5.3 Electromagnetic methods and their applications

Electromagnetic methods in geophysics use changing magnetic fields to induce currents in the ground. These methods help us understand what's beneath our feet by measuring how different materials respond to these fields. They're crucial for finding minerals, mapping groundwater, and studying environmental issues.

Geophysicists use two main types: frequency-domain and time-domain methods. Each has its strengths for different situations. By analyzing the data from these methods, we can create detailed maps and models of what's underground, helping us make informed decisions about resources and environmental management.

Electromagnetic Induction Principles

Faraday's Law and Electromagnetic Induction

Top images from around the web for Faraday's Law and Electromagnetic Induction

• 

  Magnetic Flux, Induction, and Faraday’s Law | Boundless Physics View original

  Is this image relevant?

• 

  Faraday’s Law — Electromagnetic Geophysics View original

  Is this image relevant?

• 

  Inductive sources — Electromagnetic Geophysics View original

  Is this image relevant?

• 

  Magnetic Flux, Induction, and Faraday’s Law | Boundless Physics View original

  Is this image relevant?

• 

  Faraday’s Law — Electromagnetic Geophysics View original

  Is this image relevant?

1 of 3

Top images from around the web for Faraday's Law and Electromagnetic Induction

• 

  Magnetic Flux, Induction, and Faraday’s Law | Boundless Physics View original

  Is this image relevant?

• 

  Faraday’s Law — Electromagnetic Geophysics View original

  Is this image relevant?

• 

  Inductive sources — Electromagnetic Geophysics View original

  Is this image relevant?

• 

  Magnetic Flux, Induction, and Faraday’s Law | Boundless Physics View original

  Is this image relevant?

• 

  Faraday’s Law — Electromagnetic Geophysics View original

  Is this image relevant?

1 of 3

• Electromagnetic induction is the process by which a changing magnetic field induces an electric current in a conductor
• Faraday's law of induction states that the electromotive force (emf) induced in a conductor is proportional to the rate of change of the magnetic flux through the conductor
  • The induced emf is given by: $\mathcal{E} = -\frac{d\Phi}{dt}$, where $\mathcal{E}$ is the induced emf and $\Phi$ is the magnetic flux
  • The negative sign in the equation indicates that the induced emf opposes the change in magnetic flux (Lenz's law)

Electromagnetic Induction in Geophysical Surveys

• In geophysical surveys, a primary electromagnetic field is generated by a transmitter, which induces secondary electromagnetic fields in conductive subsurface materials
  • The primary field is typically generated by a loop of wire or a long grounded wire
  • The secondary fields are generated by eddy currents induced in the conductive subsurface materials
• The secondary electromagnetic fields are measured by a receiver, and their characteristics (amplitude, phase, and frequency) provide information about the subsurface conductivity structure
  • The receiver measures the total field, which is the sum of the primary and secondary fields
  • The secondary field is typically much smaller than the primary field, requiring sensitive instrumentation and signal processing techniques
• The depth of investigation in electromagnetic surveys depends on the frequency of the primary field and the conductivity of the subsurface materials
  • Lower frequencies and more resistive subsurface materials result in greater depths of investigation
  • Higher frequencies and more conductive subsurface materials result in shallower depths of investigation

Frequency vs Time-Domain Methods

Frequency-Domain Electromagnetic (FDEM) Methods

• Frequency-domain electromagnetic (FDEM) methods use a continuous, sinusoidal primary field at a fixed frequency or a set of frequencies
  • Common FDEM systems include ground conductivity meters (EM31, EM34) and airborne systems (DIGHEM, RESOLVE)
  • FDEM systems typically operate in the frequency range of a few hundred Hz to a few hundred kHz
• In FDEM, the secondary field is measured in terms of its amplitude and phase shift relative to the primary field
  • The amplitude of the secondary field is proportional to the conductivity of the subsurface materials
  • The phase shift of the secondary field is related to the ratio of conductive to resistive properties of the subsurface materials
• FDEM methods are sensitive to the conductivity and magnetic susceptibility of the subsurface materials
  • The in-phase component of the secondary field is influenced by both conductivity and magnetic susceptibility
  • The quadrature component of the secondary field is primarily sensitive to conductivity

Time-Domain Electromagnetic (TDEM) Methods

• Time-domain electromagnetic (TDEM) methods use a pulsed primary field, which is abruptly switched off
  • Common TDEM systems include the TEM47, NanoTEM, and VTEM
  • The primary field is typically a square wave with a duration of a few milliseconds to a few seconds
• In TDEM, the secondary field is measured as a function of time during the off-time of the primary field
  • The decay rate of the secondary field is related to the conductivity of the subsurface materials
  • More conductive materials exhibit slower decay rates, while more resistive materials exhibit faster decay rates
• TDEM methods are more sensitive to the conductivity of the subsurface materials and less affected by magnetic susceptibility
  • The measured secondary field is primarily influenced by the conductivity of the subsurface materials
  • Magnetic susceptibility has a negligible effect on the TDEM response
• TDEM methods generally have a greater depth of investigation compared to FDEM methods
  • The depth of investigation in TDEM surveys can range from a few meters to several hundred meters, depending on the transmitter moment and the subsurface conductivity

Electromagnetic Applications in Geoscience

Mineral Exploration

• Electromagnetic methods are widely used in mineral exploration to detect and delineate conductive ore bodies, such as massive sulfide deposits, graphite, and nickel-copper sulfides
  • Massive sulfide deposits (e.g., volcanogenic massive sulfides) are highly conductive due to the presence of interconnected sulfide minerals
  • Graphite deposits are conductive due to the high electrical conductivity of graphite
  • Nickel-copper sulfide deposits (e.g., magmatic sulfide deposits) are conductive due to the presence of interconnected sulfide minerals
• Airborne electromagnetic (AEM) surveys are used for large-scale reconnaissance exploration and regional geological mapping
  • AEM surveys can cover large areas quickly and efficiently, providing a cost-effective means of identifying potential exploration targets
  • AEM data are often used in conjunction with other geophysical data (e.g., magnetic, radiometric) and geological information to guide exploration programs
• Ground-based electromagnetic surveys, such as moving-loop and fixed-loop methods, provide higher-resolution data for detailed target delineation and characterization
  • Moving-loop methods (e.g., SQUID, SMARTem) involve moving the transmitter and receiver along survey lines to obtain detailed profiles of the subsurface conductivity
  • Fixed-loop methods (e.g., UTEM, DEEPEM) involve a stationary transmitter loop and one or more receiver loops to detect and delineate conductive targets

Environmental Studies

• In environmental studies, electromagnetic methods are used to map the extent of groundwater contamination, identify buried waste and debris, and characterize the subsurface geology
  • Electromagnetic methods can detect conductive contaminant plumes, such as those associated with leaking underground storage tanks or landfill leachate
  • Buried metal objects, such as drums and pipes, can be identified by their strong electromagnetic response
  • The subsurface geology, including the presence of clay layers and aquitards, can be characterized based on the conductivity structure
• Borehole electromagnetic methods, such as downhole conductivity logging and cross-hole tomography, are used for near-surface investigations and monitoring of contaminant plumes
  • Downhole conductivity logging provides a detailed profile of the electrical conductivity along a borehole, which can be used to identify lithological boundaries and zones of contamination
  • Cross-hole tomography involves measuring the electromagnetic response between boreholes to create a 2D or 3D image of the subsurface conductivity distribution

Interpreting Electromagnetic Data

Data Presentation and Interpretation

• Electromagnetic data are typically presented as profiles, maps, or depth sections, depending on the survey design and objectives
  • Profiles show the variation of the electromagnetic response along a survey line, allowing for the identification of anomalies and lateral changes in conductivity
  • Maps show the spatial distribution of the electromagnetic response over the survey area, highlighting conductive zones and structural trends
  • Depth sections show the variation of conductivity with depth, providing insight into the vertical distribution of conductive targets and subsurface layers
• Conductive targets appear as anomalies with higher amplitude and/or a more pronounced phase shift compared to the background response
  • The amplitude of the anomaly is related to the conductivity and size of the target
  • The phase shift of the anomaly is related to the conductivity and geometry of the target
• The shape, size, and orientation of the anomalies provide information about the geometry and depth of the conductive targets
  • Narrow, high-amplitude anomalies typically indicate shallow, compact targets
  • Broad, low-amplitude anomalies typically indicate deep, extensive targets
  • The orientation of the anomaly can provide information about the strike and dip of the conductive target

Advanced Interpretation Techniques

• In FDEM data, the quadrature component is more sensitive to the conductivity of the subsurface, while the in-phase component is influenced by both conductivity and magnetic susceptibility
  • The quadrature component is often used to map the conductivity structure of the subsurface
  • The in-phase component can be used to identify magnetic materials and to estimate the magnetic susceptibility of the subsurface
• In TDEM data, the decay rate of the secondary field is related to the conductivity of the subsurface, with more conductive materials exhibiting slower decay rates
  • The early-time data are sensitive to shallow, conductive features
  • The late-time data are sensitive to deep, conductive features
• Forward modeling and inversion techniques are used to estimate the subsurface conductivity distribution from the measured electromagnetic data, taking into account the survey geometry and the physical properties of the subsurface materials
  • Forward modeling involves calculating the expected electromagnetic response for a given conductivity model and comparing it to the measured data
  • Inversion involves adjusting the conductivity model iteratively to minimize the difference between the calculated and measured data
  • Common inversion techniques include least-squares, regularized, and smooth-model inversions