CA1204158A - Airborne gravity surveying - Google Patents

Abstract

AIRBORNE GRAVITY SURVEYING
ABSTRACT OF THE DISCLOSURE
Method and apparatus are disclosed for airborne gravity surveying in which the airborne vehicle is stabilized with respect to speed, direction of heading and altitude, and in which the gravity meter has adequate sensitivity and signals that are recorded at a high sample rate on a magnetic tape, in which the aircraft position is computed using a multi-range navigations system that is located geo-detically.

Description

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This invention relates to airborne gravity surveying and more ~articularly to method and apparatus for such surveying with which greater accuracy is obtained than has heretofore been available.
It has heretofore been proposed to use airborne vehicles for gravity surveying as pointed out in Reviews of Geophysics, Vol. 5, No. 4, November, 1967, pages 477 to 526, published by The American Geophysical Union of 2000 Florida P~venue, N.W., Washington, D.C. 20009 commencing at page 520 to 52 with respect to fixed wing airborne vehicles.
A review of the activities with respect to helicopters can be ~ound in "Airborne ~ravity Surveying, Technical Information", published March~ 1981, by Carson ~eoscience, Perkasie, Pennsylvania, commencing at page 1-1.
Various patents have been issued which set forth apparatus for gravity surveying.
Boitnott, in U.S. Patent Nos. 3,011,347 and 3,038,338, Gustafsson, U.S. Patent No. 3,180,151 and Brede, U.S. Patent No. 3,447,293, and Hutchins, Canadian Patent No. 6S2,757 disclose instruments for measurin~ gravity or derivati~es of gravity of the earth's gravity field but do not show practical systems for accurate airborne surveying.
La Coste, U.S. Patent Nos. 2,293,437, 2,377,889,

2,964,948, 2,977,799; Heiland, U.S. Patent No. 2,626,525;
~orden, U.S. Patent Nos. 2,674,887 and 3,211,003; Graf, U.S.
Patent No. 3,019,655; Emmerich, U.S. Patent No. 3,033,037;
Slater, U.S. Patent No. 3,062,051; Hodge ~t al., U.S. Patent No. 3,194,075; ~lard, U.S. Patent No. 3,495,460; Kuzlvanov et al., U.S. Patent No. 3,501,958; Wing, U.S. Patent Mos. 3,546,943 and 3,583,225, show gravi~y meters but do not show practical systems for accurate airborne surveying.

Klasse et al., U.S. Patent No. 2,610,226, Jensen, U.S. Patent No. 2,611,802, and Rumbaugh et al., Mo. 2,611,803 ~LZ~

show method and apparatus for conducting surveys for geo-physical or magnetic explorations but do not discuss or treat airborne gravity surveying.
The proposals heretofore made for airborne surveying do not provide ade~uate stabilization for aircrat, with respect to speed, do not provide very level flight, do not provide accurate navigation and steering, do not with these other re~uirements for accurate surveying, measure the gravity, and have other shortcomings.
In accordance with the invention an improved method and apparatus are provided for airborne gravity surveying in which the airborne vehicle is stabilized with resp~ct to speed, direction of heading and altitude, and in which the gravity meter has adequate sensitivity and signals that are recorded at a high sample rate on magnetic tape, in which the aircraft position is computed using a multi-range navi-~ation system that is located yeodetically, it being pre-ferable to survey during the ni~ht hours when the air is more stable.
It is the principal object of the invention to provide an improved method and apparatus for airborne gravitv surveying with which greater accuracy of computed and recorded data is obtained.
It is a further object of the invention to provide an improved method and apparatus for airborne surveying which is preferably carried out when air conditions are relatively stable, the night hours frequentlv providing such stability.
It is a further object of the invention to provide an improved method and apparatus for airhorne surveying in which the airborne vehicle is maintained at a selected level, and is stabilized as to speed and direction.

It is a further object of the invention to provide an improved method and apparatus for airborne gravity surveying ~2~3~

in which the gravity meter is controlIed as to its sampling and specifioally the sampling rate.
It is a further object o:E the invention to provide an improved method and apparatus for airborne gravity surveyiny in which the position of the airborne vehicle is precisely known at all times.
It is a Eurther object of the invention to provide an improved method and apparatus for airborne gravity surveving in ~hich the airborne vehicle has adequate fuel supply for use at remote locations.
It is a further object of the invention to provide an improved method and apparatus in which the instruments are located and carried by the airborne vehicle at a stahle temperature and preferably in a clean environment.
It is a further object of the invention to provide an improved method and apparatus for airborne surveying in which a probe is located in such a manner as to obtain an accurate measurement of the static air pressure.
It is a further object of the invention to provide an improved method and apparatus for airhorne gravity surveying to provide a measurem4nt of the distance of the airborne vehicle from the ground.
It is a further object of the invention to provide an improved method and apparatus for airborne gravity surveying to so construct the gravity meter so that it operates more efficiently in the airborne environment.
It is a further object of the in~ention to provide an improved method and apparatus for alrborne gravity surveying including and providing a magnetic digital recording system 3Q with a high degree of sensitivity, variable sampling rate, and a capabi.lity of reading the magnetic tape in flight after data has been recorded thereon.
It is a further object of the invention to provide an 4~LS~

improved method and apparatus for airhorne gravity surveying to record on a magnetic tape multiple ranges from an elec-tronic navigation system to enhance the position accuracy.
It is a further object of the invention to provide an improved method and apparatus for airborne gra~ity surveying to describe the method of data collection to provide the necessary parameters for computing accurate gravity measure-ment.
It is a further object of the invention to provide an improved method and apparatus for airborne qra~ity surveying to simultaneously record magnetic and gravitv data.
It is a further object of the invention to provide an improved method and apparatus for airborne gravity surveying to describe the method of preplotting the reauired flight path and to require the airborne vehicle to comply with such a flight path.
It is a further object of the invention to provide for a grid pattern of lines to be flown to cover the gravity anomaly of the area to be covered.
2Q Other objects and advantageous features of the invention will be apparent from the description and claims.
The nature and characteristic features of the invention will be more readily understood from the following description taken in connection wi~h the accompanying drawings forming part hereof in which The Figuxe shows a block diagram for purposes of under-standin~ the invention.
Re~erring to the figure any suitable airborne vehicle may be employed including fixed wing aircraft, lighter than air aircraft, and helicoptersO
If a helicopter is employed one suitable helicopter is Sikorsky model 61, which is preferably e~uipped with internal fuel tanks of a capacity of up to about 8 hours of flight.

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The helicop~er preferablY has a uniquely tuned automatic flight control system that uses collective lift to control the vertical movement of the vehicle during fligh-t without changing the pitch.
For anY airborne vehicle it is essential that it have a flight control system that controls the vertical movement of the vehicle during flight, preferable to limit the ele-vation to ~ 10 feet in thirty seconds of time from a selected predetermined level.
It is preferred toempioy an environmental chamber on the vehicle which is maintained at a stable temperature and is preferably a clean environment.
A combined use of inertial navigation and electronic distance measuring equipment provide the latitude, longitude, and speed control continuously for the pilot. This total navigation packa~e allows the airborne vehicle to fly within a speed range of five knots and along a predetermined flight path to within a few hundred meters.
If a helicopter is employed the rotor blades are pre-cisely tracked and aligned for smoothness of flight.
A probe for measurement only of static air pressureand not subject to ram pressure is provided to aid in measuring the elevation of the plane and is so located that onlv static air pressure is measured, and for a helicopter it may be on a retractable probe located in front of the helicopter or a few feet above the helicopter blades at the center of ro-tation of the blades. The probe is in communication with the environmental chamber.
In order to determine the altitude of the airborne 3Q vehicle a combination of radar or laser and sensitive pressure measurements are made to establish the altitude o~ the air-craft to within ten feet. Suitable radar equi~ment is avail-able from Honeywell, Inc., ~inneapolis, ~innesota. Suitable ~Z~15~3 .

laser equipment is available from Spectra Physics, Inc., Mountain View, California.
Suitable equipment for measuring absolute pressure is available from Rosemont, Inc., Minneapolis, ~innesota.
Relative measurements are made and recorded of the alti-tude to an accuracv of -the order o 0.5 eet.
The pressure altimeters are of two types. One type is an absolute device that measures the pressure and changes that occur in the atmosphere. Ground based absolute altimeters record the changes at ground level and all of these measure-ments are combined to establish and record pressure surface changes in the survey area.
The second type comprises two bi-directional narrow range pressure transducers which are temperature stabilized in the environmental chamber and are used to measure and record minute changes in the aircraft altitude. Such transducers are available from Setra Systems, Inc., Natick, ~lassachusetts.
A static air pressure source of non-turbulent air is provided to these sensors through the pressure probe that is constructed to measure no ram pressure, only static air pressure.
In order to provide a record of the accumulated data all data is recorded at a one second or other desired interval on magnetic tape. All the analog data channels are recorded at a sensitivity of the order of 0.0001 volts. Suitable e~uipment for this purpose comprises a digital system such as the Lancer Electronics r~odel 4570, available from Lancer Electronics Corp., Collegeville, Pennsylvania, interfaced to a Kennedy Model 9800 tape transport available from Kennedy, Inc., Altadena, California. The information is read after write on the tape and displayed on a pa~er tape reader. A
digital voltmeter is available to visually monitor any channel of data.
In order to control the navigation of the airborne ~Z(~lS~3 .

vehicle a line of sight electronic distance measuring system using multiple ground stations may be~employed. One suikable type of such control system is the Motorola Miniranger, available from ~overnment Electronics Division, ~otorola, Inc., Scottsdale, Arizona.
~ nother system for controlling the navigation o~ the airborne vehicle is known as SERIES Satellite Emisslon Radio Interferometric Earth Surveying available from Jet Pronulsion Laboratory, Pasadena, California and which formed a part of Third Annual NASA Program Review, Crustal Dynamics Project, Geodynamics Research, January 26-29, 1981, Goddard Space Flight Center. An example of the system is shown in the U.S.
patent to MacDoran, No. 4,170,776. Additionally, the iconospheric calibration problem recognized in the patent has been success-fully addressed by a new technique called Satellite L-band Iconospheric Calibration (~LIC) which has demonstrated the ahility for a single 5ERIES station to derive the total electron collumnar content by cross correlation of the two broadcast Globular Positioning System (GPS) channels. An important additional data-type is Doppler occurring at an effective wave len~th of 86 cms.
A grid pattern of equally spaced lines in two directions is selected to allow a multiple number of intersections that are data check points for all of the measurements to be made by the aircraft. These lines can provide calibration inform-ation, equipment verifications and data validity certification and each of these lines is to be flown with data therealong recorded as hereinafter pointed out.
Each ground station is located on a precise geodetic marker established using the Navy transit satellite system in the translocation mode with an excellent statistical sampling of good angle passes to compute a position to less than 1 meter in latitude, longitude and elevation.

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Each transponder one of which is located at each of the ground stations is adjusted to measure a calibrated distance on a known range before being installed at the ground station.
Aftex all the ground stations are in action the airborne vehicle is flown across the centerpoint between two stations to check the base line distance. Several passes along each base line are made before the survey begins.
These calibrations and measurements are made so that the com-puted position will be known to an accuracv of the order ofa circle of three meters diameter.
- After an area of survey has bePn selected, a plot of lines to be flown is made. A computer listing of the grid forming the beginning and ending points of the lines and all of the intersection points of any two lines is made.
This listing is entered into the computer on the aircraft.
At least three unique ranges are measured every second to determine the aircraft position. An onboard computer calculates the aircraft position and supplies the d~ta to the navigator plot board and to a pllot display on the flight panel. This collected data is compared to a predetermined flight path that is located in the memory of a computer in the airborne vehicle and the airborne vehicle is guided down the required path.
For purposes of assembling the desired information a modified three axis stabilized platform gravity meter available from La Coste and Romberg, Inc., Austin, Texas, or from Bell Aerosystems, Inc., Buffalo, New York, is used.
The gravity meter is modified so that -the data is recorded with only 1.5 seconds of filtering. A further modification is made to provide a shorting switch that zeros the output from the amplifiers so that the gravity meter can be stabilized in a short period of time.

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All ~arameters of the me-ter and its ~latform are re-corded every one second on magnetic tape. The gravity meter output of the total acceleration measurement as modiied is recorded with little or no filtering. The stabilization time of the meter is therefore ver~ short as the output is kept in null stake electroncially until the aircraft is in stable flight conclitions. The meter is then allowed to accumulate the total accelerations measured by the gravity meter.
All important outputs are monitored on strip chart recorders so that the details of gravity meter operation can be observed and corrected when rec~uired. Amon~ these OUtplltS
are the cross coupling corrections, i.e. inherent and imper-fection types. These corrections are basically corrections to the meter for being slightlv off level and for the mechanical components of the meter flexing under acceleration. This is set forth in more detail in the La Coste publication previouslv referred to at pages 501 to 505.
The mode of operation will now be pointed out.
After all the sensors have been ground calibrated, the airborne vehicle takes off and goes to the flight altitude of the survey. A reference altitude from the radar or laser altimeter is preferably made over a known elevation such as a lake or airfield.
All data is ~onitored in flight by analog strip chart recorders with common time events. The analog recordings are from the gravity meter of raw beam movement, spring tension, average beam movement, cross or transverse acceler-ation, longitudinal acceleration, heading from the inertial portion of the gravity meter, and are recorded on the tape.
Analog recordings are also available from the altimeter sensors, and of the radar or laser distance, the absolute pressure reading and the relative pressure movement and are ~2~15~ .

recorded on the tape.
Analog recordings are also received from the naviga-tion system as to each ranye measurement and are recorded on the tape for whichever navigation sys-tem is employed.
Additional data is also recorded on the tape anc1 includes the line number, the time, the observed gravitv, the diyital radar measurement, the observed magnetics, -the total correction, cross coupling, the average beam movement at different levels of filtering; five different cross couplings including cross acceleration squared, vertical acceleration squared, vertical cross coupling, longitudinal cross coupling and cross acceleration; east and north gyroscope outputs, the azimuth gyroscope, the inertial navigation heading, pressure altimeter output with additional filtering, the signal ground, simultaneously signals are digitized and sampled at a one second sampling interval and put onto the tape.
Before, during and after each flight, all information is printed on paper tape to provide assurance that da-ta are being collected. Analog recorders continuously monitor all important signal parameters.
During flight, the operator of the gravity meter is able to change the sensitivity of the data recorders in order to monitor precisely the system performance. In this manner, he is able to check the platform level and the be~m position very accurately. The beam is an internal component of the La Coste gravity meter. The beam acts as a lever between the mass in the gravitv meter and the fulcrum point of the spring tension measuring screw. The zero length spring in the gravity meter is attached to the mass that is supported by the beam. The beam position is an important measurement because the automatic nulling circuit of the gravity meter requires it to be near zero or it will drive the s~ring tension away fro~ the value necessary for the best readings. If ~2~34~5i5~

-this spring is driven away from null, the meter requires 10 to 30 minutes to fullY stabilize for accurate readinys to be recorded. The beginning of lines re~uire concentration and a full coordination between the operator of the gra~itY
meter, the navigator, and the pilot to prevent any elevation, course, or speed changes that would affect the beam position.
In areas of steep gravity gradients or rough topograph~, the initial nulling of the yravity meter re~uires a skilled flight crew.
The onboard navigation computer and plotter provides a continuous monitor for the flight path of the airborne vehicle. Preplots of the proposed line spacings are made and fed into a navigational computer hereto~ore identified ~fter one of the lines of the preplot has been followed the airborne vehicle is returned to the start of the next li.ne of the preplot which is followed with data available and recorded as before.
The computer ~ectors the pilot to the beginning of the flight line and computes the around speed. If the flight path begins to deviate from the preplotted line, then slight course changes are made by the pilot.
At the end of the flight the airborne vehicle returns to the known reference elevation over the lake or airfield and calibrates the elevation before landing.

Claims (34)

The embodiments of the invention in Which an exclusive property or privilege is claimed are defined as follows..

1. Apparatus for airborne gravity surveying which comprises an airborne vehicle, means on said vehicle for controlling the flight of said vehicle along a preplotted path at a preselected level responding to vertical movement, including means to move the vehicle vertically without changing its pitch, said controlling means comprising a plurality of altimeters for determining the preselected level for correction of the path of the vehicle and providing signals for recording on a tape, said controlling means comprising navigating means providing signals for recording on a tape for indicating de-viation of the flight of the vehicle from the preplotted path vertically and horizontally and for correction of the path of the vehicle, signal means including gravity meter and magnetic means providing signals for recording on a tape, means for digitalizing the signals from said gravity meter means at a predetermined sample rate and at a high order of sensitivity, recording means including a tape for recording signals from said altimeter means, said navigation means, said signal means and said digitalizing means, and computer means for indicating the departure of the vehicle from the preplotted path.

2. Apparatus for airborne gravity surveying as defined in claim 1 in which said airborne vehicle is a helicopter.

3. Apparatus for airborne gravity surveying as defined in claim 2 in which the blades of the helicopter are precisely tracked and aligned for smoothness of flight.

4. Apparatus for airborne gravity surveying as defined in claim 1 in which said plurality of altimeters includes at least one pressure sensitive altimeter, and an altimeter operating upon wave propagation.

5. Apparatus for airborne gravity surveying as defined in claim 4 in which one of said altimeters is a radar altimeter.

6. Apparatus for airborne gravity surveying as defined in claim 4 in which one of said altimeters is a laser altimeter.

7. Apparatus for airborne gravity surveying as defined in claim 4 in which a probe is provided for the pressure sensitive altimeter located so as to be subject only to static air pressure.

8. Apparatus for airborne gravity surveying as defined in claim 1 in which the analog data is recorded at a sensitivity of the order of 0.0001 volts.

9. Apparatus for airborne gravity surveying as defined in claim 1 in which the recording means is effective for recording digitalized signals at intervals of the order of one second.

10. Apparatus for airborne gravity surveying as defined in claim 1 in which the navigating means is a line of sight electronic distance measuring device and at least three ranges are measured within a predetermined time interval.

11. Apparatus for airborne gravity surveying as defined in claim 1 in which said preplotted path has precise geodetic members thereon derived from Navy transit satellite locations in the translocation mode.

12. Apparatus for airborne gravity surveying as defined in claim 1 in which the navigating means employs satellite radio emission interferometric means.

13. Apparatus for airborne gravity surveying as defined in claim 1 in which said gravity meter provides signals at a pre-determined rate.

14. Apparatus for airborne gravity surveying as defined in claim 13 in which at least two bidirectional narrow range pressure transducers are mounted within said chamber to which air is supplied by a probe located so as to be subject only to static air pressure.

15. Apparatus for airborne gravity surveying as defined in claim 1 in which the gravity meter is capable of being stabilized in a short period of time.

16. Apparatus for airborne gravity surveying as defined in claim 1 in which a protected environmental chamber is provided within which the gravity meter is enclosed.

17. Apparatus for airborne gravity surveying as defined in claim 1 in which magnetic and gravity data are simultaneously recorded on a tape.

18. The method of airborne gravity surveying which comprises flying an airborne vehicle along a preplotted path, maintaining the stability of the airborne vehicle with respect to roll, pitch and yaw control and the elevation within a range not exceeding ten feet, the sidewise deviation of the vehicle from the preplotted path within a range of 5000 feet and the speed within a range of twenty five knots, a plurality of altimeters are employed including at least one pressure sensitive altimeter, and an altimeter operating upon wave propagation, recording data along the preplotted path including simultaneous gravity and magnetic data, altimeter data, and navigation data.

19. The method as defined in claim 18 in which the airborne vehicle is a helicopter.

20. The method as defined in claim 19 in which the stability is maintained within a range not exceeding ten feet, the sidewise deviation is maintained within a range of 500 feet and the speed within a range of five knots.

21. The method as defined in claim 19 in which the blades of the helicopter have been precisely tracked and aligned for smoothness of flight.

22. The method as defined in claim 18 in which bi-directional narrow range temperature stabilized pressure transducers measure changes in the aircraft altitude.

23. The method as defined in claim 22 in which signals from said pressure transducers are supplied for recording.

24. The method as defined in claim 18 in which one of the altimeters is a radar altimeter.

25. The method as defined in claim 18 in which one of the altimeters is a laser altimeter.

26. The method defined in claim 18 in which the pressure sensitive altimeter has an input probe located so as to be subject only to static air pressure.

27. The method defined in claim 26 in which bi-directional temperature stabilized pressure transducers measure changes in the aircraft altitude and to which said input probe is connected and which provide signals for recording.

28. The method defined in claim 18 in which the recording is of digitalized signals at intervals of the order of one second.

29. The method defined in claim 18 in which a line of sight electronic distance measuring device is employed at at least three ranges within a pre-determined time interval and provides signals for recording.

30. The method defined in claim 18 in which said gravity meter can be brought to a null position.

31. The method defined in claim 18 in which signals are provided by a gravity meter at a pre-determined rate.

32. The method defined in claim 18 in which signals are simultaneously provided of magnetic and gravity data and are recorded.

33. The method defined in claim 18 in which signals are supplied from a satellite radio emission interferometer for recording.

34. The method defined in claim 18 in which bi-directional narrow range temperature stabilized transducers provide signals for recording.