured by the optic fiber distributed sensor are
made between the locations of the i independent temperature sensors
and the location of the j fiber Bragg gratings.
30. The method as claimed in claim 28, wherein said fiber optic
distributed temperature sensor is deployed in a borehole.
31. The method as claimed in claim 28, wherein said adjustment to
temperatures measured by the optic fiber distributed sensor varies
spatially along the fiber optic distributed temperature sensor.
32. The method as claimed in claim 31, wherein the adjustment is
spatially varied based on the location of known features of the
fiber optic distributed temperature sensor.
33. A method of calibrating optical fiber distributed temperature
sensor measurements, comprising the steps of: a) providing optical
energy to a fiber optic distributed temperature sensor; b)
measuring temperature using at least one independent temperature
sensor located along said fiber optic distributed temperature
sensor; c) measuring temperature using at least one fiber Bragg
grating in said optical fiber distributed temperature sensor; and
d) using the temperature measured by the at least one independent
temperature sensor and the temperature measured by the at least one
fiber Bragg grating to adjust the temperature measured by said
fiber optic distributed temperature sensor.
34. The method as claimed in claim 33, wherein said fiber optic
distributed sensor is deployed in a borehole.
35. A method of determining temperature along a fiber optic
distributed temperature sensor, comprising the steps of: a)
providing optical energy into one end of a fiber optic distributed
temperature sensor and transmitting in the forward direction; b)
measuring the backscattered signal at Stokes wavelength; c)
determining the incremental loss in signal at Stokes wavelength at
various points along the sensor from the optical energy input in
the forward direction and populating an array; d) measuring the
backscattered signal at anti-Stokes wavelength; e) determining the
incremental loss in signal at anti-Stokes wavelength at various
points along the sensor from the optical energy input in the
forward direction and populating an array; f) providing optical
energy into the other end of said fiber optic distributed
temperature sensor and transmitting in the reverse direction; g)
measuring the backscattered signal at Stokes wavelength; h)
determining the incremental loss in signal at Stokes wavelength at
various points along said sensor from the optical energy input in
the reverse direction and populating an array; i) measuring the
backscattered signal at anti-Stokes wavelength; j) determining the
incremental loss in signal at anti-Stokes wavelength at various
points along said sensor from the optical energy input in the
reverse direction and populating an array; k) determining the
difference between the measurement incremental loss variation in
Stokes wavelength in the forward and the reverse directions for any
given point along said sensor; l) determining the difference
between the measurement incremental loss variation in anti-Stokes
wavelength in the forward and the reverse directions for any given
point along said sensor; m) correcting the measured Stokes
wavelength at i by 12 S i = Sf i - j = 0 i s j 2 ;n) correcting the
measured anti-Stokes wavelength at i by 13 A i = Af i - j = 0 i a j
2 ;o) measuring temperature using said fiber optic distributed
temperature sensor; and p) adjusting measured temperature using
corrected Stokes S.sub.i and anti-Stokes A.sub.i wavelengths.
36. The method as claimed in claim 35, wherein a tunable light
source provides said optical energy at Stokes wavelength and said
optical energy at anti-Stokes wavelength.
37. The method as claimed in claim 35, wherein said fiber optic
distributed temperature sensor is deployed in a borehole.
38. The method as claimed in claim 35, wherein said fiber optic
distributed temperature sensor further comprises a fiber Bragg
grating.
39. A method of calibrating fiber optic distributed temperature
sensor measurements, comprising the steps of: a) providing optical
energy into one end of a fiber optic distributed temperature
sensor; b) receiving backscattered signals at Stokes and
anti-Stokes wavelengths at said one end and populating an array of
signals at Stokes wavelengths and an array of signals at
anti-Stokes wavelengths; c) providing optical light into the
opposite end of said fiber optic distributed temperature sensor; d)
receiving backscattered signals at Stokes wavelength and at
anti-Stokes wavelength at said opposite end and populating an array
of signals at Stokes wavelengths and an array of signals at
anti-Stokes wavelengths; e) correcting the measured signals
received at Stokes wavelength backscattered at i by 14 S i = Sf i -
j = 0 i s j 2 ;and f) correcting the measured signals received at
anti-Stokes wavelength backscattered at i by 15 A i = Af i - j = 0
i a j 2 .
40. A method of measuring a parameter of interest along a fiber
optic distributed sensor, comprising the steps of: a) providing
optical energy into one end of a fiber optic distributed sensor and
transmitting in the forward direction; b) measuring the
backscattered signal; c) determining the incremental loss in signal
from the optical energy input in the forward direction at various
points along said sensor and populating an array; d) providing
optical energy into the other end of said fiber optic distributed
sensor and transmitting in the reverse direction; e) measuring the
backscattered signal; f) determining the incremental loss in signal
from the optical energy input in the reverse direction at various
points along the sensor and populating an array; g) determining the
average of the incremental loss in signal in the forward and the
incremental loss in signal in the reverse directions for any given
point along the fiber optic distributed sensor; and h) adjusting
parameter measurements made using the fiber optic distributed
sensor by said average.
41. The method as claimed in claim 40, wherein the backscattered
signal is measured at the Stokes wavelength.
42. The method as claimed in claim 40, wherein the backscattered
signal is measured at the anti-Stokes wavelength.
43. The method as claimed in claim 40, wherein the backscattered
signal is measured at the Stokes wavelength and the anti-Stokes
wavelength.
44. The method as claimed in claim 40, wherein the backscattered
signal is measured as the SAR.
45. The method as claimed in claim 40, wherein a tunable light
provides the optical energy.
46. The method as claimed in claim 40, wherein said fiber optic
distributed sensor is deployed in a borehole.
47. The method as claimed in claim 40, wherein said fiber optic
distributed sensor further comprises a fiber Bragg grating.
48. The method as claimed in claim 40, wherein said fiber optic
distributed sensor further comprises an independent sensor.
Description
BACKGROUND OF THE INVENTION
[0001] The invention relates to fiber optic distributed sensors and
methods of measuring parameters and calibrating parameter
measurements made using optical fiber distributed sensors. In
particular, methods of measuring temperature and calibrating
temperature measurements made using fiber optic distributed
temperature sensors are disclosed.
DESCRIPTION OF RELATED ART
[0002] Optical fibers typically include a core, a concentric
cladding surrounding the core, a concentric protective jacket or
buffer surrounding the cladding. Generally the core is made of
transparent glass or plastic possessing a certain index of
refraction and the cladding is made of transparent glass or plastic
possessing a different index of refraction. The relative refractive
indices of the core and the cladding largely determine the function
and performance of the optical fiber. As a beam of light is
introduced into the optical fiber, the velocity and direction of
the light changes at the interface of media with different
refraction indices. The angles of reflection and refraction can be
predicted using Snell's law if the refractive indices of both media
are known. It is known to alter the media with their respective
refraction indices to provide optical fiber with certain light
propagating characteristics. Typically, for minimal power loss, it
is desirable for the light to propagate mainly through the core of
the optical fiber. In addition to refraction indices, other factors
that affect the propagation of the light through the fiber optic
core include the dimensions of the core and the cladding, the
wavelength of the light, the magnetic field vectors of the light
and the electrical field vectors of the light, the configuration of
the optical fiber, the presence of imperfections, and environmental
effects such as bends, twists, creases or folds.
[0003] One advantage of optical fiber is the ability to determine
information concerning a parameter of interest relating to
environmental effects along the length of a fiber. Measurements are
made by introducing optical energy into an optical fiber and
receiving backscattered light returned from various distances along
the optical fiber. In order to relate the characteristics of
backscattered light to the parameter of interest at a particular
distance, it is known to use optical time domain reflectometry
(OTDR) to determine the distance from which the light is returned
along the fiber is required. Such methods are known and described
in U.S. Pat. No. 4,823,166 to Hartog et al. and U.S. Pat. No.
5,592,282 to Hartog, both of which are incorporated herein in the
entirety by reference. In OTDR, a pulse of optical energy is
introduced to the optical fiber and the backscattered optical
energy returning from the fiber is observed as a function of time,
which is proportional to distance along the fiber from which the
backscattered light is received. This backscattered light includes
the Rayleigh spectrum, the Brillouin spectrum, and the Raman
spectrum. The Raman spectrum is the most temperature sensitive with
the intensity of the spectrum varying with temperature, although
all three types of backscattered light contain temperature
information.
[0004] Fiber optic (FO) sensors employ the fact that environmental
effects can alter the amplitude, phase, frequency, spectral
content, or polarization of light propagated through an optical
fiber. Fiber optics sensors can be classified as intrinsic or
extrinsic. Intrinsic sensors measure ambient environmental effects
by relying on the properties of the optical fiber only while
extrinsic sensors are coupled to another device to translate
environmental effects into changes in the properties of the light
in the fiber optic. Intrinsic fiber optic distributed temperature
sensors (DTS) are known. One such device is disclosed in U.S. Pat.
No. 5,825,804 to Sai, incorporated herein in its entirety by
reference. Such sensors may be multimode fiber (MMF) or single mode
fiber (SMF). Single mode optical fibers have a relatively small
diameter and support only one spatial mode of propagation.
Multimode fibers have a core with a relatively large diameter and
permit non-axial rays or modes to propagate through the core.
[0005] Typically the Raman spectrum is used to measure temperature,
the temperature distribution is calculated based on the ratio of
between the Stokes component and the anti-Stokes component of the
Raman spectrum of the backscattered light as follows: 1 I as I s =
exp [ - hcv kT ] ( 1 )
[0006] where .beta. is a coefficient, h is Plank's constant,
.upsilon. is Raman shifted wavelength number, k is Bolzmann
constant, T is absolute temperature, I.sub.as is the anti-Stokes
component and I.sub.s is the Stokes component. The
Stokes/anti-Stokes ratio I.sub.s/I.sub.as SAR. Using Equation 1,
the temperature at the position along the optical fiber from which
the backscattering occurred can be determined.
[0007] To measure temperature along a fiber optic (FO) distributed
sensor, optical energy is introduced into the fiber and
backscattered light is excited. The backscattered signal (light)
contains information relating to the point along the fiber from
which the backscattering occurred. This light is sensed and
processed as a time-sequence signal. A one-dimensional temperature
distribution along the optical fiber is thus measured. Within the
backscattered light, typically the Raman spectrum is transferred by
an optical directional coupler to a measuring apparatus, whereby
the Stokes light and the anti-Stokes light in the Raman
backscattered light are separated by a filter, detected, and
converted to electrical signals in proportion to their associated
amplitudes by respective photo-electric converters. It is known to
calculate temperature distribution based on the ratio between these
components of backscattered light, or alternatively based on
measurement of only one component of the Raman spectrum of
backscattered light.
[0008] In optical fiber, there are losses that can affect
backscattered Stokes and anti-Stokes wavelengths differently. For
example, the optical energy introduced into the optical fiber
naturally undergoes attenuation during transmission through the
fiber. Also there may losses owing to environmental stresses like
bends or connections. These losses subtract differently from the
measured backscattered Stokes and anti-Stokes intensities. These
differences in fiber attenuation between Stokes and anti-Stokes
wavelengths must be addressed to avoid error in the measured
parameter along the FO distributed sensor.
[0009] Parameter measurements obtained using a FO distributed
sensor comprise the true parameter measurement and a measurement
error caused by deleterious influences on the fiber optic
distributed sensor. By way of example but not limitation, such
deleterious influences can include energy losses due to splices or
bends, strains in the fiber, changes in attenuation resulting from
aging or environmental conditions, drift in measurements over time,
hydrogen ingression, or environmental conditions. Such error is
cumulative with distance along a fiber. While certain measurement
errors can be predicted based on manufacturer or material
calibration information, baseline testing, or tracking of known
elements such as splice location, the occurrence and effect of
other deleterious influences and the measurement error they
introduce is difficult to assess. It is known to deploy an optical
fiber in a borehole to obtain distributed measurements of borehole
parameters and it can be appreciated that accounting for these
deleterious influences and their associated measurement error is
particularly difficult when the fiber optic distributed sensor is
deployed in a borehole. A need exists for a method of calibrating
FO distributed sensors and a particular need exists for a method of
calibrating optical fibers deployed in a borehole for use in
distributed temperature measurements.
[0010] One method for correction is presented in U.S. Pat. No.
5,102,232 issued to Tanabe et al. However this method requires
maintaining an optical fiber temperature reference point at a known
temperature. Maintaining such a reference point may not be
feasible. For example, in downhole application where an optical
fiber is disposed in a borehole, it may not be possible to maintain
a reference point at a known temperature.
[0011] Robust methods for accurately determining parameters using a
FO distributed sensor measurements in a borehole are needed. The
accuracy of parameter measurements can be limited by the algorithm
or methodology used to account for variations in the measurements
and such limitations in methodology can exist regardless of whether
an optical fiber is deployed in a borehole in a linear or loop
configuration. Methods of calibrating parameter measurements
obtained using a FO distributed sensor are useful. Methods of
measuring a parameter obtained using a FO distributed sensor that
include calibration of the parameter measurement are also useful. A
particular need exists for methods of calibrating temperature
measurements obtained using a fiber optic distributed temperature
sensor (FO-DTS) and methods of measuring temperature using a FO-DTS
that include calibration of the measurements.
SUMMARY OF THE INVENTION
[0012] The present invention comprises methods of calibrating a
parameter measurement obtained using a fiber optic distributed
sensor and methods of making a parameter measurement that include
calibration.
[0013] One embodiment of the present invention is a method of
measuring a parameter using a FO distributed sensor comprising the
steps of: measuring a parameter of interest along a FO distributed
sensor; measuring said parameter of interest using an independent
sensor located along the length of the FO distributed sensor;
determining the difference in the parameter measurement
.DELTA.T.sub.1 between the parameter measurement made by the
independent sensor and the parameter measurement made by the
distributed sensor at the location of the independent sensor; and
adjusting the parameter measurements determined by the FO
distributed sensor by .DELTA.T.sub.1. In an embodiment, the FO
distributed sensor is deployed in a borehole. In a particular
embodiment, the independent sensor is placed at a location within
an area of particular interest in a borehole. In one embodiment,
the parameter of interest is temperature, the FO distributed sensor
is a fiber optic distributed temperature sensor (FO-DTS), and the
.DELTA.T.sub.1 is the difference between the temperature measured
by an independent temperature sensor and the temperature measured
by a FO-DTS.
[0014] Another embodiment of the present invention is a method of
calibrating a fiber optic distributed sensor, comprising the steps
of: measuring a parameter of interest along a FO distributed
sensor; measuring said parameter using an independent sensor
located along the length of the FO distributed sensor; determining
the difference .DELTA.T.sub.1 between the parameter measurement
made by the independent sensor and the parameter measurement made
by the distributed sensor at the location of the independent
sensor; and using .DELTA.T.sub.1 to adjust the parameter
measurements determined by the optical fiber distributed sensor. In
a further embodiment, the FO distributed sensor is deployed in a
borehole. In yet a further embodiment, the independent sensor is
placed at a location within an area of particular interest in a
borehole. In one particular embodiment, the parameter of interest
is temperature, the FO distributed sensor is a FO-DTS, and the
.DELTA.T.sub.1 is the difference between the temperature measured
by an independent temperature sensor and the temperature measured
by a FO-DTS.
[0015] One embodiment of the present invention is a method for
calibrating a FO distributed sensor comprising providing optical
energy at the Stokes wavelength to FO distributed sensor; receiving
backscattered signal at the Stokes wavelength and measuring;
providing optical energy at the anti-Stokes wavelength to the FO
distributed sensor; receiving backscattered signal at the
anti-Stokes wavelength and measuring its intensity; calculating the
attenuation ratio between the backscattered Stokes and anti-Stokes
wavelengths at points along the FO distributed sensor using OTDR;
repeating these steps at a different time; calculating the change
in the attenuation ratio with time of the Stokes and anti-Stokes
wavelengths; and applying said calculated change with time in
attenuation ratio of the Stokes and anti-Stokes wavelengths to
parameter measurements made using a FO distributed sensor. One
particular embodiment of this method of calibration is where the
parameter is temperature and the FO distributed sensor is a
FO-DTS.
[0016] One embodiment of the present invention is a method of
measuring temperature using a FO-DTS comprising providing optical
energy at to the Stokes wavelength to the FO-DTS; receiving
backscattered signal at the Stokes wavelength and measuring its
intensity; providing optical energy at the anti-Stokes wavelength
to the FO-DTS; receiving backscattered signal at the anti-Stokes
wavelength and measuring its intensity; calculating the attenuation
ratio between the backscattered signals at the Stokes and
anti-Stokes wavelengths at points along the optical fiber using
OTDR; repeating these steps at a different time; calculating the
change in the attenuation ratio with time of the Stokes and
anti-Stokes wavelengths; and applying the calculated change in
attenuation ratio of the Stokes and anti-Stokes wavelengths to
temperature measurements along the FO-DTS.
[0017] Another embodiment of the present invention is a method to
calculate a corrected SAR comprising providing optical energy at
the Stokes wavelength to an optical fiber; receiving backscattered
signal at the Stokes wavelength and measuring its intensity;
providing optical energy at the anti-Stokes wavelength to the
optical fiber; receiving backscattered signal at the anti-Stokes
wavelength and measuring its intensity; calculating the attenuation
ratio between the backscattered Stokes and anti-Stokes wavelengths
at points along the optical fiber using OTDR; repeating these steps
at a different time; calculating the change in the attenuation
ratio with time of the Stokes and anti-Stokes wavelengths; and
multiplying a measured SAR by a correction factor, wherein the
correction factor comprises the calculated change with time in
attenuation ratio of the Stokes and anti-Stokes wavelengths.
[0018] One embodiment of the present invention is a method of
determining temperature along a FO-DTS comprising the steps of:
measuring the temperature along an FO-DTS; measuring the
temperature at one or more locations along the FO-DTS using at
least one independent temperature sensors, determining the
difference .DELTA.T.sub.1i between the temperature measured using
by each of the least one independent temperature sensor and the
temperature measuring along the FO-DTS at the respective locations
of the at least one independent temperature sensor, and adjusting
the temperatures measured by the FO-DTS by .DELTA.T.sub.1i, wherein
i is the number of independent temperature sensors provided.
[0019] Another embodiment of the present invention is a method of
calibrating a FO-DTS, comprising the steps of: measuring the
temperature along an FO-DTS; measuring the temperature at one or
more locations along the FO-DTS using at least one independent
temperature sensors, determining the difference .DELTA.T.sub.1i
between the temperature measured using by each of the least one
independent temperature sensor and the temperature measuring along
the FO-DTS at the respective locations of the at least one
independent temperature sensor, and using .DELTA.T.sub.1i to
calibrate temperature determined by the FO-DTS, wherein i is the
number of independent temperature sensors provided.
[0020] The present invention includes a method of determining
temperature comprising the steps of measuring the temperature along
an FO-DTS; measuring the temperature at one or more locations along
the FO-DTS using at least one fiber Bragg grating (FBG) in the
FO-DTS, determining the difference .DELTA.T.sub.2j between the
temperature measured using by each of the least one FBG and the
temperature measured along the FO-DTS at each respective FBG
location, and using .DELTA.T.sub.2j to adjust the temperature
measured by the FO-DTS, wherein j is the number of FBG temperature
sensors provided.
[0021] Another embodiment of the present invention is a method of
calibrating a FO-DTS, comprising the steps of: measuring the
temperature along an FO-DTS; using at least one FBG in the FO-DTS,
measuring the temperature at one or more locations along the FO-DTS
using at least one FBG, determining the difference .DELTA.T.sub.2j
between the temperature measured using by each of the least one FBG
and the temperature measuring along the FO-DTS at the respective
locations of the at least one FBG, and using .DELTA.T.sub.2j, to
adjust temperatures measured using the FO-DTS, wherein j is the
number of FBG temperature sensors provided.
[0022] An embodiment of the present invention is a method of
determining temperature along a FO-DTS, comprising the steps of:
measuring the temperature along FO-DTS; measuring the temperature
at one or more locations along the FO-DTS using at least one
independent temperature sensors, determining the difference
.DELTA.T.sub.1i between the temperature measured by each of the
least one independent temperature sensor and the temperature
measured along the FO-DTS at the respective locations of the at
least one independent temperature sensor, proving at least one FBG
in the FO-DTS, measuring the temperature at one or more locations
along the FO-DTS using at least one FBG, determining the difference
.DELTA.T.sub.2j between the temperature measured using by each of
the least one FBG and the temperature measuring along the FO-DTS at
the respective locations of the at least one FBG, and adjusting the
temperatures measured by the FO-DTS based on .DELTA.T.sub.1i and
.DELTA.T.sub.2j, wherein i is the number of independent temperature
sensors and j is the number of FBG temperature sensors
provided.
[0023] An embodiment of the present invention comprises a method of
calibrating an optical fiber distributed sensor, comprising the
steps of: measuring a parameter along a distributed sensor,
measuring said parameter using at least one independent sensor;
providing at least one FBG in the optical fiber distributed sensor
and measuring said parameter using the at least one FBG;
determining the difference .DELTA.T.sub.1i between the parameter
measured using by each of the least one independent sensor and the
parameter measuring along the FO distributed sensor at the location
of the at least one independent sensor; determining the difference
.DELTA.T.sub.2j between the parameter measured using by each of the
least one FBG and the parameter measured along the FO distributed
sensor at the location of the at least one FBG, and using
.DELTA.T.sub.1i and using .DELTA.T.sub.2j to calibrate the
parameter measurement as determined by the FO distributed sensor,
wherein i is the number of independent temperature sensors and j is
the number of FBG provided.
[0024] Another embodiment of the present invention is a method of
measuring temperature along a FO-DTS, comprising providing optical
energy into one end of the FO-DTS and transmitting in the forward
direction; measuring the optical signal at locations i along the
optical fiber distributed temperature sensor; determining the
incremental loss variation in the forward direction; providing
optical energy into the opposite end of the FO-DTS and transmitting
in the reverse direction; measuring the optical signal at locations
i along the FO-DTS; calculating the incremental loss variation at
location i in the forward and reverse directions; and adjusting the
temperatures measured by the FO-DTS by the incremental loss
variation at location i to the temperature measured by the FO-DTS
at location i.
[0025] Yet another embodiment of the present invention is a method
of calibrating a FO distributed sensor, comprising providing energy
into one end of an optical fiber and transmitting in the forward
direction; measuring the optical signal at locations i along the FO
distributed sensor, determining the incremental loss variation of a
parameter of interest in the forward direction; providing optical
energy into the opposite end of a FO distributed sensor and
transmitting in the reverse direction; measuring the optical signal
at locations i along the optical fiber distributed sensor,
calculating the incremental loss variation of a parameter of
interest in the reverse direction; and applying the incremental
loss variation at location i to the measurements of said parameter
of interest made by the FO distributed sensor at location i.
[0026] In further embodiments, the incremental loss variation is
determined at the Stokes, anti-Stokes, or Stokes and anti-Stokes
wavelengths, or with respect to the Stokes/anti-Stokes ratio.
[0027] Particular further embodiments of the above embodiments
comprise providing the fiber optic distributed sensor in a
borehole, wherein the parameter measured by the sensor is a
borehole parameter, such as temperature, pressure, or fluid
composition.
BRIEF DESCRIPTION OF THE DRAWINGS
[0028] FIG. 1 is one embodiment of the present invention wherein
light pulses at Stokes and anti-Stokes wavelengths are used.
[0029] FIG. 2 is an embodiment of the present invention wherein a
FBG is provided in a FO-DTS.
[0030] FIG. 3 shows normalized spectrum obtained using a FBG in
single mode optical fiber versus normalized spectrum obtained using
a FBG in a multimode optical fiber.
[0031] FIG. 4 shows the spectra reflected in an example case by the
FBG using single mode optical fiber and multimode optical
fiber.
[0032] FIG. 5 shows one possible configuration for a FO-DTS system
with downhole calibration utilizing FBG.
[0033] FIGS. 6, 7, and 8 contain data showing temperature and error
versus distance for an example deployment of a FO-DTS in a
borehole.
[0034] FIG. 9 shows example temperature data collected in both the
forward and reverse direction and data corrected for incremental
loss variations between Stokes and anti-Stokes wavelengths.
DETAILED DESCRIPTION
[0035] A concern in using FO distributed sensors is the possible
loss of calibration of the FO distributed sensor after deployment
due to the change in the fiber characteristics, such as non-uniform
attenuation of the fiber along the spectrum caused by micro-bending
losses or hydrogen ingression. This concern is particularly
heightened when considering long-term deployment of a FO
distributed sensor in a borehole environment as changes in the FO
cannot be easily monitored, hostile conditions such as high
temperature and pressures occur in borehole environments, and the
expense of replacing a FO can be significant. This concern can be
minimized by methods of providing at least one calibrating sensor
or FBG along the FO distributed sensor, providing measurements from
each end of a loop configuration FO distributed sensor, and
combinations thereof.
[0036] Referring now to FIG. 1, a first embodiment according to the
present invention is shown. While described in terms of a FO-DTS,
it can be appreciated that the present method is applicable for
other types of FO distributed sensors. The fiber optic distributed
temperature sensor 20 is placed so as to pass through each area
where temperature is to be measured. An optical energy source
introduces optical energy into the FO-DTS and backscattered signal
22 is generated by the return of the optical energy along optical
fiber. From the backscattered signal, the responses at the
anti-Stokes wavelength and the Stokes wavelength are determined and
temperature distribution along the FO-DTS is determined using
Equation 1.
[0037] A DTS optical electronics module 10 is shown connected to a
FO-DTS 20. A pulsed light source capable of outputting light at
Stokes wavelength 12 is connected to the DTS optical electronic
module 10 and FO-DTS 20. Similarly, a pulsed light source capable
of outputting light at the anti-Stokes wavelength 14 is connected
to DTS optical electronics module 10 and fiber optic distributed
temperature sensor 20. While the present method is described in
terms of temperature measurements and use of backscattered light at
Stokes and anti-Stokes wavelengths, it can be appreciated that the
present invention is also applicable to other distributed parameter
measurements and other wavelength spectrums.
[0038] A pulse of light is provided from the Stokes wavelength
light source 12 and the intensity of the backscattered light 22 at
the Stokes wavelength is measured by the DTS optical electronics
module 10. A pulse of light is provided from the anti-Stokes
wavelength light source 14 and the intensity of the backscattered
light 22 at the anti-Stokes wavelength is measured by the DTS
optical electronics module 10. The order in which Stokes and
anti-Stokes wavelengths are provided is unimportant. Using OTDR
techniques, the DTS optical electronics module 10 calculates the
attenuation ratio of Stokes versus anti-Stokes wavelengths at
various points along the FO-DTS 20.
[0039] This attenuation ratio is stored in memory 16. The process
is repeated again at a known later time and the attenuation ratio
from the second event is recorded and stored in memory 16. The
attenuation ratio from the second event is compared to the
attenuation ratio from the first event using processor 18 and the
change of the attenuation ratio with respect to time is
determined.
[0040] Assuming the position of the FO-DTS remains stationary, such
as when placed in a borehole for monitoring of downhole parameters,
this change of the attenuation ratio with respect to time can be
used to calibrate later measurements made using the FO-DTS. By
multiplying the change in attenuation ratio with respect to time by
the period of time that has passed since the initial measurements,
a correction factor .DELTA.AR can be determined.
[0041] Using this correction factor, a corrected SAR can be
calculated as
SAR.sub.corrected=.DELTA.AR(SAR.sub.measured) (2)
[0042] where SAR.sub.measured is the SAR measured at the later
time. The corrected SAR can be used in Equation 1 to determine more
accurately the temperature measured at later times by an in situ
FO-DTS.
[0043] In another embodiment, a tunable light source provides light
at Stokes and anti-Stokes wavelengths. In this embodiment, a single
tunable light source is used to provide light at Stokes and
anti-Stokes wavelengths rather than separate Stokes source 12 and
anti-Stokes source 14. One such tunable pulsed light source is a
pulsed tunable laser.
[0044] Another embodiment of the present invention comprises
providing a FO distributed sensor and further providing at least
one gauge capable of providing a calibrating measurement along the
length of the DTS. This calibrating measurement can be performed by
providing at least one fiber Bragg grating embedded in the FO
distributed sensor or at least one independent sensor or
combinations thereof. It is known to embed FBG in an optical fiber
to sense parameters such as temperature or strain of the structure.
Such a fiber optic grating system is described in U.S. Pat. No.
5,380,995 to Udd and Clark, incorporated herein in its entirety by
reference. It is also known that the reflection wavelength of the
grating changes with temperature due to the change in refractive
index and grating spacing over temperature.
[0045] Distributed temperature sensors systems can use MMF or SMF;
more commonly MMF is used. Single mode fibers have a core with a
relatively small diameter and MMF have a relatively large core
diameter. One embodiment of the present invention is a method of
determining temperature measurements along a FO-DTS, comprising use
of at least one FBG. A single and easily discernable temperature
peak will be indicated by the FBG if the fiber optic distributed
temperature sensor is SMF. If the fiber optic distributed
temperature sensor is MMF, multiple peaks will be indicated by the
FBG sensor, yielding a less discernable temperature reading. The
present invention contemplates use of a SMF, a MMF, or a
combination thereof.
[0046] As shown in FIG. 2, one particular embodiment is to provide
a FO-DTS comprising a segment of SMF 34 and a segment of MMF 36,
wherein at least one FBG is placed in the SMF 34. This
configuration combines the advantages of using MMF and SMF.
Referring to FIG. 2, an integration system 28 comprising a light
source 30 and a spectral analyzer 32 are connected to a SMF 34. The
light source 30 may be, e.g., a LED, a tunable laser, or a laser
diode while the spectral analyzer 32 may be, e.g., a Fabry-Perot
filter, an acouto-optical filter or an optical spectral analyzer
(OSA). A MMF 36 is connected to the SMF 34. Methods of making
connections 38 between the MMF and SSF such as splicing or using
connectors are known. At least one FBG 40 is placed in the SMF
34.
[0047] Two tests were performed to compare the performance of a SMF
with the performance of a SMF with MMF added between the
interrogation system 28 and the FBG sensor 40. FIG. 3 shows the
normalized spectrum reflected in one test where optical energy was
input into the cable and a spectrum measured using a FBG 40 for a
first case 42 comprising a SMF with FBG and a second case 44
comprising a SMF with MMF inserted between the SMF and the FBG.
FIG. 3 indicates there was no remarkable degradation of on the
shape of the spectrum or the change in center wavelength.
[0048] In another test, optical energy was input into the cable and
a spectrum measured. FIG. 4 shows the spectrum reflected by the FBG
in the two cases. In a first case 50, FBG 40 was provided in the
SMF 34, which connected directly to an integration system 28
comprising a light source 30 and an OSA 32 and a spectrum was
measured. In a second case 52, FBG 40 was provided in the SMF 34,
which connected to MMF 36 that was connected to an integration
system 28 comprising a light source 30 and an OSA 32 and a spectrum
measured. For these cases, the difference in power for the SMF
spectrum 50 and the SMF with MMF spectrum 52 was shown to be about
10 dB at the FBG center wavelength. Connection loss was estimated
to be approximately 2.5 dB.
[0049] One possible configuration for a DTS system with downhole
calibration utilizing FBG is shown in FIG. 5. The distributed
temperature sensor system comprises DTS optical electronics module
60, an OSA 62, and a FO-DTS 64 which can be either MMF or SMF. If
the FO distributed sensor is MMF, a length of SMF 66 may be
provided at the end of the MMF cable. Preferably at least one FBG
68 is provided. If a SMF length 66 is provided in conjunction with
MMF, preferably the FBG 68 is provided on the SMF length. It should
be appreciated however that the invention encompasses providing the
at least one FBG 68 on the MMF length as well as the SMF length and
encompasses providing the at least one FBG 68 on a MMF without
associated use of SMF. Each at least one FBG provides a measurement
of temperature at its particular location.
[0050] An independent temperature sensor 72 is used to measure the
temperature at a particular location. This independent temperature
sensor may be a fiber optic temperature sensor or another type of
temperature sensor such as an electrical quartz sensor or a sensor
comprising a crystal quartz gauge. In particular situations where a
highly precise or highly accurate temperature measurement is
desired, gauges of the type capable of providing a more precise and
accurate temperature measurement than a DTS can be provided. The
temperature reading of the independent temperature sensor 72 and
the temperature reading of the optical fiber distributed
temperature sensor 64 at the location of the independent
temperature sensor 72 can be compared and the difference calculated
as .DELTA.T.sub.1. This difference can be used to calibrate the
FO-DTS 64 along the length of the FO-DTS 64 by applying
.DELTA.T.sub.1 to the temperature readings obtained by the
distributed temperature sensor.
[0051] By providing at least one FBG 68, a temperature correction
can be determined at the location of the at least one FBG by
calculating the difference .DELTA.T.sub.2 of the FBG temperature
measurement and the temperature measurement of the FO-DTS at that
location. Another method is to calibrate the FO-DTS by applying the
average of .DELTA.T.sub.1 and .DELTA.T.sub.2 along the length
between the at least two sensors. Such an average may be an
arithmetic average or a weighted average based on relative location
of the temperature reading along the DTS. Alternatively
.DELTA.T.sub.1 and .DELTA.T.sub.2 may be applied spatially along
the DTS or either .DELTA.T.sub.1 or .DELTA.T.sub.2 may be applied
at various locations along the DTS. In particular .DELTA.T.sub.1
and .DELTA.T.sub.2 may be applied between the locations of the
independent sensors or the FBG or at known locations of physical
features such as splices, connection or bends.
[0052] While the example shown in FIG. 5 comprises a FBG and two
independent sensors, the present invention also contemplates use of
multiple FBG and multiple independent sensors along a FO-DTS. In
such a case, .DELTA.T.sub.1i represent the difference in the
temperature measurements between ith independent temperature sensor
and the FO-DTS at the location of the ith independent sensor, and
.DELTA.T.sub.2j represent the difference in the temperature
measurements between the jth FBG and FO-DTS at the location of the
jth FBG. Calibration of the FO-DTS can be made by applying an
arithmetic or weighted average of .DELTA.T.sub.1i and
.DELTA.T.sub.2j; applying a function of .DELTA.T.sub.1i and
.DELTA.T.sub.2j; or spatially applying .DELTA.T.sub.1i and
.DELTA.T.sub.2j based on known characteristics of the FO-DTS such
as the locations of splices in the optical fiber.
[0053] In an example of one embodiment of the present invention, a
fiber optic DTS and three independent temperature gauges were
deployed in a horizontal well, the well having both a vertical
section and a horizontal section. The vertical section was
completed with casing and gravel pack tubing was used in the
horizontal section. Packers and flow control valves were used to
isolate the horizontal section into three zones. A FO-DTS was
deployed along the entire length of the borehole. This example used
a single-ended configuration for the FO-DTS although a loop
configuration could have been used. An electrical quartz pressure
and temperature gauge was provided in each of the three horizontal
zones. In this example, the electrical quartz pressure and
temperature gauges were associated with the flow control devices,
although such association is not a limitation. Other gauge
configurations and placements are contemplated within the scope of
the present invention.
[0054] FIG. 6 shows the temperature measured by the DTS in this
example versus the distance into the borehole from the surface. The
horizontal section can be roughly correlated with portion of FIG. 6
where the temperature profile is approximately flat, from
approximately 1050 meters to total depth. In this section of the
temperature profile, three temperature spikes can be seen which
correlate to three splices in the FO-DTS. While the location of
splices along the length of a sensor can be noted during deployment
for later consideration, these splices nevertheless contribute to
loss of calibration of the distributed sensor in the localized
areas near the splices. Similar localized areas of calibration loss
may occur near bends, strains or other deleterious influences
affecting the optical fiber distributed sensor.
[0055] FIG. 7 shows a relatively constant offset of approximately
6.degree. C. throughout the entire DTS profile as compared to a
baseline temperature reference as measured in controlled conditions
or provided as manufacturer's information. This represents a
substantial variation as under ideal conditions a FO-DTS can
provide accuracy within approximately 1.degree. C. Use of
independent temperature sensors permitted corrections for this
offset. The temperature was measured using an independent
electrical temperature gauge near 1000 meters with a precision of
0.1 degree C. or less and a correction calculated as the difference
between the temperature measurement of the independent gauge and
the temperature measured by the FO-DTS. The entire DTS profile was
then offset with this correction and also set to the correct
temperature as determined by the downhole temperature gauge.
[0056] FIG. 8 shows the offset of DTS measurements taken at two
different times for the portion of the DTS in the borehole. Three
electrical quartz pressure and temperature gauges were used to
measure temperature in the borehole at locations near the splices
in the FO-DTS, as indicated by the spikes in the DTS temperature
measurements in FIG. 6. These gauges had an accuracy of
approximately 0.1.degree. C., thereby permitting correction of the
FO-DTS measurements to an level within the accuracy of FO-DTS
systems.
[0057] An embodiment of the present invention contemplates using
temperature measurements from independent temperature gauges both
to correct for gross temperature errors from physical traumas such
as splices and optical fiber loss in FIG. 6 and to increase the
accuracy of the DTS measurement, across the entire fiber length, to
the level of the independent reference gauge, that is to a level
substantially better than 1 degree C. The present invention applies
to both single ended and loop DTS configurations.
[0058] Another embodiment of the present invention is a method of
obtaining corrected temperature measurements along a FO-DTS
deployed in a loop configuration, comprising use of measurements
from both ends of the optical fiber. This method comprises
estimating the cumulative error along an optical fiber utilizing
measurements taken from back-scattered signal (light) from both
ends of the fiber loop.
[0059] In general along FO distributed sensors, the backscattered
energy E.sub.1 at a point i is: 2 E i = E input F i exp ( n = 0 i R
) ( 3 )
[0060] where E.sub.input is the input power, F.sub.i is the
coefficient of backscattering at point i and is a known function of
temperature for various optical fibers, and 3 n = 0 i R
[0061] is the additive error along the distributed fiber from the
input point to i. The input power, E.sub.input, can be measured.
Also, backscattered signal at particular wavelengths can be
measured at the points along the FO distributed sensor.
Alternatively, the ratio of particular wavelengths, such as the
SAR, can be measured at points along the FO distributed sensor.
Typically this measurement point is the energy input point. After
energy E.sub.input is input, backscattered energy E.sub.i occurs
and signals from the backscattered energy can be measured; the
backscattered signal measured at the input point is designated as
S.sub.i=ln (E.sub.i), for any point i. This measured backscattered
signal from point i includes the signal S.sub.i at point i and the
additive error or loss of signal along the distributed fiber from
the measurement point to i. This embodiment of the invention
comprises a method to estimating the additive error along the
distributed fiber to point i and calculating the true signal
S.sub.i at point i using this estimation and the measured
backscattered signal.
[0062] The relationship between backscattered signal S and input
energy is known as:
S.sub.i=ln (E.sub.inputF.sub.i) (4)
[0063] where S.sub.i is the backscattered energy at i and F.sub.i
is the coefficient of backscattering at point i. Using a method of
the present invention to determine S.sub.i, and measuring
E.sub.input, F.sub.i can be calculated. Then F.sub.i can be used
with known manufacture or baseline reference functions for various
types and configurations of optical fiber to determine the
temperature at point i.
[0064] One method of the present invention comprises providing a
pulse of optical energy at one end of the loop, end Y, and
recording backscattered wavelengths at points i along the optical
fiber. As an example, this methodology is explained in terms of
measuring the Stokes and anti-Stokes wavelengths in from each end
of the loop configuration optical fiber. It is expressly
contemplated within the scope of the present invention that this
methodology applies to other types of measurements that may
demonstrate cumulative error such as temperature measurements,
measurements of other wavelengths, or measurements of
wavelength-related variables such as SAR. It is also contemplated
that the present invention may be used with measurements of the
Stokes wavelength only, measurement of the anti-Stokes wavelength
only, or the SAR only.
[0065] The Stokes signal contribution is designated at S and the
anti-Stokes signal contribution is designated as A. This method
comprises providing optical energy to one end, end Y, of a loop
optical fiber sensor. The backscattered signals received at end Y
are recorded, and the intensity of the signals or the loss
variation between one point i and the next point i+1 is used to
populate two arrays: Sy for the signal at the Stokes wavelength and
Ay for the signal at the anti-Stokes wavelength. Then optical
energy is provided at the other end of the optical fiber loop, end
Z, and recording backscattered wavelengths at points along the
optical fiber. The backscattered signals received at end Z are
recorded, and the intensity of the signals or the loss variation
between one point i and the next point i+1 is used to populate two
arrays: Sz for the signal at the Stokes wavelength and Az for the
signal at the anti-Stokes wavelength.
[0066] The loss variation between one point, i, and the next point,
i+1, are represented by an array E.sub.S and an array E.sub.A for
Stokes and anti-Stokes signals, wherein E represents the difference
in intensity or the loss between points i and i+1.
[0067] For arrays of k points, 4 Sy i = S i + j = 0 i E S j ( 5 )
Sz i = S i + j = i + 1 k E S j ( 6 ) A y i = A S i + j = 0 i E A j
( 7 ) A z i = A S i + j = i + 1 k E A j ( 8 )
[0068] At a given point i along the optical fiber, assuming
equivalent optical energy input at ends Y and Z, the difference
between the Stokes measurement determined from end Y and the Stokes
measurement determined from end Z is: 5 Ds i = Sy i - Sz i = j = 0
i E S j - j = i + 1 k E S j ( 9 )
[0069] and the difference between the anti-Stokes measurement
determined from end Y and the anti-Stokes measurement determined
from end Z is 6 Da i = A y i - A z i = j = 0 i E A j - j = i + 1 k
E A j ( 10 )
[0070] The increment of each element gives the double of the loss
contribution for each point: 7 s i = Ds i - Ds i - 1 = j = 0 i E S
j - j = i + 1 k E S j - j = 0 i - 1 E S j + j = i k E S j = 2 E S j
and ( 11 ) a i = Da i - Da i - 1 = j = 0 i E A j - j = i + 1 k E A
j - j = 0 i - 1 E A j + j = i k E A j = 2 E A j ( 12 )
[0071] This enables the estimation of the signal contribution due
to temperature only as: 8 S i = Sy i - j = 0 i s j 2 and ( 13 ) A i
= Ay i - j = 0 i a j 2 ( 14 )
[0072] With the loss contribution to the signal extracted from both
Stokes and anti-Stokes signals, the SAR can be calculated and the
temperature at each point can be estimated using Equation 1 without
needing to know the differential loss factor for the two
wavelengths. The same method could be used to correct for the
influence of loss for data already processed to obtain the
temperature information, provided the error is cumulative.
[0073] Similarly, the present invention contemplates direct
measurement of the SAR in the forward and reverse directions and
estimation of the signal loss contribution owing to temperature as
follows:
[0074] For arrays of k points, 9 SARy i = SAR i + j = 0 i E SAR j (
15 ) SARz i = SAR i + j = i + 1 k E SAR j ( 16 )
[0075] The increment of each element gives the double of the loss
contribution for each point: 10 SAR i = DSAR i - DSAR i - 1 j = 0 i
E SAR j - j = i + 1 k E SAR j - j = 0 i - 1 E SAR j + j = i k E SAR
j = 2 E SAR j ( 17 )
[0076] This enables the estimation of the signal contribution due
to temperature only as: 11 SAR i = SARy i - j = 0 i SAR j 2 ( 18
)
[0077] FIG. 9 shows an example of data collected from energy input
at one end of an optical fiber (referred to as the forward
direction) and energy input at the opposite end of the optical
fiber (referred to as the reverse direction) and the corrected data
using the present method. In this case, the fiber passes through
cold water at approximately 305 m along its length and the fiber
passes through a hot oven at approximately 350 m along its
length.
[0078] Localized regions of high loss were induced by coiling the
fiber around a pencil at approximately 325 m and approximately 360
m along the fiber length, just before and after the oven. The
forward and backward data do not coincide due to the difference in
attenuation for the Stokes and anti-Stokes data, mainly at the high
loss points. The data were corrected even without the use of the
Stokes and anti-Stokes raw data.
[0079] It is contemplated within the scope of this invention that
the embodiments of the invention are combinable in complementary
configurations. For example, in conjunction with providing light at
Stokes and anti-Stokes wavelengths, measuring the backscattered
signals, populating arrays and calculating errors, an independent
temperature measurement could be provided to calibrate the
calculated error at the independent temperature measurement
location. The present invention contemplates measuring temperature
or calibrating borehole properties by providing such multiple
embodiments simultaneously or at different times in a borehole. The
present invention further contemplates use of FO-DTS comprising
SMF, MMF, or a combination of SMF and MMF.
[0080] There have been described and illustrated herein several
embodiments of methods and apparatus for measuring differential
temperature with fiber optic sensors distributed. While particular
embodiments of the invention have been described, it is not
intended that the invention be limited thereto, as it is intended
that the invention be as broad in scope as the art will allow and
that the specification be read likewise. It will therefore be
appreciated by those skilled in the art that yet other
modifications could be made to the provided invention without
deviating from its spirit and scope as so claimed.
* * * * *