Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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
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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.
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~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,
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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
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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)
