Lecture 7a - Electromagnetic Methods and Principles

Flashcards about the basic concepts of magnetic methods.

What fundamental principle underlies all electromagnetic (EM) methods?

Electromagnetic methods induce electrical currents in the subsurface using electromagnetic induction.

What parameters can EM methods estimate, and what insights do these provide?

EM methods estimate subsurface electrical conductivity or resistivity, providing insight into pore fluid chemistry and lithological characteristics.

What is a key advantage of EM methods over DC resistivity methods?

Unlike DC resistivity, EM methods do not require direct electrical contact with the ground through electrodes.

What capability do EM methods offer due to not needing physical contact with the ground?

Measurements can be made from remote platforms like helicopters, enabling efficient surveys over large areas.

What is one way to classify different types of EM methods?

Methods can be classified by whether they use artificially generated or naturally varying electromagnetic fields.

What is another way to classify different types of EM methods, relating to measurement?

Methods can be classified based on whether measurements are made in the frequency domain or the time domain.

What is VLF (Very Low Frequency) and what is it used for?

VLF uses distant radio signals for deep mapping of geological structures.

What is AMT (Audio Magnetotellurics) and what signal source does it rely on?

AMT uses electromagnetic signals from global thunderstorms to image deep geological structures.

What is CSAMT and how does it augment the AMT method?

CSAMT uses natural signals like AMT but adds an artificial source to cover frequency gaps.

What is HLEM and why is it a popular EM technique?

HLEM (Horizontal Loop EM) is a mobile and widely used technique.

What is TDEM/TEM and how does it differ from frequency domain methods?

TDEM/TEM (Time Domain EM) involves measuring the decay of the secondary field after the primary field is switched off.

In frequency domain EM methods, what type of current is used in the transmitter?

A constantly varying or AC electric current is driven through a wire coil.

What generates the primary magnetic field in EM methods?

The primary magnetic field is generated by the electric current in the transmitter coil.

What fundamental law of physics underpins electromagnetic induction?

According to Faraday's law, a varying magnetic flux will induce an electrical voltage, generating electrical current in a conductor.

What are secondary eddy currents in the context of EM methods?

Secondary eddy currents are induced within conducting materials in the subsurface.

Into what two components can the phase delay be split in frequency domain methods?

In phase and quadrature components.

In horizontal loop EM, how does coil separation affect the depth of investigation?

Wider coil separations allow for deeper measurements into the subsurface.

Besides coil separation, what other factor related to the coils affects the depth of investigation?

Coil orientation (vertical dipole or vertical coplanar) affects the effective depth of investigation.

What is 'skin depth' in the context of EM methods?

Skin depth is the depth at which an electromagnetic wave's amplitude decreases to about 37% of its surface magnitude.

How does frequency affect skin depth and imaging depth?

Higher frequencies result in smaller skin depths, allowing for shallower imaging.