The invention relates generally to the field of optical
acceleration and inclination sensing devices. More particularly, the invention relates
to optical accelerometers and inclinometers used for, but not limited to, sensing
Optical sensing devices for measuring parameters such as
acceleration, motion and/or pressure are used for, among other purposes, detecting
seismic energy from the Earth's subsurface. The seismic energy may be naturally
occurring, or may be imparted into the Earth by a seismic energy source for the
purpose of performing reflection seismic surveys. Detecting seismic energy may include
measuring pressure, or changes in pressure with respect to time, in a body of water.
A sensor used to measure such changes in pressure is known as a hydrophone. Detecting
seismic energy also includes detecting motion on or near the Earth's surface. Motion
may be detected using devices known as geophones or accelerometers. Geophone signals
are related to velocity of motion. Accelerometers produce signals related to the
time derivative of velocity of motion, which is acceleration. Inclinometers, which
produce signals related to the relative orientation of a device with respect to
the direction of Earth's gravitational pull, are sometimes used in association with
other sensors to determine the gravitational orientation of any device associated
with the inclinometer.
Sensors known in the art which respond to the foregoing
physical parameters generate an optical signal in response to the detected physical
parameter. The optical signal may be, for example, a change in reflected wavelength,
a change in phase or an interference pattern in response to changes in the physical
Generally, optical sensors known in the art include a selected
length of optical fiber affixed to a device that changes shape in response to changes
in the physical parameter being detected. The change in shape of the device is transformed
into a change in length of the optical fiber. Change in length of the optical fiber
may be detected by one of a number of different optical measurement techniques.
Such techniques include change in reflected wavelength of light as a result of a
change in wavelength of a Bragg grating formed in the optical fiber, or optical
coupling of a light beam transmitted through the optical fiber with a light beam
transmitted through another optical fiber, known as a "reference fiber." The reference
fiber may be disposed such that its length remains essentially unchanged irrespective
of the value of the physical, parameter. Light beams from the fiber affixed to the
device and from the reference fiber are coupled in an optical interferometer. An
interference pattern or phase change in the light generated in the optical interferometer
is related to the change in length of the fiber coupled to the device, and thus
to the physical parameter being measured. Typically the output of the interferometer
is coupled to a photodetector, which generates an electrical signal related to the
light amplitude applied to the photodetector.
A fiber optic hydrophone is disclosed, for example, in
U.S. Patent No. 5,625,724 issued to Frederick et al.
The hydrophone disclosed in the Frederick et al. '724 patent includes
a reference fiber wrapped around a rigid inner cylinder. A solid layer of compliant
material is applied over the reference fiber. The sensing arm of the interferometer
is wound over the layer of material applied over the reference fiber. The outer
material is sufficiently compliant to provide acoustic sensitivity comparable to
that of air-backed hydrophones.
Another fiber optic hydrophone is disclosed in
U.S. Patent No. 6,549,488 issued to Maas et al.
and assigned to the assignee of the present invention. A hydrophone made
according to the Maas et al. '488 patent includes a compliant sensing mandrel coaxial
with and adjacent to a rigid reference mandrel. A first optical fiber is wound around
the compliant sensing mandrel. A second optical fiber is wound around the reference
mandrel. The first and second optical fibers comprise different arms of an interferometer.
Flexible sealing members, such as O-rings, seal the compliant sensing mandrel to
the rigid reference mandrel. In one embodiment, one O-ring is disposed near each
end of the sensing mandrel. A cylindrical support member is disposed inside the
sensing mandrel. At least a portion of the support member is spaced from the sensing
mandrel so as to provide a sealed cavity between the sensing mandrel and the support
member. The sealed cavity is filled with air or similar compliant substance.
U.S. Patent No. 5,369,485 issued to Holler et al.
discloses an optical accelerometer wherein an elastic disk and a predetermined
mass are supported by a body for flexure of the disk due to acceleration, shock,
vibration and displacement of the body in a direction axially of the disk. Such
a disk, or a plurality of such disks, are wound with a pair of flat spirals of optical
fiber, each spiral being fixedly attached to a corresponding disk side so that disk
flexure lengthens a spiral on one disk side and shortens a spiral on another disk
side. Such spirals on oppositely facing disk sides are connected as opposite legs
of a fiber optical interferometer so that the interferometer provides an output
corresponding to the amplitude of the flexure. A "push-pull" pair of the spirals
may be disposed oppositely of a thermally conducting disk to minimize temperature
differences between the push-pull spiral pair. An accelerometer according to the
disclosure in the Hofler et al. patent is constructed with a centrally supported
disk having the mass distributed around the disk periphery. Such construction is
purported to be advantageous for isolation from mounting stress and for providing
a plurality of coaxially mounted disks for increased sensitivity.
U. S. Patent No.
6,650,418 issued to Tweedy et al.
discloses a fiber optic sensor that includes a flexural disk having a
pair of fiber optic coils mounted on opposite sides thereof and optically coupled
together to form an interferometer that produces an output signal in response to
acceleration of the flexural disk. The accelerometer includes a housing having first
and second end plates with a sidewall extending between the end plates. The sidewall
has an inwardly facing groove in which an outer edge portion of the flexural disk
is mounted. A compressive damper is mounted in the housing and arranged to exert
a compressive force on the flexural disk to control movement thereof in response
to acceleration of the flexural disk along a sensing axis and thereby control the
U.S. Patent No. 6,575,033 issued to Knudsen et al.
discloses a highly sensitive accelerometer, which includes a mass within
a housing suspended by opposing support members. The support members are alternately
wound around a pair of fixed mandrels and the mass in a push-pull arrangement. At
least a portion of one of the support members comprises optical fiber coils as the
support members for use in interferometric sensing processes.
More recently, multiple-direction sensitive ("multicomponent")
motion sensors disposed on a cable in conjunction with substantially collocated
hydrophones have been used on the bottom of a body of water for marine seismic surveying.
Such cables are known in the art as "dual sensor OBCs." See, for example,
U.S. Patent No. 6,314,371 issued to Monk
, which discloses a method for processing of dual sensor OBC data that
corrects for energy incidence angle, corrects for estimated reflectivity, and combines
corrected seismic sensor traces using an optimal diversity scaling technique. In
one embodiment, the disclosed method takes seismic traces from a geophone and a
hydrophone, corrects the geophone trace for the incidence angle, determines diversity
filters for optimally combining the geophone and hydrophone traces, applies the
diversity filters, estimates a reflectivity coefficient for the ocean bottom (potentially
for different angles of reflection), scales the geophone data according to the reflectivity,
and re-applies the diversity filters to obtain a combined trace. The combined trace
is expected to have various artifacts eliminated, including ghosting and reverberation,
and is expected to have an optimally determined signal-to-noise ratio.
It is important that motion sensors in general, and in
particular those sensors used in dual sensor OBCs, have good sensitivity, are relatively
insensitive to noise, and have good rejection of cross-component signal (meaning
that the motion sensors are substantially insensitive to motion along any direction
other than the sensitive axis). Accordingly, there is a continuing need for motion
and/or acceleration sensors that have improved sensitivity, reduced noise and reduced
cross-component sensitivity. More recently, an improved optical acceleration sensor
particularly suited for use with OBCs has been devised by Steven J. Maas and D.
Richard Metzbower, as more fully described in
U.S. Patent Application No. 11/095860 - filed on March 31, 2005
and assigned to the assignee of the present invention. Such improved optical
acceleration sensor includes a beam and at least one optical fiber affixed to one
side of the beam such that deflection of the beam changes a length of the optical
fiber. Means for sensing the change in length of the optical fiber is functionally
coupled to the at least one fiber.
One common limitation to substantially all motion and acceleration
sensors known in the art for use with OBCs and other submerged sensing systems is
that they are typically disposed in a pressure resistant housing. The pressure resistant
housing is adapted to exclude water under high pressure, such as caused by submersion
of the sensor at great water depth (approximately 3000 meters or more) from entering
the housing. An interior of such housings is generally maintained at surface atmospheric
pressure (about 1 bar). As a practical matter, housings having the capability of
excluding water under pressure such as at the foregoing submersion depths must be
made from steel or similar high strength material, and must have relatively thick
walled construction to avoid crushing under pressure or leakage. Such construction
is expensive, and makes any sensor system such as an OBC used therewith heavy and
difficult to deploy. Accordingly, there exists a need for improved optical motion
sensing devices that can be immersed to great water depth, while avoiding the expense
and difficulty of construction of pressure resistant housings for the sensors.
One aspect of the invention is an optical accelerometer.
An accelerometer according to this aspect of the invention includes a means for
changing a length of at least one optical fiber in response to acceleration. Means
for sensing the change in length of the optical fiber is functionally coupled to
the at least one optical fiber. The means for changing length and the at least one
optical fiber are enclosed in a pressure compensated housing. The housing is filled
with a substantially incompressible fluid or gel. In one embodiment, the means for
changing comprises a beam. The at least one optical fiber is affixed to one side
of the beam such that deflection of the beam changes the length of the at least
one optical fiber.
Another aspect of the invention is a seismic sensor system.
A system according to this aspect of the invention includes at least two accelerometers.
Each accelerometer comprises at least one optical fiber and a means for changing
the length of the at least one optical fiber in response to acceleration. Means
for sensing the change in length of the optical fiber in each of the accelerometers
is functionally coupled to each fiber. The means for changing length and the optical
fiber of each accelerometer are enclosed in a pressure compensated housing. The
housing is filled with a substantially incompressible fluid. In one embodiment,
the means for changing length includes a beam. The at least one optical fiber in
each accelerometer is affixed to one side of the beam such that deflection of the
beam changes the length of the optical fiber. The at least two accelerometers are
oriented so as to be sensitive to acceleration at least in part along mutually orthogonal
Another aspect of the invention is a gravity orientation
system. A system according to this aspect of the invention includes three accelerometers,
each accelerometer including a means for changing a length of an optical fiber in
response to Earth's gravity. The at least three accelerometers are each oriented
to be sensitive to acceleration at least in part along mutually orthogonal directions.
The at one fiber in each accelerometer comprises a Bragg grating thereon, such that
an orientation with respect to Earth's gravity of a deflecting axis of each beam
is determinable by measurement of a change in wavelength of light reflected by the
Bragg grating. By so measuring the change in length of the Bragg grating, an orientation
of each accelerometer, and thus the system, with respect to Earth's gravity is determinable.
The means for changing length and the optical fiber of each accelerometer are enclosed
in a pressure compensated housing. The housing is filled with a substantially incompressible
Other aspects and advantages of the invention will be apparent
from the following description and the appended claims.
Figure 1 shows a side view of one embodiment of an accelerometer
according to the invention.
Figure 2 shows a top view of the accelerometer shown in
Figure 3 shows a side view of another embodiment of an
Figure 4 shows an oblique view of one embodiment a multicomponent
seismic sensor system.
Figure 5 shows one embodiment of an interferometer used
to determine change in length of fibers in various accelerometer embodiments.
Figure 5A shows an alternative arrangement of interferometer.
Figure 6 shows an accelerometer beam supported at both
Figure 7 shows an embodiment of optical detection system
used to determine gravity orientation (inclinometer) of an accelerometer.
Figure 8 shows a particular embodiment of an accelerometer
Figure 9 shows an alternative embodiment of inclinometer.
Figure 10 shows the embodiment of inclinometer shown in
Figure 9 as mounted in a sensor system according to Figure 4.
Figure 11 shows an alternative embodiment of an inclinometer.
Figure 11A shows an alternative embodiment of an inclinometer
that works on a similar principle to the device shown in Figure 11,
Figure 12 shows an example multicomponent seismic sensor
system including inclinometers as shown in Figure 11.
Figure 13 shows one embodiment of a pressure compensated
Generally, accelerometers according to the various aspects
of the invention work on the principle of changing the length of an optical fiber
in response to acceleration. According to the various aspects of the invention,
a means for changing the length of an optical fiber in response to acceleration
is functionally coupled to an optical fiber. The means for changing the length of
the optical fiber and the optical fiber are enclosed in a pressure compensated housing.
The pressure compensated housing is filled with a substantially incompressible fluid.
Some embodiments of optical accelerometers that can be
used in particular embodiments of the invention work on the principle of the deflecting
beam, where the beam is typically supported at its longitudinal ends. Supporting
the beam at its longitudinal ends substantially prevents beam flexure in any direction
transverse to the plane of the beam. Figure 1 shows one embodiment of an accelerometer
beam assembly 10 including a beam 12 which may be made from plastic or other suitable
material subject to elastic strain under acceleration. The beam 12 has dimensions
shown in Figure 1 by 12X, which is the length or longitudinal dimension, and 12Z
which is the thickness dimension. The plane of the beam 12 is transverse to the
thickness dimension 12Z. The dimensions 12X and 12Z should be selected to enable
relatively free flexure in the direction of the thickness 12Z, that is, transverse
to the plane of the beam 12, while substantially preventing any flexure of the beam
along the longitudinal dimension 12X. The embodiment shown in Figure 1 includes
an optical fiber 14 affixed to one face or side of the beam 12. Affixing the fiber
14 to the beam 12 may be performed by adhesive bonding or similar technique.
In the embodiment of Figure 1, a second optical fiber 16
is shown affixed to the opposite face of the beam 12. As the beam 12 deflects under
acceleration along the direction of the thickness 12Z, the optical fibers 14, 16
are stretched or compressed, depending on the direction of deflection of the beam
12. The stretching and compression of the one fiber 14 is in opposed polarity to
that of the other fiber 16 because they are disposed on opposite sides of the beam
12. Such arrangements are known as "push-pull" connections of optical fibers.
A signal from the accelerometer related to the acceleration
applied thereto is generated by determining a change in length of the optical fiber
14, if only one fiber is used, or of both optical fibers 14, 16 if two such fibers
are used. In practical embodiments, measurement of the change in length of the fiber
may be performed by an optical interferometer. The optical connections and use of
the fibers 14, 16 as part of an optical interferometer to generate an acceleration-responsive
signal will be explained below with reference to Figures 5 and 5A. It should be
understood that only one optical fiber affixed to one face or the other of the beam,
such as fiber 14 or 16 is required to make the accelerometer function. The dual-fiber
embodiment of Figures 1 and 2 is intended to have increased sensitivity as compared
to that expected from a single fiber implementation, and to attenuate other noise
sources such as created by non-collocated reference arms or compensating interferometers.
Figure 2 shows a top view of the accelerometer beam assembly
10. The beam 12 has a width dimension 12Y. As shown in Figure 2, the optical fiber
16 may be arranged about the face of the beam 12 in a generally oval shape to maximize
the amount of fiber disposed along the longitudinal dimension (12X in Figure 1),
while minimizing the degree of bending within the fiber 16 so as to minimize optical
losses in the fiber 16. The width dimension 12Y should be selected to make the beam
12 rigid enough along the width direction to resist flexure, but no too large as
to induce any appreciable degree of bending or twisting in the beam 12 under oblique
Another embodiment of an accelerometer beam assembly, shown
in Figure 3, can include a reactive mass 18, 20 affixed to one or both faces of
the beam 12, generally in the center thereof. The masses 18, 20 increase the amount
of deflection of the beam 12 under any given amount of acceleration, and thus, increase
the overall sensitivity of the accelerometer.
A practical multicomponent seismic sensor system may be
made from a plurality of accelerometers such as explained with reference to Figures
1 through 3. Figure 4 shows one embodiment of such a multicomponent seismic sensor
system. The system includes three optical accelerometers, 10X, 10Y, 10Z, each oriented
such that its sensitive direction is along a mutually orthogonal direction from
those of the other two accelerometers, Having the accelerometers be mutually orthogonal
facilitates determining the direction from which detected seismic energy originates,
however, it should be understood that mutual orthogonality of the accelerometers
is a matter of convenience in the design of the seismic sensor system. Other arrangements
of the sensitive axes of the accelerometers may be used in different embodiments,
while maintaining the capability of determining direction of origin of seismic energy.
The accelerometers 10X, 10Y, 10Z may be mounted in a frame
22 for convenient assembly within a pressure compensated housing. Enclosing the
frame 22 and accelerometers in such a housing is for when the accelerometers are
to be submerged in water. The accelerometers would be subject to submersion in the
case when the system is used in a marine seismic survey system or in a permanent
sensor installation such as would be used on the sea floor or in a well bore. The
housing will be further explained below with reference to Figure 13.
One embodiment of an optical interferometer and associated
components used to generate an acceleration-responsive signal from beam deflection
is shown at 29 in Figure 5. The optical fibers 14, 16 attached to opposite sides
of the beam (12 in Figure 1) are each shown optically coupled at one end to a beam
splitter 26, and coupled at the other end to a combiner 28. A light source, such
as a laser diode 24 is coupled to the input of the beam splitter 26 and provides
laser light to each fiber 14, 16. A photodetector 30 is coupled to the output of
the interferometer 29, and produces an electrical signal corresponding to the optical
signal generated in the interferometer 29. Thus, deflection of the beam (12 in Figure)
under acceleration along the thickness direction (12Z in Figure 1) is converted
into an electrical signal. Depending on the particular arrangement of a seismic
sensor system, the laser diode 24 and photodetector 30 may be disposed at the Earth's
surface or water surface, and the beam splitter 26 and combiner 28 disposed near
the accelerometer(s) (12 in Figure 1). However, other embodiments may locate the
laser diode and beam splitter proximate the interferometer, such as in the frame
(22 in Figure 4). The optical interferometer system shown in Figure 5 is generally
known as a Mach-Zehnder interferometer.
Alternatively, as shown in Figure 5A, a Michelson interferometer
may be used. The Michelson interferometer 29A is made by substituting the combiner
(28 in Figure 5) with mirrors 31A and 31B at the distal ends of each fiber 14, 16.
Light passing through the fibers 14, 16 is reflected back by the mirrors 31A, 31B.
The back reflected light is recombined in the beam splitter 26A such that phase
shift and/or interference pattern may be detected by the photodector 30.
Other types of interferometers that can be used with various
embodiments of accelerometer include Fabry-Perot and Sagnac interferometers. In
embodiments which use a Fabry-Perot interferometer, the fiber (either 14 or 16 in
Figure 1) affixed to one or the opposite face of the beam (12 in Figure 1) may be
excluded. The remaining fiber (16 or 14 in Figure 1) may include a Bragg grating
thereon where the fiber is affixed to the beam (12 in Figure 1) to enable determining
a change in length of the fiber by measuring change in wavelength of back-reflected
light through the fiber. Accordingly, the particular interferometer system used
in various embodiments is not a limitation on the scope of the invention. A particular
application for a Bragg grating on one or both fibers 14, 16 will be explained below
with reference to Figure 8.
Figure 6 shows a lateral view of the beam 12 and supports
32 at the longitudinal ends of the beam 12. By supporting the beam 12 at its longitudinal
ends, and by suitable dimensions (12X, 12Z in Figure 1 and 12Y in Figure 2) flexure
of the beam 12 will be substantially limited to the thickness dimension (12Z in
Figure 1). Thus limiting flexure of the beam 12 provides the accelerometer beam
assembly (10 in Figure 1) with a high degree of cross-component rejection or insensitivity.
Initial evaluation of the accelerometer as shown in Figure 1 indicates a cross-component
rejection of greater than 30 dB.
As will be readily appreciated, rigidly, fixedly supporting
the beam 12 at both longitudinal ends can provide a high degree of cross component
rejection, but may limit the amount of beam deflection (and thus sensitivity) in
the thickness direction. Beam deflection would be limited in such cases because
the beam would necessarily have to elongate along the longitudinal direction (12X
in Figure 1) if the beam is rigidly, fixedly supported at both ends. To increase
the amount of deflection while maintaining high cross component rejection, an arrangement
such as shown in Figure 8 may be used to support the beam 12 at its longitudinal
ends. Mounting holes 13 at one end may be provided for cap screws or the like. The
other end may include elongated openings 15 such that under flexure, when the longitudinal
dimension would be reduced by a proportionate amount, the other end of the beam
12 is free to move longitudinally, but substantially not transversely to the longitudinal
Figure 7 shows a particular embodiment, which may be used
to determine an orientation of the accelerometer with respect to Earth's gravity
as well as make acceleration measurements. A fiber 14A includes a Bragg grating
14B thereon. The fiber 14A can be affixed to a beam substantially as explained with
reference to Figure 1. A light source 24A, such as a laser diode, is optically coupled
to one end of the fiber 14A through a beam splitter 25. The fiber 14A may include
a mirror 17 at its other end. A photodetector 30 is coupled to the other output
of the beam splitter 25. The output of the photodetector 30 may be coupled to a
spectral analyzer 31. Thus, the wavelength of light reflected by the Bragg grating
14B is related to the degree of elongation of the Bragg grating 14B. The accelerometer
may be used to determine the orientation thereof by calibrating the Bragg grating
reflected wavelength both at zero gravity and at unity (100% gravity). Measurements
of the reflected light wavelength can be related to orientation of the accelerometer
with respect to gravity by well known trigonometric relationships.
In the present embodiment, the accelerometer may be calibrated
to zero gravity by orienting the beam (12 in Figure 1) such that the thickness,
or deflecting, dimension of the beam (12Z in Figure 1) is oriented transversely
to Earth's gravity. A wavelength of light reflected by the Bragg grating 14B is
measured by the spectral analyzer 31. Then the beam is oriented such that its deflecting
direction (12Z in Figure 1) is directly along Earth's gravity, and the wavelength
of the light reflected by the Bragg grating 14B is again measured. The wavelength
of the light reflected by the Bragg grating 14B will change as the fiber 14A is
lengthened by deflection of the beam, and consequent elongation of the Bragg grating
14B. The relative orientation of the accelerometer with respect to Earth's gravity
will thus be related to the light wavelength reflected from the Bragg grating 14B.
The optical components described with reference to Figure 8 may be included as a
separate fiber in any particular accelerometer, or, as shown in Figure 8, may be
included in the same fiber used in the accelerometer sensor.
In a multicomponent sensor system, such as shown in Figure
4, three mutually orthogonal accelerometers may each include a fiber having a Bragg
grating thereon. Associated optical components can be used to enable determining
a change in length of the grating, as shown in Figure 9. In the embodiment of Figure
9, a single optical fiber 33 may include three separate Bragg gratings 35, 37, 39
thereon. Each Bragg grating 35, 37, 39 is affixed to one of the three accelerometer
beams, as will be explained with reference to Figure 10. Each Bragg grating 35,
37, 39 will be elongated, and thus reflect a particular wavelength of light, based
on the orientation of the corresponding accelerometer beam with respect to Earth's
gravity. Thus, the orientation of the sensor system may be inferred by measurement
of the wavelength of the Bragg grating output of each of the three Bragg gratings
35, 37, 39, and thus the orientation of each accelerometer with respect to gravity.
Orientation of the entire sensor system with respect to gravity may be determined
from the three individual accelerometer gravity component measurements using well
known trigonometric relationships. Some embodiments of the accelerometer beam according
to the embodiment of Figure 9 may include one or more reactive masses coupled thereto,
such as shown in Figure 3.
Figure 10 shows the single fiber embodiment of inclinometer
of Figure 9 in which each Bragg grating 35, 37, 39 in the fiber 33 is affixed to
a corresponding one of the accelerometer beams 12Y, 12Z, 12X. Each beam 12Y, 12X,
12Z will deflect in relation to the orientation of each beam with respect to Earth's
gravity. If a particular beam is transverse to gravity, its deflection from gravity
will be substantially zero. Maximum deflection, and corresponding change in the
length of the associated Bragg grating, will occur when an accelerometer beam's
deflection direction is substantially aligned with Earth's gravity. Orientation
can be inferred by well known formulas using measurements of orthogonal components
of Earth's gravity. In the embodiment of Figure 10, the accelerometer beams may
be oriented substantially orthogonally. Other embodiments may include a separate
fiber for each Bragg grating, or may include a Bragg grating on the same sensing
fibers used in one or more types of interferometer for sensing seismic energy, as
explained with reference to Figures 1-4.
Another embodiment of an inclinometer 50, shown in Figure
11, can provide increased strain in a fiber Bragg grating with respect to Earth's
gravitational pull by mass loading the fiber Bragg grating directly. Such direct
mass loading can increase the accuracy of the measurement of inclination. As shown
in Figure 11, linear bearings, or some other high precision constraining device,
47 enable masses 42, 43 to slide along a frame or rod 40 as a result of the force
created by Earth's gravity. Coupling a fiber 44 having a Bragg grating thereon to
the bearings 47, and thus operatively to the masses 42, 43, and adding a positive
stop or snubber 41 to each end of the portion of the rod 40 for which mass travel
is permitted enables for the Bragg grating 45 to be strained by either one of the
masses 43, 42, regardless of orientation of the device with respect to gravity.
For example, in the orientation shown in Figure 11, the upper mass 42 is stopped
by the snubber 41, while the lower mass 43 can moved when pulled by gravity so as
to strain the fiber 44. If the accelerometer is rotated so that the lower mass 43
is above the upper mass 42, the lower mass 43 will be stopped by the snubber 41,
and the upper mass 42 will move when loaded by gravity. Pulling directly on the
fiber 44, as shown in Figure 11, can induce more strain in the Bragg grating 45
creating a greater wavelength shift. Because the masses 42, 43 travel along the
rod 40 on linear bearings, the masses 42, 43 are substantially prevented from movement
other than along the rod 40. By limiting motion of the masses 42, 43 to along the
rod, 40, the inclinometer 50 is substantially sensitive only to the component of
acceleration (i.e., Earth's gravity) acting along the length of the rod 40, and
thus has high cross component rejection. The inclinometer 50 shown in Figure 11
can be calibrated substantially as explained above with reference to Figure 9.
An alternative arrangement of an inclinometer that works
generally on the same principle as the device shown in Figure 11 is shown schematically
in Figure 11A. A mass 42A is suspended along a rod 40A by linear bearings 47A, such
that the mass 42A can move along the direction of the rod 40A, but is substantially
restrained from movement in any other direction. An optical fiber 44A having a Bragg
grating 45A thereon is coupled to the mass 42A such that the mass 42A is disposed
along the fiber 44A between two fiber suspension points 44B. The fiber 44A is also
affixed to the suspension points 44B. As gravity acts on the mass 42A, it pulls
on the fiber 44A and causes its length to change, which is detectable by change
in the light reflection wavelength of the Bragg grating 45A. In principle of operation
and calibration, the device shown in Figure 11A operates substantially similarly
to the device shown in Figure 11. The embodiment shown in Figure 11A has the advantage
of being operable in any orientation with respect to gravity using only a single
mass and requiring no snubbers as does the device shown in Figure 11.
Figure 12 shows an embodiment of a multicomponent seismic
sensor system including three, mutually orthogonal inclinometers 50X, 50Y, 50Z,
and three mutually orthogonal accelerometers 10X, 10Y, 10Z. The system in Figure
12 is similar in operating principle to that shown in Figure 10, however the inclinometers
50X, 50Y, 50Z are of the kind explained with reference to Figure 11. The references
X, Y and Z relate to the individual sensitive axes of the sensor system, which by
convention may be labeled such that ordinarily horizontally disposed axes are X
and Y, and the vertically disposed axis is Z. The system may be disposed in a frame
22 as are other embodiments, such as explained with reference to Figure 4 and Figure
Any of the embodiments of optical accelerometer and inclinometer,
as well as other types of optical accelerometer, may be enclosed in a pressure compensated
housing as will be explained with reference to Figure 13. The housing 122 may be
a plastic, rubber or relatively thin-walled metal, enclosure that is adapted to
be filled with a substantially incompressible material 106 such as oil, or other
fluid, or gel. For purposes of defining the scope of the invention materials known
as "gels", such as may be formed from hydrocarbon based oil mixed with cross-linking
polymers. Materials of such type, and other materials known as "gels" are known
in the art for filling seismic streamers. The frame 22, such as may include one
or more optical accelerometers or inclinometers, including those described above
with reference to Figures 1-12, may be fixedly mounted within the interior of the
housing 122. The housing 122 includes a pressure compensator 100 operable to cause
fluid pressure inside the housing 122 to substantially match ambient pressure outside
the housing 122, In the embodiment shown in Figure 13, the pressure compensator
100 can include a piston 102 movably disposed within a cylinder 101 disposed inside
the housing 122, such that one side of the piston 102 is in fluid communication
with the outside of the housing 122, and the other side of the piston 102 is in
fluid communication with the inside of the housing 122. The piston 102 may be sealed
against the interior of the cylinder 101 by an o-ring 104 or similar sealing element
to reduce fluid leakage past the piston 102. As pressure outside the housing 122
increases, the piston 102 is caused to move inwardly, correspondingly compressing
the fluid 106 inside the housing 122. Corresponding opposite movement of the piston
102 takes place when the external pressure decreases. The pressure compensator 100
thus serves the purpose of readily communicating pressure changes outside the housing
122 to the interior of the housing 122 so as to equalize the pressures thereof,
while substantially retaining the fill material 106 within the housing 122. Other
embodiments of pressure compensator may include elastomer bladders or the like.
By maintaining fluid pressure inside the housing 122 substantially equal to fluid
pressure outside the housing 122, it is possible to build the housing 122 without
the need to make it strong enough to resist crushing under high external pressure,
as is required with conventional, pressure resistant, sealed housings having atmospheric
pressure (about 1 bar) in the interior thereof.
It is preferable in embodiments such as explained with
reference to Figure 13 for the optical accelerometer components, such as the beam
and fiber, and any interferometer components to be disposed in the housing 122 to
be encapsulated with epoxy or similar encapsulating compound to prevent fluid entry
into such components.
Optical accelerometers and sensing systems made therewith
disposed in a pressure compensated housing can provide the improved performance
of optical accelerometers for detecting such acceleration as seismic energy, while
enabling the sensors to be deployed in deep ocean water using relatively light,