Remote Sensing and Borehole Gravity Gradiometry

1. Remote Sensing and Borehole Gravity Gradiometry Wayne McRae Gravitec Instruments

2. Remote Sensing ― Purpose and Techniques Ribbon Gravity Gradiometer ― What is it? How does it work? Down a Borehole ― How far can we detect the front? The Borehole Sonde ― The shape of things to come Overview 2

3. We want to make the Earth transparent, to see the anomalies in the ground Core drilling is expensive, risky and time consuming Remote sensing locates anomalies over large areas, quickly and cheaply Remote Sensing: The Goal 3

4. Exploit material properties: Residual magnetisation Conductivity Radioactivity Seismic response (density profiling) Density contrast/Gravity & Gravity Gradient Remote Sensing: Techniques 4

5. Gravity Gradiometry: Roland Eötvös (1848-1919) Eötvös Unit (EU) is the unit of gravity gradient 1 EU = 110-9 s-2 5

6. Deployment Mobile ground surveying Static ground surveying Airborne Remote underwater vehicle/submersible Borehole Satellite Applications Petroleum/Mineral Surveying Archaeology Void Detection Passive navigation systems Customs and shipping security Unexploded ordinance (UXO) detection Mapping the planets, moons and asteroids Gravity Gradiometry: Deployment & Applications 6

7. Gravity Map of the Moon ― Lunar Prospector spacecraft in 1998-99 http://science.nasa.gov/headlines/y2008/22may_grail.htm?list712745 7

8. Principles of the Ribbon Gradiometer Sensor Characteristics • Ribbon forms the active component of an RF bridge • Actuators pulse the ribbon tension, changing it between gradient sensitive and insensitive states (~5 10 Hz) • RF drive monitors changes in ribbon deflection • Pulse modulated signals from twin bridges are demodulated to generate the gravity gradient signal 8

9. Principles of the Ribbon Gradiometer Gravity Gradient Profile 9

10. Consider a Spherical Cow P r z  d 11

11. Gravity & Gravity Gradient of a Spherical Cow 12

12. Results from Lab Prototype Suspended Platform 30  30 mmCross section 400 mm 120 40 80 kg 350 280 200 mm Sensor Sensor Head Diameter(mm) Mass(kg) Test Mass Motorized Track 13

13. Results from Lab Prototype 0 case Test mass sensitive 14

14. Responsivity Rig Gravity Gradient Simulation Rig (with Sensor Mounted) 15 Design courtesy of QinetiQ, UK

15. Constant Amplitude Piezoelectric Driver 16

16. Ingress of water into oil bearing rock r1  Thickness t r2 Cake bite model Borehole Modelling – Far Field 17 Modelling courtesy of QinetiQ, UK

17. Detection Range for 5 Eötvös sensitivity  Semi-infinite ingress of water (eff = 30 kg/m3) and gas (eff = 110 kg/m3) to oil reservoir strata 250 200 150 Layer Thickness [m] 100 50 Water to oil, 90 degree Gas to oil, 90 degree 0 0 50 100 150 200 250 300 Detection Range [m] Borehole Modelling – Far Field 18 Modelling courtesy of QinetiQ, UK

18. Borehole Tool ― Things to Come Borehole Tool • Temperature: ≈ 80°C ― 170°C • Size: 65 ―125 mm dia. • Magnetic field: Steel casing + geology (≈1 Gauss) • Inclination: Vertical to horizontal • Pressure: 14000 kPa ― 35000 kPa at 2000 m in salt water or drilling fluid 19