Lecture 4

• How do EM induction methods generate electrical currents?

  Using a transmitter coil to create alternating current
• What are the characteristics of the EM34 system?

  • Coils: 2 separate
  • Use: Crew needed
  • Penetration: 10-40m
• How does the VTEM Configuration work to measure the Earth's magnetic field?

  • Transmitter Loop generates a primary magnetic field
  • This induces eddy currents in the Earth's subsurface
  • Eddy currents generate a secondary magnetic field
  • Receiver Loop measures the secondary magnetic field
  • Magnetometer measures the Earth's ambient magnetic field
  • Data is used to map the Earth's subsurface
• How does GPR work?

  It transmits EM waves that reflect off subsurface structures
• What are the characteristics of the EM31 system?

  • Coils: 1 transmit, 1 receive
  • Use: Handheld
  • Penetration: 1-2m
• What are the factors affecting EM penetration depth and their impacts?

  Factors and impacts:

  • Source Frequency: Lower = Deeper
  • Resistivity: Higher = Deeper
  • Transmitter Power: Stronger = Deeper
• What information can be modeled using EM induction methods?

  Electricity flow (resistivity) and chargeability
• What is the relationship between magnetic fields and currents in EM induction?

  • Magnetic fields induce currents
  • Currents generate magnetic fields
  • This creates a repeating cycle
• How are the electric and magnetic field oscillations related in an electromagnetic wave?

  • The electric and magnetic field oscillations are perpendicular to each other
  • They are in phase with each other, reaching their maximum and minimum values at the same time
  • The oscillations propagate at the speed of light
• What are the characteristics of low resistivity zones in geothermal exploration?

  • Indicate presence of geothermal fluids
  • Found in ultra-deep reservoirs
  • Represented as blue areas in diagrams
  • Result from hot, saline fluids
• What are the key differences between resistivity and chargeability models?

  • Resistivity Models: Indicate resistance to electrical current flow.
  • Chargeability Models: Indicate storage capacity of electrical energy.
• How do resistivity and chargeability help in distinguishing materials?

  • Higher resistivity: Dry rocks
  • Lower resistivity: Wet, saline rocks
  • Higher chargeability: Conductive minerals
  • Lower chargeability: Non-conductive rocks
• What are the benefits of airborne EM surveys?

  • Faster coverage than land surveys
  • Cheaper per unit area
  • Can access remote locations
  • No need for ground electrodes
• What are the main differences between EM and seismic methods in terms of measurement?

  • EM Methods:
  • Measures electrical conductivity
  • Detects conductors directly
  • Sensitive to hydrocarbons in marine environments
  • Seismic Methods:
  • Measures elastic wave velocity
  • Detects changes through reflection
  • Less sensitive to hydrocarbons
• What are the key characteristics of direct and reflection arrivals in CMP surveys?

  • Direct arrival: Straight line, gradient 1/v
  • Reflection arrival: Hyperbolic curve, indicates subsurface reflections
• What is the equation for EM wave velocity?

  $v =$$c / \sqrt{\epsilon \mu}$
• What are the two main types of anomalies shown in the graph?

  • Hyperbolic anomaly from point reflector
  • Linear continuous anomaly from planar reflector
• What do EM induction surveys provide regarding subsurface features?

  • Subsurface models
  • Changes in electrical properties
  • Reveal hidden features
• What do the induced currents in subsurface rocks create?

  A secondary magnetic field detected by the receiver coil
• What are the characteristics of the Loupe system?

  • Coils: Single continuous
  • Use: High EM interference
  • Penetration: ~40m
• What does the phase angle (ϕ) indicate in EM induction?

  It identifies good conductors
• How does GPR detect subsurface features?

  • Measures reflected electromagnetic (EM) waves
  • Analyzes the time it takes for waves to return
  • Creates images based on wave reflections
• What are the key differences between the transmitter and receiver components in the radar system?

  • Transmitter generates and sends out the radar signal
  • Receiver processes the reflected radar signal that returns
• How is the time for reflected waves to return related to depth?

  It is directly proportional to the depth
• What are the key differences between constant offset and common midpoint surveys?

  • Constant Offset:
  • Fixed electrode spacing
  • Parallel measurement lines
  • Simple data profile
  • Common Midpoint:
  • Variable spacing
  • Converging lines
  • Complex analysis
• What are the key components of the CSEM system described in the image?

  • Horizontal EM component
  • Frequency domain
  • Long offset
  • Continuous pulsing
  • Moving source
  • Strong signal
  • Reduced resolution due offset
• What are the main components of GPR methods?

  • Transmitter on the surface
  • EM wave generation
  • Reflection from subsurface structures
  • Measurement of return time
• What are the key features of EM anomalies in mineral exploration?

  • Conductive minerals like metallic ores
  • Show up as strong EM anomalies (red areas)
  • Best detected with lower frequency surveys
  • Indicate low resistivity
• What components are involved in the basic setup of EM induction methods?

  • Transmitter coil
  • Receiver coil
  • Galvanometer
  • Alternating current