Transcript Teaching introductory geophysics

Teaching introductory geophysics:
what our students don’t know, need to,
and what we can do
Seth Stein
Department of Earth and Planetary Sciences
Northwestern University Evanston IL 60208
[email protected]
This presentation at
http://www.earth.northwestern.edu/people/seth/Lectures/teachinggp.ppt
Solid earth concepts
can be taught purely
descriptively at the
100-level
But this is teaching
science as magic, not
geophysics
Davidson et al
AGI: “Geophysics is a
branch of experimental
physics dealing with
the earth”
NU introductory geophysics:
-First geophysics class (200-level) for geology majors,
engineers, or other science folk
-Typically sophomores or juniors
- Some have descriptive (100-level) intro to earth science
- Math, physics, chemistry prerequisite or corequisites
- Global/planetary perspective
- Course goes beyond descriptive
- Try to challenge students & prepare them for higher level
classes
Intro geophysics classes integrate topics that we’d like
students to know from other classes, but often don’t
know well (or at all). These can include
Earth structure
Earth processes
Geologic time
GEOPHYSICS
GEOLOGY
Mechanics
PHYSICS
E&M
Waves
Heat & temperature
How we measure these
MATH
Vectors &
vector diff.
operators ( )
Spherical
geometry
Calculus
Simple ODE
& PDE
Problem solving (not just plug in) (wave, heat)
In practice, we need to “review” - or teach - much of these
This is OK since many students learn them better in context
We can’t do detailed derivations, but can give intuition &
contexts (often common to different applications)
Earth structure
Earth processes
Geologic time
GEOPHYSICS
GEOLOGY
Mechanics
PHYSICS
E&M
Waves
Heat & temperature
How we measure these
MATH
Vectors &
vector diff.
operators ( )
Spherical
geometry
Calculus
Simple ODE
& PDE
Problem solving (not just plug in) (wave, heat)
Seek to improve students’ understanding of both underlying
concepts & geophysical application
Earth structure
Earth processes
Geologic time
GEOPHYSICS
GEOLOGY
Mechanics
PHYSICS
E&M
Waves
Heat & temperature
How we measure these
MATH
Vectors &
vector diff.
operators ( )
Spherical
geometry
Calculus
Simple ODE
& PDE
Problem solving (not just plug in) (wave, heat)
Tradeoff between breath of topics covered & depth of
each (underlying math & physics) depends on class
goals & composition
Earth structure
Earth processes
Geologic time
GEOPHYSICS
GEOLOGY
Mechanics
PHYSICS
E&M
Waves
Heat & temperature
How we measure these
MATH
Vectors &
vector diff.
operators ( )
Spherical
geometry
Calculus
Simple ODE
& PDE
Problem solving (not just plug in) (wave, heat)
VECTORS
Physics independent of
coordinate system (F=ma)
Magnitude-angle vs components
Stein et al,
2007
Magnetic field
Vector operations (sum,
gradient, curl, etc.)
Plate motion
SPHERICAL GEOMETRY
Latitude & longitude
Vector components: (r,, ),
(N-S, E-W, up-down), (x,y,z)
Unit vectors in non-Cartesian system
VDOs in spherical coordinates
Operations must be done in Cartesian
Stein & Wysession,
2003
Plate motion
Stein et al,
2007
Magnetic field
SYSTEMS OF EQUATIONS/LEAST SQUARES
Can motivate w/o math or w/simple algebra
Davidson et al, 2002
GPS receiver finds position with radio signals from different satellites
Locating earthquake from arrivals at multiple seismometers
Tomography
MECHANICS
F=ma
Magnetic field
torque on
magnetic dipole
causes
Weight vs mass
Gravity on different
planets
Objects of different
mass fall at same
speed
Compass to work
Torque
Magnetic surveys
Thermal
Remnant
Magnetization
Stein et al, 2007
Paleomagnetism
Need to understand
cross product
ROTATION
(CIRCULAR MOTION)
Torque, angular velocity &
momentum, moment of
inertia
Stein et al, 2007
Precession magnetometer
Need to understand cross
product & vector nature of
acceleration
Plate motion
L=I
Moment of inertia
ELECTROMAGNETISM
B
How fields arise
Columb’s law
B from moving charges
V
Spinner
magnetometer
V=-d flux B/dt
Lorentz force
Maxwell’s equations
How we measure & use
fields
Mass spectrometer
(age dating)
F=q(E + v x B)
Circular motion
Stein et al, 2007
U(x,t)
SEISMOLOGY
Waves in space & time
Observe as either
Period, wavelength,
speed (c=/T) &
dimensions
Relation between wave
fronts and rays
t
x
Stein & Wysession,
2003
Wavefront
Ray
SEISMOLOGY
Idea of geometric optics:
when we can use rays,
when we need waves
Travel time vs amplitude
T2
Inquiring minds
want to know
Stein & Wysession,
2003
Like boat’s wake
wavefront -> ray
Huygens’
principle
graphic
T1
To
Non-geometric
optical arrivals
DIFFRACTION
Frequency
dependent (hear
but not see around
corners)
core
D/2d
Migration undoes
diffraction
Single
Interface DEPTH
slit
end
diffraction
Stein & Wysession,
2003
TIME
CONSTRUCTIVE & DESTRUCTIVE INTERFERENCE
Phase difference
between waves
Distance & time
measurement
VLBI
CMP stacking
earth.leeds.ac.uk/.../
active_tectonics/vlbi.gif
INSAR
X-ray
diffraction
http://comet.nerc.ac.uk/sc
hoolssar_how.html
Michelson–Morley
experiment (1887)
http://www.eserc.stonybrook.e
du/ProjectJava/Bragg/
Stein & Wysession, 2003
RAYS ARE
APPROXIMATIONS
TO WAVES
Stein & Wysession,
2003
GEOTHERMICS
Conservation of
energy (science
vs popular
usage)
Potential
Potential, kinetic,
chemical, heat,
electrical…
Chemical
Nuclear
Planets are heat engines
Stein et al, 2007
Most energy
available to
humans &
natural systems
is ultimately
nuclear from sun
or other stars
HEAT & TEMPERATURE
Explain physically (towel)
Heat transfer modes
Use partial derivative
(don’t have to call it grad)
Time dependant solutions
Stein et al, 2007
Cooling halfspace solution
Lava lake
Stein & Wysession, 2003
THERMAL EVOLUTION OF LITHOSPHERE
Simple models of
spreading & subduction
Tie to observations
Density anomalies
Plate driving forces
Forsyth &
Uyeda, 1975
Stein & Wysession,
2003
THERMAL
CONVECTION
Buoyancy
force
Adiabat
Convection
cell
Solid & fluid
earth
Stein et al, 2007
Tie to weather
& climate
change
VISCOSITY
Define w/o tensor
Magma & mantle
flow
Stokes’ problem
Stein et al, 2007
Rayleigh number
Stratovolcano
Shield volcano
Reflects magma chemistry; tie to hazard
Tie to glacial geology
GPS
RADIOACTIVITY
How & why it occurs
Why nuclear energy released
Age dating
Radioactive heating of earth
Nucleus
Meteorites
from early
solar
system
2
E=mc
Stein et al, 2007
SUGGESTIONS
Identify math, physics, chemistry
concepts implicitly or explicitly needed
in course
Assess which students are shaky on
“Review” - or teach - these at level
suitable for class curriculum &
composition
Seek to improve students’
understanding of both underlying
concepts & geophysical application
Develop problems & labs for this goal
Accept that some students will benefit
more than others
“You come in here
with a skull full of
mush and you
leave thinking like
a lawyer.”