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

m
1 m
2 r
G = Gravitational constant
= 6.67 X 10 -11 m 3 kg -1 s -2
Law of Mutual Attraction
Gravitational force on a mass m
2 due to the Earth’s mass
GRAVITATIONAL ACCELERATION

Combined with Newton’s Law of Mutual Attraction, we can define the acceleration of the m
2 due to the
Earth’s mass (m
1
) as:
1 gal = 1 cm/s 2 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

• + D m > +
D g anomaly
• D m > -
D g 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
 Latitude
 Elevation
 “Slab” effects
 Topographic effects

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Temporal Corrections– periodic base station readings
Spatial Corrections
◦ FREE-AIR CORRECTED
D g = g meas
– g n
+ g
FA g n g
FA
– latitude correction
– elevation correction
= -0.3086 mgal . h g
B g
TC
– slab correction
– terrain correction
◦ BOUGUER SLAB CORRECTED
D g = g meas
– g n
+ g
FA
- g
B
+ g
TC
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 !

3
3
◦ Water
◦ Sediment
◦ Sandstone
◦ Shale
1
1.7 – 2.3
2.0 – 2.6
2.0 – 2.7
◦ Limestone
◦ Granite
2.5 – 2.8
2.5 – 2.8
◦ Basalt/Gabbro 2.7 – 3.1
◦ Metamorphic Rocks 2.6 – 3.0

Measures changes in the Earth’s magnetic field produced by subsurface magnetic bodies
Controlled by mineralogy.

p
1 p
2 are the strengths of two magnetic poles – they can be negative!
m
= magnetic permeability
Note similarity to Newton’s
Law of Mutual Attraction
Magnetic Field strength
= Force per unit pole strength exerted by magnetic monopole p
2
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

Intensity of induced field is proportional to the strength of the applied external field.
=
Magnetic Susceptibility

Material
Air
Quartz
Calcite
Pyrite
Hematite
Illmenite
Magnetite
Limestone
Sandstone
Shale
Schist
Gneiss
Granite
Gabbro
Basalt
Susceptibility x 10^3 (SI)*
~0
-0.01
-0.001 - 0.01
0.05 - 5
0.5 - 35
300 - 3500
1200 - 19,200
0 - 3
0 - 20
0.01 - 15
0.3 - 3
0.1 - 25
0 - 50
1 - 90
0.2 - 175
WOW
!!!

1 st VD 2 nd VD