GL2511 Geophysics

GL2511 Geophysics

This course aims to introduce students to some of the physics that underlies Earth’s processes, and show them how geophysical methods can be used to uncover Earth’s structure and processes. It builds on material they have covered in the 1st year of their degree, and will allow them to gain skills and knowledge that will be important for the rest of their degree and future careers.

Learning outcomes

By the end of this course students should be able to:

  • Describe the main forces operating in the Earth, their behaviour, interaction and geological significance.
  • Analyse and interpret basic geophysical data to make quantitative estimates of the Earth’s physical properties
  • Apply basic mathematical and coding skills to solving geophysical problems
  • Communicate geophysical concepts to a general and specialist audience
Teaching and learning

Each week of term covers a different topic, which can be seen in more detail below.

Due to the ongoing Covid-19 pandemic and government and university guidelines, teaching for this course took place online in the 2020-2021 academic year. Each week there were pre-recorded lectures to watch and engage with. Some of these included questions for students to have a go at, others may included a multiple choice quiz to test knowledge.

Two synchronous sessions took place each week, delivered via Blackboard Collaborate. Typically, the Monday session was a tutorial session, used to discuss the lecture material and work through any questions that have been set in the lectures. The Tuesday session was a practical class where we went through exercises, mainly in the form of jupyter notebooks. Some of these formed part of the summative assessment for the course, while others were for formative assessment.

In addition to jupyter notebook based assignments, which helped to develop coding skills, students in 2020-2021 completed two further assignments designed to give them experience at communicating scientific topics. The first of these was a 2-page earthquake information sheet, aimed at interested adults without any specialist knowledge, about the 2011 Tohoku, 2015 Nepal, or 2018 Palau earthquakes. For the final assignment, students produced either a pre-recorded talk using panopto or a scientific poster on an aspect of Martian geophysics.

Level:

2nd Year Undergraduate

When:

2nd half session

Teaching:

Lectures, tutorials, computer-based practicals

Course outline 2020-2021

  • Week 1: Planetary Structure and Tectonics

    Part 1 – How do planets move in the solar system? 

    • Kepler’s Law of Planetary Motion
    • Estimating mass from orbital parameters
    • Angular momentum and Moment of Inertia
    • Proving the Earth isn’t hollow/homogenous

    Part 2 – Earth Structure

    • Seismic waves and velocities
    • Travel time curves
    • Snell’s law, refraction and reflection
    • Discovery of the core, Moho, inner core
    • Names of seismic phases in the Earth
    • Global radial Earth models 
    • Properties of the core, mantle and crust

    Part 3 – Plate Tectonics 

    • The lithosphere and how it can be defined
    • Movements of plates on a sphere 
    • Reconstructing past plate motion from paleomag data
    • Types of plate boundaries
    • Constructive, destructive and conservative boundaries 
    • What drives plate motion
    • Triple junctions
    • Cratons
    • When did plate tectonics start

    Part 4 – Other planets 

    • Properties of the Moon, Planets, Jupiter’s Moons, Pluto and Charon
    • Seismology on the Moon and Mars
    • Ice tectonics on Europa

    Practical – Installing Anaconda and opening jupyter notebooks

    General skills – putting numbers into equations, rearranging equations

  • Week 2: Heat

    Taught by Dave Healy

    Part 1 – Heat fundamentals

    • Definitions of temperature, heat, density, thermal conductivity, heat capacity, specific heat capacity, heat flow, heat production
    • Heat sources and sinks
    • Heat transfer mechanisms

    Part 2 – Heat flow and geothermal gradients

    • Fourier’s law (heat flow)
    • Data sources for heat flow and geothermal gradients
    • Heat flow in oceans
    • Heat flow in continents
    • Steady state geothermal gradients
    • Time varying geothermal gradients
    • Adiabatic geothermal gradients

    Part 3 Radiogenic heat production

    • Radioactive decay series for U, Th, K
    • Radioactive decay laws
    • Distribution of U, Th and K in the crust and mantle 
    • Secular change

    Part 4 Synthesis 

    • Relationship with plate tectonics 
    • Magma transport (advection)
    • Metamorphic and igneous rocks
    • Geothermal energy

    Practical – Jupyter notebook on oceanic heat flow (plus ‘kitchen sink’ things to try at home) 

    General skills – Differential and Exponential equations 

  • Week 3: Mechanics

    Taught by Dave Healy

    Part 1 – Mechanics Fundamentals 

    • Definitions – displacement, velocity, acceleration
    • Geological examples of displacements, velocity and accelerations
    • Definitions – mass, density, force
    • Geological of density variations
    • Plate tectonic forces
    • Newton’s 2nd law
    • Body and surface forces

    Part 2 – Stress and Pressure

    • Pressure in the Earth
    • Stress in the Earth
    • Horizontal stress
    • Vertical stress
    • Stresses on a plane – normal and shear stress
    • Mohr plots

    Part 3 – Strain 

    • Types of deformation – translation, rotation, volume change, strain
    • Examples of strain in rocks
    • Longitudinal strains
    • Shear strain (changes in angle)
    • Principal strains
    • Volumetric strain

    Part 4 – Rheology

    • Definition
    • Elasticity 
    • Hooke’s law
    • Viscosity
    • Plasticity
    • Creep regimes in olivine
    • Rheology of real rocks (elasti-visco-plastic)

    Part 5 – Synthesis 

    • Elastic properties of minerals 
    • Mechanics of earthquakes
    • Mechanics of igneous intrusions 

    Practical – Jupyter notebook exercise on Young’s modulus, extras on cornflour and water rheology and Mohr’s circle jupyter notebook

  • Week 4 Faults and Fractures

    Taught by Dave Healy

    Part 1 –Failure modes

    • Uniaxial tests
    • Triaxial tests
    • Failure modes
    • Tensile failure
    • Shear failure

    Part 2 – Tensile Fractures

    • Tensile failure criterion
    • Examples of tensile failure in rock
    • Joints
    • Veins
    • Dykes

    Part 3- Shear Fractures (faults)

    • Shear failure in rock
    • Mohr-Coulomb shear failure criterion
    • Effective stress
    • Microcracking
    • Orientation with respect to stress
    • Types of fault (normal, reverse and strike slip)

    Part 4 – Strength and Friction

    • Definitions of strength 
    • Unconfined compressive strength
    • Cohesion
    • Peak differential stress in a triaxial test
    • Frictional strength

    Part 5 – Synthesis 

    • Recap of rock mechanics 
    • Types of faults
    • Rifts
    • Transform faults
    • Thrust faults

    Practical – Jupyter notebook on Mhor envelopes, paper fault models

    Assessment – Characterising rock properties from lab measurements (jupyter notebook)

  • Week 5: Earthquakes

    Part 1 Where are earthquakes and how big are they?

    • Types of earthquakes (tectonic, volcanic and tectonic tremor, slow slip)
    • Global seismicity – plate boundaries, intraplate 
    • Earthquake intensity
    • Earthquake magnitude
    • Seismic moment
    • Energy release in an Earthquake 
    • Earthquake frequency 

    Part 2 Earthquake focal mechanisms  (DH)

    • Fault/failure types, relationship to stresses
    • Stereographic projection of planes and lines
    • Displacement patterns around faults
    • 1st motion polarities
    • compression/tension quadrants
    • nodal/anti-nodal planes
    • Focal mechanisms at plate boundaries 
    • Non-double couple sources – explosions, volcanoes 

    Part 3 Induced Seismicity

    • Examples of induced seismicity 
    • Fracking 
    • Waste water injection
    • Hydrocarbon extraction

    Part 4 Earthquake Hazard

    • Earthquake prediction (and why it isn’t possible)
    • Probabilistic seismic hazard analysis
    • Earthquake early warning
    • Landslides from earthquakes
    • Tsunamis 
    • Fires/infrastructure damage

    Practical – Jupyter notebook – looking at 1st motion polarities and estimating focal mechanisms, relating to tectonic setting. 

  • Week 6: Passive seismology

    Part 1 – How do we measure ground motion?

    • Historical seismographs (and seismoscopes)
    • Digital seismometers
    • Deploying seismometers
    • Seismic networks
    • Citizen seismometers
    • Ocean Bottom seismometers
    • DAS and nodal arrays

    Part 2 – What can seismometers detect?

    • Seismic waves 
    • Frequency content and instrument type
    • Waves from earthquakes
    • Anthopogenic signals (explosions, football, strikes, pandemics)
    • Animal signals
    • Waves and weather
    • Attenuation

    Part 3 – Locating earthquakes

    • Seismic travel-time curves 
    • Seismic phases
    • Using S-P travel times to locate an earthquake
    • Why locate earthquakes

    Part 4 –Imaging the Earth

    • Receiver functions and H-K stacking 
    • Tomography (Body and surface wave)
    • Anisotropy and shear wave splitting
    • Case study: SE Canada

    Practical – What’s that wiggle quiz (jupyter notebook)

    Practical (assessed) – Jupyter notebook – looking at S-P travel times to locate an earthquake

    Assessment – 2-page info sheet on either the 2011 Tohoku, 2015 Nepal, or 2018 Palau earthquakes

  • Week 7: Seismic refraction and reflection

    Taught by Dave Cornwell

    Part 1 – Introduction and seismic refraction

    • Refracted and reflected waves
    • Snell’s law
    • Direct waves
    • Refracted waves theory
    • Refracted waves travel times
    • Refraction analysis 
    • Multiple and dipping layers
    • Examples: Finding water, aggregate quarry, Ethiopia

    Part 2 – Seismic reflection 

    • Reflected waves
    • Travel time equations 
    • Reflection theory 
    • Hyperbola properties
    • Normal moveout (NMO)
    • Using reflections to find velocity
    • Scanning
    • Stacking flattened reflectors

    Practical – Jupyter notebook – Seismic refraction

  • Week 8: Seismic acquisition and processing

    Taught by Dave Cornwell

    Part 1 – Seismic reflection acquisition 

    • Reflection acquisition history, aims and philosophy
    • Geological objectives
    • Acquisition environment and sources of noise
    • CMP stacking 
    • Land seismic sources
    • Vibroseis
    • Marine sources
    • Receivers

    Part 2 – Seismic reflection processing

    • Seismic processing aims
    • “Raw” seismic reflection data
    • Simplified processing procedure
    • Re-processing
    • Choosing processing parameters
    • Industry processing procedure
    • Processing software
    • 2D vs 3D processing
    • Pre-processing
    • Main processing 
    • Migration
    • Post-stack Kirchhoff migration
    • Migration aperture
    • Post-stack processing
    • Interpretation

    Practical (assessed) – Jupyter notebook – Seismic reflection data processing: velocity analysis and migration

  • Week 9: Magnetism

    Part 1 – Magnetics theory

    • What is a magnetic field
    • Magnetic dipole
    • Magnetic force
    • Magnetic potential 

    Part 2 – The Earth’s magnetic field

    • Origin of the Earth’s magnetic field
    • Geodynamo
    • Geometry of the magnetic field
    • Measuring the Earth’s magnetic field
    • Geomagnetic dipole field
    • International Geomagnetic reference field 
    • Secular variation
    • External magnetic field

    Part 3 – Magnetic surveying 

    • Why do magnetic surveying
    • Magnetic susceptibility 
    • Induced and remnant magnetisation
    • Magnetic anomalies
    • Magnetometers
    • Performing a magnetic survey
    • Marine and airborne surveys
    • Regional corrections 
    • Diurnal variation

    Part 4 – Interpreting magnetic anomalies

    • Magnetic anomalies
    • Reduction to pole 
    • Depth of the anomaly 
    • Anomalies of simple geometric bodies
    • Modelling anomalies

    Practical – Jupyter notebook – modelling anomalies of simple geometric bodies

  • Week 10 Gravity

    Part 1 –  Gravity theory and the Earth’s shape

    • Law of universal gravitation
    • Gravitational acceleration
    • Gravitational potential
    • Centrifugal acceleration
    • Rotational distortion 
    • International Gravity Formula
    • Geoid

    Part 2 – Measuring gravity 

    • Gravity anomalies
    • Satellite measurements
    • Rock densities
    • Absolute gravity measurements
    • Gravimeters
    • Conducting a gravity survey
    • Network ties
    • Gravity surveying at sea and in the air

    Part 3 – Gravity corrections

    • Gravity anomalies
    • Gravity data reduction 
    • Latitude correction 
    • International Gravity Formula
    • Drift correction
    • Tide correction
    • Free-air correction
    • Bouguer correction
    • Terrain correction
    • Eotvos correction

    Part 4 – Interpreting gravity anomalies

    • Gravity anomalies
    • Removing regional trends 
    • Depth of the anomaly 
    • Anomalies of simple geometric bodies
    • Non-uniqueness
    • Examples: Crustal thickness, subduction zones, mid-oceanic ridges

    Practical (assessed) – Jupyter notebook – Gravity corrections and interpretation

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