http://foundation.aapg.org/students/undergraduate/weeks.cfm

Tim Carr - West Virginia University

3 Potential Fields Indirect Visualization Density and Magnetization Gravity and Magnetic Exploration Locate Faults and Other Structures Petroleum, Geothermal, Mineral and Water Resources Relatively Inexpensive Cover Large Areas Ground and Airborne

Mean value about 9.8 m/sec 2 = 1 g About 0.5% smaller at equator than poles Map unit = gal (for Galileo) = 1 cm/sec 2 Mean gravity = 980 gal Maps contoured in mgal = 10-6 g Modern gravimeters can detect.001 mgal variations (= 1 ppb) A gravimeter is essentially a spring balance. 4

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Centrifugal force (3400 mgal at equator) Variation of Earth s radius g(φ) = 9.78(1 + 0.0053sin 2 φ 0.0000058sin 2 2φ) 0.5% less at equator than pole = 5000 mgal At 45⁰ = 58 mgal/degree 7

Raw Gravity Latitude Corrections Altitude Mass between observer and sea level Thickness of Crust and Regional Variations Purpose is to identify features of geologic interest, not cancel everything out 8

9 Gravity decreases 0.31 mgal/m A correction for altitude only is a free-air correction However, there is also mass between the observer and sea level A correction for excess mass is a Bouguer correction

Gravity varies by latitude due to earth s equatorial bulge and centrifugal force Need altitude correction = 0.3 mgal/m = 3 x 10-7 g/m Altitude only correction = Free-Air Anomaly Map Correct for mass between you and sea level = Bouguer Anomaly Map May also need terrain corrections Correct for variations in thickness of crust = Isostatic Anomaly Map 10

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15 Earth with topography Geoid: Ideal sea-level shape of the earth Eliminate topography but keep the gravity Gravity is what determines orbits and leveling of survey instruments How do we know where the sea would be at some point inland? Datum: Ellipsoid that best fits the geoid Sphere: Globes and simple projections

16 Simple corrections for latitude and altitude Density = Lithology Can sense deep into crust Gravimeters are basically sensitive spring balances Fragile Prone to drift Discrete data points Labor intensive, low detail

Locations of anomaly sources are constrained Shapes of anomaly sources are constrained Sources cannot have geologically absurd properties Maximum depths are constrained Total masses constrained 17

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Contours at 5 mgal intervals, Color bands at 10 mgal intervals. 20

Contours at 5 mgal intervals, Color bands at 10 mgal intervals. 21

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GSA Today, January, 2015 26

27 Gravity GM-SYS Profile model of salt structure integrating seismic reflection, FTG gravity, and magnetic data (EarthExplorer, 2009).

Diamagnetism: weak repulsion from electron orbital motion, all materials Paramagnetism: moderate attraction due to unpaired electrons Ferromagnetism: strong attraction due to parallel alignment of electrons 28

29 Magnetic field of Earth = 40 micro-tesla = 40,000 nano-tesla (= 40,000 gamma) Varies from 25 to 70 micro-tesla Non-axial Not centered on the earth Varies over a human lifetime

Ferromagnetism is due to parallel electron magnetic moments Organization breaks down under heating Most materials lose magnetism around 500 C Can t pick up red hot iron with a magnet Responsible for most paleomagnetism Magnetic anomalies must be shallow Geomagnetic field has some other origin 30

Magnetic Minerals Magnetite Pyrrhotite Ilmenite Magnetite requires intermediate O activity Too much O Hematite Too little O Fe silicates Complex: Al favors biotite over Fe oxides No simple tie to lithology 31

Core is electrically conducting fluid Electric currents in core create magnetic field Motion of conducting fluid creates electric currents Currents generate magnetic field. Probable driver: convection Rotation affects flow and field orientation 32

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35 Units nanoteslas nt Main field 60,000 to 30,000 nt Core dynamo External field 10 s nt Diurnal, seasonal, ionization of atmosphere solar wind currents, storms Anomalous field 1000 s nt Magnetic susceptibility of crust

36 Corrections are complex and time variable No simple correlation with lithology Can t sense deep into crust because heat destroys magnetism Magnetism is electromagnetic phenomenon Instruments can be purely electronic Can record continuously Can be extremely detailed

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magnetic material below adds magnetism and creates positive anomaly magnetic rocks include iron ore, gabbro, granite 40

Removal of magnetic material from near surface causes negative anomaly (example is normal faulting) 41

Contours at 100-nT intervals, Color bands at 200 nt intervals. 42

Instruments measure the local gravity and magnetic field Data are processed to highlight local anomalies (residual maps) Bodies with anomalous density or magnetic susceptibility are modeled/interpreted Courtesy of ExxonMobil 43

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45 Residual gravity map (200km high pass) of the Eastern Mediterranean basin. Interpreted crustal type and key basement fabric.

Gravity, magnetic and seismic model for profile from Crete to Saudi Arabia. 46

47 Gravity maps Mechanical Instrument Discrete readings Less detail Can sense to great depths Simple corrections for latitude and elevation Density correlates with rock type Magnetic Maps Instruments are purely electronic Continuous readings Great detail Can sense only a few kilometers deep Complex corrections in time and space No simple correlation with rock type

Potential Fields Are Valuable Exploration Tools Gravity Magnetics Density and Magnetic Properties Some Ambiguity Low Cost Earth Model Valuable for Processing of Seismic Data 48

49 Assignments Complete All Homework Discuss Current Energy Events Read Today in Energy for Tuesday (3/31) at http://www.eia.gov/ Be Prepared to Discuss in Class - Wednesday Discussion Leader Connor Faccone Test Wednesday April 8 Material Through Monday (4/6) will be on Test