This invention relates to (1) improvements of conventional air-borne gravity survey systems, and (2) development of a novel type of air-borne gravity survey system, namely a "differential gravity" system, for the survey of earth formations along the flight path of an air-borne vehicle. Background of the Invention

Conventional air-borne gravity survey systems use a modified ship-borne gravity meter (which includes a gravity meter installed on a gimballed stabilized platform) as well as an independent non-inertial positioning reference, like a satellite-based

Differential Global Positioning System (ΔGPS) often in combination with a barometric altimeter, installed on¬ board an aircraft or a helicopter. An example of such installations is that developed by Carson Helicopters of Perkasie, Penn. as disclosed in U.S. patent No.

4 , 435 , 981 .

Any linear accelero eter, including a gravity meter, when installed on a moving platform, such as an aircraft, helicopter, airship, etc. will record the sum of platform motion acceleration, which is the noise, and gravity acceleration, which is the signal. Motion acceleration noise is typically four-to-five orders of magnitude stronger than the gravity signal. Both spectra strongly overlap. Hence platform motion accelerations must be determined by non-inertial means, with the help of an independent vertical reference level.

For ship-borne gravity, this vertical reference level is sea level; for off-shore air-borne gravity, it is derived from a radar or laser altitude above sea level. Above land mass this reference can be provided by an extremely accurate electronic navigation system, like a Differential Global Positioning System (ΔGPS) , often employed in combination with a high-resolution barometric altimeter.

Double differentiation of the vertical position of the aircraft with time will yield vertical (Z) motion acceleration, which, after subtraction from the total inertial (Z) acceleration, will yield gravity acceleration.

The main problem of conventional systems, however, is to obtain a resolution of at least one milligal with

at most one minute time constant, such parameters being desirable for petroleum exploration. One milligal is equivalent to the very small acceleration of 10 ^"3 cm/sec ² . A common way around this problem is to increase the time constant to, for example 60 seconds

(at the expense of spatial resolution) . This relaxes the vertical reference resolution requirement to 60 ² = 3600 x IO ^"3 cm = 3.6 cm per milligal per minute. The latest differential GPS data are beginning to approach this figure. A one minute time constant, however, represents an approximately 3.3 km spatial resolution at slow aircraft surveying speed; or half of that if a more expensive, slower flying helicopter is used. This is already acceptable for petroleum surveys but too coarse for mining and groundwater surveys.

Aircraft or helicopters are relatively noisy platforms for on-board gravity installations. In addition to vibration and other noise, in-flight vertical accelerations in survey regime typically reach .2G, which is IO ⁴ - 10 ^s higher than gravity acceleration changes.

Moreover, a one milligal gravity anomaly can be caused by a 10 m topographic high ridge (or valley) . Hence topographic contours for a one milligal survey should be known to better than ± 5m. The standard procedure subtracts radar altitude from ΔGPS or Baro-

altimeter elevation to yield the terrain profile. In forested areas, this value is degraded by unknown tree heights, which may introduce errors in excess of one milligal. Statement of the Invention

It is one object of the present invention to provide an air-borne gravity surveying system which overcomes the above drawbacks.

The air-borne gravity surveying system in accordance with the present invention comprises an acceleration/gravity sensor assembly (AGSA) which includes at least one inertial measurement unit (IMU) (such as manufactured by Honeywell Inc., Clearwater, Florida) containing accelerometers and gyroscopes capable of providing all three components of total inertial linear as well as angular accelerations, and a differential global positioning system (ΔGPS) to provide location of the aircraft for derivation of all three components of linear motion acceleration which, after substraction from the total linear inertial acceleration, will yield gravity acceleration.

The IMU unit replaces the gravity meter on a stabilized platform as used in conventional systems. The IMU consists of one or more vertical (Z) accelerometers and at least one pair of orthogonal (XY) horizontal accelerometers combined with at least two

gyroscopes for attitude control.

The IMU provides, in addition to the Z component of linear acceleration, the X and Y components and tilts relative to the inertial frame of reference. The availability of X and Y components of acceleration is useful to correct for the coupling of horizontal accelerations into the vertical accelerometer channel when this channel is not perfectly aligned with the vertical direction. The IMU can also supply magnitude of the total linear acceleration vector from which the magnitude of the total linear motion acceleration vector (as measured by ΔGPS) can be subtracted to yield the gravity vector magnitude (if gravity and motion acceleration vectors are nearly parallel, which is mostly the case) .

The IMU may be installed on-board of an aircraft, or in a towed body ("bird") suspended from an aircraft where it is subjected to lower in-flight accelerations. The IMU can be either strapped down or, if lower noise is required, installed on an active isolation table, controlled by feedback from such IMU, capable of operation in the Shuler-tuned mode.

The "bird" may be stabilized with active control surfaces, such as wings and a drag tail. The aircraft may also be stabilized by towing a dummy "bird" .

If necessary, terrain contour data for gravity terrain corrections may be acquired with sufficient precision (<<5m) even in forested area, using, for example a laser ranger or scanner. In order to suppress chaotic noise, two or more

IMU's either strapped-down, or installed on one or more platforms, can be used in an aircraft installation.

It is another object of the present invention to provide a novel "differential gravity" system. The "differential gravity" system uses two acceleration/gravity sensor assemblies (AGSA) , each preferably containing at least one IMU as disclosed above, each sensor assembly containing accelerometers and gyroscopes capable of acquiring at least the vertical but preferably all three components of total inertial linear as well as angular accelerations. Both AGSA's can be installed in separate towed bodies suspended on a cable from an aircraft at a predetermined distance from each other, or one AGSA can be mounted on- board an aircraft and the other suspended on a cable from the aircraft, or mounted on-board or towed behind and below separate aircraft flying either one above the other or in tandem at a predetermined vertical or horizontal distance, respectively. The differential gravity surveying system further comprises means for constantly monitoring vertical and horizontal distances

between the two sensor assemblies (preferably using laser ranging device and/or differential GPS) and means for subtracting the respective vertical and, optionally, horizontal components of motion acceleration and gravity of the two sensor assemblies and dividing the difference by the respective distance between the sensor assemblies to provide normalized gravity difference along the respective directions.

The advantage of the "Differential Gravity" over present "Gravity" systems is that the difference in gravity sensor positions can be measured to at least one order better resolution than the absolute position of a single gravity sensor using differential GPS alone. Since positioning error is the dominant source of error in air-borne gravity, improvement in positioning accuracy directly translates directly into improved resolution of gravity information. Furthermore, the system can operate with a ranging device only, and thus independently of GPS, should GPS become unusable for this purpose.

The vertical and/or horizontal distance between the two AGSA's may be measured by a laser or microwave range measuring device, which consists of a transmitter at one sensor assembly and a reflective surface located at the other sensor assembly. The vertical and horizontal distance between the two sensor assemblies may also be

measured by a differential Global Positioning System (ΔGPS) having an antenna at each sensor location, as well as, optionally, a barometric altimeter.

The system can be operated, at lower resolution, with ΔGPS only. In this case, the x, y, z position difference between the two sensor assemblies is monitored. A second ΔGPS antennae serves as a mobile "base station" for the first one. The distance between the two moving sensor assemblies, (length of the "baseline") is thus reduced, from several hundred kilometres for a typical regional air-borne gravity survey, to a maximum of several kilometres. Since a large portion of ΔGPS positioning error is expressed as a percentage of the baseline length, the result is a substantial improvement in the relative position resolution and thus differential gravity signal. Short Description of the Drawings

The invention will now be disclosed, by way of example, with reference to the accompanying drawings in which:

Figure 1 shows an embodiment of an air-borne gravity survey system;

Figures 2a and 2b show first and second embodiments of a "Differential Gravity" System using acceleration/gravity sensor assemblies (AGSA) , in towed bodies ("birds") suspended several hundred meters from a

helicopter or an aircraft, or mounted one in the aircraft and the other suspended from the aircraft, respectively;

Figures 3a, and 3b show third and fourth embodiments using sensor assemblies mounted on-board of separate aircraft flying one or more kilometres above or behind each other; and

Figure 4 shows sensor assemblies installed in towed bodies, flown suspended behind and below the aircraft.

Detailed Description of Preferred Embodiments

Before proceeding with the detailed description of a few embodiments of the invention let us provide the following well known definitions: Gravity Potential T

Gravity Vector T _i = T(x, y, z)

Gravity Vector magnitude |τ = (T _x ² + T _y ² + T _z ² )*

T ^x xx T ^x x T ^•■■ xz

Gravity Gradient Tensor T _{i # j} = T _yz T _yy T _yz T _zx T _zy T _2Z

T- • = T- ■

As mentioned previously, an IMU, unlike a conventional gravity meter, can provide all three components of linear as well as angular acceleration. In addition, the IMU can measure the magnitude of a total acceleration vector from which the magnitude of

total gravity vectorIT = (T _x ² + T _y ² + T- ² )* can be derived. |τ can provide valuable redundancy to T _z , since \ τ _L \ anomalies have similar amplitudes and shapes to T ₂ . Subtracting geometrically T _z and T _x (where

T _x = J T _x ,dz where z _r is a reference flight

altitutde) from |Ti| yields an estimate of absolute value of |τ _y | . A simplified procedure can be used at the peaks and troughs of anomalies of |Ti| and T _z , where T _xz and T _x = 0. At these locations |T _y | = |τ - T _z . The sign of T _y can be estimated from values of T _z and iT on adjacent lines. Knowledge of |T _y | values allows better contouring and improves detectability of targets not directly overflown.

Referring to Figure 1, there is shown an acceleration/gravity sensor assembly (AGSA) 1 mounted on a stabilized platform 2 on board an aircraft. The sensor assembly includes at least one IMU containing accelerometers and gyroscopes capable of providing all three components of total inertial linear as well as angular accelerations, and a differential global positioning system (ΔGPS) including antennas 3 capable of providing the location of the aircraft. Also mounted on board of the aircraft is a laser ranger/profiler to provide distance or terrain contour data as illustrated

by dashed lines A or B.

Referring now to Figures 2 to 4 of the drawings there are shown systems comprising two sensor assemblies AGSA _X and AGSA ₂ , which constitute variations of "Differential Gravity" systems. In Figure 2a, sensor assemblies AGSA _: and AGSA ₂ are installed in towed bodies ("bird") 6 and 7 suspended on a cable from a helicopter, and spaced several hundred or more meters apart. Towed bodies 6 and 7 are stabilized with drag tails 8. The vertical spacing between the two sensor assemblies AGSA _1# and AGSA ₂ is being constantly monitored by, for example, one or more laser ranger measuring devices, such as the G510 Laser Ranger (produced by Optech Systems Corp., Downsview, Ont.) located at one AGSA with a small reflective device located at the other AGSA as illustrated by dashed line "A" on Figures 2 - 4 and/or by a differential satellite Global Positioning System (GPS) with separate antennas at each AGSA (such as produced by the Ashtech Corp. of Sunnyvale, CA) , as well as at least one base station antenna 5 located on the ground.

Terrain profile is being acquired with, for example, a laser profiler, as illustrated with dashed line "B" in Figures 2-4. The configuration illustrated in Figure 2a measures the following parameters:

(τ _zl - T _z2 ) i ilx - |τ ₂

T _z ι, T _z2 Terrain Profile or Scan

Prefixes 1 and 2 refer to the above mentioned towed bodies

Another embodiment, illustrated in Figure 2b, is with one sensor assembly AGSA _: mounted on-board of an aircraft and the second AGSA ₂ suspended on a cable in a "bird" where the cable length may exceed one kilometre. The measurements taken are the same as in Figure 2a.

The other embodiments, illustrated in Figures 3a and 3b, may consist of two planes each equipped with a sensor assembly (AGSAi, AGSA ₂ ) flying up to ten or even more kilometres above (Figure 3a) or behind (Figure 3b) each other, respectively. Optionally, the aircraft may be stabilized with dummy payload 9 and drag tails 8. The vertical or horizontal spacing between the two planes would be monitored with the aid of one or more laser or microwave ranger measuring devices, mounted at one sensor assembly and a reflective device at the other sensor assembly, and/or by differential GPS with antennas at each sensor assembly. The measurements taken in the embodiment of Figure 3a are the same as in Figures 2a or 2b. The measurements taken in Figure 3b are T _zl , T _z2 , T _zx , |T _χ |Ti| ₂ , |T _t | , _. - |τ ₂ .

In Figure 4, the sensor assemblies are installed in towed bodies ("birds") towed behind and below their respective aircrafts. The measurements taken are the same as in Figures 2a, 2b or 3a. In order to suppress chaotic noise, two or more IMU's either strapped-down, or installed on one or more platforms, can be used in an aircraft installation.

In-flight motion accelerations of the aircraft or "birds" must be kept minimal. This can be accomplished by one or more of the following: a) an autopilot controlled by the IMU for a minimum acceleration flight regime, b) towing heavy stabilizing "bird" below the aircraft, c) a drag tail attached to the tail of the aircraft. Although the invention has been disclosed, by way of example, with reference to preferred embodiments illustrated in the drawings, it is to be understood that it is not limited to such embodiments and that other alternatives are also envisaged within the scope of the following claims.