UNIVERSITY OF SOUTH ALABAMA
Last Time
Extinctions
MAS 603: Geological Oceanography
Lecture 21:
Geophysics 1: Gravity
http://www.cartoonstock.com/newscartoons/cartoonists/for/lowres/forn441l.jpg
Geophysics
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Introduction (various techniques)
Gravity
Gravity anomalies
Geophysics
Geophysical analysis is done when…
1) you can’t get direct access to the rocks
2) you want to “model” a large area quickly
3) you know what you are looking for
Geophysics
Geophysical analysis is done when…
1) you can’t get direct access to the rocks
2) you want to “model” a large area quickly
3) you know what you are looking for
Geophysics
There are 2 scales of geophysical analysis:
1) “small-scale” (single well; shallow penetration seismic etc.)
http://www.encapgroup.com/drilling/images/oil-drilling-well.jpg
Today’s Agenda
… but every geophysical technique only
“estimates” geology (lots of interpretation)
http://www.spwla.org/library_info/glossary/reference/glossd/images/glsp32f1.gif
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Geophysics
Geophysics
There are 2 scales of geophysical analysis:
Ultimately what geophysics do
is to measure the intensity of
waves or force fields as they
pass through geological
materials.
1) “large-scale” (basin wide surveys)
Proposed well
They are looking for
unpredicted variations or
deviations from normal (i.e.,
anomalies)
anomalies)
http://www.ucmp.berkeley.edu/fosrec/images/ONeillFig8.jpg
http://miac.uqac.ca/MIAC/grav-1.jpg
http://www.seadolby.com/images/the-seafarer-images/seismic-boat.jpg
Geophysics
Geophysics
Geological interpretations are
made on the basis of these
anomalies
So what kind of geophysics is
there?
• Seismology - Study of natural [from
earthquakes] and manman-induced seismic
waves
• Gravity - Study of variations in earth's
gravitational field
• Electrical Methods - Use of electrical
conductivity / resistance of earth
• Electromagnetics - Study of induced
electromagnetic fields
An incredible 3D seismic line
across a structurally complex area
of the North Sea (once thought to
be an impact site, now thought to
be a collapse structure)
• Magnetics - Analysis of variations in
earth's magnetic field
http://www.geos.ed.ac.uk/homes/jru/Silverpitseismic.jpg
http://www.laynewater.com/images/borehole_geophysics1.jpg
Geophysics
Passive methods
Measuring spatial variations of
static or natural fields of force
Lateral subsurface material
variations give rise to spatial
variations in the fields
There is inherent ambiguity in
interpretations (multiple
solutions)
• Seismology - Study of natural [from
earthquakes] and man-induced seismic
waves
• Gravity - Study of variations in earth's
gravitational field
• Electrical Methods - Use of electrical
conductivity / resistance of earth
• Electromagnetics - Study of induced
electromagnetic fields
• Magnetics - Analysis of variations in
earth's magnetic field
• Radioactivity - Study of natural and
induced response to radioactivity
• Radioactivity - Study of natural and
induced response to radioactivity
Geophysics
Active methods
Measurement of wave field
characteristics (i.e. travel times
of elastic waves, and amplitude
and phase of electromagnetic
waves)
Energy is introduced into
subsurface - source and detector
controlled (less ambiguity)
• Seismology - Study of natural [from
earthquakes] and manman-induced seismic
waves
• Gravity - Study of variations in earth's
gravitational field
• Electrical Methods - Use of electrical
conductivity / resistance of earth
• Electromagnetics - Study of induced
electromagnetic fields
• Magnetics - Analysis of variations in
earth's magnetic field
• Radioactivity - Study of natural and
induced response to radioactivity
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Geophysical Techniques
Seismology uses the speed of seismic
waves to resolve rock density/state
Geophysical Techniques
Seismic Tomography uses heat effects on seismic wave velocities to
create 3-D images of Earth’s interior heat flow
Crust:
Crust: ~10–
~10–70 km thick, felsic to mafic in
composition
Mantle:
Mantle: ~2,800 km thick, ultramafic in
composition
Outer core:
core: ~2,200 km thick, liquid iron
Inner core:
core: ~1,500 km thick, solid iron
Geophysical Techniques
Seismic Refraction - Measures travel times of seismic waves refracted
along a velocity contrast (propagation velocity and path controlled by
density and elastic moduli of the material through which waves pass)
Geophysical Techniques
Seismic Stratigraphy uses wave reflections and refractions to resolve
surfaces between rock types (Acoustic Impedance)
Geophone
Geophysical Techniques
Electromagnetics (as well as electrical resistivity, groundgroundpenetrating radar etc.) have useful application in the environmental
environmental
field (e.g., to resolve underground chemical, physical, or
lithological changes)
Geophysical Techniques
Gravity (Gravimetry) uses a gravitometer to measure spatial
variations in the strength of Earth’s gravitational field (due to
lateral variations in mass density)
http://www.nce.gc.ca/images/geoide6.jpg
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Geophysical Techniques
Magnetics uses a magnetometer to measure spatial variations in
strength / intensity of Earth's magnetic field (lateral variations in
susceptibility to magnetism)
Geophysical Techniques
Electromagnetics uses induction and detection of electromagnetic
fields (force fields exerted by electrically charged particles in
motion)
Idea behind metal detectors
Geophysical Techniques
Well logging uses multiple techniques to detect lithological changes
and oil/gas
Gravity
One of 4 known fundamental forces in nature (and the weakest of
them)
Responsible for:
1) galaxies, stars and planets
2) human weight problems
3) apples falling on our head
http://www.alexhughescartoons.co.uk/Home/uploaded_images/Gravity-748796.jpg
Chalk Board
Gravity
The first indirect use of gravimetry
was in the late 1800’s during the
Great Land Survey of India.
Gravity
The first indirect use of gravimetry
was in the late 1800’s during the
Great Land Survey of India.
Two survey methods were used:
Two survey methods were used:
1) Astronomical (angle of stars
relative to the observer)
1) Astronomical (angle of stars
relative to the observer)
2) Plane Table
2) Plane Table
Kalianpur
Kaliana
600 km
The two methods ended deviating by 150 m
over the survey line (very significant)
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Gravity
Gravity
Plane table surveying uses a level
to site along.
Plane table surveying uses a level
to site along.
The Astronomical method needed
to be positioned using a plumb
line
http://www.nadar-gis.com/namibia-mapuserguide/cartography.htm
http://www.nadar-gis.com/namibia-mapuserguide/cartography.htm
http://www.gingersrus.com/Greenhouse/resized/10PlumbBobResized.jpg
Gravity
Gravity
Isostasy depressed the heavier
rocks and reduced the
gravitational attraction of the
mountains.
The plumb bob was deflected by
the mass of the Himalayan
Mountains, but only 1/3 what it
should have been given the height
of the mountains and the suspected
density of the rocks.
Gravity
Unfortunately, there are several
considerations that must be made
before you start using gravimetry
to map out rock density.
http://www.nadar-gis.com/namibia-mapuserguide/cartography.htm
Gravity
Unfortunately, there are several
considerations that must be made
before you start using gravimetry
to map out rock density.
1) Radius of the Earth: the polar radius is
flattened by 41km compared to the
equatorial radius.
http://www.nmm.ac.uk/server/show/nav.3115
Requires a latitude adjustment
g = 978,049 mGal (1+ 0.0052884Sin2Φ – 0.0000059Sin22Φ) where Φ is your latitude
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Gravity
Gravity
3) Topography/Rock Mass (Bouguer
anomaly): to isolate the effects of lateral
2) Elevation (free air correction):
mathematically convert observed gravity
values to ones that look like they were all
recorded at the same elevation
variations in density on gravity, it is also
necessary to correct for the gravitational
attraction of the slab of material between the
observation point and the mean sea level.
FA = go - gt + (δg/δz) h
where:
go = observed gravity (mGal)
gt = theoretical gravity (mGal)
δg/δz = vertical gradient of gravity (0.3086
mGal per m)
h = elevation above mean sea level (m).
In General: requires an elevation adjustment of -0.3086 mGal per m of elevation above
sea level
BA = go - gt + (δg/δz - 2πGρc) h
where:
go = observed gravity (mGal)
gt = theoretical gravity (mGal)
δg/δz = vertical gradient of gravity (0.3086
mGal·m-1)
G = gravitational constant (6.672 x 10-11
m³·kg-1s-2 or 6.672 x 10-6 m²·kg-1·mGal
ρc = density of crustal rock (kg·m-3)
h = elevation above mean sea level (m).
Gravity
http://pubs.usgs.gov/of/2000/of00-304/htmldocs/chap02/index.htm
Although we tend to think of the corrections as “corrections” it is important to note
that they are doing different things and may actually provide different (useful)
information
Or whole planets
This adjustment also compensates for
water
Gravity
FREEAIR
Gravity
http://ess.nrcan.gc.ca/2002_2006/nrd/slavecomp/gravity_e.php
TOPOGRAPHY
By mixing which corrections you do, you
can produce different gravity maps of whole
countries…
BOUGUER
Gravity
And even their moons!
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Gravity
Gravity
Positive anomaly
(greater attraction
than normal: higher
rock density)
Positive anomaly
(greater attraction
than normal: higher
rock density)
Negative anomaly
(less attraction than
normal: lower rock
density)
Bouguer Gravity
Bouguer Gravity
Now
consider our
back yard
Consider the colour variation: positive anomalies dominate oceanic basins… why?
Applied Geophysics
Applied Geophysics
Salt is a low density material and should show a
strong negative anomaly when present below the
surface.
http://www.glossary.oilfield.slb.com/files/OGL98094.gif
Distribution of salt domes and salt sheets south of Louisiana
Chalk Board
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Applied Geophysics
Upcoming Stuff
You can also look for near-surface metal deposits
Wednesday: Side Scan Sonar Readings (Riikki)
Next Week:
Monday : Seismology and BIFs (Kate)
Wednesday: (Justin) Need to move….
Work on your Earth History Papers
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