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GEOL 414 Applied Geophysics
Fall Semester 2006Principles of various geophysical methods and their
application to geologic problems. Prerequisites: Geology 101, Mathematics 213, Physics 204 or 206. 3 credits.
Applied GeophysicsTextbook: Burger, Sheehan, and Jones, Introduction to Applied Geophysics, 2006, W.W. Norton & Co. Inc., ISBN 0393-92637-0
Course Goals: To give students an understanding of the physical principles of geophysical methods so that they will appreciate the strengths and limitations of the methods. After certain fundamentals have been mastered, the students study the procedures used in data acquisition and use the department's equipment to conduct geophysical surveys. The final phase of each section of the course gives students training in interpretation of geophysical data.
Geol 414 Topic Sequence
• Section 1 Introduction to Geophysics -Methods, Units, Applications, Geophysical Societies, Geophysical literature
• Section 2 Gravity fundamentals• Gravity surveying • Gravity data reduction and analysis• Section 3 Magnetic fundamentals• Magnetic surveying• Magnetic data reduction and analysis• Section 4 Combined gravity and magnetic survey field
project• Section 5 Electrical Methods and Surveying• Section 6 Seismology fundamentals• Section 7 Refraction seismology • Section 8 Reflection seismology• Section 9 Thermal methods
American Geological Institute IndexGeology Disciplines
• Geology• Economic Geology
• Geochemistry• Geophysics
• Paleontology• Hydrology
• Soil Science• Engineering Geology
• Oceanography• Planetology
American Geological Institute IndexSub-disciplines in Geophysics
• General Geophysics• Experimental Geophysics• Exploration Geophysics
• Geodesy• Geomagnetism & paleomagnetism
• Gravity• Heat Flow
• Seismology• Marine Geophysics
American Geological Institute IndexSub-disciplines in Geophysics
• Extraterrestrial Geophysics*
• Soil Physics*
• Mathematical Geology*
• Mineral Physics*
• Stable Isotopes*
American Geophysical Union Sections
• Atmospheric Sciences• Biogeosciences
• Geodesy• Geomagnetism and Paleomagnetism
• Hydrology• Ocean Sciences
• Planetary Sciences• Seismology
• Space Physics and Aeronomy• Tectonophysics
• Volcanology, Geochemistry, and Petrology
Topics in some Geophysics Textbooks: Applied GeophysicsTelford, Geldart, Sheriff
• Gravity Methods• Magnetic Methods• Seismic Methods
• Electrical Properties of Rocks and Minerals• Natural Electrical Sources• Electromagnetic Methods
• Resistivity Method• Induced Polarization• Radioactivity Method
• Geophysical Well Logging
Applied and Environmental Geophysics
Reynolds
• Potential Field Methods
• Applied Seismology
• Electrical Methods
• Electromagnetic Methods
Gravity
• Units - gals 1 gal = 1 cm s-2
• Absolute gravity
• Relative gravity
• Range 983164 to 978049 mgals
• Property of Earth – density distribution
• F = Gmm r-2
• g = Gm r-2
Heat Flow• Units – mW m-2
• Q = ΛΚ
• Q = Q0 + Ab
• A = A0 e-z/b
• Range 103 to 0
• Mean ~ 84 mW m-2
2
2
22
0
1
U
U f x y z
UU
t
( , , )
Laplace’s equation
Poisson’s equation
The diffusion equation
Magnetics
• Units – nanoTeslas nT• 7 measurable
components– Declination– Inclination– F (total field)– X (north)– Y (east)– Z (vertical)– H (horizontal)
Fm m
r
HF
m
m
rI kH
1 1 22
2
Coulomb’s Law
Field Strength
Magnetic Intensity
Magnetics
• Units – nanoTeslas nT• Main field 60,000 to 30,000 nT
– Core dynamo
• External field 10’s nT– Diurnal, seasonal, ionization of atmosphere
solar wind currents, storms
Anomalous field 1000’s nT– Magnetic susceptibility of crust
Seismology
• Measured units – time & distance
• Interpreted units – velocity
• Application interpretation – rock type, structure
• Refraction
• Reflection
• Earthquakes
22
2
2
1U
v
U
t
The Wave Equation
Resistivity
• Units – ohm m
• V = iR
• R = rA/L
• Various arrays with characteristic electrode spacing
Bouguer Gravity of the Lake St. Martin Impact Structure
Residual Gravity of the Lake St. Martin Impact Structure
Surprise Valley, California
Surprise Valley, California
1400
1450
1500
1550
1600
1650
1700
1750
1800
1850
1900
1950
2000
2050
2100
2150
2200
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2300
E levation
Surprise Valley, California
Digital elevation model
-120.225 -120.215 -120.205 -120.195
41.65
41.655
41.66
41.665
41.67
41.675
41.68
41.685
41.69
41.695
41.7
Surprise Valley, California
Bouguer Gravity contours
Red lines are seismic lines
Black lines are gravity profiles seen in next two slides
Complete Bouguer Anomaly
Surprise Valley, California
Surprise Valley, California
Surprise Valley, California
Bouguer gravity profiles
Surprise Valley, CaliforniaVertical derivative of the Bouguer gravity profiles
The Geoid
Gravity at UND
• The predicted gravity is given in units of 'milligals': 1 gal = 1 cm sec-2 1 kilogal = 1000 gal 1 milligal = 1/1000 gal Latitude: 47.91877432
• Longitude: 97.06790622 • MSL Height: 252.9 • Predicted gravity: 980798 +/- 2 milligals
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