Use of Geophysics in Geologic Maps

1. Use of Geophysics in Geologic Maps With Assistance from Nigel Wattrus GEOL 3000

2. Geophysical Data allows Buried Geology to be interpreted from its Physical Properties

3. Common Geophysical Methods and their applications • 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 user-induced field • Electrical Resistivity – electrical conductivity (commonly related to water content or metal content)

4. Gravity 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

5. m2 m1 r Law of Mutual Attraction Physics of Gravity Newton’s Law of Gravity G = Gravitational constant = 6.67 X 10-11 m3kg-1s-2 Gravitational force on a mass m2 due to the Earth’s mass GRAVITATIONAL ACCELERATION

6. How a geologist sees the world How a geophysicist sees the world Gravitational Acceleration - g 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 Changes in g due to near surface changes in mass/density

7. Gravity Anomalies • +Dm > +Dg anomaly • -Dm > -Dg anomaly

8. Mass on a spring Timing pendulums Timing falling objects Measuring Gravity Anomalies • RELATIVE measurement – much easier to do! • Spring extension is proportional to the applied gravitational force • k is the spring constant

9. Gravitymeters • Factors affecting Readings • Temporal • Instrument drift • Tides • Spatial • Latitude • Elevation • “Slab” effects • Topographic effects

10. Gravity Corrections • Temporal Corrections– periodic base station readings • Spatial Corrections • FREE-AIR CORRECTED Dg = gmeas – gn + gFA • BOUGUER SLAB CORRECTED Dg = gmeas – gn + gFA - gB+ gTC gn – latitude correction gFA – elevation correction = -0.3086 mgal . h gB – slab correction gTC – terrain correction Sea Level

11. Local vs Regional Anomalies

12. Affect of Source Depth • 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 !

13. Density of Common Earth Materials • 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

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

15. Magnetic Field strength Physics of Magnetism Coulomb’s Law = 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 p1 p2are the strengths of two magnetic poles – they can be negative! m= magnetic permeability Note similarity to Newton’s Law of Mutual Attraction

16. Intensity of induced field is proportional to the strength of the applied external field. • = Magnetic Susceptibility Magnetic Induction

17. WOW!!! Magnetic Susceptibilities

18. Magnetic Anomalies

19. Using Geophysics for Geologic Intepretations

20. Geology of Northeastern Minnesota