CA2261510A1 - Gravitational survey system and method - Google Patents

Abstract

In the following paper we propose a new and novel "SYSTEM" for making hyperfine airborne gravity-gradiometry measurements. The "SYSTEM" consists of both a "PROCESS", and several possible "DEVICES" for carrying out the proposed process. The method involves making high-resolution measurements of the gravitational redshift of photons while the measuring instrument is put into momentary freefall. The idea is revolutionary because it could theoretically eliminate all of the external accelerations that normally arise in such surveys, including those from the aircraft itself. These external accelerations are completely indistinguishable from a superimposed g-field, and are thus the limiting factors to the survey resolution in current airborne gravity exploration. The method can thus potentially give gravitational gradient resolutions several orders of magnitude better than what has been possible up to now.

Description

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.

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 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.

(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.

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 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 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.

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.

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.

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 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.

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

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.

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.

described above, but encompass all such variations thereof as come within the scope of the appended claims below.

Claims (18)

1. Making gravitational gradient measurements while the measuring device is itself in momentary freefall during the data acquisition process.

2. Computer regulated magnetic levitation of the measuring device (which is itself housed in a vacuum chamber) as a means for initiating and regulating instrument freefall.

3. Using the gravitational redshift of photons (or any other suitable entity that has measurable quantities which change in a gravitational field) in a freefall device in order to infer the local gravitational gradient.

4. A method of inferring the local gravitational gradient comprising the steps of:
(g) 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.
(h) Momentarily suspending the instrument on magnetic cushions or other suitable means, the suspension itself being regulated by an onboard computer.
(i) Momentarily putting the instrument into freefall by computer regulation of the instrument suspension.
(j) Generating a source beam while in freefall, and monitoring the beam frequency (or other suitable quantities).
(k) Allowing the beam to travel a height h through the earth's (or other body's) gravitational field while the instrument is in freefall.
(l) Comparing a quantity (for example v) of the source beam, to its value at the detector.

5. A method as claimed in (4) whereby the change in v is determined through:
(v) Converting the beam which has traveled through the g-field into a flux current.
(vi) Amplifying the flux current.
(vii) Passing the amplified flux current through a coil loop, in turn inducing an EMF in the loop.
(viii) Measuring the induced EMF and using this to calculate .DELTA.v.

6. A method as claimed in (4) whereby the change in v is determined through:
(vi) Converting two originally identical energy beams into two separate flux currents after one beam has traveled a height h through the g-field.
(vii) Amplifying the two respective flux currents.
(viii) 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.
(ix) Measuring the quantum current induced in the loop in order to calculate this net EMF.
(x) Using the calculated EMF to infer .DELTA.v.

7. A method based on the photoelectric effect for generating opposing flux currents through a double coil loop as described within.

8. A method for determining changes in v (or other suitable quantities) from a source, by converting an energy beam into a measurable current or EMF at a spatially separated detector and using this measured EMF in order to calculate .DELTA.v.

9. A method for inferring the local gravitational gradient by measuring this change in v (or other suitable quantity) as described in 8.

10. A method for inferring local gravitational gradients by measuring slightly different flux currents through a double coil loop; the difference in flux owing to the fact that one energy beam did work in rising through the gravitational field.

11. A superconducting double coil loop mechanism that responds to opposing flux currents in each respective loop by reaching a momentarily measurable quantum steady state.

12. A housing for such types of devices as described above, with gyros for inertial stabilization, and which is encased on the inner surface of a spherical cavity, the spherical cavity being free to roll, twist, and turn around the device while the device maintains an almost fixed direction in space.

13. A method for maintaining the device in an almost fixed direction in space by securing it on low-friction rollers that make contact with the inner surface of a spherical cavity, and stabilizing the device with high-inertia gyros.

14. A method for momentarily suspending the device as a precursor to initiating freefall by using computer regulated magnetic fields or other suitable equivalent means.

15. A machine for carrying out the detailed procedure described in claims 4 and 5 above and having the essential features described therein.

16 16. A machine for carrying out the detailed procedure described in claims 4 and 6 above and having the essential features described therein.

17. A machine as in claim 15, capable of carrying out the detailed procedure as described in claims 4 and 5, along multiple independent axes.

18. A machine as in claim 16, capable of carrying out the detailed procedure as described in claims 4 and 6, along multiple independent axes.