CA1127417A - Dipole mass laser-based gravity gradiometer - Google Patents
Dipole mass laser-based gravity gradiometerInfo
- Publication number
- CA1127417A CA1127417A CA328,425A CA328425A CA1127417A CA 1127417 A CA1127417 A CA 1127417A CA 328425 A CA328425 A CA 328425A CA 1127417 A CA1127417 A CA 1127417A
- Authority
- CA
- Canada
- Prior art keywords
- laser beam
- laser
- polarization modes
- biasing element
- torque
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Expired
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Classifications
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01V—GEOPHYSICS; GRAVITATIONAL MEASUREMENTS; DETECTING MASSES OR OBJECTS; TAGS
- G01V7/00—Measuring gravitational fields or waves; Gravimetric prospecting or detecting
- G01V7/08—Measuring gravitational fields or waves; Gravimetric prospecting or detecting using balances
- G01V7/10—Measuring gravitational fields or waves; Gravimetric prospecting or detecting using balances using torsion balances, e.g. Eötvös balance
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- Physics & Mathematics (AREA)
- Life Sciences & Earth Sciences (AREA)
- General Life Sciences & Earth Sciences (AREA)
- General Physics & Mathematics (AREA)
- Geophysics (AREA)
- Investigating Or Analysing Materials By Optical Means (AREA)
- Geophysics And Detection Of Objects (AREA)
Abstract
DIPOLE MASS LASER-BASED GRAVITY GRADIOMETER
ABSTRACT OF THE DISCLOSURE
A gravity gradiometer using at least one mass dipole mounted in a laser force-measurement system to detect the torque on the mass dipole generated by gravity gradient. The mass dipole is mounted on one end of a photoelastic modulator element positioned in the laser beam to differentially modulate circular polarization modes in response to application of a torque. In a preferred form, two mass dipoles are mounted on opposite ends of the modulator element which is rigidly sup-ported at its center to improve noise rejection.
ABSTRACT OF THE DISCLOSURE
A gravity gradiometer using at least one mass dipole mounted in a laser force-measurement system to detect the torque on the mass dipole generated by gravity gradient. The mass dipole is mounted on one end of a photoelastic modulator element positioned in the laser beam to differentially modulate circular polarization modes in response to application of a torque. In a preferred form, two mass dipoles are mounted on opposite ends of the modulator element which is rigidly sup-ported at its center to improve noise rejection.
Description
B~CKGROUND OF THE INV~NTION
This invention relates to laser-based gravity gradiometers and m~re particularly to a gravity gradiometer in which the torque caused by ~ravity gradient on a mass dipole is sensed by a laser force-measurement device.
The prior art believed to be most relevan~ to the present invention is U.S. Patent 3,786,~81, issued to Kiehn, on January 22, 1974, and entitled "Electromagnetic Wave ~odulation and Measurement Sys-tem and Method." The preferred embodiment disclosed by the patent includes a ring laser having a plurality of circular polarization modes and a modulator element within the laser cavity which produces differen-tial frequency shifts between the polarization modes in response to application of a torque. One use taught for such a device is a gravity meter in which the force of gravity on a known mass is converted to a torque by being attached to the modulator element by means of a lever arm. Such a device is suggested for use in a borehole for measuring the earth's gravity. As is well known and as is taught in the patent, such a device is also an accelerometer and responds to any acceleration of the measuring instrument. Thus, to measure gravity at a location in the borehole, the measuring device must be stationary for a time period long enough to allow the device to stabilize. It would be much more advanta-geous to have a gravity-measuring device which would allow the required gravity measurements to be taken while the device is moved through the borehole.
In normal gravity meter borehole logging, the parameter of most importance is the difference in gravity at known, closely spaced locations in the borehole, that is, the gravity gradient over short intervals. Gravity gradiometers of various types are well known in the art but have not been used in a borehole due to their large size, sensi-tivity to motion, and long settling times. On form of such gravity gradiometer involves a mass dipole suspended at a 45 angle relative to the direction of the gravity gradient to be measured. It is known that a gravity gradient which is neither parallel nor perpendicular to the axis of the mass dipole will exert a small torque on the mass dipole.
Horizontal gradiPnts have been detected by supporting the dipole on a fine filament which allows the dipole to turn slightly in response to a gradient. Other efforts have been made to support the dipol~ on a bear-ing, ideally frictionless, which would allow the dipole to rotate in response to the torque with the rotation9 and therefore the torque, being sensed by electrostatic sensors. As a result, most of these sys-tems have been either very complicated or fragile and tend to have very long settling times so that their use in the borehole would still require stationary reading, and, therefore, long time periods in which the borehole must be out of use.
SUMMARY OF THE INVENTION
Accordingly, an object of the present invention is to provide an improved gravity gradiometer suitable for use in a borehole.
A gravity gradiometer according to the present invention com-prises at least one mass dipole connected to a photoelastic modulator element positioned within a laser cavity in which circular polarization modes are differentially modulated by the application of a torque to the modulator element. Means are provided for detecting the beat frequency between the circular polarization modes and providing an output indicat-ing the beat frequency, and, therefore, the level of gravity gradient.
BRIEF DESCRIPTION OF THE DRAWING
-The present invention may be more fully understood by reading the following description of the preferred embodiments with reference to the attached drawing which illustrates a gravity gradiometer according to the present invention.
This invention relates to laser-based gravity gradiometers and m~re particularly to a gravity gradiometer in which the torque caused by ~ravity gradient on a mass dipole is sensed by a laser force-measurement device.
The prior art believed to be most relevan~ to the present invention is U.S. Patent 3,786,~81, issued to Kiehn, on January 22, 1974, and entitled "Electromagnetic Wave ~odulation and Measurement Sys-tem and Method." The preferred embodiment disclosed by the patent includes a ring laser having a plurality of circular polarization modes and a modulator element within the laser cavity which produces differen-tial frequency shifts between the polarization modes in response to application of a torque. One use taught for such a device is a gravity meter in which the force of gravity on a known mass is converted to a torque by being attached to the modulator element by means of a lever arm. Such a device is suggested for use in a borehole for measuring the earth's gravity. As is well known and as is taught in the patent, such a device is also an accelerometer and responds to any acceleration of the measuring instrument. Thus, to measure gravity at a location in the borehole, the measuring device must be stationary for a time period long enough to allow the device to stabilize. It would be much more advanta-geous to have a gravity-measuring device which would allow the required gravity measurements to be taken while the device is moved through the borehole.
In normal gravity meter borehole logging, the parameter of most importance is the difference in gravity at known, closely spaced locations in the borehole, that is, the gravity gradient over short intervals. Gravity gradiometers of various types are well known in the art but have not been used in a borehole due to their large size, sensi-tivity to motion, and long settling times. On form of such gravity gradiometer involves a mass dipole suspended at a 45 angle relative to the direction of the gravity gradient to be measured. It is known that a gravity gradient which is neither parallel nor perpendicular to the axis of the mass dipole will exert a small torque on the mass dipole.
Horizontal gradiPnts have been detected by supporting the dipole on a fine filament which allows the dipole to turn slightly in response to a gradient. Other efforts have been made to support the dipol~ on a bear-ing, ideally frictionless, which would allow the dipole to rotate in response to the torque with the rotation9 and therefore the torque, being sensed by electrostatic sensors. As a result, most of these sys-tems have been either very complicated or fragile and tend to have very long settling times so that their use in the borehole would still require stationary reading, and, therefore, long time periods in which the borehole must be out of use.
SUMMARY OF THE INVENTION
Accordingly, an object of the present invention is to provide an improved gravity gradiometer suitable for use in a borehole.
A gravity gradiometer according to the present invention com-prises at least one mass dipole connected to a photoelastic modulator element positioned within a laser cavity in which circular polarization modes are differentially modulated by the application of a torque to the modulator element. Means are provided for detecting the beat frequency between the circular polarization modes and providing an output indicat-ing the beat frequency, and, therefore, the level of gravity gradient.
BRIEF DESCRIPTION OF THE DRAWING
-The present invention may be more fully understood by reading the following description of the preferred embodiments with reference to the attached drawing which illustrates a gravity gradiometer according to the present invention.
- 2 ~
~7~
DESCRIPTION ~F TXE PREFERRED EMBODIMEN~
The drawing illustrates a gravity gradiometer according to the present invention in essentially the same way as illustrated in the abové-referenced U.S. Patent 3,786,681. A laser amplifier tube 2 gener-ates a laser beam indicated by the dotted line 3. The beam 3 travels mainly within the cavity formed by mirrors 4, 6, 8, and 10. Mirror 8 is partially transmissive to allow a portion of the beam to pass to a beat detection portion of the system more fully described below. A modulator element 12, comprising a quartz rod, is bonded at its center to a rigid support 14 which is part of the housing containing the gradiometer sys-tem. Bonded to a first end of the quartz rod 12 is a first mass dipole 16 comprising, as illustrated, two masses connected by a beam. A second substantially identical mass dipole 18 is connected to the opposite end of the modulator element 12. The dipoles, as illustrated, are posi-tioned at substantially right angles to each other. Also illustrated as part of the basic system is a bar magnet 20 positioned near amplifier tube 2 to insure the production of circular polarization modes. Other known means for generating circular polarization may be used instead.
As stated in the background of the invention section, it is known that a gravity gradient which has a direction neither parallel nor perpendicular to the axis of a mass dipole, such as dipoles 16 or 18, will generate a torque in that dipole. The mass dipoles illustrated in the figure will respond to vertical and horizontal gravity gradients in the plane perpendicular to the axis of element 12 but not to those par-allel to either beam of the two mass dipoles 16 and 18. In the drawing, a vertical gravity gradient which would tend to rotate dipole 16 clock-wise would at the same time try to rotate dipole 18 counterclockwise.
As a result, the torques are symmetrical and the differential modulation of the circular polarization modes of the laser beam are additive. It can be seen, therefore, that it is not essential that both dipoles as
~7~
DESCRIPTION ~F TXE PREFERRED EMBODIMEN~
The drawing illustrates a gravity gradiometer according to the present invention in essentially the same way as illustrated in the abové-referenced U.S. Patent 3,786,681. A laser amplifier tube 2 gener-ates a laser beam indicated by the dotted line 3. The beam 3 travels mainly within the cavity formed by mirrors 4, 6, 8, and 10. Mirror 8 is partially transmissive to allow a portion of the beam to pass to a beat detection portion of the system more fully described below. A modulator element 12, comprising a quartz rod, is bonded at its center to a rigid support 14 which is part of the housing containing the gradiometer sys-tem. Bonded to a first end of the quartz rod 12 is a first mass dipole 16 comprising, as illustrated, two masses connected by a beam. A second substantially identical mass dipole 18 is connected to the opposite end of the modulator element 12. The dipoles, as illustrated, are posi-tioned at substantially right angles to each other. Also illustrated as part of the basic system is a bar magnet 20 positioned near amplifier tube 2 to insure the production of circular polarization modes. Other known means for generating circular polarization may be used instead.
As stated in the background of the invention section, it is known that a gravity gradient which has a direction neither parallel nor perpendicular to the axis of a mass dipole, such as dipoles 16 or 18, will generate a torque in that dipole. The mass dipoles illustrated in the figure will respond to vertical and horizontal gravity gradients in the plane perpendicular to the axis of element 12 but not to those par-allel to either beam of the two mass dipoles 16 and 18. In the drawing, a vertical gravity gradient which would tend to rotate dipole 16 clock-wise would at the same time try to rotate dipole 18 counterclockwise.
As a result, the torques are symmetrical and the differential modulation of the circular polarization modes of the laser beam are additive. It can be seen, therefore, that it is not essential that both dipoles as
3 ~Z~ 7 illustrated be employed. For given masses and element dimensions, half of ~he same signal could be derived by using a single dipole connected to one end of the modulator ~od 12. The dual dipole arrangement is pre-ferred because it provides larger signal and, most importantly, because it eliminates sources of noise such as rotational acceleration of the de~ice about the axis of the laser beam passing through modulator ele-ment 12.
Since the dipoles 16 and 18 are rigidly attached to the quartz rod 12, the only motion which the dipoles experience is that allowed by the elasticity of the quartz rod in response to the torque applied to it. As a result, the settling time of this arrangement is extremely short and the device is very rugged so that it is ideally suited for use in borebole exploratory work.
In use, the torque applied to the dipole or pair of dipoles causes a differential change in the frequency of the circular polari~a-tion modes and a portion of the laser beam 3 passes through mirror 8 to the beat detection portion of the system. In a preferred form, this beat detection portion includes polarizers 21 and 22, mirrors 24 and 26, and a beam splitter 28 for recombining the beams from mirrors 24 and 26.
The recombined beam indicated by numeral 2~ passes to a beat detector 30 for providing an electrical output indicating the frequency difference between the beam directed to it. This electrical output is therefore an indication of the gravity gradient to which the dipoles 16 and 18 have been exposed.
It is noted in the above-referenced U.S. Patent 3,786,681 that this type of force-measurement system has a nonlinear region caused by phase locking near the zéro beat frequency. It can be seen that the gravity gradients experienced by mass dipoles 16 and 18 will be rather small and will generate small torques in the modulator 12 and thus will result in operation nearer the zero beat frequency point. As a result, it is preferred that a biasin~ element be provlded in the laser cavlty for moving the normal operating point away from the zero beat frequency level. This element 32 is a quartz rod similar to the modulator element 12. Rod 32 is pre-torqued and bonded to rigid supports 34 and 36 whlch resist the torque and thereby maintain the element in a constant torque condition. As a result, bias element 32 produces a steadystate frequency difference between the various polarization modes of the laser beam 3.
The output of beat detector 30 is then equal to the signal produced by modulator 12 plus the bias level set by element 32.
To maintain the blas level of elemen~ 32 essentially constant under all environmental conditions, it is preferred that the torque applied to element 32 be supplied by means of a stressing element 38 made of the same material and of essentially identical dimensions. As illustrated in the figure, stressing element 38 is also bonded to supports 34 and 36 so that if one of the supports 34 or 36 is free-floating, the torque in stressing element 38 is equal in magnitude but opposite in sense to that experienced by biasing element 32. Since both elements are made of the identical material and experience the same conditions~ they should respond symmetrica]ly to, for example, temperature changes to maintain a constant torque in both elements.
While amorphous quartz is the preferred material from which elements 12, 32, and 38 are made, lt is clear that other materials may be substituted. As noted in U.S. Patent 3,786,681, other photoelastic materials which exhibit force-responsive birefringent effects may also be used.
For the best precision, it is important to know whether the bias level generated by element 32 changes at all during the taking of gravity gradient readings. This can be accomplished by the further addition of a second ring laser cavity having a laser beam passing through stressing element 38. The second ring laser comprises a second ^~
laser amplifier tube ~0 and the mirrors 4, ~, 8, and 10. The same magnet 20 can be arranged to provide a magnetic field for amplifier tube 40 or other means of insuring circular polarization ~an be provided.
The laser beam 42 generated by amplifier tube 40 passes through stress-ing element 38 but is otherwise unaffected by any modulating element which could differentially vary the frequency of the circular polari~a-tion modes. As a result, the beat frequency induced by stressing ele-ment 38 in beam 42 is a direct indication of the bias level produced in beam 3 by bias element 32. A portion of laser beam 42 passes from the ring laser cavity through mirror 8 to a beat detection section. This section comprises the polarizers 21 and 22, mirrors 24 and 26, beam splitter 281 and a second beat detector 44. The electrical output of bea~ detector 44 is therefore an indication of bias level provided by biasing element 32.
The output of beat detector 44 may be used in several ways.
If the output of beat detector 30 is recorded on a stripchart recorder or possibly on magnetic tape, the output of beat detector 44 may be recorded as a second trace on the stripchart recorder or a second track on magnetic tape. This would allow comparison of the bias level to the signal output so that any ~hanges in the bias level could be taken into consideration in analyzing the signal. Alternatively, the output of beat detector 44 could be subtracted from the output of beat detector 30 to provide a single corrected output which indicates only the desired gradient signal.
Since the reflecting mirrors can-be used for both ring lasers - with very little increase in size, only a small amount of additional equipment is needed to add the monitoring ring laser. It would be pos-sible to pass the beam 42 through biasing element 32 itself without interfering with beam 3. This can be done either by physical separation of the paths of beams 3 and 42 through element 32 or by use of different ~12'~7 frequencies for beams 3 and 42 with separation accomplished by filters.
The arrangement illustrated in the drawing is believed to be more prac-tical and is therefore preferred.
The monitor system is not essential to operation of the laser gravity gradiometer and can be used only for calibration of the system if desired. In addition, the monitor system ma~ be used during qualifi-cation of the device for a particular environment and may be eliminated if the results show that the biasing element 32 provides a stable bias level. It will be preferred in essentially all cases to use a stressing element 38 made of the same ma~erial and of essentially identical dimen-sions to bias element 32 so that the bias level will be as stable as possible.
It is believed that the best way to improve the ruggedness of a gravity gradiometer according to the present invention will be to con-struct most of the elements illustrated in the drawing from or within a solid section o~ quartz. Such an assembly method is illustrated in Fig-ure 3a and 3b of U.S. Patent 3,517,560, issued to Jacobs, et al., on June 30, 1970. Such a construction method reduces the number of gas-to-solid interfaees through which the laser beam must pass.
While the present invention has ~eçn shown and illustrated in terms of specific apparatus, it is apparent that other variations and modifications can be made within the scope of the pres~nt invention as defined by the appended claims.
ACM:el (6)
Since the dipoles 16 and 18 are rigidly attached to the quartz rod 12, the only motion which the dipoles experience is that allowed by the elasticity of the quartz rod in response to the torque applied to it. As a result, the settling time of this arrangement is extremely short and the device is very rugged so that it is ideally suited for use in borebole exploratory work.
In use, the torque applied to the dipole or pair of dipoles causes a differential change in the frequency of the circular polari~a-tion modes and a portion of the laser beam 3 passes through mirror 8 to the beat detection portion of the system. In a preferred form, this beat detection portion includes polarizers 21 and 22, mirrors 24 and 26, and a beam splitter 28 for recombining the beams from mirrors 24 and 26.
The recombined beam indicated by numeral 2~ passes to a beat detector 30 for providing an electrical output indicating the frequency difference between the beam directed to it. This electrical output is therefore an indication of the gravity gradient to which the dipoles 16 and 18 have been exposed.
It is noted in the above-referenced U.S. Patent 3,786,681 that this type of force-measurement system has a nonlinear region caused by phase locking near the zéro beat frequency. It can be seen that the gravity gradients experienced by mass dipoles 16 and 18 will be rather small and will generate small torques in the modulator 12 and thus will result in operation nearer the zero beat frequency point. As a result, it is preferred that a biasin~ element be provlded in the laser cavlty for moving the normal operating point away from the zero beat frequency level. This element 32 is a quartz rod similar to the modulator element 12. Rod 32 is pre-torqued and bonded to rigid supports 34 and 36 whlch resist the torque and thereby maintain the element in a constant torque condition. As a result, bias element 32 produces a steadystate frequency difference between the various polarization modes of the laser beam 3.
The output of beat detector 30 is then equal to the signal produced by modulator 12 plus the bias level set by element 32.
To maintain the blas level of elemen~ 32 essentially constant under all environmental conditions, it is preferred that the torque applied to element 32 be supplied by means of a stressing element 38 made of the same material and of essentially identical dimensions. As illustrated in the figure, stressing element 38 is also bonded to supports 34 and 36 so that if one of the supports 34 or 36 is free-floating, the torque in stressing element 38 is equal in magnitude but opposite in sense to that experienced by biasing element 32. Since both elements are made of the identical material and experience the same conditions~ they should respond symmetrica]ly to, for example, temperature changes to maintain a constant torque in both elements.
While amorphous quartz is the preferred material from which elements 12, 32, and 38 are made, lt is clear that other materials may be substituted. As noted in U.S. Patent 3,786,681, other photoelastic materials which exhibit force-responsive birefringent effects may also be used.
For the best precision, it is important to know whether the bias level generated by element 32 changes at all during the taking of gravity gradient readings. This can be accomplished by the further addition of a second ring laser cavity having a laser beam passing through stressing element 38. The second ring laser comprises a second ^~
laser amplifier tube ~0 and the mirrors 4, ~, 8, and 10. The same magnet 20 can be arranged to provide a magnetic field for amplifier tube 40 or other means of insuring circular polarization ~an be provided.
The laser beam 42 generated by amplifier tube 40 passes through stress-ing element 38 but is otherwise unaffected by any modulating element which could differentially vary the frequency of the circular polari~a-tion modes. As a result, the beat frequency induced by stressing ele-ment 38 in beam 42 is a direct indication of the bias level produced in beam 3 by bias element 32. A portion of laser beam 42 passes from the ring laser cavity through mirror 8 to a beat detection section. This section comprises the polarizers 21 and 22, mirrors 24 and 26, beam splitter 281 and a second beat detector 44. The electrical output of bea~ detector 44 is therefore an indication of bias level provided by biasing element 32.
The output of beat detector 44 may be used in several ways.
If the output of beat detector 30 is recorded on a stripchart recorder or possibly on magnetic tape, the output of beat detector 44 may be recorded as a second trace on the stripchart recorder or a second track on magnetic tape. This would allow comparison of the bias level to the signal output so that any ~hanges in the bias level could be taken into consideration in analyzing the signal. Alternatively, the output of beat detector 44 could be subtracted from the output of beat detector 30 to provide a single corrected output which indicates only the desired gradient signal.
Since the reflecting mirrors can-be used for both ring lasers - with very little increase in size, only a small amount of additional equipment is needed to add the monitoring ring laser. It would be pos-sible to pass the beam 42 through biasing element 32 itself without interfering with beam 3. This can be done either by physical separation of the paths of beams 3 and 42 through element 32 or by use of different ~12'~7 frequencies for beams 3 and 42 with separation accomplished by filters.
The arrangement illustrated in the drawing is believed to be more prac-tical and is therefore preferred.
The monitor system is not essential to operation of the laser gravity gradiometer and can be used only for calibration of the system if desired. In addition, the monitor system ma~ be used during qualifi-cation of the device for a particular environment and may be eliminated if the results show that the biasing element 32 provides a stable bias level. It will be preferred in essentially all cases to use a stressing element 38 made of the same ma~erial and of essentially identical dimen-sions to bias element 32 so that the bias level will be as stable as possible.
It is believed that the best way to improve the ruggedness of a gravity gradiometer according to the present invention will be to con-struct most of the elements illustrated in the drawing from or within a solid section o~ quartz. Such an assembly method is illustrated in Fig-ure 3a and 3b of U.S. Patent 3,517,560, issued to Jacobs, et al., on June 30, 1970. Such a construction method reduces the number of gas-to-solid interfaees through which the laser beam must pass.
While the present invention has ~eçn shown and illustrated in terms of specific apparatus, it is apparent that other variations and modifications can be made within the scope of the pres~nt invention as defined by the appended claims.
ACM:el (6)
Claims (15)
1. A gravity gradiometer comprising:
a laser means for generating a laser beam having a plurality of polarization modes;
a modulator including a photoelastic element posi-tioned in the path of the laser beam for differentially altering the characteristics of the polarization modes in response to the application of a force;
at least one mass dipole secured to the modulator with the axis of the mass dipole perpendicular to the path of the laser beam, whereby a gravity gradient in the plane perpendicular to the laser beam produces a force on the mass dipole which is coupled to the modulator to produce a fre-quency difference between the polarization modes; and means for detecting the frequency difference.
a laser means for generating a laser beam having a plurality of polarization modes;
a modulator including a photoelastic element posi-tioned in the path of the laser beam for differentially altering the characteristics of the polarization modes in response to the application of a force;
at least one mass dipole secured to the modulator with the axis of the mass dipole perpendicular to the path of the laser beam, whereby a gravity gradient in the plane perpendicular to the laser beam produces a force on the mass dipole which is coupled to the modulator to produce a fre-quency difference between the polarization modes; and means for detecting the frequency difference.
2. A gravity gradiometer according to Claim 1 wherein the photoelastic element is fixedly supported intermediate its first and second ends relative to the path of the laser beam for differentially altering the characteristics of the polarization modes in response to the application of a torque to the photoe-lastic element and further including first and second mass dipoles secured to the respective first and second ends of the photoelastic element, the mass dipoles being positioned at sub-stantially right angles to each other, whereby a gravity gradient in a plane perpendicular to the path of the laser beam produces forces on the mass dipoles which are coupled to the modulator to produce a torque on the photoelastic element for differentially altering the characteristics of the polarization modes to cause a frequency difference between the polarization modes.
3. A gravity gradiometer according to Claim 1 or 2 in which the laser means includes a laser tube and at least three reflectors forming a ring laser cavity for the laser tube.
4. A gravity gradiometer according to Claim 1 or 2 in which the laser means includes a gas laser tube.
5. A gravity gradiometer according to Claim 1 further including a biasing element positioned within the path of the laser beam, the biasing element consisting of photoelastic material pre-stressed by the application of a permanent torque wherein the axis of the torque is parallel to the axis of the laser beam, whereby the biasing element causes an essentially constant frequency difference between the polarization modes in addition to that caused by the modulator and causes the gravity gradiometer to operate in a linear response range.
6. A gravity gradiometer according to Claim 5 wherein the biasing element is pre-stressed by being secured to a stressing element consisting of photoelastic material and further including a second laser means for generating a second laser beam having a plurality of polarization modes, the stressing element being positioned so that the second laser beam passes through the stressing element along the axis of the torque applied to the stressing element, whereby the torque produces a frequency dif-ference between the polarization modes of the second laser beam, and also further including second means for detecting the fre-quency difference between the polarization modes of the second laser beam.
7. A gravity gradiometer according to Claim 5 wherein the biasing element is pre-stressed by being secured to a stressing element consisting of the same photoelastic material as the biasing element and having essentially the same dimensions and orientation as the biasing element, whereby the level of stress in the biasing element is constant over a range of envi-ronmental conditions.
8. A gravity gradiometer according to Claim 2 further including a biasing element positioned within the path of the laser beam, the biasing element consisting of photoelastic material pre-stressed by the application of a permanent torque wherein the axis of the torque is parallel to the axis of the laser beam, whereby the biasing element causes an essentially constant frequency difference between the polarization modes in addition to that caused by the modulator and causes the gravity gradiometer to operate in a linear response range.
9. A gravity gradiometer according to Claim 8 wherein the biasing element is pre-stressed by being secured to a stressing element consisting of photoelastic material and further including a second laser means for generating a second laser beam having a plurality of polarization modes, the stressing element being positioned so that the second laser beam passes through the stressing element along the axis of the torque applied to the stressing element, whereby the torque produces a frequency dif-ference between the polarization modes of the second laser beam, and also further including second means for detecting the frequency difference between the polarization modes of the second laser beam.
10. A gravity gradiometer according to Claim 8 wherein the biasing element is pre-stressed by being secured to a stressing element consisting of the same photoelastic material as the biasing element and having essentially the same dimensions and orientation as the biasing element, whereby the level of stress in the biasing element is constant over a range of envi-ronmental conditions.
11. A method for determining gravity gradients com-prising the steps of:
generating a laser beam having a plurality of cir-cular polarization modes;
positioning a modulator comprising a photoelastic element in the path of said laser beam for differentially altering the characteristics of the polarization modes in response to the application of a stress, said modulator being fixedly supported at its center relative to the path of said laser beam;
attaching first and second mass dipoles to first and second ends of said modulator, said dipoles being posi-tioned at substantially right angles to each other, whereby a gravity gradient in a plane perpendicular to the path of said laser beam will produce torques on said mass dipoles which are coupled to said modulator, thus producing a fre-quency difference between said modes; and detecting said frequency difference.
generating a laser beam having a plurality of cir-cular polarization modes;
positioning a modulator comprising a photoelastic element in the path of said laser beam for differentially altering the characteristics of the polarization modes in response to the application of a stress, said modulator being fixedly supported at its center relative to the path of said laser beam;
attaching first and second mass dipoles to first and second ends of said modulator, said dipoles being posi-tioned at substantially right angles to each other, whereby a gravity gradient in a plane perpendicular to the path of said laser beam will produce torques on said mass dipoles which are coupled to said modulator, thus producing a fre-quency difference between said modes; and detecting said frequency difference.
12. The method of Claim 11, including the step of employing at least three reflectors and a laser tube to form a ring laser cavity to generate said laser beam.
13. The method of Claim 12, wherein said laser tube comprises a gas laser tube.
14. The method of Claim 11, including the step of positioning a biasing element within the path of said laser beam, said biasing element comprising photoelastic material pre-stressed by application of a permanent torque wherein the axis of the torque is parallel to the axis of the laser beam, whereby said biasing element causes an essentially constant frequency difference between said polarization modes in addition to that caused by said modulator, thus causing said method to provide a linear response to said gravity gradients.
15. The method of Claim 11, 12, or 14, wherein said method is employed for determining gravity gradients in bore holes.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US91097178A | 1978-05-30 | 1978-05-30 | |
| US910,971 | 1978-05-30 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1127417A true CA1127417A (en) | 1982-07-13 |
Family
ID=25429588
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA328,425A Expired CA1127417A (en) | 1978-05-30 | 1979-05-25 | Dipole mass laser-based gravity gradiometer |
Country Status (7)
| Country | Link |
|---|---|
| JP (1) | JPS54164173A (en) |
| CA (1) | CA1127417A (en) |
| DE (1) | DE2921823A1 (en) |
| FR (1) | FR2427618A1 (en) |
| IT (1) | IT1116874B (en) |
| NL (1) | NL7903808A (en) |
| SE (1) | SE7904268L (en) |
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CN108919370A (en) * | 2018-07-25 | 2018-11-30 | 赛特雷德（重庆）科技有限公司 | A kind of positioning device and method based on gravitation field measurement |
Family Cites Families (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US3517560A (en) * | 1965-03-23 | 1970-06-30 | North American Rockwell | Accelerometer |
| US3786681A (en) * | 1971-03-04 | 1974-01-22 | Sci Systems Inc | Electromagnetic wave modulation and measurement system and method |
-
1979
- 1979-05-15 SE SE7904268A patent/SE7904268L/en not_active Application Discontinuation
- 1979-05-15 NL NL7903808A patent/NL7903808A/en not_active Application Discontinuation
- 1979-05-25 IT IT49185/79A patent/IT1116874B/en active
- 1979-05-25 CA CA328,425A patent/CA1127417A/en not_active Expired
- 1979-05-29 JP JP6670879A patent/JPS54164173A/en active Pending
- 1979-05-29 DE DE19792921823 patent/DE2921823A1/en not_active Withdrawn
- 1979-05-29 FR FR7913679A patent/FR2427618A1/en active Pending
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CN108919370A (en) * | 2018-07-25 | 2018-11-30 | 赛特雷德（重庆）科技有限公司 | A kind of positioning device and method based on gravitation field measurement |
Also Published As
| Publication number | Publication date |
|---|---|
| SE7904268L (en) | 1979-12-01 |
| JPS54164173A (en) | 1979-12-27 |
| IT7949185A0 (en) | 1979-05-25 |
| NL7903808A (en) | 1979-12-04 |
| FR2427618A1 (en) | 1979-12-28 |
| IT1116874B (en) | 1986-02-10 |
| DE2921823A1 (en) | 1979-12-06 |
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