Electromagnetic Methods in Mineral Exploration

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Questions and Answers

What are the two fundamental principles that electromagnetic survey methods are based on?

Faraday's law of electromagnetic induction and Ampère's law (correct)

The law of magnetic permeability and electrical permittivity

Ampère's law and the law of electrical resistance

Faraday's law of electromagnetic induction and Ohm's law

What is the magnetic flux 𝜙𝐵 proportional to?

The electrical resistance of the circuit

The magnetic field in free space (correct)

The electromotive force in the circuit

The rate of change of magnetic flux through the circuit

What is the direction of the normal vector 𝑛^ in the equation for magnetic flux 𝜙𝐵?

Tangential to the loop

Outward-pointing normal vector (correct)

Perpendicular to the magnetic field

Inward-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 Signup and view all the answers

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 Signup and view all the answers

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 Signup and view all the answers

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 Signup and view all the answers

What is the main physical property of interest in electromagnetic induction methods?

Electrical conductivity Signup and view all the answers

What is the primary physical property that electromagnetic methods are sensitive to?

Electrical conductivity Signup and view all the answers

What is the unit of measurement for electrical conductivity?

Siemens/meter Signup and view all the answers

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 Signup and view all the answers

What is the range of scales at which EM data can be acquired?

Centimeters to kilometers Signup and view all the answers

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 Signup and view all the answers

What is the purpose of an electromagnetic survey?

To measure the electrical conductivity of the Earth's materials Signup and view all the answers

What happens when the secondary field is in the same direction as the primary field?

The response is plotted as a positive value Signup and view all the answers

What is the relationship between electrical conductivity and electrical resistivity?

They are inversely related Signup and view all the answers

What is the unit of measurement for electrical resistivity?

Ohm-meters Signup and view all the answers

What is the orientation of the primary and secondary fields when they pass through the receiver loop?

They point in opposite directions Signup and view all the answers

What is the result when the receiver coil is over the buried loop?

The response is zero Signup and view all the answers

What is the primary factor that affects the electrical conductivity of the Earth's materials?

All of the above Signup and view all the answers

What is the location of the datum in a frequency domain horizontal loop survey?

At the midpoint between the transmitter and receiver coils Signup and view all the answers

What is the energy source in an electromagnetic survey?

Either an electric field or changing magnetic field Signup and view all the answers

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 Signup and view all the answers

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 Signup and view all the answers

What is the primary component of the magnetic field generated by a small loop transmitter?

Dipolar Signup and view all the answers

What is the frequency range of electromagnetic induction transmitters?

10^1 Hz - 10^4 Hz Signup and view all the answers

What type of transmitters are used in GPR surveys?

High-frequency transmitters Signup and view all the answers

What is the purpose of the receiver coil in electromagnetic induction?

To record the voltage proportional to the rate of change of magnetic flux Signup and view all the answers

What is the condition required for current to be generated in a loop?

The magnetic flux must be normal to the loop Signup and view all the answers

What is the quantity that depends on the shape of the loop?

Inductance Signup and view all the answers

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 Signup and view all the answers

What is the type of current variation in the small loop transmitters used in this course?

Sinusoidal Signup and view all the answers

What is the reason for the minus sign in Faraday's law?

The magnetic field of the induced current opposes the change in flux Signup and view all the answers

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 Signup and view all the answers

What is the primary magnetic field due to a frequency domain transmitter?

A magnetic field that varies sinusoidally in time Signup and view all the answers

What is the magnetic moment of a loop?

m→=Ian^ Signup and view all the answers

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 Signup and view all the answers

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 Signup and view all the answers

What is the effect of the speed of the magnet on the induced voltage?

The magnitude of the induced voltage increases Signup and view all the answers

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 Signup and view all the answers

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.

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Description

This quiz covers the role of electromagnetic methods in mineral exploration and engineering problems, including their scalability and various configurations.