university of south alabama last time · 2008-03-31 · seismic refraction - measures travel times...

$: UNIVERSITY OF SOUTH ALABAMA Last Time · 2008-03-31 · Seismic Refraction - Measures travel times of seismic waves refracted along a velocity contrast (propagation velocity and path$
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MAS 603: Geological OceanographyMAS 603: Geological Oceanography

Lecture 21: Lecture 21: Geophysics 1: GravityGeophysics 1: Gravity

UNIVERSITY OF SOUTH ALABAMA Last TimeLast Time

Extinctions

http://www.cartoonstock.com/newscartoons/cartoonists/for/lowres/forn441l.jpg

Geophysics

• Introduction (various techniques)• Gravity• Gravity anomalies

TodayToday’’s Agendas Agenda

Geophysical analysis is done when…

1) you can’t get direct access to the rocks2) you want to “model” a large area quickly3) you know what you are looking for

GeophysicsGeophysics

Geophysical analysis is done when…

1) you can’t get direct access to the rocks2) you want to “model” a large area quickly3) you know what you are looking for

… but every geophysical technique only “estimates” geology (lots of interpretation)


There are 2 scales of geophysical analysis:

1) “small-scale” (single well; shallow penetration seismic etc.)


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There are 2 scales of geophysical analysis:

1) “large-scale” (basin wide surveys)


http://www.seadolby.com/images/the-seafarer-images/seismic-boat.jpghttp://www.ucmp.berkeley.edu/fosrec/images/ONeillFig8.jpg

Proposed well

Ultimately what geophysics do Ultimately what geophysics do is to measure the intensity of is to measure the intensity of waves waves or forceor force fields as they fields as they pass through geological pass through geological materials.materials.

They are looking for They are looking for unpredicted variations or unpredicted variations or deviations from normal (i.e., deviations from normal (i.e., anomaliesanomalies))


http://miac.uqac.ca/MIAC/grav-1.jpg

Geological interpretations are Geological interpretations are made on the basis of these made on the basis of these anomaliesanomalies


http://www.geos.ed.ac.uk/homes/jru/Silverpitseismic.jpg

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)

So what kind of geophysics is So what kind of geophysics is there?there?

GeophysicsGeophysics•• SeismologySeismology -- Study of natural [from Study of natural [from

earthquakes] and manearthquakes] and man--induced seismic induced seismic waves waves

•• GravityGravity -- Study of variations in earth's Study of variations in earth's gravitational fieldgravitational field

•• Electrical MethodsElectrical Methods -- Use of electrical Use of electrical conductivity / resistance of earthconductivity / resistance of earth

•• ElectromagneticsElectromagnetics -- Study of inducedStudy of inducedelectromagnetic fieldselectromagnetic fields

•• MagneticsMagnetics -- Analysis of variations in Analysis of variations in earth's magnetic fieldearth's magnetic field

•• RadioactivityRadioactivity -- Study of natural and Study of natural and induced response to radioactivityinduced response to radioactivityhttp://www.laynewater.com/images/borehole_geophysics1.jpg

GeophysicsGeophysics• Seismology - Study of natural [from

earthquakes] and man-induced seismic waves

•• GravityGravity -- Study of variations in earth's Study of variations in earth's gravitational fieldgravitational field

• Electrical Methods - Use of electrical conductivity / resistance of earth

• Electromagnetics - Study of inducedelectromagnetic fields

•• MagneticsMagnetics -- Analysis of variations in Analysis of variations in earth's magnetic fieldearth's magnetic field

•• RadioactivityRadioactivity -- Study of natural and Study of natural and induced response to radioactivityinduced response to radioactivity

Passive methodsPassive methods

Measuring spatial variations of Measuring spatial variations of static or natural fields of forcestatic or natural fields of force

Lateral subsurface material Lateral subsurface material variations give rise to spatial variations give rise to spatial variations in the fieldsvariations in the fields

There is inherent ambiguity in There is inherent ambiguity in interpretations (multiple interpretations (multiple solutions)solutions)

GeophysicsGeophysics•• SeismologySeismology -- Study of natural [from Study of natural [from

earthquakes] and manearthquakes] and man--induced seismic induced seismic waves waves

• Gravity - Study of variations in earth's gravitational field

•• Electrical MethodsElectrical Methods -- Use of electrical Use of electrical conductivity / resistance of earthconductivity / resistance of earth

•• ElectromagneticsElectromagnetics -- Study of inducedStudy of inducedelectromagnetic fieldselectromagnetic fields

• Magnetics - Analysis of variations in earth's magnetic field

• Radioactivity - Study of natural and induced response to radioactivity

Active methodsActive methods

Measurement of wave field Measurement of wave field characteristics (i.e. travel times characteristics (i.e. travel times of elastic waves, and amplitude of elastic waves, and amplitude and phase of electromagnetic and phase of electromagnetic waves)waves)

Energy is introduced into Energy is introduced into subsurface subsurface -- source and detector source and detector controlled (less ambiguity)controlled (less ambiguity)

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CrustCrust: ~10: ~10––70 km thick, 70 km thick, felsicfelsic to mafic in to mafic in compositioncomposition

MantleMantle: ~2,800 km thick, ultramafic in : ~2,800 km thick, ultramafic in compositioncomposition

Outer coreOuter core: ~2,200 km thick, liquid iron: ~2,200 km thick, liquid iron

Inner coreInner core: ~1,500 km thick, solid iron: ~1,500 km thick, solid iron

Seismology uses the speed of seismic waves to resolve rock density/state

Geophysical TechniquesGeophysical Techniques

Seismic Tomography uses heat effects on seismic wave velocities to create 3-D images of Earth’s interior heat flow


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)

GeophoneGeophone


Seismic Stratigraphy uses wave reflections and refractions to resolve surfaces between rock types (Acoustic Impedance)


Electromagnetics (as well as electrical resistivity, ground(as well as electrical resistivity, ground--penetrating radar etc.) have useful application in the environmepenetrating radar etc.) have useful application in the environmental ntal field (e.g., to resolve underground chemical, physical, or field (e.g., to resolve underground chemical, physical, or lithological changes)lithological changes)


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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Magnetics uses a magnetometer to measure spatial variations in strength / intensity of Earth's magnetic field (lateral variations in susceptibility to magnetism)


Electromagnetics uses induction and detection of electromagnetic fields (force fields exerted by electrically charged particles in motion)

Idea behind metal detectorsIdea behind metal detectors


Well logging uses multiple techniques to detect lithological changes and oil/gas


Chalk Board

One of 4 known fundamental forces in nature (and the weakest of them)

Responsible for:

1) galaxies, stars and planets2) human weight problems3) apples falling on our head

GravityGravity

http://www.alexhughescartoons.co.uk/Home/uploaded_images/Gravity-748796.jpg

The first indirect use of gravimetrywas in the late 1800’s during the Great Land Survey of India.

Two survey methods were used:

1) Astronomical (angle of stars relative to the observer)

2) Plane Table

GravityGravity

The first indirect use of gravimetrywas in the late 1800’s during the Great Land Survey of India.

Two survey methods were used:

1) Astronomical (angle of stars relative to the observer)

2) Plane Table

GravityGravity

Kaliana Kalianpur

600 km

The two methods ended deviating by 150 m over the survey line (very significant)

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Plane table surveying uses a level to site along.

GravityGravity

http://www.nadar-gis.com/namibia-mapuserguide/cartography.htm

Plane table surveying uses a level to site along.

The Astronomical method needed to be positioned using a plumb line

GravityGravity

http://www.gingersrus.com/Greenhouse/resized/10PlumbBobResized.jpg


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.

GravityGravity


Isostasy depressed the heavier rocks and reduced the gravitational attraction of the mountains.

GravityGravity

Unfortunately, there are several considerations that must be made before you start using gravimetryto map out rock density.

GravityGravity

Unfortunately, there are several considerations that must be made before you start using gravimetryto map out rock density.

1) Radius of the Earth: the polar radius is flattened by 41km compared to the equatorial radius.

GravityGravity

Requires a latitude adjustment

g = 978,049 mGal (1+ 0.0052884Sin2Φ – 0.0000059Sin22Φ) where Φ is your latitude

http://www.nmm.ac.uk/server/show/nav.3115

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2) Elevation (free air correction): mathematically convert observed gravity values to ones that look like they were all recorded at the same elevation

GravityGravity

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

3) Topography/Rock Mass (Bougueranomaly): to isolate the effects of lateral 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.

GravityGravity

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·mGalρc = density of crustal rock (kg·m-3)h = elevation above mean sea level (m).

This adjustment also compensates for water

http://ess.nrcan.gc.ca/2002_2006/nrd/slavecomp/gravity_e.php

GravityGravity

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

GravityGravity

FREEAIR

BOUGUER

TOPOGRAPHY

By mixing which corrections you do, you can produce different gravity maps of whole countries…

GravityGravity

Or whole planets

GravityGravity

And even their moons!

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GravityGravityPositive anomaly (greater attraction than normal: higher rock density)

GravityGravityPositive anomaly (greater attraction than normal: higher rock density)

Negative anomaly (less attraction than normal: lower rock density)

BouguerBouguer GravityGravity

Consider the colour variation: positive anomalies dominate oceanic basins… why?

BouguerBouguer GravityGravity

Now consider our back yard

Applied GeophysicsApplied Geophysics

Distribution of salt domes and salt sheets south of Louisiana

http://www.glossary.oilfield.slb.com/files/OGL98094.gif

Applied GeophysicsApplied GeophysicsSalt is a low density material and should show a strong negative anomaly when present below the surface.

Chalk Board

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Applied GeophysicsApplied GeophysicsYou can also look for near-surface metal deposits

Upcoming Stuff

Wednesday: Side Scan Sonar Readings (Riikki)

Next Week:Monday : Seismology and BIFs (Kate)Wednesday: (Justin) Need to move….

Work on your Earth History Papers

university of south alabama last time · 2008-03-31 · seismic refraction - measures travel times...

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