Geological Mapping of Sabah, Malaysia, Using Airborne Gravity ...
Gravity and Magnetic Mapping
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Transcript of Gravity and Magnetic Mapping
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Gravity and Magnetic Mapping
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Gravity Map of
Wisconsin
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Gravity• Mean value about 9.8 m/sec2 = 1 g• About 0.5% smaller at equator than poles• Map unit = gal (for Galileo) = 1 cm/sec2
• Mean gravity = 980 gal• Maps contoured in mgal = 10-6 g• Modern gravimeters can detect .001 mgal
variations (= 1 ppb)• A gravimeter is essentially a spring balance.
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Gravimeter
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Gravity and Latitude• Centrifugal force (3400 mgal at equator)• Variation of Earth’s radius• g(φ) = 9.78(1 + 0.0053sin2 φ –
0.0000058sin22φ)• 0.5% less at equator than pole = 5000 mgal• At 45⁰ = 58 mgal/degree
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Gravity and Motion• Earth is a sphere (sort of)• Moving on the earth results in centrifugal
force• Centripetal acceleration = v2/r• v at equator = 465 m/sec, r = 6,371,000 m
a = 0.03394 m/sec2 = 3394 mgal• v at equator = 466 m/sec, r = 6,371,000 m
a = 0.03409 m/sec2 = 3409 mgal• Moving 1 m/sec = 15 mgal at equator
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Gravity Corrections• Raw Gravity• Latitude Corrections• Altitude • Mass between observer and sea level• Thickness of Crust and Regional Variations• Purpose is to identify features of geologic
interest, not cancel everything out
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Gravity and Altitude
• Gravity decreases 0.31 mgal/m• A correction for altitude only is a free-air
correction • However, there is also mass between the
observer and sea level• A correction for excess mass is a Bouguer
correction
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Gravitational Attraction of a Plate• Attraction of a plate = 2 πG ρ t• Note there is no elevation term• If ρ = 1000 kg/m3 and t = 1 m• 2 πG ρ t = 41.93 × 10-8 m/s2 = 0.042 mgal• For ρ = 2700 kg/m3, correction = 0.11
mgal/m• Combined with altitude correction, total
correction = 0.19 mgal/m
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Gravity Maps• Gravity varies by latitude due to earth’s
equatorial bulge and centrifugal force• Need altitude correction = 0.3 mgal/m = 3 x 10-7
g/m• Altitude only correction = Free-Air Anomaly Map• Correct for mass between you and sea level =
Bouguer Anomaly Map– May also need terrain corrections
• Correct for variations in thickness of crust = Isostatic Anomaly Map
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Raw Gravity, Canada
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Colorado Bouguer Map
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Colorado Isostatic Map
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Chicxulub Impact Basin Gravity Map
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Venus Gravity Maps
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Gravity Map of Mars
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Sea Floor Seen from Space
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Shape of the World
• Earth with topography• Geoid: Ideal sea-level shape of the earth
– Eliminate topography but keep the gravity– Gravity is what determines orbits and leveling of
survey instruments– How do we know where the sea would be at some
point inland?• Datum: Ellipsoid that best fits the geoid• Sphere: Globes and simple projections
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The Geoid
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Gravity Mapping
• Simple corrections for latitude and altitude
• Density = Lithology• Can sense deep into
crust
• Gravimeters are basically sensitive spring balances
• Fragile• Prone to drift• Discrete data points• Labor intensive, low
detail
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Inherent Ambiguity
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The Gaussian Myth
• Gravity and Magnetic data are inherently ambiguous
• There are an infinite number of possible interpretations
• Therefore we can’t conclude anything useful from gravity or magnetic maps
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The Gaussian Myth
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The Gaussian Myth Debunked
• Locations of anomaly sources are constrained
• Shapes of anomaly sources are constrained• Sources cannot have geologically absurd
properties• Maximum depths are constrained• Total masses constrained
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Magnetic Map of
Wisconsin
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Gravity Map of
Wisconsin
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Magnetic Field (Dipole)
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Why Lines of Force?
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Magnetic Poles
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Geomagnetism
• Magnetic field of Earth = 40 microtesla = 40,000 nt (= 40,000 gamma)
• Varies from 25 to 70 microtesla• Non-axial• Not centered on the earth• Varies over a human lifetime
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Magnetism• Diamagnetism: weak repulsion from
electron orbital motion, all materials• Paramagnetism: moderate attraction due
to unpaired electrons• Ferromagnetism: strong attraction due to
parallel alignment of electrons
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Curie Point• Ferromagnetism is due to parallel electron
magnetic moments• Organization breaks down under heating• Most materials lose magnetism around 500 C• Can’t pick up red hot iron with a magnet• Responsible for most paleomagnetism• Magnetic anomalies must be shallow• Geomagnetic field has some other origin
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Source of Geomagnetic Field• Global and Approximately a Dipole• Must be in center of earth• Changes rapidly on a scale of years• Rules out a Permanent Magnet• Most Likely a Geodynamo
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Dynamo Effect• Generator: Wire coil spinning in magnetic
field to generate current• Uses own current to power electromagnets• Not perpetual motion: needs a starter and
continuing source of energy (wind, steam, etc).
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Geodynamo• Core is electrically conducting fluid• Electric currents in core create magnetic
field• Motion of conducting fluid creates electric
currents• Currents generate magnetic field….• Probable driver: convection• Rotation affects flow and field orientation
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Geodynamo
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World Magnetic Declination
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Overall Magnetic Field of Earth
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Magnetic Field 1900-2000
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What Makes Rocks Magnetic?• Magnetic Minerals
– Magnetite– Pyrrhotite– Ilmenite
• Magnetite requires intermediate O activity– Too much O Hematite– Too little O Fe silicates
• Complex: Al favors biotite over Fe oxides• No simple tie to lithology
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Magnetic Mapping
• Corrections are complex and time variable
• No simple correlation with lithology
• Can’t sense deep into crust because heat destroys magnetism
• Magnetism is electromagnetic phenomenon
• Instruments can be purely electronic
• Can record continuously
• Can be extremely detailed
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Gravity and Magnetic MappingGravity maps Magnetic Maps
Mechanical Instrument Instruments are purely electronic
Discrete readings Continuous readings
Less detail Great detail
Can sense to great depths Can sense only a few kilometers deep
Simple corrections for latitude and elevation
Complex corrections in time and space
Density correlates with rock type No simple correlation with rock type
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Magnetic Anomalies
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Anomaly due to Induced Magnetism
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Anomaly due to Remanent Magnetism
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Magnetic Anomalies in the Atlantic