Note: Descriptions are shown in the official language in which they were submitted.
CA 02261510 1999-02-11
BACKGROUND OF THE INVENTION
FIELD OF THE INVENTION
This invention relates to a gravitational survey system and method, and in
particular, to a
system and method for making hyperfine airborne gravity-gradiometry
measurements, as well as
apparatus for conducting the same.
DESCRIPTION OF THE PRIOR ART
Gravitational measuring devices have been investigated for over 30 years now.
Instrument resolution has increased over this period of time from just a few
milligal in the earliest
instruments, to a nanogal or even better in present devices. These devices
themselves are
important in a wide range of fields, but they are of particular importance to
the oil industry where
gravitational anomalies can be indicative of oil deposits beneath the surface
of the earth.
Unfortunately, making ground-based gravitational measurements has proved to be
impractical to
the oil industry because of the extreme length of time it takes to accumulate
just a small amount
of data.
CA 02261510 1999-02-11
Typically, for example, a ground-based gravimeter must be set up and allowed
to
stabilize itself for half an-hour or so. A measurement is then made, and the
instrument is then
moved to a new location where it must be allowed to stabilize itself once
again. In a twelve-hour
survey day therefore, and with data points spaced at, say 250 metre intervals,
one might expect to
accumulate 24 data points covering 6 line kilometres in total. This is
completely inadequate
considering that a typical oil survey must cover 20,000 line kilometres or
more.
One might consider spacing the data points at greater distances with respect
to each other,
but this automatically sacrifices the resolution of the survey, which is based
on both the
sensitivity of the instrument as well as the data sample spacing interval.
In order to circumvent this problem, attempts have been made at housing
gravity
measuring-instruments in aircraft which can then be used to fly over
predetermined survey lines
in a relatively short time. The instruments are set up to take measurements at
regular intervals,
say once every second, over the duration of the flight. The sample interval
corresponds to a data
spacing of between 20 metres and 40 metres on the ground, depending on the
actual speed of the
aircraft.
Just like in ground surveys, inherent gravimeter stability is a problem when
using typical
instruments in airborne setups, but an even greater problem is the current
inability to accurately
account for the accelerations of the instruments themselves at each and every
point of the survey.
These accelerations are a direct consequence of the continuously changing
motion of the aircraft,
whether it be in speed or in direction, and they are indistinguishable from
gravitational
accelerations, thus superimposing themselves on the data acquired. To get a
feel for just how
precisely one must account for the motion of the aircraft, consider the
following example.
Suppose that the speed of the aircraft changed slowly and smoothly over a span
of part of
the survey, say by 3km/h over a duration of one hour. Furthermore, suppose
that the aircraft was
so stable that it never wavered by even a centimetre from a linear flight
direction. Even in this
extremely unrealistic example, the acceleration superimposed on the gravimeter
would amount to
almost 10 miligal. If we could not account for this acceleration, then the
survey resolution would
be limited by this amount, a full seven orders of magnitude less than the
capabilities of the
instrument. Consider now, that in a real survey the speed of the aircraft can
change by as much as
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several km/h in just a few seconds! And furthermore, this change in speed is
virtually never in a
"smooth" way as in the idealized scenario above.
Also, because of air currents and other effects, the airplane can never
maintain an
unwavering direction at every moment as was assumed above. This motion too,
adds additional
unwanted accelerations into the data. The aircraft is, in fact, constantly
rising, falling, pitching,
rolling, twisting, and turning. These effects may seem small to a passenger
onboard the aircraft,
but in terms of survey data, these effects are absolutely huge. In fact, if we
weren't able to
account for any of the changes in speed and direction that typically occur in
a flight, then the
resolution of the data would be billions of times less then the resolution of
the instruments!
To circumvent this problem, a number of techniques have been employed to try
and
"account" for the external accelerations that typically occur. They range
from, amongst other
things, inertial stabilization of the instruments, to the use of multiple GPS
receivers mounted all
over the aircraft. However, even using a combination of all the current
techniques available, the
best airborne gravity surveys are still only able to produce data to within a
resolution of about one
miligal, and the situation is even worse for gravity gradiometry. This is
completely inadequate for
the oil industry. A new method must be found to account for the external
accelerations. What we
propose here is a method which will actually eliminate them completely!
SUMMARY OF THE INVENTION
It is an object of the invention to provide a system and method for conducting
airborne
gravity and gravity-gradiometry surveys which reduce or eliminate completely
the effects of the
aircraft motion on the data obtained, leading to data of increased resolution
and reliability.
The invention, in a first aspect, comprises a method of inferring the local
gravitational
gradient comprising the steps of:
(a) Stabilizing a measuring instrument in a spherical cavity, the spherical
cavity being free to
move around the instrument while the instrument stays essentially fixed along
a given
axis.
(b) Momentarily suspending the instrument on magnetic cushions or other
suitable means,
the suspension itself being regulated by an onboard computer.
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(c) Momentarily putting the instrument into freefall by computer regulation of
the instrument
suspension.
(d) Generating a source beam while in freefall, and monitoring the beam
frequency (or other
suitable quantities).
(e) Allowing the beam to travel a height h through the earth's (or other
body's) gravitational
field while the instrument is in freefall.
(f) Comparing a quantity (for example v) of the source beam, to its value at
the detector.
Preferably, the change in v is determined through:
(i) Converting the beam which has traveled through the g-field into a flux
current.
(ii) Amplifying the flux current.
(iii) Passing the amplified flux current through a coil loop, in turn inducing
an
EMF in the loop.
(iv) Measuring the induced EMF and using this to calculate Ov.
Alternatively, the change in v is determined through:
(i) Converting two originally identical energy beams into two separate flux
currents after one beam has traveled a height h through the g-field.
(ii) Amplifying the two respective flux currents.
(iii) Passing the amplified flux currents through adjacent coils of a
superconducting double coil loop in order to induce opposing (non-equal)
voltages and hence a net EMF within the loop.
(iv) Measuring the quantum current induced in the loop in order to calculate
this net EMF.
(v) Using the calculated EMF to infer Ov.
The invention, in a second aspect comprises a device for making hype~ne
airborne
gravity-gradiometry measurements.
The invention, in a third aspect comprises a system for making hyperfine
airborne
gravity-gradiometry measurements.
Further features of the invention will be described or will become apparent in
the course
of the detailed description below.
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BRIEF DESCRIPTION OF THE DRAWINGS
In order that both the method, and the device for carrying out the proposed
method, may be
more clearly understood, the preferred embodiment thereof will now be
described in detail by
way of example, with reference to the accompanying drawings, in which:
Diagram I is a perspective view of the housing source and detector assemblies
of
a preferred embodiment of the present invention;
Diagram 2 is a planar view of the preferred embodiment;
Diagram 3 is a perspective view of the housing source and detector assemblies
of
an alternative embodiment; and
Diagram 4 is an elevation view of the detector assembly of the alternative
embodiment.
DETAILED DESCRIPTION
The Method
NEWTON'S LAW OF GRAVITY VS. GENERAL RELATIVITY
Newton's law of gravity is the usual starting point for investigations into
gravitational
surveying methods and devices. It is so accurate a theory that we can use it
to land a man on the
moon to within a few hundred metres of a given target. But Newton's law is not
the correct
theory of gravity, general relativity is.
Although in many cases Newton's law of gravity and general relativity predict
the same
quantity to a degree which can be said to differ by only an insignificant
amount, it cannot be said
that this is always the case or for that matter, that the two theories predict
all the same
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phenomena. Indeed, general relativity predicts certain phenomena which are not
addressed in the
Newtonian theory at all.
One important prediction of general relativity is that of the gravitational
redshift. Briefly
stated, general relativity predicts that as a photon of energy E=by rises
through the curved space-
time of a "gravitational field", it will lose energy as it struggles against
the field. This lose of
energy will be reflected by a shift in the photon's frequency vw' to the red
end of the
electromagnetic spectrum. Thus, by knowing the frequency of photons emitted
from a source, and
measuring the frequency of the same photons after they have risen through the
gravitational field
by a height h, we can infer the gravitational field in that localized region.
This fact will be utilized
in the process, as we shall see shortly.
Another key aspect of the general theory of relativity comes through what is
known as
the principle of equivalence. Briefly stated, this principle asserts that an
object in freefall in a
uniform gravitational field will behave completely equivalently to an object
free of all external
forces such as those that arise in airborne gravity surveys from the motion of
the aircraft. We can
utilize this aspect as follows. If we put our on-board instrumentation into an
aircraft in such a
manner that will allow it to go into momentary freefall, say for 1/1000000 of
a second or so,
(which is a much longer time period than it will take for the photon to rise
say 1.5 metres to a
detector), all external forces, and in particular, the acceleration of the
aircraft during the freefall
time, will be absent. We can thus make a direct measurement of the photon
energy (or the
frequency) at the detector, confident that the effects of the aircraft have
been eliminated during
the measurement process. The time period of freefall may seem small, but in
fact the principle of
equivalence does not depend on how long an object has been in freefall, only
that it is in fact in
freefall.
Now, it is a fact that in the short amount of time that it takes for the
photons to rise from
the source to the detector, the detector will have accelerated in freefall
very slightly relative to its
original speed when the photons were emitted by the source. In a uniform
gravitational field, the
relative velocity acquired by the detector would create a Doppler shift that
would exactly
compensate for the redshift of the g-field, (again, by the principle of
equivalence). However,
since the earth's field is a non-uniform one, this exact compensation will not
occur. Irregularities
in the earth's density distribution cause gravitational gradients, and it is
in fact these irregularities
which are of primary interest to the oil industry. In fact, even if the earth
was completely
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homogeneous and isotropic, the structure of the field is such that there would
still be a
gravitational gradient. Any non-uniformities in the density of the earth only
add to this effect, and
any difference in the photon's frequency at the detector relative to the
frequency at the source will
be wholly and completely due to this gradient in the g-field.
Thus, the method proposed here is to make sensitive measurements of the change
in
energy (or shift in frequency) of photons rising through the earth's
gravitational gradient while
the measuring instrument itself is put into momentary freefall. A more
detailed description of
how that is to be done accompanies the description of the instrument proposed
to carry out this
process below.
The Device
The Source
Diagram 1 shows a device for carrying out the airborne gravity gradiometry
measurements,
according to the present invention. A source laser 1 for generating photons of
a well-defined
frequency vs as measured by a "momentarily co-moving reference frame" is
mounted onto the
housing (described in more detail below). vs is an intrinsic property of the
laser itself and for each
different laser is a well-defined and easily measurable quantity. The choice
of laser (and therefore
frequency) is somewhat arbitrary, but a laser with a low frequency and a high
power output is
more desirable than one with a high frequency and low power output because the
former
intrinsically emits more photons per unit time than the latter, and therefore
is less vulnerable to
statistically errors in the number of photons received at the detector. ~
An electromagnetic (e.m.) filter 2, preferably having an extremely small
bandwidth, is
mounted just above the source laser 1. Although the laser emits e.m. radiation
at a well-defined
frequency, this filter makes sure that only e.m. radiation at the desired
frequency actually makes it
through to the detector, and for example, harmonics are blocked. Or if the
frequency at the source
were somehow altered, that no photons would be emitted into the detector.
' Although the words "laser" or "laser light" shall be used throughout the
above description, it should not
be implied that the device is necessarily emitting photons in the visible
spectrum.
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A beam sputter 3 mounted above the e.m. filter 2 partially reflects the light
passing through
the e.m. filter at 2 to a conventional high-resolution frequency monitor 4.
The remaining light is
transmitted to the detector at the top of the device. Monitoring both the
source beam-frequency
and the power output will allow us to ensure that the source frequency and
power do not change
throughout the survey, and that the laser is therefore functioning correctly.
Having both the beam
splitter and frequency monitor close to the source is desirable so that
gradient effects on the
photons here will be negligible. The opposite will of course be desired over
the length from the
source to the detector. Here we want to maximize the distance in order to
enhance the gradient
effects on the photons in the z-direction.
The Detector
Many possible types of detectors are conceivable for measuring or inferring
the
frequency of the photons at the top of the detector. One obvious possibility
is to use a
conventional frequency monitor identical to the one we use in measuring the
source frequency at
the bottom of the instrument. Unlike the source frequency however, which we
expect to be
constant, the frequency at the detector will be constantly changing as the
aircraft flies over the
earth. Any frequency monitor at the detector would therefore have to be
endowed with the ability
to sweep a broad range of frequencies in a very short time (i.e. in the time
of instrument freefall
which is about 0.000001 seconds). This may prove to be impractical, or at the
very least, a
significant engineering problem. Other possibilities may be more fruitful and
we discuss a few of
these below. The first is based on an application of the photoelectric effect,
and the second is
based on a modified design for a quantum interference device. We will discuss
these separately in
what follows, and we shall also mention other possibilities at the end of our
discussion.
Referring again to Diagram I, a photoelectric surface 5 is mounted at the top
of the
device. Photons from the source, and which have risen a height h through the
gravitational field,
will hit this surface and eject electrons whose kinetic energy will be related
to the incident photon
frequency through the relationship
Ek=~+hv~
Here ~ represents the work function for the photoelectric surface.
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Note that the frequency v~ of the photons, which are incident on the
photoelectric surface, is
not the same as the frequency v5 at the source, having changed because the
photons have lost
energy in rising through the gravitational field.
A pre-amp and multiplier combination 6 is positioned above the photoelectric
surface 5 and is
preferably configured to cleanly amplify the signal from the electrons ejected
at the photosurface
5. Both the pre-amp and the multiplier are housed in a Faraday cage (not
shown) to eliminate the
possibility of outside signal noise.
A current loop 7 mounted directly above the pre-amp and multiplier combination
at 6 is
directly coupled to a high-resolution voltage-measuring device at 8. Electrons
leaving the pre-
amp and multiplier combination at 6 pass almost immediately through this
current loop and
induce an EMF in the loop. The voltage measured at 8 will be directly related
to the flux current
passing through the surface area of the loop, and for a given number of
electrons will be directly
proportional to their speed, (J=nv). The speed of these electrons will
obviously be related to their
kinetic energy Ek, and this quantity in turn is related to the frequency v~ of
the electromagnetic
radiation hitting the photoelectric surface as shown by the equation above.
Since for a given
photoelectric surface, the number of electrons released per unit time depends
only on the number
of photons hitting the surface per unit time and not on their energy, any
differences in the energy
of the photons at the detector would show up as differences in the kinetic
energy of the electrons
released, but not their number. This is because the cross section for such
events is an intrinsic
property of the surface. Thus, if we use a source laser with a fixed power
output, the different
currents passing through the current loop at the detector will be caused
solely from the different
electron velocities. This is in turn due to their different energies, and this
is in turn due to the
different frequencies v~ of the e.m. radiation at the detector. This is why we
want to monitor the
power output and frequency at the source. If they are kept constant, then
changes in the voltage at
8 can be attributed to the frequency shifts in the e.m. radiation as the
gravitational field of the
earth changes from region to region.z
Note that we can amplify the apparent current flux at 7 and hence the induced
EMF at 8 by
making the current loop at 7 consist of multiple windings instead of, for
example, just one
winding. This is because the induced EMF is directly proportional to the
surface area to which the
Z Even if the source power cannot be kept exactly constant, by monitoring the
power output at the source
we could apply a suitable compensation factor to the data in the post-flight
processing of it later.
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electrons pass through. Having 100 windings for example, increases this
surface area 100 fold,
and hence voltages at 8 will be 100 times as great. Immersing the current loop
in liquid nitrogen
or using a super conducting material is also preferable in order to reduce
system noise and give
extremely high-resolution measurements at 8.
It will be understood by those skilled in the art that a wide variety of other
detectors are
conceivable for measuring or inferring the frequency of the photons at the top
of the detector, and
such variations are within the scope of this invention.
The Housing
The frame and housing of the instrument are important parts in its successful
implementation.
As shown in Diagram 1, the source and detector assemblies are mounted on a
base 9 consisting of
a massive weight. The majority of the weight according to the present
invention is located
directly at the source. Preferably, the base consists of at least 95% of the
total weight of the
device. The mass distribution is important because the machine, which is
assumed to be a rigid
structure, will accelerate when put into freefall with a net acceleration that
is due to the
gravitational force acting on each mass element. Having almost all the mass
right at the source
ensures that the whole instrument (including the detector at the top) will
accelerate at a rate given
bY gs°°«e. In the time it takes photons to reach the detector at
the top (t ~ h/c), the detector will
have increased in velocity by Ov = gso"r~e (Wc). Any frequency change not
compensated by the
Doppler shift here will then be solely the result of the change in g from the
source to the detector
(i.e. the gravitational gradient) and will show up as a voltage at 8.
The device is mounted with a plurality of slots l0a-lOh defined within a
preferentially rigid
rectangular casing 12 (described in more detail below). The main rigid frame
of the apparatus fits
into the slots. Preferably, there are 8 slots in total, 4 at the top and 4 at
the bottom, and each slot
has a slightly bigger radius than the support rods 21 which fit into them.
Within each slot, and
also on the end of each support rod, are current loops wired to a main current
generator (not
shown) which is itself regulated by an onboard computer 22.
When the current is on, opposing magnetic fields are generated between the rod
loops and
each of the 8 rod slots, which act to put the instrument into momentary
magnetic suspension.
Internal sensors within each of the 8 slots confirm for the computer when
there is no physical
CA 02261510 1999-02-11
contact between the rods and slots during these times of magnetic suspension,
and the computer
can adjust the current to each slot (and hence the magnetic field there) as
may be necessary.
When all 8 slots show nil contact, the current can be momentarily shut off by
the computer thus
terminating the magnetic field within each slot. At this point the machine
will go into momentary
freefall. This time of current shut-off need only be of the order of 0.000001
seconds, which is
more than enough time to account for the current decay in the loops, and then
the subsequent time
for photons to rise from the source laser to the detector at the top.
Preferably, the laser is
constantly and continuously "on" and emitting photons, but measurements are
only made at
discrete times, say every 0.1 seconds, and when the computer has determined
that the machine is
truly in freefall. Note also, that in the extremely short time it takes to
make a single measurement
the instrument itself will only fall about 5x10-'Z metres. Even a million such
measurements would
only give a cumulative freefall distance of about S microns. Also, since the
acquisition time is so
short, the source can be thought of as at a fixed location in space for each
acquired data point.3 A
GPS receiver 11 at the source will allow one to identify this location over
the earth at the time of
measurement. The measured change in photon energy is then a reflection of the
change in g in the
z-direction at the point in space where the source has been localized.4
The apparatus is encased in a preferably rigid rectangular casing 12 (partly
shown in Diagram
1), and a vacuum pump 13 partially inset into one of the walls ensures that
the inside of the
container is free of air, dust, and all other foreign materials.
Referring to Diagram 2, the casing 12 is mounted into a spherical cavity 14,
and eight low-
friction rollers 15a-15h on the corners of the casing make contact with the
inner surface of the
cavity allowing the sphere to rotate around the casing about any axis. The
spherical cavity is
preferably secured within the frame of the aircraft conducting the survey
using the riveting bolts
at 16a-16d, and high-inertia gyros 17a-17f attached to the outer casing allow
it to maintain an
almost fixed direction in space as the aircraft and spherical cavity rotate
around it. In addition, the
spherical shell is preferably made from a highly conductive material.
Accordingly, the spherical
shell acts like a Faraday cage; and so long as the low friction rollers are
made from low
3 In the acquisition time of 0.000001 seconds, an aircraft travelling at
typical survey speeds of 100 km/h
will only move about 3 /100 of a millimetre! The acquisition time of a single
measurement must be
distinguished here from the data-sampling period, which, if chosen to be say,
every tenth of a second,
would correspond to a data spacing of approximately every 3 metres on the
ground.
4 A 2-dimensional lattice of such measurements as is typically made in
standard geophysical surveys, will
allow one to model the earth beneath, and in particular, will allow one to
identify anomalous regions of
high or low density where oil deposits or other materials of interest may be
found.
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conductivity materials, all external electric and magnetic fields will be
eliminated without
affecting the internal circuitry of the device itself.
It will be appreciated that the above description related to the preferred
embodiment by way
of example only. Many variations of the invention will be obvious to those
knowledgeable in the
field, and such variations are within the scope of the invention as described
and claimed, whether
or not expressly described.
For example, an alternative embodiment of the invention described in detail
below has many
of the essential features of the preferred embodiment discussed above, but
differs in how
frequency changes can be inferred at the top of the device. It is based on an
application of ideas in
superconducting quantum interference, and it represents a new design with
modifications specific
to this type of survey and its needs. The alternative embodiment is shown in
Diagram 3 where
like parts are given like numbers, but only those components which are
different than those in the
preferred embodiment are specifically described.
An Alternative Device
A second laser 14 identical in every way to that described above is mounted
transversely at
the top of the device. In particular, the frequency of e.m. radiation it emits
in a momentarily
comoving reference frame (which is an intrinsic property of the laser itself)
is exactly the same as
that for the source laser 1 described above. A second small bandwidth filter
15, similar to the first
bandwidth filter 2, ensures that harmonics are blocked as described
previously. A frequency
monitor and power source regulator at 16 are coupled to the on-board system
computer which
monitors both lasers and ensures that they are both functioning at the same
power output and
intrinsic frequency. The computer can adjust the power output to either if
necessary. Both e.m.
energy beams are then incident on identical photoelectric surfaces, the
difference being that the
source photons from laser 1 have traveled a vertical height h through the
earth's gravitational
field and have therefore lost energy by the time they hit their corresponding
photosurface at 17a,
while the photons from the second laser at 14 impact the second photosurface
17b without having
been redshifted. Ejected electrons are then multiplied at the first and second
pre-amp/multiplier
systems 18a and 18b respectively, and pass through the respective cross-
sections A and B of the
coupled double-coil-loop 19, as shown in Diagram 4. The number of electrons
passing through A
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will of course be identical to the number passing in the opposite direction
through B.5 If the
electron velocities were the same then the induced EMF in the double-coil-loop
would be zero.
However, since the electrons at A will have a lower energy and hence lower
velocity than those at
B due to the fact that the incident photons that ejected them were redshifted,
there will be a net
EMF in the coil loop at 19. If the coil loop is made superconducting, then
over the time scale of
measurement (which is much longer than the time scale for quantum
stabilization in the loop
itself) the current induced will reach a temporarily measurable steady state.
This quasi-steady
state is described most adequately by the laws of quantum mechanics. In
particular, the induced
current in the loop behaves in all ways like a wave, which can in fact
interfere with itself unless it
wraps itself an integral number of times around the loop. As such, the wave
can only take on
discrete values of 7~.". Any time-varying change in the flux current through A
will act like a time-
varying external field-perturbation and cause transitions between the possible
quantum states of
the system. Although these different possible states are quantized and
therefore discrete,
- differences between successive states are so small that for all practical
purposes they form a
continuum allowing for highly precise measurements to be made 6
Extensions of the Idea
It should be clear from the discussions above that if the axis of the
instrument were aligned
along the direction of the field, then gZ would be the only non-zero component
of the field itself,'
and hence any analysis of the vector field g could therefore be reduced to an
analysis of the scalar
function gZ(x,y,z) alone.
Thus far, we have talked only about a practical method for measuring the
quantity dgZ/dz.
This quantity is important in and of itself, and is in fact sufFcient for
identifying buried
anomalous mass distributions or deficiencies. However, it should be clear from
the foregoing
discussions, that by aligning similar source-detector configurations along
other axes, we could
measure other components relating to changes in g. Suitably measured, a
knowledge of all
5 Again, because the cross section for such events is an intrinsic property of
the photoelectric surface itself.
The number of electrons ejected therefore, depends only on the surface and the
number of incident photons
impinging upon it, not upon their energy (provided of course that the energy
of these photons is greater
than the work function for the particular surface in question).
6 Also, just as in the previous example, having each loop of the double-coil
consist of multiple windings is
desirable since it will inherently increase the sensitivity of the device and
therefore the precision of the
measurements made.
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CA 02261510 1999-02-11
components of the gradient of gZ (namely, dgZ/dx, dgZ/dy, dgZ/dz) would allow
for a numerical
integration of the ~gZ data in order to produce the function gZ(x,y,z), which
in some cases may be
more desirable than having a knowledge of dgZ/dz alone. Thus, a logical
extension to the ideas
presented so far would be to have multiple detectors set up along appropriate
axes and then to
make simultaneous measurements of the redshifts at each of the detectors while
the instrument
itself is put into momentary freefall.
The Scope Of This Patent Application
While in the foregoing description of the "PROCESS", proposals have been made
for two
different "DEVICES" as means of measuring the change in frequency of e.m.
radiation in the
presence of the earth and hence inferring its local gravitational gradient, it
will of course be
appreciated that other means could also be utilized and substituted for the
specific ones proposed
within. For example, having a source beam rise through a height h of the g-
field and pass through
a diffraction grating, then recording the diffraction pattern; or having two
beams interfere after
one of them has redshifted in rising a height h and then using an
interferometer to determine Ov,
are just two other possibilities. It is therefore not to be implied that the
invention, which
encompasses both a "PROCESS" and a "DEVICE" for carrying out the said process,
is limited to
only the specific elements described above. Thus, for example, whenever used
herein, "source",
"detector", and other like language are intended to incorporate all such
suitable equivalent means
of generating a measurable quantity at a source, which, when measured at a
spatially separated
point, is changed by the gravitational gradient of the earth or other body,
the measurement
process having taken place while all or part of the apparatus is in momentary
or perpetual freefall.
Thus, for example, source photons of frequency vs could be replaced by a
source particle beam of
energy ES.g
The foregoing then, is a description only of the preferred embodiment of the
"DEVICE" for
carrying out the proposed "PROCESS" and is given here by way of example only.
Both the
"PROCESS" and the "DEVICE(S)" for carrying out the process are separate claims
as described
below, and the proposed devices are not to be taken as limited to any of the
specific features as
Since g = (gX~ gr~ gZ) ~ (0~ O~gZ )
8 Also, although in the foregoing discussion we have been referring to both
the "PROCESS" and the
"DEVICES" in the context of airborne surveys alone, this has been for
illustrative purposes only, and it will
be recognised by those skilled in the art that all of the ideas presented are
equally applicable to shipborne
surveys, satellite surveys, and static ground-based surveys alike.
14
CA 02261510 1999-02-11
described above, but encompass all such variations thereof as come within the
scope of the
appended claims below.
