Electromagnetic Methods (EM)• Measurement of varying electromagnetic fields• Induced by transmitter antennas, recorded by receiver

antennas• Alternative measurement of subsurface conductivity• Advantage is no contact electrodes are required• EM surveys are faster, and can be carried out from aircraft• Useful in a wide range of applications:

– Mineral prospecting– Mapping of faults, shear zones– Detection/location of underground pipes, cables– Mapping of conductive contaminants – Mapping of (conductive) clays in agricultural studies

Electromagnetic Methods (EM)

Basic principle:

Transmitter current (Ip) generates primary field (P), which generates ground emf, leading to subsurface “eddy” currents.

Subsurface eddy currents then generate a secondary field (S), finally both P and S are measured by the receiver.

Sources of time-varying EM fields

Natural sources (“Magneto-telluric fields”, or MT)• Interaction of solar wind with ionosphere• From 10-5 Hz, up to 20 kHz• Low MT frequencies (.001 Hz to 1 Hz) used to

investigate upper mantle and lower crust• “Audio-magnetotelluric” (AMT) (1-20 kHz) to

investigate to 1 – 2 km depth• Good at detecting/resolving conductive layers• Poor where shallow section is very conductive

Sources of time-varying EM fields

Controlled MT source:

• Magnetotelluric signal is weak at certain frequencies, no control on direction

• Controlled source AMT (CSAMT) uses electric field bipolar transmitter, 10 m to 30 m in length

Sources of time-varying EM fields

Controlled MT source:

Sources of time-varying EM fields

“VLF” systems:• Source is provided by the US military for communication

channels• “Very Low Frequency” is actually 15 – 25 kHz (these are

high frequencies in geophysical EM)• VLF transmitters are located around the world• Consist of long, vertical wire carrying AC current• Signal from several stations is detectable in most places

around the world• Local variations in conductivity change the local orientation

of the VLF field

VLF systems

Primary field, P is horizontal

• where a conductor is present this changes (“tilts”) the total field (P + S)

• tilt angle survey will “crossover” over a conductor

Facing the transmitterSchematic view

VLF systems

Example of a tilt-angle survey

Sources of time-varying EM fields

Controlled source systems• Time-varying primary EM fields

generated by currents in a transmitter loop

• Receiver coil is physically separated 1 m – several hundred metres

• Reference signal provided to receiver by cable link

• Ground systems, airborne systems differ only in scale, principles are the same

Sources of time-varying EM fields

EM theory: basic quantities

- Dielectric permittivity

EM theory: time varying relationships

Time varying terms (these were assumed zero in the earlier part of this course)

EM theory: time varying relationships

EM theory: time varying relationships

Substituting:

Basic equations for propagation of EM fields

Notes on complex numbers … see course web pages, or your Math textbook

Complex numbers

Complex numbers

Complex numbers

Because:

Therefore:

Complex numbers

Complex numbers

Multiplication:

Division:

Complex numbers