Transcript With Assistance GEOL 3000 from Nigel Wattrus

GEOL 3000
With Assistance
from Nigel Wattrus
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Seismic Reflection – subhorizontal geologic structures
Siesmic Refraction - subhorizonal changes in density
or elasticity
Gravity - contrasts in density (sees deep into the crust)
Magnetics – magnetic properties created by the earth’s
magnetic field
Electromagnetic – magnetic properties created by userinduced field
Electrical Resistivity – electrical conductivity (commonly
related to water content or metal content)
Measures changes in the Earth’s gravity field
produced by subsurface variations in density
Rock density – primarily controlled by mineralogy.
Mafic rocks typically have unusually high
densities producing positive gravity anomalies
m2
m1
r
G = Gravitational
constant
= 6.67 X 10-11
m3kg-1s-2
Law of Mutual Attraction
Gravitational force on a mass m2 due to the Earth’s mass
GRAVITATIONAL ACCELERATION
Combined with Newton’s Law of Mutual Attraction, we
can define the acceleration of the m2 due to the
Earth’s mass (m1) as:
1 gal = 1 cm/s2
mgal = 0.001 gal
1 “gravity unit” = 0.1 mgal
How a geologist sees the
world
How a geophysicist sees
the world
Changes in g due to
near surface changes
in mass/density
• +Dm > +Dg anomaly
• -Dm > -Dg anomaly
Timing falling objects
Mass on a spring
Timing pendulums
• RELATIVE measurement – much easier to do!
• Spring extension is proportional to the applied
gravitational force
• k is the spring constant
Factors affecting Readings
Temporal
 Instrument drift
 Tides
Spatial
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Latitude
Elevation
“Slab” effects
Topographic effects
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Temporal Corrections– periodic base station
readings
gn – latitude correction
Spatial Corrections
◦ FREE-AIR CORRECTED
Dg = gmeas – gn + gFA
gFA – elevation correction
= -0.3086 mgal . h
gB – slab correction
gTC – terrain correction
◦ BOUGUER SLAB CORRECTED
Dg = gmeas – gn + gFA - gB + gTC
Sea Level
Increasing depth REDUCES
amplitude of anomaly and
INCREASES it’s width
 Non-unique results
The deep target’s anomaly
can be reproduced by a
larger, less dense shallow
target. USE GEOLOGIC
CONSTRAINTS !
Units:
g/cm3 or kg/m3
Typical values:
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Water
Sediment
Sandstone
Shale
Limestone
Granite
Basalt/Gabbro
Metamorphic Rocks
1
1.7
2.0
2.0
2.5
2.5
2.7
2.6
–
–
–
–
–
–
–
2.3
2.6
2.7
2.8
2.8
3.1
3.0
Measures changes in the Earth’s magnetic field
produced by subsurface magnetic bodies
Controlled by mineralogy.
Coulomb’s Law
Magnetic Field strength
= Force per unit pole strength exerted
by magnetic monopole p2
p1 p2 are the strengths of two
magnetic poles – they can be
negative!
m= magnetic permeability
Note similarity to Newton’s
Law of Mutual Attraction
H is magnetic analog of g
Unit measure - N/Amp.m = tesla (T)
Use nanotesla (nT) = 10-9 T
Average strength of the Earth’s field is
~50,000 nT
Magnetic Induction
Intensity of induced field is
proportional to the strength
of the applied external field.
= Magnetic Susceptibility
Material
Susceptibility x 10^3 (SI)*
Air
~0
Quartz
-0.01
Calcite
-0.001 - 0.01
Pyrite
0.05 - 5
Hematite
0.5 - 35
Illmenite
300 - 3500
Magnetite
1200 - 19,200
Limestone
0-3
Sandstone
0 - 20
Shale
0.01 - 15
Schist
0.3 - 3
Gneiss
0.1 - 25
Granite
0 - 50
Gabbro
1 - 90
Basalt
0.2 - 175
WOW!!!
1st VD
2nd VD
Geology of
Northeastern
Minnesota
Aeromagnetic Data over the Central Duluth Complex
Aeromagnetic Data over the
Southern Beaver Bay Complex
Aeromagnetic Data at a
Quadrangle Scale