Transcript American Geological Institute Index

GEOL 414 Applied Geophysics
Fall Semester 2006
Principles of various geophysical methods and their
application to geologic problems. Prerequisites:
Geology 101, Mathematics 213, Physics 204 or 206. 3
credits.
Applied Geophysics
Textbook: 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 Index
Geology Disciplines
• Geology
• Economic Geology
• Geochemistry
• Geophysics
• Paleontology
• Hydrology
• Soil Science
• Engineering Geology
• Oceanography
• Planetology
American Geological Institute Index
Sub-disciplines in Geophysics
• General Geophysics
• Experimental Geophysics
• Exploration Geophysics
• Geodesy
• Geomagnetism & paleomagnetism
• Gravity
• Heat Flow
• Seismology
• Marine Geophysics
American Geological Institute Index
Sub-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 Geophysics
Telford, 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
 U0
Laplace’s equation
 2U  f ( x, y, z)
1 U
2
 U 2
 t
Poisson’s equation
2
The diffusion equation
Magnetics
• Units – nanoTeslas nT
• 7 measurable
components
–
–
–
–
–
–
–
Declination
Inclination
F (total field)
X (north)
Y (east)
Z (vertical)
H (horizontal)
1 m1m2
 r2
F m
H
 2
m r
I  kH
F
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
1 U
 U 2 2
v t
2
2
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
Surprise
Valley,
California
Digital
elevation
model
41.7
41.695
Surprise Valley, California
41.69
Bouguer Gravity contours 41.685
Red lines are seismic
lines
Black lines are gravity
profiles seen in next two
slides
41.68
41.675
41.67
41.665
41.66
41.655
41.65
-120.225
-120.215
-120.205
-120.195
Surprise
Valley,
California
Surprise
Valley,
California
Bouguer
gravity
profiles
Surprise Valley, California
Vertical 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