Last Time. Today s s Agenda. Geophysics. Geophysics. Geophysics. MAS 603: Geological Oceanography. Lecture 21: Geophysics 1: Gravity

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UNIVERSITY OF SOUTH ALABAMA Last Time MAS 603: Geological Oceanography Extinctions Lecture 21: 1: http://www.cartoonstock.com/newscartoons/cartoonists/for/lowres/forn441l.

what you are looking for 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 but every

1 UNIVERSITY OF SOUTH ALABAMA Last Time MAS 603: Geological Oceanography Extinctions Lecture 21: 1: Today s s Agenda Introduction (various techniques) anomalies 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 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 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.) 1

There are 2 scales of geophysical analysis: 1) large-scale (basin wide surveys) Proposed well Ultimately what geophysics do is to measure

They are looking for unpredicted variations or deviations from normal (i.e., anomalies) http://miac.uqac.ca/miac/grav-1.jpg http://www.ucmp.

jpg Geological interpretations are made on the basis of these anomalies An incredible 3D seismic line across a structurally complex area of

Seismology - Study of natural [from earthquakes] and man-induced seismic waves - Study of variations in earth's gravitational field

Magnetics - Analysis of variations in earth's magnetic field http://www.geos.ed.ac.uk/homes/jru/silverpitseismic.jpg http://www.laynewater.

jpg Radioactivity - Study of natural and induced response to radioactivity Passive methods Measuring spatial variations of static or

interpretations (multiple solutions) Magnetics - Analysis of variations in earth's magnetic field Radioactivity - Study of natural and

response to radioactivity Active methods Measurement of wave field characteristics (i.e. travel times of elastic waves, and amplitude and

Analysis of variations in earth's magnetic field Radioactivity - Study of natural and response to radioactivity 2

2 There are 2 scales of geophysical analysis: 1) large-scale (basin wide surveys) Proposed well Ultimately what geophysics do is to measure the intensity of waves or force fields as they pass through geological materials. They are looking for unpredicted variations or deviations from normal (i.e., anomalies) Geological interpretations are made on the basis of these anomalies 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 there? Seismology - Study of natural [from earthquakes] and man-induced seismic waves - 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 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 - 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 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 man-induced seismic waves - 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 2

Seismology uses the speed of seismic waves to

heat effects on seismic wave velocities to create

~10 70 km thick, felsic to mafic in composition

composition Outer core: : ~2,200 km thick, liquid

$Seismic Refraction - Measures travel times of$ (propagation velocity and path controlled by

$reflections and refractions to resolve surfaces$ between rock types (Acoustic Impedance) Geophone

resistivity, ground- penetrating radar etc.

lithological changes) (Gravimetry) uses a

3 Seismology uses the speed of seismic waves to resolve rock density/state Seismic Tomography uses heat effects on seismic wave velocities to create 3-D images of Earth s interior heat flow Crust: : ~10 70 km thick, felsic to mafic in composition Mantle: : ~2,800 km thick, ultramafic in composition Outer core: : ~2,200 km thick, liquid iron Inner core: : ~1,500 km thick, solid iron 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) Seismic Stratigraphy uses wave reflections and refractions to resolve surfaces between rock types (Acoustic Impedance) Geophone Electromagnetics (as well as electrical resistivity, ground- penetrating radar etc.) have useful application in the environmental ntal field (e.g., to resolve underground chemical, physical, or lithological changes) (Gravimetry) uses a gravitometer to measure spatial variations in the strength of Earth s gravitational field (due to lateral variations in mass density) 3

Magnetics uses a magnetometer to measure spatial variations in strength / intensity of Earth's

induction and detection of electromagnetic fields (force fields exerted by electrically charged

lithological changes and oil/gas One of 4 known fundamental forces in nature (and the weakest of

on our head Chalk Board http://www.alexhughescartoons.co.uk/home/uploaded_images/-748796.

Kaliana Kalianpur Two survey methods were used: Two survey methods were used: 600 km 1)

4 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 detectors Well logging uses multiple techniques to detect lithological changes and oil/gas 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 Chalk Board The first indirect use of gravimetry was in the late 1800 s during the Great Land Survey of India. The first indirect use of gravimetry was in the late 1800 s during the Great Land Survey of India. Kaliana Kalianpur Two survey methods were used: Two survey methods were used: 600 km 1) Astronomical (angle of stars relative to the observer) 2) Plane Table 1) Astronomical (angle of stars relative to the observer) 2) Plane Table The two methods ended deviating by 150 m over the survey line (very significant) 4

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.

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

nadar-gis.com/namibia-mapuserguide/cartography.htm Unfortunately, there are several considerations that must be made before you start using gravimetry to map out rock density.

5 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 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. Unfortunately, there are several considerations that must be made before you start using gravimetry to map out rock density. 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. Requires a latitude adjustment g = 978,049 mgal ( Sin 2 Φ Sin 2 2Φ) where Φ is your latitude 5

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

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 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.

usgs.gov/of/2000/of00-304/htmldocs/chap02/index.

6 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 ( mgal per m) h = elevation above mean sea level (m). In General: requires an elevation adjustment of 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 ( mgal m-1) G = gravitational constant (6.672 x m³ kg-1s-2 or x 10-6 m² kg-1 mgal ρc = density of crustal rock (kg m-3) h = elevation above mean sea level (m). 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 FREEAIR TOPOGRAPHY By mixing which corrections you do, you can produce different gravity maps of whole countries BOUGUER And even their moons! 6

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

rock density) Bouguer Bouguer Now consider our back yard Consider the colour variation: positive

Applied Applied Salt is a low density material and should show a strong negative anomaly when

7 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 Bouguer Now consider our back yard Consider the colour variation: positive anomalies dominate oceanic basins why? Applied Applied Salt is a low density material and should show a strong negative anomaly when present below the surface. Distribution of salt domes and salt sheets south of Louisiana Chalk Board 7

8 Applied You 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 8

Note that gravity exploration is different to seismic exploration in the following way:

224B3 Other factors that cause changes in g and need to be corrected Note that gravity exploration is different to seismic exploration in the following way: In a seismic survey, the travel time depends