U.S. patent application number 11/899173 was filed with the patent office on 2009-03-05 for fiber optic system for electromagnetic surveying.
Invention is credited to Steven J. Maas, Stig Rune Tenghamn.
Application Number | 20090058422 11/899173 |
Document ID | / |
Family ID | 39866071 |
Filed Date | 2009-03-05 |
United States Patent
Application |
20090058422 |
Kind Code |
A1 |
Tenghamn; Stig Rune ; et
al. |
March 5, 2009 |
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.
Inventors: |
Tenghamn; Stig Rune; (Katy,
TX) ; Maas; Steven J.; (Pfugerville, TX) |
Correspondence
Address: |
E. Eugene Thigpen;Petroleum Geo-Services, Inc.
P.O. Box 42805
Houston
TX
77242-2805
US
|
Family ID: |
39866071 |
Appl. No.: |
11/899173 |
Filed: |
September 4, 2007 |
Current U.S.
Class: |
324/337 ;
324/344 |
Current CPC
Class: |
G01V 3/083 20130101;
G01V 3/12 20130101; G01V 2003/085 20130101; G01R 33/0327
20130101 |
Class at
Publication: |
324/337 ;
324/344 |
International
Class: |
G01V 3/12 20060101
G01V003/12 |
Claims
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.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] Not applicable.
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
[0002] Not applicable.
BACKGROUND OF THE INVENTION
[0003] 1. Field of the Invention
[0004] The invention relates generally to the field of
electromagnetic surveying of the Earth's subsurface. More
specifically, the invention relates to electromagnetic survey
systems including optical output sensors to reduce effects of
electromagnetic noise on signals measured by the system.
[0005] 2. Background Art
[0006] Electromagnetic geophysical surveying of the Earth's
subsurface includes "controlled source" and "natural source"
electromagnetic surveying. Controlled source electromagnetic
surveying includes imparting an electric field or a magnetic field
into subsurface Earth formations, such formations being below the
sea floor in marine surveys, and measuring electric field amplitude
and/or amplitude of magnetic fields induced in response to the
imparted field. Such measurements are performed by measuring
voltage differences induced between spaced apart electrodes,
induced in antennas and/or interrogating magnetometers disposed at
the Earth's surface, or near or above the sea floor. The electric
and/or magnetic fields from which the above measurements are made
are induced in response to the electric field and/or magnetic field
imparted into the Earth's subsurface, as stated above, and
inferences about the spatial distribution of electrical
conductivity of the Earth's subsurface are made from recordings of
the induced electric and/or magnetic field measurements.
[0007] Natural source electromagnetic surveying includes deploying
multi-component ocean bottom receiver stations and by taking the
ratio of perpendicular field components, one can eliminate the need
to know characteristics of the natural source.
[0008] Controlled source electromagnetic surveying known in the art
includes imparting alternating electric current into the subsurface
formations. The alternating current has one or more selected
frequencies. Such surveying is known as frequency domain controlled
source electromagnetic (f-CSEM) surveying. Another technique for
electromagnetic surveying of subsurface Earth formations known in
the art is transient controlled source electromagnetic surveying
(t-CSEM). In t-CSEM, electric current is imparted into the Earth at
the Earth's surface (or sea floor), in a manner similar to f-CSEM.
The electric current may be direct current (DC). At a selected
time, the electric current is switched off, switched on, or has its
polarity changed, and induced voltages and/or magnetic fields are
measured, typically with respect to time over a selected time
interval, at the Earth's surface or water surface. Alternative
switching techniques are possible. Structure of the subsurface is
inferred by the time distribution of the induced voltages and/or
magnetic fields. For example, U.S. Patent Application Publication
No. 2004/232917 and U.S. Pat. No. 6,914,433 Detection of subsurface
resistivity contrasts with application to location of fluids
(Wright, et al) describes a method of mapping subsurface
resistivity contrasts by making multichannel transient
electromagnetic (MTEM) measurements on or near the Earth's surface
using at least one source, receiving means for measuring the system
response and at least one receiver for measuring the resultant
earth response. All signals from each source-receiver pair are
processed to recover the corresponding electromagnetic impulse
response of the earth and such impulse responses, or any
transformation of such impulse responses, are displayed to create a
subsurface representation of resistivity contrasts. The system and
method enable subsurface fluid deposits to be located and
identified and the movement of such fluids to be monitored.
[0009] The above methods for f-CSEM and t-CSEM have been adapted
for use in marine environments. Cable based sensors have been
devised for detecting electric and/or magnetic field signals
resulting from imparting electric and/or magnetic fields into
formations below the bottom of a body of water. See, for example,
U.S. Patent Application Publication No. 2006/0238200 filed by
Johnstad. The amplitude of electric field signals detected by
electrodes on cables such as described in the Johnstad publication
may be on the order of fractions of a nanovolt. Accordingly, a
particular consideration in the design and implementation of
electromagnetic survey receiver systems is reducing the amount of
noise that may be induced in the signals detected by the various
sensing elements in the receiver system. One example of such noise
reduction is to include batteries at each of a plurality of
receiver system stations where signal amplification devices may be
located. By eliminating the need to transmit operating power along
a cable associated with the receiver system, induced noise may be
reduced. Battery power does not eliminate induced noise resulting
from electrical signal telemetry in cable type systems such as
shown in the Johnstad publication, however, because electric
current carrying such signals, representative of the voltages
and/or magnetic field amplitudes measured, may induce noise in the
measured signals.
[0010] There is a continuing need for electromagnetic survey
devices that reduce noise induced in the measured signals caused by
electric power and signal transmission along receiver cables.
SUMMARY OF THE INVENTION
[0011] An electromagnetic survey sensing device according to one
aspect of the invention 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.
[0012] An electromagnetic survey system according to another aspect
of the invention includes 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 includes a light source and a photodetector
therein in optical communication with the associated fiber. At
least two electrodes are disposed at spaced apart locations along
the receiver cable. The electrical to optical converter is
electrically coupled to the at least two electrodes. The converter
is configured to change a property of light from the source in
response to voltage imparted across the at least two electrodes.
The optical fiber is optically coupled to an output of the
electrical to optical converter. The optical fiber is in optical
communication with the detector in the recording device.
[0013] A method for sensing an electromagnetic field according to
another aspect of the invention includes exposing an electric
dipole antenna to the electromagnetic field. Voltage imparted to
the antenna is conducted to an electrical device that changes a
property of light imparted thereto in response to the voltage. A
property of light conducted from a light source to the device and
from the device to a photodetector along an optical fiber is
varied, so that a signal corresponding to the voltage is optically
communicated to the photodetector.
[0014] An electromagnetic survey system according to another aspect
of the invention includes a survey vessel configured to tow a
receiver cable through a body of water. The system includes 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 are
each configured to change a property of light from source
associated with the recording device. A signal detector is
associated with the recording device and is configured to convert
the changed property of light into a signal corresponding to a
property of a magnetic field proximate each sensor.
[0015] A method for electromagnetic surveying of formations in the
Earth's subsurface includes imparting an electromagnetic field into
the formations. A magnetostrictive material is exposed to an
electromagnetic field produced in response to the imparted
electromagnetic field. A change in dimension of the
magnetostrictive material caused by the responsively produced field
is transferred to an optical fiber. The transferring causes 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.
[0016] Other aspects and advantages of the invention will be
apparent from the following description and the appended
claims.
BRIEF DESCRIPTION OF THE DRAWINGS
[0017] FIG. 1A shows an example marine electromagnetic survey
system having a towed receiver cable.
[0018] FIG. 1B shows an example marine electromagnetic survey
system including an ocean bottom receiver cable.
[0019] FIG. 2A shows one example electrical to optical converter
that can be used with the systems shown in FIG. 1A and FIG. 1B.
[0020] FIG. 2B shows light source and signal detection components
of a recording system as in FIG. 1 that may be used with various
examples of a system according to the invention.
[0021] FIGS. 3 through 6 show other examples of devices that can
change a length of an optical path in response to a voltage
imparted across electrodes.
[0022] FIGS. 7A and 7B shown an example of a magnetic field sensing
system using fiber optic sensors.
DETAILED DESCRIPTION
[0023] An example electromagnetic survey system is shown
schematically in FIG. 1A. The system shown in FIG. 1A is intended
for marine use, however it will be appreciated by those skilled in
the art that the invention is equally applicable to land-based
electromagnetic surveys. A survey vessel 10 moves along the surface
of a body of water 11 such as a lake or the ocean. The vessel 10
may include equipment, shown generally at 12 and referred to for
convenience as a "recording system" that includes devices (none
shown separately) for applying electric current to a source cable
14 towed by the vessel, for navigating the vessel and for recording
signals detected by one or more sensors on a receiver cable 16.
[0024] The source cable 14 in the present example includes two
source electrodes 18 disposed at spaced apart positions along the
source cable 14. At selected times the equipment (not shown
separately) in the recording system 12 conducts electric current
across the source electrodes 18. Such electric current produces an
electromagnetic field that propagates through the water 11 and into
the formations below the water bottom 22. The particular type of
current conducted across the source electrodes 18 may be single- or
multi-frequency alternating current, or various forms of switched
direct current, such that either or both transient and frequency
domain controlled source electromagnetic surveying may be
performed. It should also be understood that the arrangement of
source electrodes 18 shown in FIG. 1A, referred to as a horizontal
electric dipole transmitter antenna, is not the only type of
electromagnetic transmitter that may be used with the invention.
The source cable 14 could also tow, in addition to or in
substitution of the horizontal electric dipole transmitter antenna
shown in FIG. 1A, any one or more of a vertical electric dipole
antenna, and horizontal or vertical magnetic dipole antenna.
Accordingly, the source antenna configuration shown in FIG. 1A is
not intended to limit the scope of the invention.
[0025] In the example shown in FIG. 1A, the vessel 10 also tows a
receiver cable 16. The receiver cable 16 includes at least one pair
of electrodes 20 disposed at spaced apart positions along the
receiver cable 16. An electric field resulting from interaction of
the induced electromagnetic field in the formations below the water
bottom 22 induces voltages across the at least one pair of
electrodes 20. In the present example, the pair of electrodes 20
may be associated with an electrical to optical converter 24
disposed at a selected position along the receiver cable 16,
typically, but not necessarily between the electrodes 20. The
electrical to optical converter 24 generates an optical signal that
is related to the voltage induced across the electrodes 20 in
response to the electromagnetic field imparted by the transmitter
antenna (source electrodes 18). The optical signal is transmitted
along an optical fiber (see 27 in FIG. 2A) associated with the
receiver cable 16 to the recording unit 12, or as will be explained
below with reference to FIG. 1B to a recording device.
[0026] FIG. 1B shows an arrangement similar to that of FIG. 1A,
however, the receiver cable 16A is deployed on the water bottom 22.
The receiver cable 16A in FIG. 1B may include at least one pair of
electrodes 20A and an electrical to optical converter 24A
associated therewith similar to those shown in FIG. 1A. The
receiver cable 16A in FIG. 1B may have a recording device 17
associated therewith to record signals produced by the electrical
to optical converter 24A and transmitted along an optical fiber
(see FIG. 2A) from the converter 24A to the recording device 17.
During acquisition using a receiver cable on the water bottom such
as shown in FIG. 1A, there is typically no direct electrical or
other type of connection between the receiver cable 16A and the
recording system 12 on the vessel 10, thus it is convenient to
provide a recording device 17 associated with the receiver cable
16A. Other than the manner of deployment of the receiver cable 16A,
acquisition of electromagnetic signals may be similar to that
explained above with reference to FIG. 1A.
[0027] An example of an electrical to optical converter 24 is shown
in more detail in FIG. 2A. The electrical to optical converter 24
may include a piezoelectric element or crystal 28 that is
electrically coupled to each of the electrodes 20. When the
piezoelectric element 28 is so coupled to the electrodes 20, any
voltage imparted across the pair of electrodes 20 will be conducted
to the piezoelectric element 28. The piezoelectric element 28 will
change shape to an extent corresponding to the voltage imparted
across the electrodes 20. The piezoelectric element 28 may be in a
convenient shape such as a cylinder that is configured to change
diameter in response to the imparted voltage. The piezoelectric
element 28 may be made from piezoelectric materials having high
charge constants and high electromechanical coupling coefficients,
such as lead zirconate titanate ("PZT"), lead magnesium
niobate-lead titanate ("PMN-PT") and lead zirconate niobate-lead
titanate ("PZN-PT").
[0028] An optical interferometer, which in the present example may
include an optical coupling 26, a sensing fiber 30 and a reference
fiber 32 coupled to output terminals of the optical coupling 26 may
be arranged as shown in FIG. 2A. The sensing fiber 30 may be wound
around the piezoelectric element 28 such that change in dimensions
of the piezoelectric element 28, related to the voltage across the
electrodes 20, will cause a corresponding change in the length of
the sensing fiber 30. The reference fiber 32 may be provided such
that changes in ambient conditions (e.g., temperature, pressure)
will substantially equally affect both the sensing fiber 30 and the
reference fiber 32. Each of the fibers 32, 30 may be terminated at
an end opposite the optical coupling 26 by a mirror 39. The
combination of optical elements shown in FIG. 2A may result in a
phase shift in light passing through the sensing fiber 30, which
when combined with light passing through the reference fiber 32 may
produce an optical phase shift in the first optical coupling 26.
The phase information may be conducted along an optical fiber 27 to
the detection equipment (e.g. in the recording device 17 in FIG. 1B
or in the recording system 12 in FIG. 1A) where the optical phase
can be demodulated to reconstruct a signal corresponding to the
imparted voltage with respect to time.
[0029] An example of a light source and detection devices used in
association with the recording unit (12 in FIG. 1A) or the
recording device (17 in FIG. 1B) will new be explained with
reference to FIG. 2B. In FIG. 2B, a light source 34 such as a laser
diode may provide light at one or more wavelengths to the optical
fiber 27 associated with the receiver cable (e.g., 16 in FIG. 1A).
The light source 34 may be disposed in the recording system 12, if
a towed receiver cable is used, or may be disposed in the recording
device (17 in FIG. 1B) if the receiver cable (e.g., 16A in FIG. 1B)
is disposed on the water bottom. The output of the light source 34
may pass through a modulator 36 to cause the light to be modulated,
for example, to be pulsed at one or more selected frequencies.
Output of the modulator 36 may be conducted to an optical coupling
42, one output of which is coupled to the optical fiber 27. A
return tap on the optical coupling 42 may be coupled to a
photodetector 40, which converts the returning light into an
electrical signal corresponding to the amplitude of the light.
Output of the photodetector 40 may be coupled to a demodulator 38
to recover the signals from the electrical to optical converter
(e.g., 24 in FIG. 2A)
[0030] The general principle of the electrical to optical converter
(24 in FIG. 2A) is to convert a voltage imparted across the
electrodes (20 in FIG. 2A) to a corresponding change in properties
of light from the source (34 in FIG. 2B). The change in properties
of the light caused by the converter is such that a signal
corresponding to the imparted voltage may be optically communicated
to the recording system or the recording device without the need
for electrical power to be transmitted along the receiver cable
either as operating power or as signal telemetry. One type of
change in properties of the light from the source is to cause a
phase shift in the light. In such examples, the phase shift is
related to the length of an optical path along the sensing fiber
and/or optical components associated with the sensing fiber. The
electrical to optical converter elements shown in FIG. 2A, for
example cause a change in length of the optical path through the
sensing fiber (30 in FIG. 2A) by changing the fiber length
corresponding to change in diameter of the piezoelectric element
(28 in FIG. 2A) as explained above.
[0031] Other examples of electrical to optical converters that can
change the length of an optical path in response to a voltage
imparted across the electrodes (20 in FIG. 2A) will now be
explained with reference to FIGS. 3 through 6.
[0032] In FIG. 3, the converter 24 can include a cylindrically
shaped piezoelectric element or crystal 28 as in the previous
examples. The piezoelectric element 28 may be etched or otherwise
have a feature 28A approximately in its longitudinal center that
causes one longitudinal segment of the piezoelectric element 28 to
operate independently of the other longitudinal segment. A first
sensing fiber 30A may be wound around one longitudinal segment, and
a second sensing fiber 32A may be wound around the other
longitudinal segments of the piezoelectric element 28. The sensing
fibers 30A, 32A may each include a mirror 39 or similar reflective
surface at the terminal ends thereof, and may be coupled at the
opposite ends thereof to an optical coupling as in FIG. 2A. The two
longitudinal segments of the piezoelectric element or crystal 28
are electrically coupled to the electrodes (20 in FIG. 2A) in
opposed polarity as shown in FIG. 3. Arranged as shown in FIG. 3,
one longitudinal segment of the piezoelectric element 28 will
contract in diameter and the other segment will expand in diameter
when a voltage is imparted across the electrodes (20 in FIG. 2A).
Therefore, one sensing fiber will increase length and the other
will decrease length. When the fibers 30A, 28A change length, an
interference pattern may be generated in the optical coupling 26.
The arrangement shown in FIG. 3 may thus provide increased
sensitivity because of the opposite change in length of each
sensing fiber 30A, 28A in response to voltage imparted across the
electrodes (20 in FIG. 1A). The principle of the device shown in
FIG. 3 may also be implemented by using two separate piezoelectric
elements electrically coupled to the electrodes in opposite
polarity.
[0033] Another example shown in FIG. 4 may include a sensing fiber
30 and a reference fiber 32 as in the example of FIG. 2A. The
example of FIG. 4 may include a piezoelectric element 28A including
a stack of piezoelectric wafers disposed proximate the terminal end
of the sensing fiber 30. A collimating lens 29 may be fixed to the
end of the sensing fiber 30. One side of the piezoelectric element
28A may include a mirror 28B facing the lens 29. The piezoelectric
element 28A is electrically coupled to the electrodes (20 in FIG.
2A) such that a voltage imparted across the electrodes changes the
length of the piezoelectric element 28A and correspondingly changes
a distance between the mirror 28B and the lens 29. Changing the
foregoing distance changes the optical path length of the sensing
fiber, and results in an interference pattern in an optical
coupling 26, which is coupled to both the sensing fiber 30 and the
reference fiber 32. The reference fiber 32 may itself include a
mirror 39 at the terminal end thereof to reflect light back to the
optical coupling 26 for creating the interference patter with light
from the sensing fiber.
[0034] Another example of an electrical to optical converter is
shown in FIG. 5 that is similar in operating principle to the
example shown in FIG. 4. In the example of FIG. 5, a first sensing
fiber 30B is arranged as part of an interferometer substantially as
explained above with reference to FIG. 4 and is electrically
coupled to the electrodes (20 in FIG. 2A) so that the element 28A
changes length in response to voltage imparted across the
electrodes (20 in FIG. 2A). Rather than using a reference fiber as
in FIG. 4, the example of FIG. 5 includes a second sensing fiber
32B that has a collimating lens 29 at the terminal end thereof. A
piezoelectric element 28A with mirror 28B thereon, similar to the
piezoelectric element proximate the end of the first sensing fiber
30A, is disposed proximate the end of the second sensing fiber 32A.
The piezoelectric element 28A proximate the end of the second
sensing fiber is electrically coupled to the electrodes (20 in FIG.
2A) in opposed polarity to the coupling of the element 28A
proximate the first sensing fiber 30A. Thus, a voltage imparted
across the electrodes (20 in FIG. 2A) causes the two piezoelectric
elements 28A to oppositely change length. The optical path length
of the first sensing fiber 30A changes length in an opposite manner
to the change in length of the second sensing fiber 32A. The
arrangement shown in FIG. 5 may have increased sensitivity as
compared to the arrangement shown in FIG. 4.
[0035] Another example of an electrical to optical converter is
shown in FIG. 6. The sensing fibers 30C, 32C forming part of an
optical interferometer as in the previous examples may each include
a micro electromechanical sensing material, such as a lithium
niobate etched into each fiber as shown at 40. Each etching has a
mirror (not show separately) associated therewith. The etchings 40
may be electrically coupled proximate the ends thereof to the
electrodes (20 in FIG. 2A). When voltage is imparted across the
electrodes, the etchings 40 will change length, and thereby move
the associated mirror. Such movement will change the optical path
length of each fiber 30C, 32C so that an interference pattern may
be generated in the optical coupling 26.
[0036] The foregoing examples are directed to electric field
sensors that convert voltage imparted across spaced apart
electrodes into a change in optical properties of light passed
through an optical fiber. In other examples, described below with
reference to FIG. 7A and FIG. 7B, a magnetic field resulting from
imparting an electromagnetic field into the subsurface formations
may be detected using a magnetostrictive sensor associated with an
optical fiber. First referring to FIG. 7A, a survey system that is
configured similarly to the system shown in FIG. 1A includes a
survey vessel 10 including a recording system 12 thereon. The
vessel 10 tows at least one electromagnetic receiver cable 16
according to the present example. The receiver cable 16 may include
a plurality of spaced apart magnetostrictive sensors 24A at spaced
apart positions along the receiver cable 16. An electromagnetic
field source (not shown in FIG. 7A) may be towed by the vessel 10
or by another vessel (not shown).
[0037] The sensors 24A respond to changes in the amplitude of a
magnetic field by changing a dimension of a magnetostrictive
material. Such dimensional change causes a corresponding change in
a dimension of an optical fiber.
[0038] Referring to FIG. 7B, a sensing fiber 32 and a reference
fiber 30 form two arms of an interferometer. Such interferometer
may operate similarly to the example shown and explained with
reference to FIG. 4, although such example is not a limit on the
scope of this invention. The sensing fiber 32 may be coupled to a
magnetostrictive material 132 such than changes in magnetic field
proximate the magnetostrictive material cause change in length of
the sensing fiber 32. Each of the sensing fiber 32 and the
reference fiber 30 may be terminated by a mirror 39. The principle
of such sensors is described, for example in U.S. Pat. No.
4,376,248 issued to Giaollrenzi et al. Other magnetostrictive
transducers and sensing systems are described, for example, in U.S.
Pat. Nos. 4,600,885; 4,653,915; 4,881,813; 4,889,986; 5,243,403;
5,305,075; 5,396,166; 5,491,335; 5,986,784; 6,081,633 and 6,285,806
B1.
[0039] Electromagnetic sensing devices and systems made therewith
may provide measurements responsive to electric fields induced in
the Earth's subsurface without the need to supply electrical power
to sensing devices and/or amplification devices, and without the
need for electrical signal telemetry. Such sensing devices and
systems may have reduced sensitivity to electrical noise than
conventional systems that transmit electrical power and signal
telemetry along sensing cables.
[0040] While the invention has been described with respect to a
limited number of embodiments, those skilled in the art, having
benefit of this disclosure, will appreciate that other embodiments
can be devised which do not depart from the scope of the invention
as disclosed herein. Accordingly, the scope of the invention should
be limited only by the attached claims.
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