GEO369 Introduction to Geophysics Andrew M' Goodliffe Assistant Professor Department of Geological S

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GEO369 Introduction to GeophysicsAndrew M.
GoodliffeAssistant ProfessorDepartment of
Geological Sciencesoffice Bevill 202office
phone [phone]e-mail amg_at_ua.eduwww
http//www.geo.ua.edu
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Logistics

• Turn off cell phones
• Hand out syllabus
• Class will be lecture and lab based
• Web page associated with this class
• www.geo.ua.edu/AMG/GEO369
• Class notes, links, class reading lists, etc.
• For those of you that do not have Sun/PC logins
  (Bevill 266) please make an appointment to see me
  at the end of class.
• We will be using the Sun server and PC
  extensively.
• We will meet next Tuesday in Bevill 266
• Would a visit to the library be useful?
• My field schedule
• 21 OCT  SP1/South Pacific/     Transit/UHI/      
  Suva, Fiji  08/NSF/F
• 09 NOV  Transit                UH 05             
  Lae, PNG      

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What is Geophysics??
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Goals

• To make you aware and capable of
  implementing/discussing as many geophysical
  techniques as possible
• Radar Interferometry
• Satellite altimetry
• Isostasy
• Earthquakes
• Gravity
• Magnetism
• Paleomagnetism
• Bathymetry
• Sidescan
• GPS
• Seismic Refraction
• Time series analysis
• Seismic Reflection
• Heat Flow
• Down-hole logging
• Vertical seismic profiles
• Geophysical analysis of rock samples

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Methodology

• Rather than a conventional lecture based class, I
  will strive to make it as hands-on as possible.
• We will largely be studying the data from one
  region of the world Papua New Guinea, and will
  use that data to better understand the
  geology/tectonics of that area
• There will be plenty of examples from elsewhere.

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Papua New Guinea
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Geophysics at the Coarsest Scale

• What techniques do we have to look at a region at
  the coarsest scale?
• Satellite imagery ie Landsat, SPOT
• Satellite altimetry
• Satellite gravity
• DEMs
• Digital elevation models
• DEMs from space SRTM
• Shuttle radar topography mission

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Some Gravity Basics

• We will cover gravity in more detail later in the
  class but for now here are the important
  concepts

Newtons Law of Gravitation states that the force
of attraction F between two masses m1 and m2
whose dimensions are small with respect to the
distance r between them is given by the equation
below, where G is the Gravitational Constant
(6.67x10-11 m3kg-1s-2).
r
m2
m1
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Some Gravity Basics
For the case of a mass above the earth we can
re-write the previous equation using M as the
mass of the Earth, m as the mass of the object,
and R as the distance between the centers of the
objects
R
m
M
Force is related to mass by acceleration, and the
term g is known as the gravitational
acceleration. On this theoretical Earth gravity
would be constant, however, the Earths
ellipsoidal shape, rotation, irregular surface
relief and internal mass distribution cause
gravity to vary over its surface.
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Some Gravity Basics
The gravitational field can be defined in terms
of the gravitational potential, U
Gravitational acceleration g is a vector
quantity, having both magnitude and direction.
The gravitational potential U is a scalar, having
only magnitude. The first derivative of U in any
direction is the component of gravity in that
direction. Equipotential surfaces are those on
which the gravitational potential is constant,
ie. the geoid. The ocean surface is an
equipotential surface, and defines the geoid.
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Shape of the Earth

• The geoid is one way of defining the shape of the
  earth, but we can also use the ellipsoid. The
  earth is not a perfect sphere, it is an oblate
  spheroid
• We can approximate the shape of the earth by an
  ellipsoid
• Radius from center to equator 6378.16 km
• Radius from center to pole 6356.77 km
• Polar shortening of 1 part in 298.25 often
  referred to as flattening
• The geoid and the ellipsoid are not coincident as
  the earth is not homogeneous

From http//rst.gsfc.nasa.gov/Intro/Part2_1b.html
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Shape of the Earth
Similarly, a satellite orbiting the earth moves
up and down along its orbit as it is affected by
the same gravitational forces that produce the
geoidal surface.
From http//rst.gsfc.nasa.gov/Intro/Part2_1b.html
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Geoid
Sandwell et al, bathy workshop.
Large geoid low over India (100 m) mass
defficency A ship going from Darwin, Australia,
to the southern tip of India is going
downhill! Geoid highs over trenches old and
cold subducted slabs
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Satellite Altimetry

• Satellites do not carry accelerometers
• Gravity variations can be calculated from changes
  in the position (shifts in orbital height) of a
  satellite as it orbits due to variations in
  gravity.
• Tracking of radio signals (using Doppler shifts
  in frequency) from the satellite help to
  determine these variations
• Locating the position of the satellite with
  satellite laser ranging
• Measure the changing height of the surface (sea
  level with reference to the ellipsoid) with radar
  or laser altimetry

The presence of extra mass on the seafloor is to
cause a deviation of gravitational attraction
such that water above the seamount bunches up.
Sandwell et al, bathy workshop.
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Satellite Altimetry
The satellite uses a radar pulse to measure the
distance to the sea surface. Repeating this pulse
every 0.001 seconds allows the noise levels to be
reduced (waves etc). The difference between this
distance and that to the theoretical ocean
surface is the geoid anomaly. Note that each
height value represents an average of
observations taken during 1 second when the
satellite moves about 7 km over the ground.
Height precision is on the order of 3 cm.
Sandwell et al, bathy workshop.
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Gravity from Space
As the geiod (gravitational potential) and the
gravity field are related, the gravity field can
be calculated from a map of the geoid. This is
one of the first methods used to get a detailed
image of the seafloor. Deriving the gravity
field from space is only good for looking at
coarse detail, mainly in unexplored regions.
From http//rst.gsfc.nasa.gov/Intro/Part2_1b.html
Geoid and gravity maps of part of the Gulf of
Mexico
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Gravity from Space
From http//rst.gsfc.nasa.gov/Intro/Part2_1b.html
Tonga-Kermadec Trench, Louisville Seamount Chain
Southern Oceans around Antarctica
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Bathymetry
Sandwell et al, bathy workshop.
The obvious way to collect bathymetry data for
the oceans is using acoustic methods along sparse
ship tracks. The above map shows survey ship
tracks in the South Pacific at the scale of the
continental US. Making a map of the US using only
data from along these tracks would obviously be
very ineffective. A systematic survey of the
oceans by ships would take more than 200 years of
survey time at a cost of billions of U.S. dollars.
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Bathymetry from Space

• In contrast A complete satellite survey can be
  made in five years for under 100M. This has not
  yet been done, but we are part of the way there.
• Gravity is correlated with bathymetry at short
  wavelength
• By examining the correlation between bathymetry
  along sparse ship tracks and the corresponding
  satellite gravity values, a function can be
  derived to convert satellite gravity data to
  bathymetry where there are no ship tracks.
• This function varies depending on the geology
• Highly sedimented
• No sediment
• etc

From http//topex.ucsd.edu/marine_topo/text/topo.h
tml
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Bathymetry from Space
From http//topex.ucsd.edu/marine_topo/text/topo.h
tml
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Topography

• There are a number of sources of topography
  many are the domain of geographers, ie
• Maps (USGS, etc)
• Conventional Digital elevation models (DEM)
• Many of these are derived from contour maps
  (digitized).
• Popular GIS programs can display and analyze
  these.
• Geophysicists get involved when the technology
  gets a little more complicated. For example
• Imaging radar systems (for example the SIR-A, -B,
  and C flown on the Space Shuttle) typically use
  variations in the signal bounced back from the
  ground to create an image

From http//spaceplace.jpl.nasa.gov/en/kids/srtm_m
akemap3.shtml
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Imaging Radar
In this image, the color scene comes from a
Landsat image from the Sahara desert in NW Sudan.
The diagonal strip is from an image acquired from
SIR-A flown in Nov. 1981. Because the sand is dry
and has a low dielectric constant the radar waves
penetrate up to 3 m, imaging not just the sand,
but also the bedrock below it
From http//rst.gsfc.nasa.gov/Sect8/Sect8_7.html
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Radar Interferometry

• Radar imagery taken on two different dates can be
  compared, and the phase difference determined to
  calculate the distances to point targets. This
  was taken to the next level with the Space
  Shuttle Topography Mission (SRTM) in 2000.
• Rather than compare two images taken on different
  dates, two images were collected simultaneously
  from slightly different locations.
• What is phase difference?

• This is an example of the use of phase difference
  when collecting interferometric sidescan data.
• Two adjacent transducer arrays
• Both receive at the same time.
• As they are next to each other they are receiving
  basically the same signal
• However, the signal coming to array A may have
  traveled slightly further than that arriving at
  array B.
• This translates into a phase difference.
• The phase difference can be used to determine the
  angle from which the signal came.
• Combined with travel time, this tells us the
  distance to that point on the seafloor

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SRTM
A similar principle can be used from space, but
the transducers (antenna) have to be further
apart in this case 60 m.
A transmit antenna illuminates the terrain with a
radar beam which is scattered by the surface. Two
receive antennas with a fixed separation between
them (baseline) record the backscattered radar
echo from slightly different positions resulting
in two different radar images. The two signals
received at both ends of the baseline show a
phase shift due to different signal paths.
Through the calculation of the relationship
between target-receiver distances and the phase
difference one obtains elevation information
which can be turned into digital elevation models
and maps.
From http//spaceplace.jpl.nasa.gov/en/kids/srtm_m
ake2.shtml
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SRTM
http//www2.dlr.de/oeffentlichkeit/specials/sonder
seiten/srtm/srtm_folder_02.pdf
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SRTM
USGS G-TOP30 and digitized data
SRTM
Using this method, near global topography has
been generated at a pixel size of 1 arc-second
(approximately 30 m). So far, data at this
resolution is only available for the continental
US. For other regions the data is available at a
pixel size of 3 arc-seconds (90 m). The above
image compares 90 m SRTM data for the Papuan
Peninsula with the previously available 30
arc-second data (1 km grid size).
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References Used

• Basic gravitational theory
• Kearey, P., M. Brooks, and I. Hill, An
  Introduction to Geophysical Exploration.
• The geoid, satellite altimetry
• http//rst.gsfc.nasa.gov/Intro/Part2_1b.html
• Calculating bathymetry from satellite gravity
• http//topex.ucsd.edu/marine_topo/text/topo.html
• Smith, W. H. F. and D. T. Sandwell, Bathymetric
  prediction from dense satellite altimetry and
  sparse shipboard bathymetry, J. Geophys. Res.,
  99, 21803-21824, 1994.
• Sandwell, D.T., Gille, S.T., and W.H.F. Smith,
  eds., Bathymetry from Space Oceanography,
  Geophysics, and Climate, Geoscience Professional
  Services, Bethesda, Maryland, June 2002, 24 pp.,
  www.igpp.ucsd.edu/bathymetry_workshop.
• Imagaing Radar (and a small amount of
  interferometry).
• http//rst.gsfc.nasa.gov/Sect8/Sect8_7.html
• Radar Interferometry (SRTM mission)
• http//www2.jpl.nasa.gov/srtm/
• http//spaceplace.jpl.nasa.gov/en/kids/srtm_make2.
  shtml
• http//www2.dlr.de/oeffentlichkeit/specials/sonder
  seiten/srtm/srtm_folder_02.pdf