Frank van Kann School of Physics The University of Western Australia

1. Requirements and general principles of airborne gravity gradiometers for mineral exploration Frank van Kann School of Physics The University of Western Australia

2. Outline • Airborne gravity - can it work? • Airborne gravity gradiometry - is it better? • Geological models - the evidence!

3. Contains information about sub-surface geology

4. gz is commonly used to access this information

5. Gravity signal of a 100MT anomaly at 1000m (about 0.7mgal) 0.1mgal

8. How to measure Gravity? Compare the force of gravity With the force of a spring

9. Attach a mass to the spring to measure g

11. The equivalence principle Confuses measurement of gz with the aircraft motion

13. “Principle of equivalence” Can’t distinguish gravity from acceleration Aircraft motion 100,000 times bigger than anomaly

14. The aircraft motion is equivalent to about 106 GU = 100,000 mgal . Must be corrected by an additional non-inertial measurement of the aircraft motion. Use GPS! GPS noise limits the precision of airborne gravity.

19. Avoid the equivalence principle: measure the “gravity gradient” Fundamentally, no dependence on acceleration.

20. Gravity gradient signals of a 100MT anomaly at 1000m (about 13Eö)

21. Gravity gradient signals of a 100MT anomaly at 1000m (about 13Eö)

22. US military has invested $1b to develop an airborne gradiometer (why?) Abandoned and partially de-classified after end of “cold war” BHP-Billiton acquired licence and invested further $50m to develop the Falcon system (not good enough for mineral exploration) Why is it so hard?…..

23. Fundamentally, there is no dependence on acceleration but instrumental imperfections may give rise to errors due to acceleration. “Common mode rejection” is the key. A signal of 1Eö corresponds to: 10-10 m/s2 differential acceleration. This must be detected in the presence of: 1 m/s2 aircraft acceleration. 10 orders of magnitude…...

25. This is the falcon system

26. m1 m2 Beam balance sensors soft for this mode rigid beam stiff for this mode ideal pivot and for this mode Different mechanisms for stiffness to common force and differential force • low differential force stiffness  3 Hz  high sensitivity • high common force stiffness  5 kHz  high CMRR possible •  CMRR depends on balance (centre of mass at centre of pivot) •  tune CMRR by balancing during assembly to > 130 dB Maximum sensitivity to anomalies here

27. The actual prototype beam balance

28. Close up view of the micropivot (or elastic hinge) 30mm 200µm 50µm

30. Rotational motion also produces errors. Centrifugal force Motion of the hour hand of a clock is 21 Eö Angular acceleration Bringing the hour hand of a clock to rest in 1 day is 2 Eö

31. Aircraft rotational motion Expressed in terms of equivalent gradient error

32. Centrifugal force Motion of the hour hand of a clock is 21 Eö Aircraft motion is 4x105 Eö eliminate by rotational stabilisation Angular acceleration Bringing the hour hand of a clock to rest in 1 day is 2 Eö Aircraft motion is 2x108 Eö reduce by rotational stabilisation and eliminate using two orthogonal sensors

33. Strategy for dealing with 200 million Eo 10-3 radians 3-axis motion Pitch, Roll, Yaw C Gimbals (attached to dewar) A Housing and Bars Pitch shown in this illustration 10-6 radians (within required range) 10-3 radians Dewar B Platform (attached to aircraft) Housing 1 radian Matched OQR Bars C Internal Gimbals remove residual motion and support the 2 OQR bars A Aircraft Platform with other geophysical sensors B External Rotational Stabilisation Platform (keeps instrument level)

34. Rotational motion also produces errors. Centrifugal force Motion of the hour hand of a clock is 21 Eö Angular acceleration Bringing the hour hand of a clock to rest in 1 day is 2 Eö

36. m1 m2 m1 m2 Rejecting aircraft rotation Common Mode Rejection: Use 2 gradiometers to reject angular acceleration. Angular acceleration makes beams move in the same direction. It does not change the angle between them. Measure the angle between them.

37. m1 m2 m1 m2 Measuring the gravity gradient Common Mode Rejection: Use 2 gradiometers to reject angular acceleration. A Gravity Gradient makes the beams move in opposite directions. This is what we measure. Q. How much does it move (per Eö)? A. 2.5x10-12 radians (~2x10-13 metres!!) (which is less than 1/1000th atom size!) WE CAN MEASURE THIS NOW A 30 Megatonne orebody 1km deep produces a gravity gradient of 1 Eö

40. 7km Key to Geology Map Broken Hill Group Rasp Ridge Gneiss Cues Formation Allendale Metasediments Parnell Formation Freyers Metasediments Alma Gneiss 0 Hores Gneiss 0 12km (a) Structural geology map for the Broken Hill mining area. Sundown Group

41. Broken Hill AGG Simulations (Dransfield, 1994; van Kann, 2004) Model response

42. Broken Hill AGG Simulations (Dransfield, 1994; van Kann, 2004) Model response 12 Eo/√Hz noise and 500m filter

43. Broken Hill AGG Simulations (Dransfield, 1994; van Kann, 2004) Model response 12 Eo/√Hz noise and 500m filter Falcon data (2003)

44. Broken Hill AGG Simulations (Dransfield, 1994; van Kann, 2004) 1 Eo/√Hz noise and 100m filter Model response 12 Eo/√Hz noise and 500m filter Falcon data (2003)

45. Conclusions • Airborne gravity only for regional work • Require 1Eö in 50m (1Hz) for detailed • mineral exploration. • Instruments with 1Eö in 50m are proven on the ground, • awaiting committed commercial collaboration to fly.