Method and apparatus for regional gravity surveys using a GPS receiver

• US 5,869,759 A
• Filed: 08/23/1996
• Issued: 02/09/1999
• Est. Priority Date: 08/23/1996
• Status: Expired due to Fees

• Current Assignee: Scintrex Ltd. (LaCoste & Romberg Scintrex, Inc.)
• Original Assignee: Scintrex Ltd. (LaCoste & Romberg Scintrex, Inc.)
• Inventors: Seigel, Harold O.
• Primary Examiner(s): Oda, Christine K.
• Application Number:
  US08/701,871
• Time in Patent Office: 900 Days
• Field of Search: 73/382 R, 73/382 G, 364/449.7, 364/449.9, 342/457
• US Class Current: 73/382.00R
• CPC Class Codes: G01V 7/16 specially adapted for use on moving platforms, e.g. ship, aircraft

Abstract

Method and apparatus for conducting regional gravity surveys, comprising a gravimeter module suspended below a helicopter and adapted to be lowered to the earth'"'"'s surface for generating gravimeter readings. The gravimeter module includes an automatic reading gravimeter sensor and a mechanism for self-levelling of the sensor. A ground contact sensor is provided for determining when the gravimeter module contacts the earth'"'"'s surface as a result of being lowered thereto and for determining when the gravimeter module ceases contacting the earth'"'"'s surface as a result of being raised therefrom. The ground contact sensor generates a time marker indicative of these events. A GPS receiver is mounted in fixed relationship with the helicopter for generating periodic position coordinates. A system is provided for receiving and recording the subsurface gravimeter readings, the periodic position coordinates, and the time marker on a common time base. The system further includes circuitry for calculating the position coordinates of the gravimeter module when the gravimeter module contacts the earth'"'"'s surface and when the module ceases contacting the earth'"'"'s surface, by means of interpolating between respective values of the periodic position coordinates at the generated time markers.

Claims

1. Apparatus for conducting helicopter-supported gravity surveys of a survey area, comprising:

a gravimeter module suspended below a helicopter and adapted to be lowered to the earth'"'"'s surface for generating gravimeter readings, said gravimeter module including an automatic reading gravimeter sensor and means for self-levelling said sensor;

a ground contact sensor for determining when said gravimeter module contacts the earth'"'"'s surface as a result of being lowered thereto, and in response generating a time marker indicative thereof;

a GPS receiver mounted in fixed relationship to said helicopter for generating periodic position coordinates; and

a system for receiving and recording said gravimeter readings, said periodic position coordinates and said time marker on a common time base.

3. The apparatus of claim 1, wherein said system further includes means for calculating the position of said gravimeter module when said gravimeter module contacts the earth'"'"'s surface, by means of interpolating between respective values of said periodic position coordinates at said time marker.

5. The apparatus of claim 1, further comprising three legs projecting from a bottom surface of said gravimeter module, and three contact switches connected to respective ones of said legs, whereby upon closure of all of said contact switches said time marker is generated.

7. The apparatus of claim 1 or 2, further including a proximity sensor mounted to said gravimeter module for indicating distance of said gravimeter module above the earth'"'"'s surface.

8. The apparatus of claim 3 or 4, further including a farther GPS receiver located at a predetermined fixed location on the earth'"'"'s surface and in the vicinity of said survey area for providing information in response to which said system corrects its calculation of said position of the gravimeter module to account for ionospheric effects.

9. The apparatus of claim 1 or 2, further including a water pressure monitor for determining water depth of said gravimeter module when said module is lowered into water below the earth'"'"'s surface.

10. The apparatus of claim 1 or 2, further including a water immersion sensor for generating further time markers upon entry of said gravimeter module into water and emergence of said module therefrom.

11. The apparatus of claim 1 or 2, wherein said means for self-levelling said sensor includes pendulum means for rapid coarse levelling and motor driven means controlled by the output of level sensors rigidly mounted on said gravimeter sensor, to achieve fine levelling.

2. Apparatus for conducting helicopter-supported gravity surveys of a survey area, comprising:

a gravimeter module suspended below a helicopter and adapted to be lowered to the earth'"'"'s surface for generating gravimeter readings, said gravimeter module including an automatic reading gravimeter sensor and means for self-levelling said sensor;

a ground contact sensor for determining when said gravimeter module ceases contacting the earth'"'"'s surface as a result of being raised therefrom subsequent to generation of said gravimeter readings, and in response generating a time marker indicative thereof;

a GPS receiver mounted in fixed relationship to said helicopter for generating periodic position coordinates; and

a system for receiving and recording said gravimeter readings, said periodic position coordinates and said time marker on a common time base.

4. The apparatus of claim 2, wherein said system further includes means for calculating the position of said gravimeter module when said gravimeter module ceases contacting the earth'"'"'s surface, by means of interpolating between respective values of said periodic position coordinates at said time marker.

6. The apparatus of claim 2, further comprising three legs projecting from a bottom surface of said gravimeter module, and three contact switches connected to respective ones of said legs, whereby upon opening of any one of said contact switches said time marker is generated.

12. A method of conducting helicopter-supported gravity surveys, comprising the steps of:

suspending a self-levelling automatic reading gravimeter sensor from a helicopter, said sensor being capable of generating gravimeter readings when in contact with the earth'"'"'s surface;

lowering said gravimeter sensor to the earth'"'"'s surface;

determining the GPS relative coordinates of said gravimeter sensor while said gravimeter sensor is being lowered;

generating a time marker in response to said sensor contacting the earth'"'"'s surface;

levelling said gravimeter sensor and obtaining said gravimeter readings therefrom while said gravimeter sensor is in contact with the earth'"'"'s surface and said helicopter hovers overhead; and

recording said gravimeter readings, said GPS relative coordinates and said time marker on a common time base.

14. The method of claim 12 or 13, further comprising the step of determining the elevation of said gravimeter sensor on the earth'"'"'s surface by interpolating said GPS relative coordinates before and after said time marker.

15. The method of claim 14, further comprising the step of correcting said determining of the elevation of said gravimeter sensor to account for ionospheric effects.

16. The method of claim 14, wherein said step of interpolating comprises fitting at least two of said GPS relative coordinates before and after said time marker onto a polynomial function of elevation versus time and calculating for said elevation at a time equal to said time marker.

17. The method of claim 12 or 13, further comprising the steps of immersing said gravimeter sensor in water below the earth'"'"'s surface and determining water depth by means of a water pressure monitor.

18. The method of claim 17, further comprising the step of recording when said gravimeter sensor is immersed in said water.

19. The method of claim 17, further comprising the step of lifting said gravimeter sensor out of said water.

20. The method of claim 19, further comprising the step of recording when said gravimeter sensor emerges from said water.

21. The method of claims 12 or 13 wherein said gravimeter sensor is levelled in two stages, firstly, in a coarse but rapid fashion by pendulum action and then by motor driven means, controlled by the output of level sensors which are rigidly mounted on the gravimeter sensor.

13. A method of conducting helicopter-supported gravity surveys, comprising the steps of

suspending a self-levelling automatic reading gravimeter sensor from a helicopter, said sensor being capable of generating gravimeter readings when in contact with the earth'"'"'s surface;

lowering said gravimeter sensor to the earth'"'"'s surface;

levelling said gravimeter sensor and obtaining said gravimeter readings therefrom while said gravimeter sensor is in contact with the earth'"'"'s surface and said helicopter hovers overhead;

raising said gravimeter sensor from the earth'"'"'s surface subsequent to generation of said gravimeter readings;

determining the GPS relative coordinates of said gravimeter sensor while said gravimeter sensor is being raised;

generating a time marker in response to said sensor ceasing to be in contact with the earth'"'"'s surface; and

recording said gravimeter readings, said GPS relative coordinates and said time markers on a common time base.