Remote Sensing and Borehole Gravity Gradiometry

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• Remote Sensing and Borehole Gravity Gradiometry

Wayne McRae Gravitec Instruments
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Overview

• 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

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Remote Sensing The Goal

• 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

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Remote Sensing Techniques

• Exploit material properties
• Residual magnetisation
• Conductivity
• Radioactivity
• Seismic response (density profiling)
• Density contrast/Gravity Gravity Gradient

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Gravity Gradiometry Roland Eötvös (1848-1919)
Eötvös Unit (EU) is the unit of gravity
gradient 1 EU 1?10-9 s-2
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Gravity Gradiometry Deployment Applications

• 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

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

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Principles of the Ribbon Gradiometer
Gravity Gradient Profile
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Borehole Modelling Near Field
Reasonable Worst CaseMaterial Density
(?) Porosity (?)Rock 2600 kg/m3 Rock
0.3Concrete 2300 kg/m3 Concrete 0.1 Fluid
1000 kg/m3Density Contrast??eff ??Rock ?
??Concrete 500 kg/m3
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Modelling courtesy of QinetiQ, UK
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Consider a Spherical Cow
P
r
z
?
d
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Gravity Gravity Gradient of a Spherical Cow
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Results from Lab Prototype
Suspended Platform
30 ? 30 mmCross section
40
kg
80
120
400 mm
200
mm
280
350
Sensor
Sensor Head
Diameter(mm)
Mass(kg)
Test Mass
Motorized Track
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Results from Lab Prototype
0? case Test mass sensitive
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Responsivity Rig
Gravity Gradient Simulation Rig (with Sensor
Mounted)
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Design courtesy of QinetiQ, UK
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Constant Amplitude Piezoelectric Driver
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Borehole Modelling Far Field
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Modelling courtesy of QinetiQ, UK
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Borehole Modelling Far Field
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Modelling courtesy of QinetiQ, UK
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Borehole Tool ? Things to Come

• Borehole Tool
• Temperature 80C ? 170C
• 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

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