Geophysical Methods in Geology Lecture 1 Introduction - Community

Geophysical Methods in Geology Michaelmas Term Gravity & Magnetics

Prof. G. R. Foulger

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Lecture 1

•  •  •  • 

Introduction

The global gravity field

The spheroid and geoid

Instruments for measuring gravity

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Introduction

•  Involves passive potential fields only

•  Gravity simpler than magnetics

•  Geophysics: Interested in variations in gravity from place to place

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Introduction

•  Everything hinges on Newton’s Law:

Sir Isaac Newton (1642-1727) 4


Why does gravity vary from place to place?

–  First, because the Earth is not a sphere

–  It is an ellipsoid of revolution

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A sphere - all 3 axes equal

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An ellipsoid of revolution - 2 axes equal, 1 different

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The Earth is an oblate spheroid

–  flattened at the poles

–  so gravity is higher at the poles

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Why else does gravity vary from place to place?

–  Second, the mass distribution varies from place to place

Note that the density contrast is what controls the anomaly, not the absolute density of the body

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Global-scale gravity anomalies

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Definition:

–  The geoid is an equipotential surface corresponding to mean sea level.

–  On land it corresponds to the level that water would reach in canals connecting the seas.

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The geoid height (= “geoid anomaly”) varies because gravity varies from place to place as a result of variable mass & density distribution

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Global-scale geoid anomalies plotted as map

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Global-scale geoid anomalies

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Local-scale anomalies

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Measuring gravity

• 

Units

–  –  –  – 

• 

1 Gal (after Galileo) = 1 cm/s2

thus g (at Earth’s surface) ~ 103 Gals

anomalies measured in milliGals

1 mGal = 10-3 Gals = 10-5 m/s2

Gravimeters sensitive to ~ 0.01 mGal - 10-8 of Earth’s field

Usually relative gravimeters used

• 

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Measuring gravity

• 

Units

–  –  –  – 

• 

1 Gal (after Galileo) = 1 cm/s2

thus g (at Earth’s surface) ~ 103 Gals

anomalies measured in milliGals

1 mGal = 10-3 Gals = 10-5 m/s2

Gravimeters sensitive to ~ 0.01 mGal - 10-8 of Earth’s field

Usually relative gravimeters used

• 

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Gravity measuring instruments

• 

On land: Can be done using:

a)  relative gravimeters

•  • 

stable gravimeters

unstable gravimeters

b)  absolute gravimeters

•  • 

pendulums

falling masses

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Relative measurements: stable gravimeters

Work on the principle of a force balancing the force of gravity. Example: the Gulf gravimeter

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Relative measurements: unstable gravimeters

e.g. Worden gravimeter

Advantages

Disadvantages

need not lock the mass

may not be overturned - contains

open saucer of

desiccant - can spill

no power is needed for temperature compensation

has only small range (~ 60 mGal) - thus must be adjusted for each survey

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Relative measurements: LaCoste gravimeter

Advantages

wide range

Needs power to keep

at constant temperature. Uses a

Disadvantages

lot of charge & takes hours to warm up.

0.01 mGal sensitivity

very quick to use

mass must be clamped during transport

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•  Cunning mechanical devices

•  increases in g cause extension of spring

•  extension magnified by mechanical geometry

•  Examples: the Wordon and the LaCosteRomberg gravimeters

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Relative measurements: LaCoste gravimeter

Use:

1.  Put on charge overnight

2.  Set up and level

3.  Unclamp mass

4.  Adjust to centre meter pointer

5.  Make reading

6.  Clamp mass

7.  Pack away & move to next station

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Relative measurements: borehole gravimeter

•  Can be used to obtain density

of formations

•  Main problems:

–  temperature control

–  deviation from vertical

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Absolute measurements: pendulums

First done by Pierre Bouguer in 1749

L = pendulum length

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Absolute measurements: falling bodies

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Absolute measurements: processing data

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Lecture 2

•  •  •  •  • 

Instruments for measuring gravity (cont.)

Gravity surveys on land

Reduction of data

Bouguer & Free-Air anomalies

Examples

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• 

At sea: Can be done by:

a)  lowering meter onto sea floor

b)  onboard ship

Sea floor measurements:

–  0.1 mGal accuracy

–  remote control

–  ship must stop so very expensive� Gulf gravimeter, 1956

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•  onboard ship

–  0.2 mGal accuracy

–  ship accelerations up to 100,000 mGal

–  horizontal motion corrected gyroscopically

–  vertical by averaging e.g. over 5 min

–  only good up to sea state 4-ish

Shipboard Gravimeter (“KAIREI”)� 41


• 

In the air:

–  resolution claims:

•  • 

0.3 mGal @ 1 km for helicopters at ~ 50 knots

0.2-1 mGal @ 2 km for fixed-wing at ~ 100 knots

–  GPS helped a lot

–  potentially very economical

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• 

In space:

a)  involves measuring height of satellite above sea level by radar altimetry

b)  Skylab, SEASAT, Geosat and ERS 1 & 2

SEASAT

ERS 2

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ERS 2 flyby

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SEASAT system

Cross tracks

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The amazing results

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Gravity from space

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Gravity measurements on the Moon

•  Apollo 17 Traverse Gravimeter Experiment •  measured variations in gravity near TaurusLittrow landing site •  to obtain information about subsurface structure there •  gravity measurements made at 12 traverse stops on the 3 surface excursions •  results were radioed by the crew back to Earth 49

Comparative accuracies

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1.  Include station where absolute g is known. 2.  Station spacing must fit anomaly scale. 3.  Heights of all stations must be known or measured to ~ 10 cm. 4.  Latitudes must be known to 50 m. 5.  Topography affects the measurements - locate stations where little topography. 6.  Access - keep stations to existing roads or waterways if there are no roads. 7.  Design survey well. Computer processing cannot compensate for poor experiment design.

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Method 1.  Measure base station, 2.  Measure more stations, 3.  Remeasure base station approximately every two hours. 4.  Record in log book: –  time of measurement –  reading –  terrain

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Reduction of observations 1.  Drift - plot graph of measurement of base station throughout the day

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Reduction of observations 2.  Meter calibration –  provided by manufacturer, and converts scale units to mGal

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Reduction of observations 3. Latitude correction – needed because g increases with latitude because of flattening of Earth at poles

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Reduction of observations 4. Free-Air correction

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Gravity surveys

Reduction of observations 5.  Bouguer correction –  accounts for mass of rock between station and sea level

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Reduction of observations 6.  Terrain corrections –  both hills and valleys reduce gravity

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Reduction of observations 7.  Tidal correction –  normally absorbed in drift correction, but necessary for ultra-accurate surveys

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Reduction of observations 8.  Eötvös correction –  necessary on moving platform

V = speed in knots

α = vehicle heading

φ = latitude

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Reduction of observations 1.  Errors –  –  –  –  –  –  –  –  – 

reading error drift error meter calibration constant subtraction of gφ

Free-Air & Bouguer corrections - height needed

Bouguer & Terrain corrections - density needed

Terrain corrections - topography needed

Eötvös correction - speed & bearing needed

satellites - position of satellite 61

Examples

1.  A gravity survey of Iceland whole country surveyed 1967-1985, with assistance of US military

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Examples

1.  A gravity survey of Iceland –  –  –  –  –  – 

1610 stations, with 10-km spacings. Locations, elevations, gravity required 46 gravity base stations used tied to stations in the USA and Scandinavia both land and sea measurements needed a massive project - terrain corrections took years

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Examples

1.  A gravity survey of Iceland • 

Problems: –  bathymetry of surrounding sea floor needed for terrain and Bouguer corrections –  problems where ice caps exist –  problems with transport - roads limited, helicopters dangerous, snowmobiles on glaciers, blizzards 64

Examples

1.  A gravity survey of Iceland Example of a gravity station description sheet

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Examples

1.  A gravity survey of Iceland

Sea survey sections

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Examples

1.  A gravity survey of Iceland –  a massive project that took several decades, consumed several careers and cost several million dollars

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Examples

2.  Microgravity surveying of active volcano –  –  – 

study of changes in gravity at Pu’u O’o, Hawaii mass changes sought correlated with eruptive activity

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Examples

2.  Microgravity surveying of active volcano: Requirements –  very accurate height measurements, to ~ 3 cm - requires precise levelling GPS not accurate enough –  explicit corrections for Earth tides –  local value for Free-Air correction –  multiple readings at each site –  0.01 mGal precision sought 69

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Examples

2.  Microgravity surveying of active volcano: Results Location of gravity measurement sites

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Examples

2.  Microgravity surveying of active volcano: Results

Looking for non-correspondence between gravity and elevation changes

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Examples

2.  Microgravity surveying of active volcano: Conclusions –  Mass changes at Kilauea’s summit much smaller than the erupted mass –  erupted magma not supplied mainly from storage in the summit magma reservoir –  summit magma chamber is only a waypoint for magma en route to eruption.

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Lecture 3

•  •  •  •  • 

Gravity anomalies

Lithosphere & Asthenosphere

Isostasy

Rock densities

Examples

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Gravity anomalies

Anomalies 1.  The Bouguer anomaly –  represents gravity at sea level with everything above sea level stripped away

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Gravity anomalies

Anomalies 2.  The Free-Air anomaly –  represents gravity at sea level with everything above sea level flattened into an infinitesimally thin layer at sea level

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Isostasy

•  The study of how loads e.g. mountains, are compensated at depth

•  First studied in 1740

•  Experiment to measure vertical deviation due to the Andes

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Isostasy

•  Same observed at Himalaya

•  Suggested compensating mass beneath mountain chain

•  Application of Archimedes principle to the Earth

•  Concept of rigid lithosphere overlying viscous asthenosphere

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Rheology: How matter flows and changes shape

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Rheology: How matter flows and changes shape

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Isostasy

• 

2 basic theories suggested:

1.  Pratt theory

2.  Airy theory

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Isostasy

1.  Pratt theory

•  Depth of compensation constant

•  Density varies laterally

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Isostasy

2.  Airy theory

•  Depth of compensation variable

•  Density constant laterally

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Isostasy

•  Examples:

–  Airy compensation

•  lateral variation between oceanic and continental lithosphere

–  Pratt compensation

•  lateral variation within continental lithosphere

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Rock densities

•  •  •  •  • 

Density (ρ) varies little

Range ~ 1,500 - 4,500 kg/m3

= factor of 2 - 3

c.f. earthquake magnitudes, which vary by ~ 30 orders of magnitude!

Density increases with

–  –  – 

age

depth (compaction)

basicity (e.g., basalt denser than granite)

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Rock densities

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Rock densities

Common rock densities

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Rock densities

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7 basic methods of measuring density:

1.  Direct measurement - weigh sample in air & water

Issues: small samples may not be representative of whole formation

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Rock densities

Weight of sample in air = 1.0 kg

Weight of sample in water = 0.6 kg

What is its density?

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Rock densities

Answer: 2,500 kg/m3

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Rock densities

• 


2.  Using a borehole gravimeter

•  measure gravity at two heights in well

•  relevant equation involves 2 x the Bouguer term:

g1 − g2 = 0.3086h − 4G ρ h Issues: representative of whole formation

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Rock densities

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3.  Using a borehole density logger (gammagamma logger)

•  comprises Co60 source and Geiger counter

•  radiation returned depends on rock density

•  tool held against rock by spring

•  empirically calibrated

Issues: max penetration only

~ 15 cm - may be unrepresentative of formation

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Rock densities

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4.  Nettleton’s method

•  conduct gravity survey over topographic feature

•  reduce data using suite of densities

•  choose density that results in gravity looking least like topography

Issues: density of the topographic feature may be anomalous

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Rock densities

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5.  Rearranging the Bouguer equation

Issues: a lot of gravity data are required for accurate result

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Rock densities

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6.  Nafe-Drake curve

•  relates seismic velocity to density

•  empirical

Issues: only gives very rough idea of density

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Rock densities

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7.  Tables of rock density

•  a last resort if all else fails

Material Density (gm/cm3) Air Water Sediments Sandstone Shale Limestone Granite Basalts Metamorphic Rocks

~0 1 1.7-2.3 2.0-2.6 2.0-2.7 2.5-2.8 2.5-2.8 2.7-3.1 2.6-3.0

Issues: highly inaccurate

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Example

•  •  •  • 

Sulphur exploration, Orla, Texas

Gravity found to be most cost-effective geophysical technique

Strategy: reconnaissance coverage + detailed local gravity surveys

Densities from samples and boreholes

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Example

• 

dominant lithologies limestone, dolomite, sands, gypsum, salt, anhydrite

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Example

Reasons why gravity was suitable:

1.  Good lateral density contrasts - 2,500 - 3,000 kg/m3

2.  Measurable gravity anomalies

3.  Anomaly recognition

4.  Logistics and economy

5.  Deposits reliably associated with gravity anomalies

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however, proper interpretation needed to correctly assess the reserve associated with the anomaly

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Example

Procedure:

•  Reconnaissance surveys ~ 1 station/km2

•  Promising anomalies studied with detailed, highaccuracy surveys with ~ 10 stations/km2, accurate terrain corrections out to Hammer “D” ring

•  The promising anomalies were more sharply defined and zones of steep gradient detected, which can be enhanced by various analytical methods

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Example

Reconnaissance ~ 1 station/km2

Detailed - ~ 10 stations/km2

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Example

Density determination:

1.  Measurement of drill cuttings and cores

2.  Neutron borehole logs (a porosity well log which measures mainly hydrogen density) and lithologs

3.  Gamma-gamma logs

4.  Borehole gravimeter measurements

see

http://www.igcworld.com/PDF/sulfur_seg_ref8.pdf for discussion of advantages and disadvantages 101

Example

Borehole gravimeter results

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Example

Gravity results for “Prospect B”

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Example

Borehole gravimeter and neutron log density results for “Prospect B”

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Example

Final modelling results for “Prospect B”

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Lecture 4

•  Data processing

•  Data interpretation

•  Examples

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Data processing

1.  Removal of regional trend

–  the deeper the body the broader the trend –  we may be interested in the deeper trend, e.g., sedimentary basin thickness –  or the shallower trend, e.g., an ore body

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Data processing

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http:// dotearth.blogs.nytimes.com/ 2012/01/11/can-bettercommunication-of-climatescience-cut-climate-risks/? emc=eta1

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Data processing

1.  Removal of regional trend: Methods

a)  by eye b)  digitally –  1-dimensional

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Data processing

• 

Removal of regional trend: Methods

c)  2-dimensional Griffin’s method –  calculate average value of anomaly at surrounding points, and subtract from gravity value of point

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Data processing

• 

Removal of regional trend:

d)  Trend surface analysis –  fit low-order polynomial

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Data processing

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Removal of regional trend:

e)  Spectral analysis

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Data processing

e)  Spectral analysis : A global example

Short wavelengths

Long wavelengths

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Data processing

2.  Depth effect

–  –  – 

the shallower the body the higher the anomaly amplitude the shorter the wavelength

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Data processing

2.  Taking the vertical derivative (i.e. the gradient)

–  –  – 

gravity falls off as r2 1st derivative falls off as r3 2nd derivative falls off as r4

Taking the 1st or 2nd derivative: –  –  –  – 

enhances shallow bodies and suppresses deep ones removes the regional can reveal the sense of contacts can be used to calculate limiting depths (the “Smith rules”)

Disadvantage: Enhances noise 116

Data processing

2. 

The vertical derivative: Example - the Texas Gulf Coast

Gravity field

2nd derivative

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Data processing

2.  The vertical derivative:

–  Example of how it removes the regional

–  a cement field, Oklahoma

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Data processing

Other methods:

3.  Isostatic anomalies may be calculated: –  IA = BA – effect of root 4.  Maximum horizontal gradient –  can enhance near-vertical geological boundaries 5.  Upward & downward continuation –  can suppress or enhance local or regional signals 6.  Presentation –  Can make a huge difference to what can be seen and what not

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Data Interpretation

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Two basic approaches: 1.  Indirect (inverse) interpretation – 

use the data to draw conclusions about the causative body

2.  Direct (forward) interpretation – 

erect a model based on geologic knowledge, calculate the predicted gravity field, compare with observations & iterate the model to fit

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Data interpretation

Fundamental problem: Ambiguity

Many combinations of density & size give same gravity

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Data interpretation

1.  Indirect (inverse) interpretation: several approaches are available 1.  Maximum depth to top of body

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Data interpretation

1.  Indirect (inverse) interpretation: several approaches are available 2.  Excess mass: Gauss theorem Gravity flux from body

Gravity flux from body through Earth’s surface

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Data interpretation

1.  Indirect (inverse) interpretation: several approaches are available 2.  Excess mass: Gauss theorem In practice - grid gravity map, calculate

area x anomaly

for all boxes, and sum them

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Data interpretation

1.  Indirect (inverse) interpretation: 3.  Nature of upper corners of body •  May be critical to identify sedimentary basins

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Data interpretation

1.  Indirect (inverse) interpretation: several approaches are available 3.  Nature of upper corners of body •  May be critical to identify sedimentary basins

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Data interpretation

3.  Nature of upper corners of body: Example •  EW profile across northern England •  structure of alternating granite intrusions and sedimentary basins

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Data interpretation

1.  Indirect (inverse) interpretation: several approaches are available 4.  Approx. thickness : rearrange the SLAB FORMULA

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Data interpretation

1.  Indirect (inverse) interpretation: several approaches are available 4.  Approx. thickness : rearrange the SLAB FORMULA • 

gives reasonable rough estimate if anomaly fairly wide

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Direct (forward) interpretation

1.  Erect a model based on geology,

2.  Calculate the predicted gravity field,

3.  Compare with observations & iterate the model to fit

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Data interpretation

2.  Direct (forward) interpretation: formulae for simple shapes 1.  Buried sphere

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Data interpretation

2.  Direct (forward) interpretation: formulae for simple shapes 2.  Infinite horizontal cylinder

Imagine the point source extending in and out of the screen

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Data interpretation

2.  Direct (forward) interpretation: formulae for simple shapes 3.  Horizontal sheet

σ = mass/unit area of sheet

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Data interpretation

2.  Direct (forward) interpretation: formulae for simple shapes 4.  Infinite sheet

σ = mass/unit area of sheet

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Data interpretation

2.  Direct (forward) interpretation: formulae for simple shapes 5.  Infinite slab

= the SLAB FORMULA

ρ = density contrast of slab with surroundings

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Data interpretation

2.  Direct (forward) interpretation: formulae for simple shapes 6.  Vertical cylinder

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Data interpretation

2.  Direct (forward) interpretation: Computer modelling

•  Erect model

•  Compute

•  Adjust until it fits the gravity

•  Theoretically, and infinite number of models will fit

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Data interpretation

2.  Direct (forward) interpretation: Computer modelling

•  2D

•  2.5D

•  2.75D methods available

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Data interpretation

2.  Direct (forward) interpretation: 3D computer modelling •  Define body by series of contours and layer thicknesses

•  Computer calculates gravity anomaly of each and adds

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Lecture 5: Examples

1.  2.  3.  4.  5. 

A rift valley: The Owens Valley, California

An ore body, Canada

An oil field, Oklahoma

A salt dome, Gulf of Mexico

An active volcano: Long Valley caldera, California

–  –  –  – 

Tectonic background

The gravity field

Pre-processing and display of data

Problems in modelling & interpretation

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Examples

1.  Owen’s Valley, southern California – 

a sediment-filled, deep rift valley

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Examples

1.  Owen’s Valley, southern California – 

a sediment-filled, deep rift valley

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Examples

2. 

Pine Point lead-zinc ore body, Canada

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Examples

3. 

Altus Pool oil field, Oklahoma

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Examples

4. 

Salt dome in the Gulf of Mexico gravity map

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Examples

4. 

Salt dome in the Gulf of Mexico model

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Long Valley Caldera, California

•  Regional context

–  M > 3 earthquakes in California: 1990 1999

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Tectonic map

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•  Volcanic history

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•  Bishop Tuff •  Cataclysmic eruption of 600 km3 of highsilica rhyolite 760,000 years ago •  VEI 7/8

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•  Bishop Tuff 600 km3

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Current earthquake activity: M > 3 earthquakes:1978-1999 154


Earthquake seismometer network, 1997 155


Improving earthquake locations 156

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Three-dimensional tomography images the present-day seismic structure

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Gas emissions:

COSPEC SO2 monitoring at Pu’u O’o, Hawaii

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Tree kill from CO2

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Long Valley Caldera, California Gravity data

Complete Bouguer anomaly map, including terrain corrections

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Long Valley Caldera, California Gravity data

•  Pre-processing and display of data

–  Bouguer, Free-Air, Isostatic anomalies

–  wavelength filtering

–  directional filtering

–  1st and 2nd vertical gradients

–  horizontal gradients

–  upward & downward continuation

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Complete Bouguer anomaly map, no terrain corrections

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Complete Bouguer anomaly map, including terrain corrections

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Low-pass filtered Bouguer anomaly map, retaining wavelengths > 50 km

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High-pass filtered Bouguer anomaly map

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2nd vertical derivative of Bouguer anomaly map

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Free-Air anomaly map

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Application of directional filter to Free-Air anomaly

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Upward continuation of isostatic anomaly map to 5 km

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Contoured horizontal gradients of isostatic anomaly map

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Ridge crests on the horizontal gradient map

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Speculative structural cross section

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The primary problem: Is there a magma chamber, and if so how deep and large is it?

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Lecture 6: Magnetics

•  Introduction

•  Earth’s main field

•  Rock magnetism

•  Rock susceptibility

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Magnetics

• 

3 fundamental differences with gravity:

1.  Vector fields - cannot assume field vertical

2.  Magnetic poles attractive or repulsive

3.  Dependent on mineralogy, not bulk properties

•  • 

simple to make measurements

complex to understand & interpret

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Magnetics

•  The oldest geophysical profession!

•  But now usurped by younger methods e.g. seismic reflection

•  Still most widely used in terms of line-km measured each year

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Magnetics

•  Used for

– oil & minerals

• depth to basement

• sedimentary structures

• igneous bodies

• kimberlite pipes

• geothermal

– archaeology

• fire pits

• kilns

• disturbed earth

– hazardous waste

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Magnetics

Basic concepts

– Field lines around bar magnet

– “North-seeking” poles are +ve and are called “south poles”

– Poles always occur in pairs

–

+

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Magnetics

Parameters & variables

–  Magnetic force (F) = force between two poles

–  Intensity of induced magnetisation (I) = strength of field induced when body placed in external field

–  Susceptibility (k) = the degree to which a body can be magnetised

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Units

– Field strength: Newtons/Amperemetre (N/A-m), known as a Tesla (T)

– For surveying the nanoTesla (nT) is used, also known as a gamma (γ).

– 1 nT = 10-9 T

– Average strength of Earth’s field is ~ 50,000 nT

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Induced Magnetisation

•  When magnetic material, e.g. iron, is in a magnetic field, it will produce its own magnetization.

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The Earth’s main field

•  99% internal

•  simple dipole ~ 80%

•  rest can be modelled as dipoles around CMB

•  real source is convection in Earth’s outer core

Good example of how model can fit data but be unrelated to the truth

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Earth’s main field

•  How known origin in outer core?

•  Curie temperature

–  temperature above which materials lose their magnetisation = 578˚C for magnetite

–  reached at 5-10 km depth under continents

•  Before discovered outer core liquid (using seismology), Einstein, described problem as one of the 5 most important unsolved problems in physics. 183

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•  The external 1%

–  caused by electric currents in ionosphere

–  11-year periodicity (sunspot activity)

–  diurnal periodicity up to 30 γ (due to SUN)

–  monthly variation up to 2 γ (due to MOON)

–  random variations (magnetic storms) up to 1,000 γ (due to solar flares)

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•  The rest due to local, geological bodies

–  interesting to geologists

•  Examples

–  dykes

–  folded/faulted sills

–  lava flows

–  basic intrusions

–  metamorphic basement

–  ore bodies w/magnetite

–  archaeological targets e.g. fire pits, kilns

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Earth’s main field: Declination

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Earth’s main field: Declination

Change in declination (D), deg E 1590-1990

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Earth’s main field: Total intensity

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Earth’s main field: Total intensity

Change in total intensity (microTesla), 1590-1990

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Earth’s main field: Annual change in total intensity

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Earth’s main field: Reversals

•  •  •  • 

geologically sudden

no regular pattern

last one at 0.7 Ma

used for palaeomagnetic dating

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•  also used for dating the sea floor and deducing continental drift

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•  1st successfully modelled in 1995 using Cray supercomputer

•  presence of solid inner core key to stabilising field

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•  modelling a reversal of the Earth’s field

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http://www.youtube.com/watch? v=SJDcyyY01p4

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Summary: The history of understanding Earth’s main field

•  1600

–  W. Gilbert absolved pole star of responsibility

•  ~ 1700

–  Halley rejected magnetized surface rock because field changes with time

•  early 20th century

–  Einstein’s famous quote

•  1995

–  reversal modelled

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•  Instruments

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Magnetics

Rock magnetism: Kinds of magnetism 1.  Diamagnetism –  all electron shells full, electrons spin in opposite directions & magnetic effects cancel. –  in inducing field, opposite induced field produced, i.e., k -ve, e.g., quartzite, salt

quartzite

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Magnetics

Rock magnetism: Kinds of magnetism 2.  Paramagnetism –  electron shells incomplete, & magnetic effects don’t fully cancel. –  in inducing field, same-sense induced field produced, i.e., k +ve, e.g., 20Ca – 28Ni element series

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Magnetics

Rock magnetism: Kinds of magnetism 3.  Ferromagnetism •  paramagnetic minerals where groups of atoms align to make “domains” •  very large k •  only 3 mineral types: Iron, Nickel & Cobalt minerals •  3 kinds of ferromagnetism pyrite

nickeline

cobaltite

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Magnetics

Rock magnetism: Kinds of magnetism 3.  Ferromagnetism: 3 kinds a)  pure ferromagnetism •  directions of spin in domains aligned to inducing field - very large k. •  Fe, Ni, Co only

202

Magnetics

Rock magnetism: Kinds of magnetism 3.  Ferromagnetism: 3 kinds b)  antiferromagnetism •  directions of spin in domains alternate - very small k, e.g. hematite

hematite

203

Magnetics

Rock magnetism: Kinds of magnetism 3.  Ferromagnetism: 3 kinds c)  ferrimagnetism •  domain spin directions alternate, but one direction weaker - small but +ve k, e.g. magnetite

magnetite

204

68

Magnetics

How do rocks acquire their magnetism?

Types:

•  Chemical remnant

•  Detrital remnant

•  Isothermal remnant, e.g., lightning

•  Thermo-remnant

•  Viscous remnant

205

Magnetics

Induced vs. remnant magnetism

•  Induced can be calculated from direction and strength of Earth’s field

•  May know nothing of remnant magnetisation of a rock

–  can be measured using Astatic or Spinner magnetometer but often assumed to be zero

206

Magnetics

Magnetic susceptibility

•  The intensity of magnetization, I, is related to the strength of the inducing field, H, through a constant of proportionality k – the magnetic susceptibility.

I = kH •  analogous to density in gravity surveying 207

69

Magnetics Magnetic susceptibility

•  Highest in basic igneous rocks •  Lowest in sedimentary rocks

208

Magnetics: Instruments

1.  Observatory instruments

–  measure all three components of Earth’s field, i.e., N, E and vertical

209

Magnetic observatories

Leirvogar, Iceland

Eskdalemuir, Scotland

210

70

Global Observatories

211


2.  Magnetic balance

–  e.g., the torsion balance

–  now obsolete, subsequent to development of flux-gate magnetometer for detecting submarines in WW II

–  relative instruments

212

The Fluxgate Magnetometer

213

71


3.  Fluxgate magnetometer

–  –  –  –  –  –  – 

continuous measurements

2 coils wound in opposition around ferrite cores

current passed through primary

induced current in secondary measured

cancel out in absence of Earth’s field

Don’t if Earth’s field present

precision 0.5 – 1 γ

214



215



Disadvantages

not absolute instrument

liable to drift

not sufficiently accurate for modern work

216

72



–  3-component instrument used on the moon

217


4.  Proton precession magnetometer

–  nuclear resonance

–  protons precess around magnetic field

218



–  bottle containing protons & coil

–  50-100 oersted current

–  removed abruptly

–  precession frequency measured

–  takes 2-3 s

–  precision 1 γ (some 0.1 or 0.01 γ)

219

73



220



Advantages

Disadvantages

great sensitivity

each measurement takes several seconds - bad for aeromagnetic

measures total field

if large gradient, there will be gradient in bottle

absolute

do not work where AC e.g. under power lines

does not require levelling

no moving parts

221


5.  Overhauser effect proton magnetometer

•  • 

similar principle to proton-precession magnetometer but paramagnetic ions added

resonate at free-electron resonant frequency, in VHF radio frequency range

Advantages over proton-precession magnetometer

continuous effect - can make 8-10 readings per second

signal:noise ratio better

222

74


6.  Optical pump (alkali vapour) magnetometer

–  sub-γ sensitivity useful for sedimentary work

–  cell of helium, cesium, rubidium or some alkali-metal vapour

–  energy states of electrons affected by magnetic fields

–  excited by light from source of same material

223


6.  Optical pump (alkali vapour) magnetometer

–  –  –  –  –  – 

depopulation of energy states by light absorption unequal

repopulation to lower states by emission of energy equal

thus unequal populations in various energy states results

= optically-pumped state

gas more transparent in this state

0.005 γ precision!

224


7.  Magnetic gradiometer

• 

two magnetometers separated by some distance vertically or horizontally

Advantages

•  •  • 

diurnal variations corrections not needed

shallow sources with steep gradients accentuated compared with deep sources with gentle gradients

very popular for archaeological work

225

75


7.  Magnetic gradiometer

226


8.  SQUID system (Superconducting QUantum Interference Device)

•  high sensitivity (10-5 γ), 3-components

•  measures both direction & magnitude of Earth’s field

•  insensitive to frequency of changing field

227



–  operates at liquid helium temperatures

–  fairly big and thus less portable than other types

–  used to measure

•  magnetotelluric field

•  drift of Earth’s field

•  remnant and induced magnetization of samples in laboratory

228

76



229

Summary


– 3 kinds, 3 sub-kinds


•  Instruments

– 8 principal types

230


•  Magnetic surveys

•  Interpretation

231

77

Magnetic surveys

1.  On land

–  usually proton precession magnetometer

–  profiles or grids of points

–  tie back to base at 2-3 hr intervals or continually recording instrument:

•  • 

diurnal drift

magnetic storms

232

Magnetic surveys

1.  On land

–  at each station record time of day and reading

–  stay away from magnetic objects:

•  •  • 

wire fences

railway lines

roads

–  discard metal objects e.g. mobile phones, keys

–  take multiple readings to assess repeatability

233

Magnetic surveys

1.  Land surveys: reduction of observations

–  if magnetic storm - discard data

234

78

Magnetic surveys


–  diurnal correction - up to 100 γ

235

Magnetic surveys


–  regional trend - similar to gravity

–  other “gravity” corrections not needed for magnetic surveys

236

Magnetic surveys

2.  Air surveys

–  –  –  – 

most magnetic surveying

aeroplane or helicopter

good for inaccessible areas

usually proton precession magnetometer towed (the “BIRD”)

–  can also be on tail stinger, wing tips, or inboard

–  gradiometers can be used

237

79

Magnetic surveys

2.  Air surveys

–  –  –  – 

accuracy used to be limited by navigation

problem removed by GPS

previously done by radio beacon or air photos

air photos don’t work e.g. over jungle or sea

238

Magnetic surveys

2.  Air surveys

•  layout of flight lines to suit survey target & strike

•  if lines criss-cross can reduce errors by minimising cross-point differences

•  100s to 1,000s feet flying heights - should be constant

239

Magnetic surveys

2.  Air surveys

•  • 

0.5 and 0.25-mile flight-line spacings

Mattagami sulphide area, Quebec, Canada

240

80

Magnetic surveys

2.  Air surveys: Advantages:

–  –  –  – 

very cheap per line-km

can filter out shallow anomalies

this could also be a disadvantage

onboard processing - quasi-real-time results

241

Magnetic surveys

3.  Sea surveys

–  –  –  – 

magnetometer towed behind ship (“FISH”)

measurements at 10-15 m intervals

GPS

gradiometers can be used

242

Magnetic surveys

3.  Sea surveys

–  Often simultaneous with seismics

–  Ship’s course thus not optimised

243

81

Magnetics: Data display

• 

Pre-processing

–  remove outliers

244


• 

Pre-processing

–  remove outliers

245


•  • 

The most basic

plot raw data on flight lines

246

82


• 

Must decide on height

247


•  anomalies can merge

248


•  vertical profiles

249

83


•  contour maps

250

Magnetics: Interpretation

•  Problems

1.  strength may not reflect geological importance

251


• 

Problems

2.  ambiguity

much

greater than gravity

252

84


5 basic approaches:

1.  qualitative

2.  parametric

3.  forward modeling

4.  inverse methods

5.  data enhancement

253


•  5 basic approaches:

1.  qualitative

2.  parametric

3.  forward modelling



254


1.  Qualitative

•  much interpretation goes no further

•  depth to basement

•  gives thickness of sedimentary basins

255

85


1.  Qualitative

– 

dyke swarm, Ontario

256


1.  Qualitative

– 

dyke swarm, Ontario

257


1.  Qualitative

–  magnetic field does not equate to lithology

–  differences in mineralogy overwhelm differences in size

258

86



1.  qualitative

2.  parametric




259


2.  Parametric

–  deductions from curves give helpful starting point for computer modelling

–  sketching lines of force

260


2.  Parametric

–  formulae, e.g.,

z = 2w1/2 (sphere)

z = 2.05w1/2 (horizontal cylinder)

z = depth to centre

w1/2 = anomaly half-width

261

87


2.  Parametric

– 

Peter’s methods

262


2.  Parametric

–  Many other methods:

•  Vacquier method (vertical prisms)

•  Werner method (dykes)

•  Naudy method (dykes)

•  Hutchinson’s method (dykes, scarps & thin beds)

263



1.  qualitative

2.  parametric




264

88


3.  Forward modelling

–  Gay’s master curves

–  series of curves for dykes of various dips, strikes, intensity of magnetisation, depth & width

–  obsolete - computers now used

265


3.  Forward modeling

–  Computer modeling

–  various programs available

–  can be used for simple shapes, or for bodies of arbitrary shape–2D and 3D

–  because of great ambiguity, sensible to work with shapes as simple as possible

266

Magnetised spheres

At N pole

In N hemisphere

At equator

267

89



2.  Dykes

•  simple case of vertical dyke

268


3.  Forward modeling

2.  Dykes

–  variable orientations of dyke and Earth’s field

269



2.  Dykes

–  variable orientations of dyke and Earth’s field

270

90

Lecture 9


–  –  –  –  – 

qualitative

parametric

forward modelling

inverse methods

data enhancement

•  Examples

271


• 

5 basic approaches:

1.  qualitative

2.  parametric




272


4.  Inverse modelling

–  extremely difficult

–  must reduce number of variables

–  a few methods exist, but little used

273

91


• 

5 basic approaches:

1.  qualitative

2.  parametric




274


5.  Data enhancement

–  display data in different ways

–  may reveal features otherwise invisible

275



1.  reduction to pole

–  transform anomaly to one which would occur if magnetisation were vertical (like at the north pole)

–  simplifies anomalies & centres them over their causative bodies

276

92



2.  upward & downward continuation

•  upward continuation attenuates highfrequency, shallow anomalies

•  downward continuation enhances them, but blows up noise

277

Magnetics Interpretation


3.  Pseudogravity

–  if assumed magnetization distribution = density distribution, then magnetic field proportional to gradient of gravity field

–  rarely used because not usually a valid assumption

278

Examples

1.  2.  3.  4. 

Eastern Mediterranean

North Atlantic

Long Valley

sedimentary basins, ore bodies, archaeological

5.  Dixie Valley, Nevada

279

93

Magnetics: Examples

• 


Interpretation:

Bouguer gravity

magnetics

280

Magnetics: Examples

• 


Interpretation:

•  Gravity high in Mediterranean extending to Dead Sea rift suggests thin crust

Bouguer gravity

magnetics

281

Magnetics: Examples

• 


Interpretation:

•  Magnetics suggest transform nature of Dead Sea rift

•  left-lateral

Bouguer gravity

magnetics

282

94

Magnetics: Examples

• 


Interpretation:

•  Magnetics suggest Gulf of Suez associated with arcuate lineament

•  extending to Cyprean Arc

•  ancient shear zone?

magnetics

Bouguer gravity

283

Magnetics: Examples

• 

A spreading ridge

284

Magnetics: Examples

North Atlantic region

285

95

Magnetics: Examples

Plate tectonics on Mars?

Map of the magnetic field of Mars observed by the MGS satellite at a nominal 400-km altitude

286

Magnetics: Examples

• 

California

287

Magnetics: Examples

• 

Long Valley

288

96

Magnetics: Examples

• 

Tectonic trends

– 

Southern Cross, Western Australia

289

Magnetics: Examples

• 

Basin topography

– 

Bass Basin, Australia

290

Magnetics: Examples

• 

Iron ore deposit

– 

Northern Middleback Range, South Australia

291

97

Magnetics: Examples

• 

Kimberlite pipe

– 

Koffiefontein area, South Africa

292

Magnetics: Examples

• 

Archaeological prospecting

– 

human settlements

293

Magnetics: High-resolution aeromagnetic surveys

•  •  • 

A recent development

can detect sedimentary structure

requires very detailed surveys

294

98

High-resolution Magnetics

survey factor

line spacing

tie spacing

precision

system noise

navigation

flying height

conventional

high res

2-5 km

5-10

x line

< 1 km

3 x line

0.1 nT

2 nT

± 200 m

> 300 m

0.005 nT

< 0.2 nT

± 5 m

80 m

295


•  Processing

–  sophisticated methods needed


–  ground truthing critical

296


•  Faults in a geothermal area

–  Dixie Valley, Nevada

297

99


•  Tectonic history: –  December 1954 –  M ~ 7 earthquakes activated fault 80 km long with 5 m displacement.

298


•  Economic significance – geothermal energy exploitation

299


•  Location of Dixie Valley aeromagnetic survey

300

100


•  Dixie Valley survey specifications

301


•  Data after basic processing

302


•  Several sophisticated data processing methods applied –  reduction-to-pole –  gradient window –  anomaly separation –  depth estimation using horizontal gradients

303

101


•  Data reduced to pole

304


•  Shaded relief image horizontalgradients

305


•  Separation of reduction-to-pole data •  different depth components from matched filtering

306

102


•  Interpreted faults: –  < 100 m –  100 – 250 m –  250 m – 1.5 km

307

103

Geophysical Methods in Geology Lecture 1 Introduction - Community

Geophysical Methods in Geology Lecture 1 Introduction - Community

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