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

2250

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