GEOL 3000 With Assistance from Nigel Wattrus

 Seismic Reflection  Seismic Reflection – subhorizontal geologic structures  Siesmic Refraction  Siesmic Refraction - subhorizonal changes in density or elasticity  Gravity  Gravity - contrasts in density (sees deep into the crust)  Magnetics  Magnetics – magnetic properties created by the earth’s magnetic field  Electromagnetic  Electromagnetic – 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 m 3 kg -1 s -2 Law of Mutual Attraction r m1m1 m2m2 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 = 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 Readings Temporal  Instrument drift  Tides Spatial  Latitude  Elevation  “Slab” effects  Topographic effects

 Temporal Corrections– periodic base station readings  Spatial Corrections ◦ FREE-AIR CORRECTED  g = g meas – g n + g FA ◦ BOUGUER SLAB CORRECTED  g = g meas – g n + g FA - g B + g TC g n – latitude correction g FA – elevation correction = mgal. h g B – slab correction g TC – 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/cm 3 or kg/m 3 Typical values: ◦ Water1 ◦ Sediment1.7 – 2.3 ◦ Sandstone2.0 – 2.6 ◦ Shale2.0 – 2.7 ◦ Limestone2.5 – 2.8 ◦ Granite2.5 – 2.8 ◦ Basalt/Gabbro2.7 – 3.1 ◦ Metamorphic Rocks2.6 – 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!  = magnetic permeability Note similarity to Newton’s Law of Mutual Attraction = 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) = 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 Induction

Material Susceptibility x 10^3 (SI)* Air~0 Quartz-0.01 Calcite Pyrite Hematite Illmenite Magnetite ,200 Limestone0 - 3 Sandstone Shale Schist Gneiss Granite Gabbro Basalt WOW !!!

Geology of NortheasternMinnesota