GEOL 3000With Assistance from Nigel Wattrus

Seismic Reflection Seismic Reflection – subhorizontal geologic structures Siesmic Refraction Siesmic Refraction - subhorizonal changes in density

or elasticity GravityGravity - contrasts in density (sees deep into the

crust) MagneticsMagnetics – magnetic properties created by the

earth’s magnetic field ElectromagneticElectromagnetic – magnetic properties created by

user-induced field Electrical Resistivity 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

G = Gravitational constant

= 6.67 X 10-11 m3kg-1s-2

Law of Mutual Attraction

r

m1 m2

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

• +m > +g anomaly

• -m > -g anomaly

Timing falling objects

Timing pendulums

• RELATIVE measurement – much easier to do!

• Spring extension is proportional to the applied gravitational force

• k is the spring constant

Mass on a spring

Factors affecting ReadingsTemporal

Instrument drift Tides

Spatial Latitude Elevation “Slab” effects Topographic effects

Temporal Corrections– periodic base station readings

Spatial Corrections◦ FREE-AIR CORRECTED g = gmeas – gn + gFA

◦ BOUGUER SLAB CORRECTEDg = gmeas – gn + gFA - gB + gTC

gn – latitude correctiongFA – elevation correction

= -0.3086 mgal . h

gB – slab correction gTC – terrain correction

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:◦ Water 1◦ Sediment 1.7 – 2.3◦ Sandstone 2.0 – 2.6◦ Shale 2.0 – 2.7◦ Limestone 2.5 – 2.8◦ Granite 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.

p1 p2 are the strengths of two magnetic poles – they can be negative!

= magnetic permeability

Note similarity to Newton’s Law of Mutual Attraction

= Force per unit pole strength exerted by magnetic monopole p2

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 Field strength

Coulomb’s Law

Magnetic Susceptibility

Intensity of induced field is proportional to the strength of the applied external field.

Magnetic InductionMagnetic Induction

Material Material Susceptibility x 10^3 (SI)*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

WOWWOW!!!!!!

Geology of Geology of Northeastern Northeastern Minnesota Minnesota