40 Questions
What are the two fundamental principles that electromagnetic survey methods are based on?
Faraday's law of electromagnetic induction and Ampère's law
What is the magnetic flux 𝜙𝐵 proportional to?
The magnetic field in free space
What is the direction of the normal vector 𝑛^ in the equation for magnetic flux 𝜙𝐵?
Outward-pointing normal vector
What is the relationship between the magnetic flux through the surface bordered by the loop and the induced EMF in the loop?
The magnetic flux is directly proportional to the rate of change of the induced EMF
What is the purpose of the voltmeter in the example of a permanent magnet moving through a coil of wire?
To register a signal when the magnet is moving
What is the cause of the current flowing in the wire in the example of a permanent magnet moving through a coil of wire?
The electric field generated by the moving magnet
What is the relationship between the EMF and the current in a circuit, according to Ohm's law?
The EMF is equal to the current multiplied by the resistance
What is the main physical property of interest in electromagnetic induction methods?
Electrical conductivity
What is the primary physical property that electromagnetic methods are sensitive to?
Electrical conductivity
What is the unit of measurement for electrical conductivity?
Siemens/meter
What is the primary factor affecting the coupling between the source and receiver in a frequency domain horizontal loop survey?
The direction of the primary magnetic flux relative to the normal of the buried loop
What is the range of scales at which EM data can be acquired?
Centimeters to kilometers
What is the purpose of the two-stage process in sketching the expected responses in a frequency domain horizontal loop survey?
To sketch the characteristic curve and determine the relative amplitude of the in-phase and out-of-phase parts
What is the purpose of an electromagnetic survey?
To measure the electrical conductivity of the Earth's materials
What happens when the secondary field is in the same direction as the primary field?
The response is plotted as a positive value
What is the relationship between electrical conductivity and electrical resistivity?
They are inversely related
What is the unit of measurement for electrical resistivity?
Ohm-meters
What is the orientation of the primary and secondary fields when they pass through the receiver loop?
They point in opposite directions
What is the result when the receiver coil is over the buried loop?
The response is zero
What is the primary factor that affects the electrical conductivity of the Earth's materials?
All of the above
What is the location of the datum in a frequency domain horizontal loop survey?
At the midpoint between the transmitter and receiver coils
What is the energy source in an electromagnetic survey?
Either an electric field or changing magnetic field
Why is the response positive when both loops are to the left or to the right of the buried loop?
Because the primary and secondary fields are in the same direction
What is the purpose of the frequency domain horizontal loop survey?
To determine the response of a buried conductor to a time-varying magnetic field
What is the primary component of the magnetic field generated by a small loop transmitter?
Dipolar
What is the frequency range of electromagnetic induction transmitters?
10^1 Hz - 10^4 Hz
What type of transmitters are used in GPR surveys?
High-frequency transmitters
What is the purpose of the receiver coil in electromagnetic induction?
To record the voltage proportional to the rate of change of magnetic flux
What is the condition required for current to be generated in a loop?
The magnetic flux must be normal to the loop
What is the quantity that depends on the shape of the loop?
Inductance
Why is the buried body approximated by a wire coil with resistance and inductance?
To understand the coupling between the transmitter, buried body and receiver
What is the type of current variation in the small loop transmitters used in this course?
Sinusoidal
What is the reason for the minus sign in Faraday's law?
The magnetic field of the induced current opposes the change in flux
What is the condition for the dipole approximation to hold for a magnetic field of a closed loop source?
When the distance from the loop is more than five times its diameter
What is the primary magnetic field due to a frequency domain transmitter?
A magnetic field that varies sinusoidally in time
What is the magnetic moment of a loop?
m→=Ian^
What is the goal of using an inductive EM source in the context of circuit model for EM induction?
To locate a conductive body buried in a relatively non-conductive host material
What is the effect of the direction of motion and orientation of the magnet on the induced voltage?
The sign of the induced voltage changes
What is the effect of the speed of the magnet on the induced voltage?
The magnitude of the induced voltage increases
What is the comparison between the voltage induced in a four coil loop and a two coil loop?
The voltage induced in a four coil loop is larger than in a two coil loop
Study Notes
Electromagnetic (EM) Methods
- EM methods have been used in mineral exploration and engineering problems since the 1960s.
- EM systems vary in size, source functions, and receiver-transmitter configurations, making them scalable.
- EM data can be acquired at different scales, from centimeters (mine tunnels, structures) to kilometers (airborne).
Electromagnetic Survey
- A generic EM survey involves an energy source (electric field or changing magnetic field) and measures physical properties such as magnetic permeability, electrical permittivity, and conductivity.
- The data can be either electric or magnetic field values.
Conductivity
- Electromagnetic methods are sensitive to electrical conductivity (σ), which characterizes the ease of current flow through a material.
- Conductivity is measured in Siemens/meter (S/m).
- Electrical resistivity (ρ) is the inverse of conductivity, measured in ohm-meters (Ωm).
- Conductivity varies widely across different Earth materials, depending on factors such as rock type, porosity, and fluid/metal content.
EM Induction Principles
- EM induction methods rely on Faraday's law of electromagnetic induction and Ampère's law.
- Faraday's law states that a changing magnetic field induces an electromotive force (EMF) in a closed circuit.
- The magnetic flux (φB) that crosses a closed loop is proportional to the rate of change of magnetic flux through the circuit.
Magnetic Field and Current
- The magnetic field of a closed loop source is approximately that of a perfect magnetic dipole when observed far enough from the loop.
- The magnetic moment (m) of a loop is given by m = I A n^, where I is the current, A is the area, and n^ is the unit vector perpendicular to the plane of the loop.
- The magnetic field of a dipole is given by the magnetization multiplied by a geometric factor.
Circuit Model for EM Induction
- The circuit model is used to understand the coupling between the transmitter, buried body, and receiver.
- Eddy currents are generated in the buried body by changing magnetic flux passing through the body.
- The buried body can be approximated by a wire coil with resistance (R) and inductance (L).
Transmitter and Primary Field
- Small loop transmitters with current varying sinusoidally in time are used in EM induction.
- These transmitters operate at frequencies ranging from 10^1 Hz to 10^4 Hz.
- The primary magnetic field generated by the transmitter is approximately dipolar.
Receiver
- The receiver is typically a wire coil that measures the rate of change of magnetic flux through the loop.
- The voltage recorded in the receiver coil is proportional to the rate of change of magnetic flux.
Coupling between Transmitter and Buried Loop
- The coupling between the transmitter and buried loop depends on the geometry of the system.
- The closer the direction of the primary magnetic flux is to the normal of the buried loop, the better the coupling.
Measured Responses
- The measured responses in a frequency domain horizontal loop survey depend on the geometry of the system and the properties of the buried conductor.
- The response can be sketched by considering the geometry of the source and receiver and the knowledge of whether the conductor is good or poor.
This quiz covers the role of electromagnetic methods in mineral exploration and engineering problems, including their scalability and various configurations.
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